CNL-18-044, Tennessee Valley Authority Response to NRC Request for Additional Information Related to Topical Report TVA-NPG-AWA16, TVA Overall Basin Probable Maximum Precipitation.
| ML18117A225 | |
| Person / Time | |
|---|---|
| Site: | Browns Ferry, Watts Bar, Sequoyah  |
| Issue date: | 04/19/2018 |
| From: | James Shea Tennessee Valley Authority |
| To: | Document Control Desk, Office of Nuclear Reactor Regulation |
| References | |
| CNL-18-044 | |
| Download: ML18117A225 (73) | |
Text
{{#Wiki_filter:Tennessee Valley Authority , 1101 Market Street, Chattanooga, Tennessee 37402 CNL-18-044 April 19, 2018 10 CFR 50.4 ATTN : Document Control Desk U.S. Nuclear Regulatory Commission Washington , D.C. 20555-0001 Browns Ferry Nuclear Plant, Units 1, 2, and 3 Renewed Facility Operating License Nos. DPR-33, DPR-52, and DPR-68 NRC Docket Nos. 50-259, 50-260 , and 50-296 Sequoyah Nuclear Plant, Units 1 and 2 Renewed Facility Operating License Nos. DPR-77 and DPR-79 NRC Docket Nos. 50-327 and 50-328 Watts Bar Nuclear Plant, Units 1 and 2 Facility Operating License Nos. NPF-90 and NPF-96 NRC Docket Nos. 50-390 and 50-391
Subject:
Tennessee Valley Authority Response to NRC Request for Additional Information Related to Topical Report TVA-NPG-AWA16, "TVA Overall Basin Probable Maximum Precipitation and Local Intense Precipitation Analysis, Calculation CDQ0000002016000041" References : 1. TVA Letter to NRC , "Request for Review and Approval of Topical Report TVA-NPG-AWA16, 'TVA Overall Basin Probable Maximum Precipitation and Local Intense Precipitation Analysis , Calculation CDQ0000002016000041 '," dated September 20, 2016 (ML16264A454)
- 2. Electronic Mail from NRC to TVA, "Request For Additional Information Related to TVA Fleet Topical Report TVA-NPGAWA16 (EPIC : L-2016-TOP-0011 )," dated February 23, 2018 (ML18057A637)
By letter dated September 20, 2016 (Reference 1), Tennessee Valley Authority (TVA) submitted topical report TVA-NPG-AWA16, "TVA Overall Basin Probable Maximum Precipitation and Local Intense Precipitation Analysis , Calculation CDQ0000002016000041 " for review and approval. Approval of this Topical Report was requested to support Probable Maximum Flood D calculatio_ns and associated with planned License Am~ndment Requests for S~qu~yah Nuclear -(1>/ Plant, Units 1 and 2, and Watts Bar Nuclear Plant, Units 1 and 2, and a potential License Amendment Request for Browns Ferry Nuclear Plant, Units 1, 2, and 3. l) D 30
U.S. Nuclear Regulatory Commission CNL-18-044 Page 2 April 19, 2018 In Reference 2, the NRC transmitted a Request for Additional Information (RAI) related to the TVA Topical Report. As described in the Reference 2 email, TVA agreed to provide responses to the RAls by April 20 , 2018. The enclosure to this letter contains TVA's response to the RAIS . As discussed in the Enclosure, a revision of the Topical Report will be submitted to incorporate the required changes needed as a result of these RAls . There are no regulatory commitments associated with this submittal. Please address any questions regarding this request to Russell Thompson at 423-751-2567 . Respectfully , J. W . Shea Vice President, Nuclear Regulatory Affairs & Support Services
Enclosure:
Response to NRC Request for Additional Information Related to Topical Report TVA-NPG-AWA16, "TVA Overall Basin Probable Maximum Precipitation and Local Intense Precipitation Analysis , Calculation CDQ0000002016000041" cc (Enclosure) : NRC Regional Administrator - Region II NRC Senior Resident Inspector - Browns Ferry Nuclear Plant NRR Project Manager - Browns Ferry Nuclear Plant NRC Senior Resident Inspector - Sequoyah Nuclear Plant NRR Project Manager - Sequoyah Nuclear Plant NRC Senior Resident Inspector - Watts Bar Nuclear Plant NRR Project Manager - Watts Bar Nuclear Plant
Enclosure Response to NRC Request for Additional Information Related to Topical Report TVA-NPG-AWA16, "TVA Overall Basin Probable Maximum Precipitation and Local Intense Precipitation Analysis, Calculation CDQ0000002016000041" By a letter dated September 20, 2016 (Agencywide Documents Access and Management System (ADAMS) Accession Number ML16264A454), the Tennessee Valley Authority (TVA) submitted the a fleet topical report (TR) TVA-NPG-AWA 16 "Overall Basin probable Maximum Precipitation and Local Intense Precipitation Analysis. " This TR will be used for future licensing actions for Browns Ferry Units 1, 2 and 3, Sequoyah Units 1 and 2 and Watts Bar Units 1 and 2. The U.S. Nuclear Regulatory Commission (NRG) staff is reviewing your submittal and has determined that additional information is required to complete the review. The specific information requested is attached to this email. The proposed questions were emailed in draft form and a clarification call was held on January 22, 2018. Your staff confirmed that these draft questions did not include proprietary or security-related information and agreed to provide a response April 20, 2018 to this request for additional information (RA/) . The NRG staff considers that timely responses to RA ls help ensure sufficient time is available for staff review and contribute toward the NRC's goal of efficient and effective use of staff resources. Please note that if you do not respond to this request by the agreed-upon date or provide an acceptable alternate date, we may deny your application for amendment under the provisions of Title 10 of the Code of Federal Regulations, Section 2. 108. If circumstances result in the need to revise the agreed upon response date, please contact me at (301) 415-8480 or via e-mail Andrew.Hon@nrc.gov. Regulatory Basis 10 CFR Part 50, Appendix A, General Design Criterion (GDC) 2, states, in part, that structures, systems, and components important to safety shall be designed to withstand the effects of natural phenomena such as floods without loss of capability to perform their safety functions. The design bases for these structures, systems, and components shall reflect appropriate consideration of the most severe of the natural phenomena that have been historically reported for the site and surrounding area, with sufficient margin for the limited accuracy, quantity, and period of time in which the historical data have been accumulated. NUREG-0800, "Standard Review Plan for the Review of Safety Analysis Reports for Nuclear Power Plants: LWR Edition, Section 2.4.3, "Probable Maximum Flood (PMF) On Streams and Rivers," states that to meet the requirements of GDC 2 with regards to design bases for flooding in streams and rivers, the probable maximum precipitation (PMP) on the drainage area that contributes to runoff on the stream network adjacent to the plant site should be determined. Similarly, NUREG-0800 Section 2.4.2, "Floods," states that estimates of potential local flooding on the site and drainage design should be based on estimates of local intense precipitation or local PMP. Page 1 of 71
RAI #1: Complete Storm Analysis Information for All Short List Storms Technical Deficiency: Information in the Topical Report is incomplete; additional information is necessary for the staff to make its regulatory finding. Request: Provide the analysis information for all short list storms that were used for PMP calculation. The detailed storm analysis information should include:
- Storm calculation spreadsheet
- Depth-area-duration values and chart
- Storm cumulative mass curve chart
- Total storm isohyetal analysis map
- HYSPL/T moisture trajectory map
- In-place storm representative dew point (or sea surface temperature) analysis map TVA Response:
The detailed storm analysis information requested above for the short list storms used in the PMP Calculation CDQ0000002016000041 is provided in Attachment 1 - Folder RAI 1. The information is located in the Storm Precipitation Analysis System (SPAS) folder for each storm . Note, no Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model output is available for storms that occurred prior to 1948. Also , for the SPAS 1299 Zone 1 (Alta Pass, NC-July 1916) storm , the storm representative dew point analysis data provided in Tennessee Valley Authority Floods and Flood Control document, Figure 47 (Tennessee Valley Authority, Technical Report 26, 1961) are utilized and included within the RAl1 folder. RAI #2: TVA Observed Hourly Dew Point Data Sheet for All Short List Storms Technical Deficiency: Information in the Topical Report is incomplete; additional information is necessary for the staff to make its regulatory finding. Request: For each short list storm, provide an individual spreadsheet documenting the hourly dew point data that were used for storm representative dew point selection. If publicly-accessible dew point data were used (e.g., NGOC ISO), the unique station identifier (e.g., USAF, WBAN, and/or /CAO) and the starting/ending dew point date and hour (used for the calculation of average 6-, 12-, or 24-hour dew points) should be clearly specified. Provide detailed meteorological reasoning if the selection of storm representative dew point location deviated significantly from the HYSPLIT trajectories. If sea surface temperature is used as a surrogate of surface dew point observation, the sea surface temperature observation should be provided. Provide the relevant data or source information used to determine the storm representative dew point for short list storms for which hourly dew point data were unavailable or not used. Page 2 of 71
TVA Response: Individual spreadsheets containing the hourly dew point or sea surface temperature (SST) data used in the selection of the storm representative dew point for short list storms used in PMP Calculation CDQ0000002016000041 are provided in Attachment 1 - Folder RAl2 . The publicly-accessible dew point station identifier information and a table cross-referencing the starting/ending dew point timeframe for each duration investigated (i.e., 6-, 12-, and 24-hours) are provided within each spreadsheet. In each of the storms evaluated, the region used as the storm representative dew point or SST location is within the general region suggested by HYSPLIT, if available. Therefore, no significant deviations from HYSPLIT occurred . As a result, detailed meteorological reasoning is not provided . In situations where SSTs were used as a surrogate for dew point observations, that data is provided in daily format with the data source identified. Note, many previous United States Army Corps of Engineers (USACE) / National Weather Service (NWS) storm representative values can be found in hydrometeorological report (HMR) 25A (storms prior to 1948), HMR 51 , HMR 52, HMR 53, TVA Technical Report 26, and/or the USACE Storm Studies documentation . These sources are identified when they were used to derive storm representative dew point information where updated hourly and/or SST data was not available. RAI #3: TVA Storm Adjustment Factor Feature Class Table for All Short List Storms Technical Deficiency: Information in the Topical Report is incomplete; additional information is necessary for the staff to make its regulatory finding. Request: For each short list storm, provide the storm adjustment factor feature class table developed for the TVA PMP study (as documented in Figure 24 of Calculation CDQ0000002016000041 ). The data layers should be in a common GIS data format that can be processed by ESRI ArcGIS, and should cover all of the information shown in Figure 24 including STORM, LON, LAT, ZONE_, ELEV, IPMF, MTF, OTF, TAF, and TRANS. TVA Response: Geographic Information System (GIS) database files with all the storm adjustment factor feature classes developed for the TVA PMP study are provided in Attachment 1 - Folder RAl3 . The data layers are in a common GIS data format that can be processed by ESRI ArcGIS and cover all of the information requested including STORM , LON , LAT, ZONE_, ELEV, IPMF , MTF, OTF, TAF , and TRANS. Page 3 of 71
RAI #4: TVA Dew Point Climatology Data and GIS Layers Technical Deficiency: Information in the Topical Report is incomplete; additional information is necessary for the staff to make its regulatory finding. Request: Provide the digital dew point climatology GIS data layers used for PMP development. The digital dew point climatology GIS data layers should be provided for the monthly 6-, 12-, and 24-hour, 100-year recurrence interval dew point maps provided in Appendix C of Calculation CDQ0000002016000041. In addition, provide the corresponding monthly dew point climatology values at each gauge that was used to develop the maps provided in Appendix C of Calculation CDQ0000002016000041. TVA Response: The GIS data used for the digital dew point climatology GIS data layers applied in the TVA PMP development are provided in Attachment 1 - Folder RAl4 . The digital dew point climatology GIS data layers provide the monthly 6-, 12-, and 24-hour, 100-year recurrence interval dew point maps shown in Appendix C of Calculation CDQ0000002016000041 . The data also includes the monthly dew point climatology values at each gauge used to develop the maps provided in Appendix C of Calculation CDQ0000002016000041. After further review and discussion with the NRC staff in regard to the most appropriate dew point climatology database, TVA has agreed to conservatively revise the dew point climatology applied in Calculation No. CDQ0000002016000041 and to utilize the National Center for Environmental Information (NCEI) TD3505 hourly dew point database. This will extend the period of record and provide additional dew point observational data for use in developing updated dew point cl imatology. The updated climatology will replace the previously used GIS layers. The updated storm adjustments will be processed and applied to each storm used for the TVA PMP development. Updates to this data set are anticipated to affect Sections 5.1.1 , 6.1.1 , 6.5.1, 6.8.1, 6.8.4, and Appendix C of Calculation No. CDQ0000002016000041 and will be submitted as Revision 1 to Topical Report TVA-NPG-AWA16 (TVA Calculation CDQ0000002016000041 ). Page 4 of 71
RAI #5: TVA Probable Maximum Precipitation Data and GIS Layers Technical Deficiency: Information in the Topical Report is incomplete; additional information is necessary for the staff to make its regulatory finding. Request: Provide the final digital PMP GIS data layers (across all durations and areas) developed for the TVA Overall Basin Probable Maximum Precipitation and Local Intense Precipitation Analysis. The digital PMP GIS data layer should cover the full TVA Basin for which PMP values have been determined. TVA Response: - Folder RAl5 contains 3 folders , one for each storm type (i.e ., general , local , and tropical) used in the TVA PMP calculation for the domain at a spatial resolution of 90 arc-seconds or .025 x .025 decimal degrees. GIS raster files are included for the entire domain and all durations. After further review and discussion with the NRC staff in regard to the most appropriate dew point climatology database and storm representative dew points for some storms , TVA has agreed to conservaHvely revise Calculation No. CDQ0000002016000041 as further discussed in the TVA responses to RAls #11 and #12 . Those changes are anticipated to affect Section 2.5, Section 5.1, Section 5.7 and Appendices A, C and F of Calculation No. CDQ0000002016000041 and will be submitted as Revision 1 to Topical Report TVA-NPG-AWA16 (TVA Calculation No.CDQ0000002016000041). RAI #6: Reasonableness of OTF Values Technical Deficiency: Staff's review of Orographic Transposition Factor (OTF) application examples demonstrates discrepancies between expected OTF values and OTF values used by the licensee for selected storms. The OTF is applied after an observed precipitation event is 1) moisture maximized using the in-place maximization factor (IPMF) and 2) geographically transpositioned (on a Lat-Lon plane) using a moisture transposition factor (MTF) . The MTF captures geographic differences in moisture availability through comparison of dew point climatology. While it captures spatial variation in moisture, the MTF may not adequately capture the effects of terrain, hence the need for a terrain adjustment (e.g. , the OTF, Barrier Adjustment Factor, or Storm Separation Method). Staff interprets from the licensee 's descriptions in the Technical Report that the OTF is intended to capture the impact that terrain will have on rainfall depths when transpositioning a storm from the original location to a new location. Therefore, staff believes that the OTF should be independent of geographical moisture influence (which is captured through use of the MTF) . Consequently, staff believes that the OTF should not be significant when moving storms between regions with similar orographic characteristics (i.e., such regions should have a calculated OTF close to 1.00). To assess whether the OTF calculation process produces OTF values close to 1.00 in cases where the original and transpositioned storm paths have similar orographic characteristics, staff evaluated the OTF for a series of case studies using data supplied by the licensee for TVA short Page 5 of 71
list storms. NRG Table 1 includes a summary of the rationale for evaluating these case studies and the subsequent observations. NRC T a bl e 1 OTF cases tud"1es eva ua tdb e ,y stffa Storm Rationale Observations Warner, OK The storm center location and TVA Zone 1 Avg . Zone 1 OTF: 0.80 (Example 1) share similar orographic characteristics (e.g ., both locations exh ibit minimal barriers, are of similar elevation , and are located at a sim ilar distance-to-coast). Fall River, KS The storm center location and TVA Zone 1 Avg . Zone 1 OTF : 0.75 (Example 2) share similar orographic characteristics (e.g., both locations exhibit minimal barriers, are of similar elevation , and are located at a similar distance-to-coast) . Smethport, PA The storm center location and TVA Zone 4 Avg . Zone 4 OTF*: 0.66 (Example 3a) share sim ilar orographic characteristics (e.g., both locations exh ibit orographic influence and are a similar distance-to-coast) . TVA Zone 4 has higher overall orographic influence than the storm center due to higher terrain elevation and complexity. Smethport, PA The storm center location and TVA Zone 2 Avg. Zone 2 OTF*: 0.59 (Example 3b) share sim ilar orograph ic characteristics (e.g. , both locations exh ibit orographic influence, are of similar elevation , and are a sim ilar distance-to-coast) .
- Note: the OTF for the Smethport, PA storm was manually adjusted by the licensee to include rescaling to a maximum value of 1. 00. An additional question related to the Smethport, PA storm is included in RA/ #7.
For each of the examples included in NRG Table 1, staff believes that the orographic adjustments should be minimal and the OTF should be close to 1.00; instead, the licensee 's analysis results in OTF values significantly less than 1.00 and large reductions in the adjusted rainfall depths. Request: Provide a justification for the departure of O TF from 1. 00 when transpositioning storms across orographically similar zones (examples provided in NRG Table 1), and discuss whether the reductions in OTF are reasonable. Provide a justification for applying the OTF to the transposition of all storms throughout the TVA Basin, given the example results provided in NRG Table 1. Page 6 of 71
TVA Response: Orographic Transposition Factor (OTF) Background The OTF adjustment process is being used to not only capture the difference in terrain effects between two locations but also to capture all processes that result in precipitation reaching the ground at one location versus another location . The OTF is a mathematical representation of the ratio of the precipitation frequency climatology at one location versus another location . The precipitation frequency climatology is derived from actual observed precipitation events. The largest of these storm events each year is then used to define the Annual Maximum Series (AMS) at a given station. These actual precipitation events and their observed precipitation are a result of all precipitation producing processes that occurred during a given storm event. In HMR terms , the resulting observed precipitation represents both the convergence-only component and the orographic component. The gridded precipitation frequency climatology was produced using gridded mean annual maxima (MAM) grids developed with the Parameter-elevation Regressions on Independent Slopes Model (PRISM) . PRISM utilizes geographic information such as elevation, slope, aspect, distance from coast, and terrain weighting for weighting station data at each grid location . Therefore , the use of the precipitation frequency climatology grids is reflective of all precipitation producing processes. The use of the gridded precipitation climatology represents an optimal combination of factors, including representing extreme precipitation events equivalent to the level of rainfall utilized in TVA's storm selection process , and providing the most robust statistics given the period of record used in the development of the precipitation frequency climatologies . Differences in these values between regions of similar topography reflects the variances in other precipitation producing processes, such as access to moisture, seasonality, general synoptic conditions that produce rainfall , and other meteorological parameters, as well as variances in statistical interpolation . The variability between similar topographic regions provided in NRC Table 1 reflects these differences. National Oceanic and Atmospheric Administration (NOAA) Atlas 14 precipitation frequency cl imatology is the best available data set to utilize in quantifying the differences in precipitation processes between two locations within the same transposition region. It provides a reproducible and explicitly quantifiable data set. The limitations of the data are known . The processes used to develop the data are known and have been reviewed and accepted . The assumptions involved in applying the data are known and can be quantified . This includes use in highly orographic and non-orographic regions . Non-TVA examples of this process include use in the entire region covered by the Virginia statewide PMP study (Kappel , et al. , 2015), the Texas statewide PMP study (Kappel, et al. , 2016) , and the entire region covered by the Colorado-New Mexico Regional PMP (in progress). Many of the regions are similar to less orographic regions of TVA zones 1-3 and to the regions discussed in NRC Table 1. The review process of previous and ongoing AWA PMP studies includes representation from the National Weather Service, Corps of Engineers, Bureau of Reclamation, Federal Energy Regulatory Commission, Natural Resource Conservation Service and United States Geological Society, as well as meteorologists on faculty at major universities, private industry meteorologists, and state dam safety regulators . The review boards of these studies have concurred that the OTF process as utilized for TVA is reasonable and acceptable for PMP calculation purposes. Similarly, the Storm Separation Method (SSM) applied in HMRs 55A, 57, and 59 utilized the 100 year 24-hour precipitation frequency climatology from NOAA Atlas 2 in a similar fashion Page 7 of 71
to TVA's appl ication . Like TVA, these HMRs applied the process to all locations including non-orographic regions of those studies (e.g. , HMR 59 Figure 6.4). Specific examples of the use of NOAA Atlas 2 precipitation frequency data to all regions for use in calculating the HMR orographic factor (K factor) include the following :
- HMR 59 Section 6.6.1 states: "The K-factor is derived from two relationships: 1) The first involves the one-percent chance ( 100-year return period) precipitation amount in proximate areas of large and small topographic variation . Th is relationship is represented by TIC where Tis the 100-year, 24-hour return-frequency precipitation ."
- HMR 57 applied the NOAA Atlas 2 100-year, 24-hour precipitation to address orographics and determined it was important to maintain a close spatial correlation of maximum index values of total PMP and maximum values of 100-year, 24-hour precipitation (HMR 57 - pg 88).
- In HMR 57 , a TIC (total 100 yr precipitationl100 yr convergence component) value less than 1 resulted in some areas. In the Snake River plain , where physiographic features could likely account for the low TIC values , the values were accepted .
Values as low as 0.84 to the lee of the Olympic Mountains of Washington where the mountains were believed to disrupt the resupply of boundary-layer moisture were also accepted (HMR 57 - pg 76) .
- HMR 56 Section 2.2.3 states: "Topography is known to play an important role in rainfall in the Tennessee River watershed ."
- HMR 56 Sections 3.5.2 and 3.5.3 discuss the application of the terrain stimulation effect in smooth and intermediate regions and an additional broad scale orographic factor in the mountainous eastern region . These are some of the additional factors NOAA Atlas 14 climatologies captured as part of the OTF process. The most important aspect is defining appropriate and reasonable transposition limits to place storms within similar regions when considering topographical and meteorological interactions. The assumption is the NOAA Atlas 14 climatologies capture the effects of terrain , upwind barriers, access to moisture, preferred moisture inflow directions, seasonal variation of synoptic meteorological environments, etc. These factors are explicitly captured because the NOAA Atlas 14 climatologies are built from observed precipitation events, which inherently included these factors .
- HMR 56 Figures 67 and 68 show topographically significant terrain throughout TVA zone 1, 2, and 3.
These discussions and examples demonstrate that the OTF represents more than the difference in topographic effects between two locations. The OTF represents the difference in all precipitation processes between two locations, such as access to moisture, seasonality, and synoptic conditions, in addition to topographic effects. Page 8 of 71
General Orographic Discussion The orographic component of the topographic effects refers to the influence that terrain has on precipitation production and accumulation , both in-place and upwind/downwind . Orographic effects include many processes; some of the important ones include the following :
- Terrain can help release atmospheric instability by initiating lift (releasing conditional instability through forced ascent), providing extra lift to already rising motions, or producing the opposite effects through descending air. Lifting processes can be triggered by a rise in terrain or upstream blocking terrain (causing downstream convergence) . These forced ascent processes result in rising motions, cooling of the air mass, increased saturation , and enhanced precipitation. Forced descent has the opposite effect, attenuating the precipitation producing processes by warming and drying the atmosphere , resulting in a more stable atmosphere and less precipitation .
- Higher terrain receives more of the precipitation than adjacent lower elevations because there is a better chance the precipitation will reach the ground at higher elevations or experience less evaporation before reaching the ground than adjacent lower elevations.
- Orographic effects depend on many factors such as slope, aspect, angle of interaction, width of barrier, height of barrier, moisture advection duration, moisture depth in the atmosphere, atmospheric profile (e.g ., the amount of instability) , wind speed/direction , the size of the storm , and the combination of these and other factors through space and time.
- For any given storm event, the barrier and upwind topography are constant with changing atmospheric parameters.
There are many orographic processes and interactions related to terrain interactions that are not well understood or quantified . Therefore, observed data (precipitation accumulations) are used as a proxy, where it is assumed that the observed precipitation represents all the precipitation processes associated with a storm event. Given this , it is logical that observed precipitation at a given location represents a combination of all factors that produced the precipitation, including what would have occurred without any terrain influence and what actually occurred because of the terrain influence (if any). The best proxy would be to have thousands of observed events of the storm type being analyzed at any given location and then be able to compare those storm events to a similar number of storm events at another location where the topography and meteorology between the two locations is the same. However, that extensive data set of observed storm events does not exist. Therefore, judgments are made regarding regions that are considered as having the same meteorology and topography and then utilize statistical analyses provided in NOAA Atlas 14 as the comparable data set. As part of the OTF process, the following is assumed :
- NOAA Atlas 14 precipitation frequency climatology represents all precipitation producing factors that have occurred at a location . This is based on the fact that the NOAA Atlas 14 data is derived from AMS values at individual stations that were the result of an actual storm event. That actual storm event included both the amount of precipitation that would have occurred without topography and the amount of precipitation that occurred because of topography (if any) .
- Comparing the precipitation frequency climatology at one point to another will produce a ratio that shows how much more or less efficient the precipitation producing processes are between the two locations.
Page 9 of 71
If there is no orographic influence at either location being compared or between the two locations, then the differences should be a function of ( 1) storm precipitation producing processes in the absence of topography (thermodynamic and dynamic) , (2) how much more or less moisture is available from a climatological perspective, and (3) elevation differences at the location or intervening barriers. Discussion Related to Departure of OTF from 1.00 in Orographically Similar Zones It would be reasonable to expect a near constant 1.00 over orographically similar zones if the OTF only represented the orographic effect. However, as discussed previously, there are other atmospheric components inherent in the precipitation frequency outputs that are carried th rough into the OTF. Example 1 and 2 in NRC Table 1 address the OTF reduction exhibited in storms transposed from eastern Kansas and Oklahoma to TVA transposition zone 1, a region of similar orographic characteristics and elevation . The average OTFs are 0.80 and 0.75, respectively. TVA Figure 1 illustrates the spatial pattern of the NOAA Atlas 14 1,000-year 24-hour rainfall over the region. The climatological precipitation over TVA transposition zone 1 is significantly lower than the storm centers west of the Mississippi River. The meteorological reason for the variation is primarily because the Warner, OK and Fall River, KS storm centers receive their moisture directly from the Gulf of Mexico at a location where air masses originating from the High Plains to the west interact preferably with the low-level jet that is common in the region . These factors combine in this region to produce more efficient thermodynamic contrast , higher instability, and more frequent high-intensity rainfall. In contrast, the frequency of occurrence of the low-level jet is much lower over TVA transposition zone 1 and zone 2 and the thermodynamic contrast in the TVA region is therefore not as extreme from a climatological perspective. Similar types of storms can occur over the Ohio River/Tennessee River Valleys, but are less common and less intense, especially from a frequency of occurrence perspective. The horizontal (reduced to sea level) climatological difference in moisture availability is addressed with the moisture transposition factor (MTF) by comparing the moisture levels associated with the 100-year recurrence interval dew point values . These values are only sl ightly lower over the TVA location than the storm center locations. The more significant reduction exhibited by the OTF reflects individual storm precipitation producing processes inherent in the precipitation climatology, as opposed to the climatological maximum moisture differences used in the MTF. Examples 3 and 4 in NRC Table 1 exhibit special cases as described in the TVA PMP report. This issue is discussed in depth in the response to RAI #7 below. As noted in the above discussions, the OTF captures all the precipitation producing processes, including the effect of topography (orographic effect), if any. The use of this process is relevant in both orographic and non-orographic regions. Therefore, the use of the OTF process is applicable for all locations within the TVA domain and for all storms used in the TVA PMP analysis (except as noted in the RAI #7 response) . Page 10 of 71
1,000..year 24-hr Pr<cfpltation (in)
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'* llll n ,~ 1111 ,'!> 1f TVA Figure 1 - Fall River, KS and Warner, OK storm centers over NOAA Atlas 14 1,000-year 24-hour precipitation RAI #7: OTF Reduction for Smethport, PA and Simpson, KY Technical Deficiency: OTF values for two local storms which control PMP estimates were manually rescaled to a maximum of 1.00 (i.e., all original OTF values were divided by the maximum calculated OTF, resulting in widespread reductions and a maximum value of 1.00).
This rescaling greatly reduces the Local Storm PMP. As described in Section 6. 1. 1.5 of the Topical Report, the OTF values for two local storms (Smethport, PA and Simpson, KY) were rescaled to a maximum of 1. 00. Following discussions with the Review Board and the licensee, "it was determined that the factors leading to extreme levels of moisture and instability combined with terrain influences" which produced extreme rainfall at Smethport and Simpson "were similar to what could occur over the eastern foothills and mountainous terrain in the TVA basin. " As a result, the licensee decided it was "unreasonable to further adjust the events upward based on the OTF", and "the OTF factors for these events were normalized to a maximum of 1.00." Page 11 of 71
Staff's review of the data provided by the licensee suggests that the maximum original (i.e. , unadjusted) 0 TF values for the Smethport and Simpson events are 2. 15 and 2. 09, respectively. In comparison, the average Zone 4 original OTF is 1.39 for Smethport and 1.35 for Simpson. After rescaling the original OTF, the average Zone 4 OTF is reduced to 0. 66 for Smethport and
- 0. 65 for Simpson - approximately a 50% reduction. These modifications to the OTF result in a significant reduction in the adjusted DAD values for these storms. In addition, since these storms control PMP estimates, the resultant PMP values are significantly reduced.
NRG Figure 1 provides a comparison of TVA 's rescaled OTF values and the original (i.e. , unadjusted) OTF values for the Smethport storm. Similarly, NRG Figure 2 provides a comparison of TVA 's rescaled O TF values and the original O TF values for the Simpson storm. NRC Figure 1. Comparison of Smethport OTF using TVA's rescaling approach (left) and original approach (right) for TRANS=1 grids (i.e., transpositionable zone) NRC Figure 2. Comparison of Simpson OTF using TVA's rescaling approach (left) and original approach (right) for TRANS=1 grids (i.e., transpositionable zone) Request: Provide justification for adjusting the Smethport, PA and Simpson, KY OTF values to a maximum of 1.00, and for using significantly reduced OTF values throughout the transpositionable zone. Page 12 of 71
TVA Response: OTF Re-scaling Background Discussion The OTF re-scaling is to preserve the spatial distribution of the adjusted rainfall for these storms over the transposed areas without unreasonably inflating the rainfall beyond the maximized in-place depths. These storm's point rainfall depths were at, or near, the world record curve for their respective critical durations. The intent of the original OTF cap of 1.0 (instead of 1.5) was to prevent exceedance of the world record depths (TVA Figure 2) after storm transposition . The cap of 1.50 was applied to these storms initially, but the resulting precipitation was far greater than the world record rainfall amounts. The intent of the normalization process was to preserve the maximum orographic adjustment for the location(s) with the most orographic impact (with a value of 1.0) while decreasing the values elsewhere so that the spatial pattern of the 100-year Atlas 14 6-hour precipitation climatology (TVA Figure 7) was maintained . Further, rainfall amounts associated with both of these events were highly questionable, because no hourly rainfall accumulation data were recorded at or near the storm center locations. Therefore, hourly incremental rainfall data provided were not based on observed data and instead were derived from surrounding stations with significantly lower total precipitation amounts or inferred from depth-duration curves . This results in having low confidence in the incremental hourly rainfall amounts. In fact, the Simpson, KY July 1939 storm is not used in HMR 51 (no working papers or notes are available as to why this storm was left out of that document) . Given these considerations, extensive discussions with the TVA Review Board and AWA evaluations took place regarding the use of both storms . A lthough it is possible that the Smethport, PA storm is not transpositionable to any part of the TVA basin , no conclusive data existed to eliminate the storm . However, several data adjustments suggested this possibility. This included anomalously high OTF values and high MTF values . In these types of situations where there is no clear argument for inclusion or exclusion, the TVA choice is made to include the storm. In doing so, adjustment factors and fit with other adjusted storms must be considered, similar to the discussions in HMR 51 Section 3.2.2 and Section 3.2.4. Use of the storm and the storm adjustment values as calculated, resulted in unreasonably high rainfall depths when transposing the storm from north central Pennsylvania to the TVA basin. An MTF value greater than 1.00 results from the fact that the target grid locations in the TVA basin are closer to the moisture source region than the area associated with the storm center in north central Pennsylvania. More significantly, much of the target region has greater precipitation frequency depths due to more frequent extreme rainfalls and enhanced orographic influence. This results in calculated total adjustment factors at the target grid locations that are greatly inflated where the OTF values can exceed 2.00. Generally, when OTF values are > 1.50, the storm transposition limits are re-evaluated . This is because OTF values greater than 1.50 or less than 0.50 are an indication that the storm may not be transposable to that location since the physical characteristics may be too different from the source location . However, the storm must still be transposed to these locations. In these cases, the practice is to cap all OTF values at a maximum of 1.50. The Smethport event was a world record at its critical durations. Increasing rainfall by an areal-average MTF of 1.12 and applying an OTF in the traditional manner (much of the target cells would be capped at 1.50) would result in an adjusted rainfall over TVA transposition zone 4 that was much too high and not physically possible . In addition, the 1.50 OTF cap would result in constant spatial fields of PMP depth because almost the entire region is greater than the 1.50 OTF. TVA Figure 2 illustrates the adjusted rainfall depths in relation to the world record rainfall depth-duration curve . In summary, typical application of the OTF and MTF to the Smethport, PA storm over transposition zone 4 created unreasonably high rainfall depths without an appropriate spatial distribution . The Simpson, KY storm transposition resulted in a similar problem, although not to the magnitude of Smethport. Page 13 of 71
The two most reasonable approaches to these problems were to either remove the storms from the database as not transposable (this was evaluated and discussed internally and in review board meetings) or to adjust the transposition factors so that the resulting rainfall levels were at a more reasonable level. In reality , the transposition limits for these storms should be very limited , but the project decision was to keep the storms and adjust the OTF. This is a more conservative application . For these storms, the OTF is useful to determine the spatial distribution of gridded PMP over orographic target regions, but cannot be used to provide reasonable adjusted rainfall magnitudes. Thus , it was determined that the OTF should not further increase rainfall beyond the in-place depth . The OTF was re-scaled to a maximum of 1.00, rather than capped at 1.00, to allow the spatial distribution of the precipitation climatology to transfer through the OTF. As a result, the areal-average total adjustment factor (TAF) for Smethport, PA was 0.74, with a maximum of 1.13. For the Simpson, KY event, the areal-average TAF was 0.72 , with a maximum of 1.21. Limiting the adjustment for these storms to an increase of 13% and 21 %, respectively, was determined to be a reasonable constraint, keeping the adjusted depths to a minimum above the world record curve . The adjusted transposed rainfall for Smethport was very similar to the transposed rainfall depths fo r the Johnson City, TN event, a storm that also occurred over the target region (TVA Figures 3 and 4) . This storm provided additional confirmation that the Smethport constraints were reasonable . The maximum 6-hour 10-square mile rainfall is 29.3", which is in line with the world point maximum-recorded rainfall curve . Again , assuming the world-record rainfall curve demonstrates a physical upper limit to rainfall accumulation over time , this suggests the adjusted rainfall is reasonable , if not conservative . Furthermore, the ratio of 24-hour 10-square mile PMP to the 100-year 24-hour Atlas 14 precipitation depth was 3.4 for transposition zone 3 and 3.1 for transposition zone 4. These ratios are consistent with previous AWA and HMR PMP studies for orographic regions and provide further evidence that the constraints are reasonable . However, after further review and discussion with the NRC staff in regard to this question , TVA has agreed to add further conservatism and revise the OTF determination methodology used in LI P calculations . This methodology change ensures the OTF used in transpositioning local storms to the SQN, BFN and WBN plant sites is not less than 1.0. Note, this change only affects the Simpson , KY storm because the Smethport, PA storm was not transpositioned to the plant sites. Updates to this data set will affect Sections 6.1.1.5 and 6.4.4 of Calculation No . CDQ0000002016000041 and will be submitted as Revision 1 to Topical Report TVA-NPG-AWA16 (TVA Calculation CDQ0000002016000041) . Page 14 of 71
Maximum observed point rai1nfall as a functilon of duration 10 000 , - - - - - - - - -
* ~ ortd ,lax,mum US .iaximu 1 000 S1mp.son I ad sted)
Holt
- c i:.
s: i
- Holt (adjusted)
Q 00 Smatl'1C ~ J" * . ... 0 **
- minutes ..---+,o---,--
ears 1 0 1000000 10,000 000 Dura on (min) TVA Figure 2. Smethport, PA and Simpson, KY adjusted rainfall plotted on the world record depth-duration curve Page 15 of 71
Controlling torms hour Loca l Stonn PMP ( 10 111?)
.... I'"-', "\\
T A tudy Area IS"A M'W ll'W aw
.t*
- r.**
Contrtbudn9 Storms
- Johnson City. TN 1924
- Smeth port. PA 1042 flb"N ..., '"'
llW Qoc,oir,cci,~ 12"W t'9,A~~l:oi*"'.. ec-- P.,qett0" .Ahn
- r:=:aaa:==--======------======M*'
0 50 100 150 200 es No,!:l'~i,,tsJ TVA Figure 3. Controlling storms for the 6-hour 10-sqmi PMP. Smethport, PA controls a portion over southern Zone 4 Page 16 of 71
6-hour Local Stonn P IP ( LO mi:) T VA Study Arca l<'W Averoge Depth : 23.16" Maximum Depth: 29.29" ,,.. Minumum Depth : 14 .31*
,.., PMI' O.pth
{lo.chH,
-
- 2 12
- 14 26
- 2 -4 - 14 6 - 26 - 26
- * -8 1 8 30 6-8 20 32 8 - 10
- 20 - 22 32 - 34 10 22 - 2* CJ> 34
-~ - -==--==-=====
0 50 100
- - - --=====M***
150 200 TVA Figure 4. 6-hour 10-square mile PMP resulting from the Johnson City, TN and Smethport, PA adjusted rainfall RAI #8: OTF Calculation using NWS Atlas 14 Technical Deficiency: The OTF is intended to accurately capture localized spatial variation in orography. However, the NWS Atlas 14 data used to calculate OTF are inherently regionalized, which poses a concern whether the original intention of developing an OTF was fully captured. Section 4.5 of the Topical Report states that, in comparison to the topographic adjustments used in the TVA HMR, "the OTF, along with hourly gridded rainfall data from SPAS analyses, is able to evaluate and quantify ... variations over a much more refined scale both spatially and temporally. " Localized refinement is achieved through use of Atlas 14 precipitation frequency (PF) data, which were developed using L-moment regional frequency analysis. However, during the development of the PF data, Atlas 14 identified homogeneity zones (i.e., regional groups) for data pooling. Based on Section 4. 2. 2 of Atlas 14, Volume 2, the regional application of L-moments derives "the shape parameters from all stations in a homogeneous region rather than from each station individually. " From Section 4.4 of Atlas 14, Volume 2: "effort was made during the subdivision process to mitigate discrepancies that could be caused by (1) sampling error due to small sample sizes, or (2) regionalization that does not reflect a local situation." NRC Figure 3 shows the 24-h through 60-day regional groupings identified in Atlas 14, Volume 2. Page 17 of 71
Generally speaking, all precipitation data within a homogeneity zone were first locally normalized, and then pooled together for probabilistic density function fitting. Therefore, it is important to understand that the NWS Atlas 14 values do not only capture the local precipitation features. It is jointly influenced by the local mean (of annual maximum series at each gauge}, regional probability density distribution, and final interpolation by PRISM. X"I', 24-HOUR THROUGH 60-0AY L..MOMENT REGIONS NRC Figure 3. Regional groupings for daily data used to prepare NOAA Atlas 14 Volume 2 Section 3. 1. 4 of the WMO-No. 1045 Manual on Estimation of Probable Maximum Precipitation states: "Precipitation-frequency values represent an equal probability level of rainfall. The values for the rarer recurrence intervals, for example the 50-year or 100-year recurrence interval, are associated with severe weather systems. Therefore, they are better indicators of the geographic variation of PMP than mean seasonal or annual precipitation maps. " Thus, staff believes that the specific features of Atlas 14 are important artifacts influencing the OTF and are worth considering. Given that the Atlas 14 method scales station PF data by the mean of the annual maximum series and uses PRISM for base map smoothing, the final rainfall estimates would induce spatial smoothing based on averages rather than rarer recurrence intervals associated with severe weather systems. Page 18 of 71
OTF Best Fit Linear Trend Method The licensee used a 6-h precipitation frequency climatology to compute local storm OTF and a 24-h precipitation frequency climatology to compute general and tropical storm OTF. For each short list storm, the OTF calculation approach used for the TVA Topical Report used linear regression to estimate the ratio between precipitation frequency depths for the recurrence interval associated with the storm's maximum point rainfall at either 6-h or 24-h. OTF 100-year Ratio Method Other AWA PMP studies have calculated the OTF using the 100-y precipitation frequency ratio rather than the linear regression approach. Since longer recurrence interval estimates may be more representative of PMP-type storms but may lack reliable estimates, AWA has used the 100-year precipitation frequency ratio to compute OTF in other studies (e.g., the PMP study for Texas) . NRG staff has also conducted limited sensitivity analysis and finds that the 100-year ratio is more stable than the regression approach. For example, precipitation frequency data provide higher precipitation depths at BFN than at WBN and SQN; however, the linear regression method can result in lower OTF values at BFN than at WBN and SQN. Request: a) Considering Atlas 14 's regional features, provide a justification regarding whether the Atlas 14 PF data represent reasonable spatial variation representative of orographic PMP effects or PMP in general. b) Provide a justification for using the best fit linear trend method in lieu of the 100-y ratio method for determining LIP and basin-wide PMP values. TVA Response - 8(a}: NOAA Atlas 14 is based on several layers, decisions and assumptions that result in the final PF estimates ultimately used for OTF calculations . A high level of review on regional PF analysis is provided to ensure all aspects of the NOAA Atlas 14 PF estimates are completely understood . A regional frequency analysis approach utilizes L-moments, decreases the uncertainty of rainfall frequency estimates for more rare events and dampens the influence of outlier precipitation amounts from extreme storms as compared to site-specific station analysis. The basis of a regional frequency analysis is that data from sites with in a homogeneous region can be pooled to improve the rel iability of the magnitude-frequency estimates for all sites (especially the upper tail of the distribution) . A homogeneous region may be a geographic area delineated by meteorological climatologies or may be a collection of sites having similar characteristics pertinent to the phenomenon being investigated. The definition of a homogeneous region is the condition that all sites can be described by one probability distribution having common distribution parameters after the site data are rescaled by their at-site mean. Thus, all sites within a homogeneous region have a common regional magnitude-frequency curve , termed a regional growth curve , that becomes site-specific after scaling by the at-site mean of the data . Quantile estimates at a site are estimated by: Equation 1 where Qi(F) is the at-site inverse Cumulative Distribution Function (CDF), Ui is the estimate of the at-site mean, and q(F) is the reg ional growth curve , regional inverse CDF. Page 19 of 71
NOAA performed measurements of heterogeneity and station discordancy tests (Hosking and Wallis , 1997) to ensure all sites meet the criteria of a "homogenous region", meaning one probability distribution having common distribution parameters after the site data are rescaled by their at-site mean . Regionalization is captured through the regional growth curve for the specified homogenous region , which is then localized by the at-site scaling factor "MAM". Note, that NOAA HDSC group utilized different reg ionalization approaches for the various Atlas 14 volumes. The PRISM group utilized the PRISM model to derive MAM grids based on the MAM of station data. The PRISM group used similar methods to derive 30-year climatologies when creating the MAM grids; predictor variables are listed in TVA Table 1 (NOAA Atlas Table 2) . The resulting MAM PRISM grids served as the basis for deriving precipitation frequency estimates at different recurrence intervals using a spatial interpolation procedure called the Cascade, Residual Add- Back (CRAB) derivation procedure. The level of smoothing applied in orographic areas in NOAA Atlas 2 was "LIGHT" as described below. Additional text and TVA Figure 5 from NOAA Atlas 14 are below and provide details on the amount of smoothing applied to the final PF estimates. Page 20 of 71
TVA Table 1. PRISM redictors used to derive MAM rids From NOAA Atlas 14 v2 Table __ Values of rele :-ant PRISM pmnme '"or mo
- fuig of 1- and ""-1_Jho
- m~x flood <itatistics foi- the ORB (Om Ri~~ Ba :in). See aly e al. 200_) fonietails oo. PRIS I pai-an:ieteP.>.
D . rion 1-ho hour\ alues Ra : lS f i.n:lluence 60170 km* s l\,finimum mm r f on-face "!.I _ s atioo.s* s, 2 _osta: "oos* desired in re~ession P1 I\ifiniml.ID.li va *d re~s ion slope 0.6/ __+
/3111 Maximum** *dreg,* si slope 30.0t Jo.o-P1a Ikfault :alid regression :;lope ~ .5.1 5.9-Distance Weiehtin~
Di.sranc:r weighting exponent 2.0CO Impomw.c:e fa or distance o.: lo.5 1.'i'"eighfmg l\.fini.mum all wab e dis ce 50l50 km MAP wei~;hting expooent .0/1 .0 Importance fa ,o r or :MAP o.: lo.5 W\:i gl!rtmg Mininnun station-grid. oell MAP 50}50 ,o di.fJerenre below which M..\P
,.,reigbting
- maxinnun
- Maxim.um station-grid c-e* l\iLA..P 500/500%
differenc-.e abo-\.-e which :M:...i\P ,,.~ *gbt ts zero Facet Meimltine: C face weighting exponent 0.5/0.S: CT Minimum mter-ce ele , *on / 01/ re grad! belo\.V which a cell is
- t Mrurimum DEl\.f filtetma 80/80 bn waveletlt:,oth oc topographic facet amioation Coasrai Proximitv eimtin2
- 1. .a.=
- Optimized .l. *
- cross- *alida
- s * *cs see Ta e 4).
- S .o pes are exp1'essed mtlmi.s are normalized b, the a,..-erage o red value of the pl'ecipim *on in the l'egression da a se ii the g cell. :ni
- here are l scp: ,LAP mm))* 000]-
** Nonnal.1, referred o a: elei.*.ati ,.:.reigh.ting
- ~ la.,wmim value: .actu ..11 v ne raried dynanlically ti*
- ilie ni.ode .
Page 21 of 71
go*w 85"W OO"W 75"W 40"N 40"N 85°N 85"N
- Heavy smooth ing D Moderate smoothing
- Light smoothing 85"W OO"W 75"W 0 55 110 220 130 Coastal Proximity and Effe cti ve Terrain Height-based Sm oothing Thre sholds Ohio River Basin and Surrounding States TVA Figure 5. A map of areas receiving different degrees of spatial smoothing based on PRISM's effective terrain height and coastal proximity grids from (NOAA Atlas 14 v2 Figure 4.8.4)
- 1. HEAVY: Flat areas were determined if effective terrain height is less than 100 m (328 ft) ,
and then a 17x17 grid cell (approximately 15 miles by 15 miles) , center-weighted filter was used at the longer durations and a 25x25 grid cell (approximately 25 miles by 25 miles) filter at the shorter (<24-hour) durations . The shorter durations were subjected to greater smoothing because the lower stati on density was prone to cause unnatural variabil ity.
- 2. MODE RATE: Moderately complex terrain areas were determined if effective terra in height was greater than 100 meters (328 feet) and less than 200 meters (656 feet), and then a 11 x11 grid cell (approximately 5.5 miles x 5.5 miles), center weighted filter was used for all durations.
- 3. LIGHT: Complex terrain areas and coastl ines were determined if effective terrain height was greater than 200 meters (656 feet) or if the coastal proximity grid (a grid of values indicating distance from coast) was <=5, and then no filter was used at th is stage.
However, light smoothing was conducted during the next stage. Page 22 of 71
The CRAB process used in NOAA Atlas 14 v2 uses the previously derived PF grid to derive the next PF grid in a cascading fashion . From NOAA Atlas 14 v2 , "The technique derives grids along the frequency dimension with quantile estimates for different durations being separately interpolated . Hence, duration-dependent spatial patterns evolve independently of other durations." The initial PRISM MAM grid is used to estimate the 2 year grid , the final 2 year grids are used to estimate the 5 year grid, and this continues through all frequencies . A comparison of the 2-year PF (analogous to the MAM grid) to the 100 year grid are provided for the 6 hour and 24 hour (TVA Figures 6 through 9) . For each frequency and duration , the grids were normalized to the highest PF value; the normalized grids are used to illustrate the variability in the spatial distribution for the 2-year and 100-year frequencies (TVA Figures 10 through 13). These figures illustrate that the variability of a more frequent event (2 year) is not that different from a more rare event (100 year). 2-year 6-hour OAA Alla 14 Precipitation estimate (inchc ) Tcncsscc Valley Authority Drainage Arca
-W *w M*.-.. l:'"Vf tt'W 2..,...- e-nour , r.cip1tat1on (indtes l
< 25 -
2 5* 3 -
*5 - 5 5-55 0 3 - 3 5 - 5 5-6 0 35 6 - 6.5
- * * *5 C] > 65 8'"# *N .,w
~ ! - . f.... 'ffl.oS191WIJ'"Mlcnt1""'
Pr ~Mel{--
-==-ic:::i-====----====Mi 0 50 100 150 200 les OIIUtn WGS ll&I TVA Figure 6. 2-year 6-hour NOAA Atlas 14 spatial pattern Page 23 of 71
IOO-year 6-hottr 1 0 A tla 14 Precipitation - timate (inche ) Tcncsscc Valley Aut hority Drainage Arca
...w ,,*~
100.year 6-hour Precipitation (inches)
. ,o . 35c:J.s - 5 * *.o -o5 C] 7.5-B
- 35., - 50
- 55 - 65-7 CJ ,o .,s . s.s.o - 10 . 15
~~flffl~ll&4l.lTt.tZ~1fM 0
- =- c:::ma:======------=====::::iMlles 50 100 150 200
~ ,, M<:,f#(I' QllllrloWGSt_.
TVA Figure 7. 100-year 6-hour NOAA Atlas 14 spatial pattern Page 24 of 71
_-year 24-hour OAA Alias 14 Prec ipitation st imate (inches) Tcncsscc Valley Authority Drainage Arca w .... .,., 1l"t,1 2-yur 24-hour PrK1P4t1tion (Inches) 0
< 175 175 2 - 225 -
- 275 - 3 3-325 325 - 35 0 225 35 - 3.75 Alabama - 25 - 275 0 > 375
...,. p*w .,.,. IJW ,>-w
~S.-*an WGS191MllfM1r,w,,eN Pr(NQOl'l ! 1.1_..,... M t'ftlllOI' 0
- = - = =--======------=====:::iM1l 50 100 150 200 es ~VWGS'lfll,I TVA Figure 8. 2-year 24-hour NOAA Atlas 14 spatial pattern Page 25 of 71
IOO-year 24-hour NOAA Atla 14 Precipitation Estima tes (inchc ) Tcnc cc Valley rnhority Drainage Arca
, ~ - - ~ WOSI ... IJTMlfM wt
""~t-..u~.,.
- =- =- =====-----=====Mjes 50 100 150 200 Dllll.,,;'I WO$ta81 TVA Figure 9. 100-year 24-hou r NOAA Atlas 14 spatial pattern Page 26 of 71
_-year 6-hour OAA Atlas 14 Precipitation stimates ormalizcd to a Maximum of t .00
..-w ...., M"W IO"W ... w e,*w
""" ,,*w wast lO"N
- < 035 - 065-070
- 035 -0.40 - 0 70 -0 75 040 -0.45 - 0.75 -0 60
,.. CJ 045 - 0 .5 0 - 0.60 - 0.85 C) 050 - 0 55 - 085 - 090 Georg*
- 055 - 0 .60 0 .90 - 0 95
- 060 - 065 0 095 - 100 t'..ocrdlN!M S,.~em wGS 1984 VTM J,:,oe 'l{IH
~
- _c::_c::_c::====-----====:::::iMtles 50 100 150 200 Plqecll(llll1~MeR'..,,
o.anwos,-. TVA Figure 10. 2-year 6-hour NOAA Atlas 14 Normalized spatial pattern Page 27 of 71
100-year 6-hour OAA Atlas 14 Prec.ipitation stimates onnalizcd to a Maximum of 1.00 Nomulized 100-yeu
~oul' Precipitation
- < 0 .35 - 065-070
- 0.35 -0 40 -
0.40 - 0 .45 - 0.70
- 0 75 0.75
- 0 80 r:::J 045 -0.50 - 0.80
- 0 85 C) 050 - 0 55 - 085
- O 90
- 055-060 090-095
- 060 -0650095-100 CrW C-...<<* S~*- WCS tilM UTM llllM 1&N
- c::-c:::a-=:=====------======Mdes 50 100 150 200 Pr~T~~
0...... WGS1'il84 TVA Figure 11. 100-year 6-hour NOAA Atlas 14 Normalized spatial pattern Page 28 of 71
2-year 24-hour , A Arla 14 Precipitation tima tes ormalizcd to a Maximum of 1.00
,rw J&' pj
- < 0 35 - 065-0 70
- 035 - 040 0 70 - 0 75 o*o-o *5
- o 75 - 080 JI 0 045 . o 50-080
- 0.85 c:) 050 - 0 55 - 085
- O 90
- 055 - 0 60 0.90 - 0 95
- 060 - 065 0 095 - 100 C000 ~ " IIGS11'4U'lMl~~
..._~,,~u
- =- =- -======------=====:: :i M 0 50 100 150 200 oles a<<
OIIM',\WOS1Mt TVA Figure 12. 2-year 24-hour NOAA Atlas 14 Normalized spatial pattern Page 29 of 71
100-year 24-bour Oi\A A 1la l4 Precipitation E ti mate* onnalizcd to a Maximum of 1.00
... rw ...w A01Qr. . s,,on WC.Sl ttllU.Ic,,*1flf 1-'t~l~Mflf<C'.9'
..::=ac::-=====-----=====Mlles 0 50 100 150 200 0..,., WG41!1t,1 TVA Figure 13. 100-year 24-hour NOAA Atlas 14 Normalized spatial pattern In summary, the NOAA Atlas 14 PF data provide reasonable spatial variation representative of orographic PMP because:
- 1. A regional approach decreases the uncertainty of rainfall frequency estimates for more rare events (upper end of distribution tail) , trading space for time.
- 2. At-site mean or MAM is used to capture local rainfall influence, while utilizing regional distribution to attain better estimates at more rare frequencies .
- 3. PRISM MAM development, spatial interpolation , and smoothing provide realistic representation of spatial precipitation patterns. MAM grid is based on Mean Annual Precipitation (MAP) and other climate parameters.
- 4. The concern of "spatial smoothing of averages rather than rarer events" is not an issue as NOAA Atlas 14 states "duration-dependent spatial patterns evolve independently of other durations" which is evident by looking at TVA Figures 6 through 9.
- 5. As compared to NOAA Technical Papers 40 and 49, the regional approach (vs . site- specific) and spatial interpolation methods (PRISM and CRAB method vs . iso-contours) , the NOAA Atlas 14 datasets provide a more realistic representation of orographic precipitation and the spatial distribution .
Page 30 of 71
TVA Response - 8(b): After further review and discussion with the NRC staff in regard to this question , TVA has agreed to conservatively revise OTF determination methodology used in Calculation No. CDQ0000002016000041 LIP calculations . For this aspect, TVA will utilize the 6-hour 100-year precipitation frequency climatology from NOAA Atlas 14 to adjust storms during the transposition process. This approach will be applied to the storms moved to the SQN , BFN and WBN plant sites. This update replaces the use of the linear fit method of the NOAA Atlas 14 precipitation frequency climatology . This change does not affect the Simpson , KY storm or the Smethport, PA storm as those OTF values were held to 1.00 as discussed in response to RAI #7 . Updates to this data set will affect Sections 6.4 of Calculation No. CDQ0000002016000041 and will be submitted as Revision 1 to Topical Report TVA-NPG-AWA16 (TVA Calculation CDQ0000002016000041) . Note, that TVA also performed sensitivity of using the 100-year only data versus the linear fit method in all other regions of the TVA basin . Differences between using the 100-year only values and the best fit linear trend are minimal for the TVA basin and within the margin of error associated with the uncertainty in the overall PMP development. Therefore , implementation as currently applied is acceptable with the exception noted above. RAI #9: OTF Calculation Issues Technical Deficiency: Potential issues with the OTF calculations in certain regions were identified by staff and require clarification. Staff's review of the Total Adjustment Factor (TAF) Excel files provided in response to RA/ #1 revealed some anomalies in how the OTF was calculated. For a select set of grid cells, the OTF was calculated using an absolute cell value in the Excel spreadsheet rather than using the OTF regression-based formula used in other cells. Visualization of the areas using the absolute cell reference value is provided in NRG Figure 4 (for general and tropical storms) and NRG Figure 5 (for local storms). NRC Figure 4: Grid cells for which OTF calculation used an absolute cell reference value for General & Tropical storms (the red grid cell indicates the location of the grid cell used for assigning an OTF value for all yellow colored grid cells) Page 31 of 71
NRC Figure 5: Grid cells (in yellow) for which OTF calculation used an absolute cell reference value for Local storms (the red grid cell indicates the location of the grid cell used for assigning an OTF value for all yellow colored grid cells) In addition, staff compared the Excel-based OTF values from the RA/ #1 response and the GIS-based OTF values from RA/ #3. The comparison revealed a discrepancy in calculated OTF values for local storms which was confined to a region of the southern Appalachians. Visualization of the areas affected by this discrepancy is provided in NRG Figure 6 . NRC Figure 6: Grid cells (in red) for which OTF differs between RAI #1 & RAI #3 for Local storms Page 32 of 71
Request: a) Provide an explanation for why the OTF was calculated using an absolute cell reference value for the grid cells identified in NRG Figure 4 and NRG Figure 5 rather than using the OTF regression-based formula used for the other cells. b) Provide an explanation for why the OTF values provided in RAJ #1 and RAJ #3 differ, as illustrated in NRG Figure 6. TVA Response - 9(a): The primary reason for applying a constant OTF based on an absolute cell reference rather than using the OTF regression-based formula over the regions highlighted in TVA Figures 14 and 15 was to address a significant discontinuity in OTF values for 35° N latitude. This discontinuity is a direct result of boundary discrepancies between NOAA Atlas 14 Volume 2, which covers the project area north of 35° latitude, and Volume 9, which covers the area south of 35° latitude . The boundary issues are briefly acknowledged in Atlas 14 Volume 9: "Precipitation frequency estimates for each volume of NOAA Atlas 14 were computed independently using all available data at the time. Some discrepancies between volumes at project boundaries are inevitable and they will generally be more pronounced for rarer frequencies" (Perica, et al., 2013, pg . 4). A secondary reason for applying the constant OTF was to smooth out "bubbles" that occurred over the non-orographic western portion of the project area, primarily south of 35° N, that do not necessarily reflect orography, terrain features , or elevation . General storm and tropical storm PMP utilize the 24-hour Atlas 14 precipitation frequency grids for OTF calculations . At the 24-hour duration, the boundary issue is most prevalent over northeast Mississippi/northwest Alabama and northeast Alabama/northwest Georgia , as shown in TVA Figure 14. For general and tropical storm PMP, the discontinuity is less of a concern than local storm PMP as general and tropical PMP tends to control the PMF for larger basins. The discontinuity tends to dissolve through the basin average over very large basins. Page 33 of 71
JOO-year 24-hour NOAA Atlas 14 Precipitation Estimates (inches) Teoessee Valley Authority Drainage Area 37'N 3N
'8'W 1/T'W f!lj"VJ 8','W ... w .,.... srw CoolQlnllle SySl8TI WGS 1i64 VTM lane 16N Prq<<11e1n Tranwe,w Meru:ot Mnes on.,n, WGS198.t 0 50 100 150 200 TVA Figure 14. 24-hour 100-year precipitation illustrating discontinuity between volumes for 35° N Page 34 of 71
Local storm PMP utilizes the 6-hour Atlas 14 precipitation frequency grids for OTF calculations . At the 6-hour duration , the boundary issue exists along the 35° N state boundaries similarly to the 24-hour duration . Furthermore, there is also a significant discontinuity over the Tennessee/North Carolina/Central Georgia state border. The depth of rainfall over the Hiwassee River drainage basin south of 35°N (Atlas 14 Volume 9) is significantly less than north of 35°N (Volume 2) , as shown in TVA Figure 15. 100-year &-hour OA A Atla 14 Precipitation Estim ates (inches) Tenessee Valley Authority Drainage Area
~*w vw "W 16-W 64 ' W .,.w a,*w 81 ' W JI'
- ,;*N 3N E4'W . .,, ,_,,W lO'W CclcrOr\a Systan WGS 1~ VTM 2ai.1GN
~qec>>onlrl 'ISW'WW~
-==--==-=== ==-----=====Mies 0 50 100 150 200 Dnm WGS 198A TVA Figure 15. 6-hour 100-year precipitation illustrating discontinuity between volumes for 35° N The western portion of the project area is non-orographic and predominantly lacking terrain features that could influence rainfall. Ideally there would be very little variation in the precipitation frequency estimates used to determine the OTF in this region . To correct for the significant variation occurring on either side of 35°N, a decision was made to recalculate the OTF over the 35°N region using a constant OTF from a representative point chosen in western Tennessee. This process was employed to remove the most significant portion of the OTF discontinuity resulting from the Atlas 14 boundary issues . Secondarily, the recalculation would smooth out any variations in the OTF over this non-orographic region that might occur from variations in the underlying Atlas 14 datasets.
Page 35 of 71
There are two areas of subjectivity involved in the recalculation process; defining the region to be recalculated to a constant OTF, and selecting an absolute cell reference as a representative location from which to take the OTF to assign to the recalculation area rather than using normal OTF regression-based formula . For general and tropical storm PMP, the recalculated region is shown in yellow in NRC Figure 4. The region was manually delineated in a manner that encompassed the problem area and followed the isopleth of the OTF spatial pattern consistent with the representative point chosen . The point location of 35.2° N, 87 .3° W was chosen to represent the non-orographic general and tropical storm recalculation region. For each storm , the OTF at each grid point inside the recalculation area was set to match the OTF value at the representative location . For local storm PMP, the recalculated region is shown in yellow in NRC Figure 5. The general process that was applied to the general and tropical storm OTF recalculations was applied to the local storm OTF. The local storm recalculation area covers a somewhat different area and different shape than the general/tropical storm area due to the spatial pattern of the 6-hour precipitation frequency differing from the 24-hour patterns. For local storm OTF recalculation, the point of representation is located at 35.475° N, -88.175° W. In addition, the area in red shown in NRC Figure 6 was reevaluated due to the significant disparity between the Atlas 14 volumes over the Hiwassee drainage area. The OTF for this area was recalculated in a similar way to the process described above for the western region of the project area where a representative location was chosen and each grid point within the area of interest (AOI) was assigned the OTF from the representative location . For this area , the absolute cell reference representative location was chosen as a point within the basin near the outlet of the basin at 35.15° N, -84.45° W. Due to the highly orographic nature of the AOI, a constant OTF reassignment alone is not sufficient; therefore an elevation adjustment factor was applied to the constant OTF to estimate the orographic effect over the AOI. The elevation adjustment factor was determined as the ratio of elevation at the target location to the elevation at the representative location , which is 1,624'. A sixth root is applied to sufficiently mute the ratio to be consistent with the surrounding OTF values outside the AOI. 6 htarget ElevationAdjustmentFactor = -h-- rep.
- where, htarget = elevation at the target location hrep = elevation at the representative location (1 ,624')
An example map of general storm PMP before the constant OTF adjustments over the western portion of the project area is provided in TVA Figure 16. TVA Figure 17 shows the same PMP map after the general/tropical storm OTF adjustments are implemented . Page 36 of 71
72-hour General Stonn PMP ( L0,000 mi") - Before OTF Adju tme nt TVA Study Area Ol'W erw eft"W 115-W (WW ID'W rrrw J7'N l7'N ,.. l'>'N
- M' N PM P Depth 3N (In ches)
- <2 1111 12 24 - 26
- 2 14 26 - 28 4-6
- 16-18
- 26-30 6 18 30 - 32 o s - 10
- 20-22 0 32 . 34 0 10 22-24 0 > 34
..... ,,,... a.:rw &rw Coord11'1Mi1S1'Stf11'1 USA~Abl!t1~1Ar.. eor.c:
llli(t(ICll(ln~
-c:=--==--======------======M 0 50 100 150 200 iles OIi.im NOnh A.Intl 1CM1 t983 TVA Figure 16. Example of general storm PMP before constant OTF adjustment Page 37 of 7 1
72-hour General Storm PMP ( I0,000 m?) - After OTF Adjustments TVA Stud y Area
~ fJIJ"W ~"W !W'W a:rw ,rrw J7"N 34* PMPOepth ,. ..
(lncM s)
- <2 14 26
- 2 14-16-26-28
- 4 16-1 8 - 28 - 30 6 18 30 - 32 6 20- 22 32 - 34 0 10 ..-w 22-24 0 > 34 vw .
-::::11-==--=== =::::11----i:====:::iM 0 50 100 150
..-w 200 iles
~ S , s H m USA~~E~mt!Coni< _
OIIUffl North "menc*n1983 TVA Figure 17. Example of general storm PMP after constant OTF adjustment An example map of local storm PMP before the constant OTF adjustments over the western portion of the project area and the elevation-based OTF adjustments over northern Georgia is provided in TVA Figure 18. TVA Figure 19 shows the same PMP map after the local storm OTF adjustments are implemented . Page 38 of 71
6-hour Local Stonn PM P ( IO mi2) - Before OTF Adjustment TVA tudy Arca IO'W erw arw ~-w M-w m*w ,,.., V'N .6'N ,.. PIIP Depth ,.... (Inches)
- <2 - 12 24 - 26
- 2 14-16 28
- 4 16 28 - 30 6 18 30 - 32 8 20 - 22 32 . 34 0 10 .....
22-24 0 > 34 _c:::. . 0 ic:=-a:::::=====------======Miles 50 100 150 200
... w 12",\i C.OO,....,S.514'1'1 USA~,Atlft1Eo,i,AIMCoril< _..
OP.,rrl No11'1¥Wftt.M 19') TVA Figure 18. Example of local storm PMP before constant and elevation-dependent OTF adjustments Pag e 39 of 71
6-hour Local Stom1 PMP ( 10 mi~) - Afl er OTF AcU u tment T A tudy Arca
**w arw 81'J"W ar.*w &a'W a:,-w 3TN ,, .
~*N
.M' PUP Depth pnches)
- <2 - 12 24 - 26
- 2-4 - 14 26-28
- 4-6 - 16 28 - 30 6-8 - 18 30 - 32 8-10 20 - 22 32-34 10* 12 - 22-24 0 > 34 erw
*+
M"W MW 111a:::::::a-===-a:==:==:==::=::a--------===:==:==:::::i Moos 0 50 100 150 200 TVA Figure 19. Example of local storm PMP after constant and elevation-dependent OTF adjustments TVA Response - 9(b): (Note: NRC 9(b) question references RAI #1 and RAI #3 . These RAI references actually refer to informal information provided in response to NRC's audit Information Need #1 and #3 , respectively .) TVA inadvertently provided an older version of the local storm OTF values for Information Need
- 1 . The Total Adjustment Factor spreadsheets provided for Information Need #1 included the constant OTF adjustments made over the western portion of the project area , but did not yet include the elevation-based adjustments that were applied over the Hiwassee drainage as described above. The GIS files provided in Information Need #3 were the final version of the local storm OTF and included all adjustments and therefore were different than the values provided for Information Need #1 for the grid points highlighted in NRC Figure 6.
Page 40 of 71
RAI #10: Custom Transposition Limits Technical Deficiency: Based on staff's review of information provided in response to RA/ #1 , the majority of storms included transposition limits that conform to the TVA Zone boundaries. However, at least four storms appeared to contain custom transposition limits, as listed in NRG Table 2 that don 't conform to the TVA Zone boundaries. NRC T a bl e 2 S ummary of storms su b"11ec ted t o cus t om t ranspos1T10n I"1m1*ts Storm SPAS No. Storm Type Transposition Limits ' Elba , AL 1305 General South of 35 deg N (exclusive of Zone 4) Americus , GA 1317 Tropical Based on TSR L-Cv 0.24 contour* Larto Lake, LA 1182 Tropical Based on TSR L-Cv 0.24 contour* Big Rapids , Ml 1206 General North of 36.5 deg N (exclusive of Zone 4)
- Note: information from TAF Excel file, OTF sheet Request:
a) Provide a justification as to why each of the storms listed in NRG Table 2 was subjected to custom transposition limits. b) Provide a justification for the use of custom transposition limits for the Americus, GA and Larto Lake, LA storm using TSR L-Gv 0.24 contour. Provide the physical basis used to justify this custom approach. TVA Response - 10(a): (Note: The NRC question references RAI #1 . This RAI reference actually refers to informal information provided in response to NRC's aud it Information Need #1 .) Each storm on the final storm short list was evaluated for explicit transposition limits . TVA transposition Zones (1-4) were used as initial reference for all storms. Further refinements between and within zones took place only as required based on unique individual storm characteristics and/or maintaining spatial continuity of adjustment factors and PMP depths. Discussions took place between AWA and other TVA study participants during the Review Board meetings to evaluate specific storm transposition limits. Extensive discussions regarding transposition limits and specifically refined boundaries or constraints were requ ired because of the meteorological judgment that is applied in developing and assigning the transposition limits for a given storm . Specific to the four storms listed in the request, the following response is provided :
- The Elba , AL storm was limited to areas south of 35°N latitude based on the synoptic meteorology associated with the storm , including direct access to Gulf of Mexico moisture without any intervening topography , the combination of synoptic meteorological factors that led to the storm versus what could occur over the TVA basin , and previous transposition limits applied by the NWS (TVA Figure 20) . The synoptic meteorology associated with this event was directly related to the moisture Page 41 of 71
and thermal environment associated with the interaction of the front moving through the region and the relatively warm waters of the Gulf of Mexico. This combination would not occur further north during this season of occurrence. Note that the NWS transposition limits map explicitly shows that the storm is only transpositionable to 34°N latitude. It is assumed the NWS considered the synoptic meteorological environment to be a limiting factor of not moving the storm further north . For final application , TVA applied a more conservative transposition limit to this storm (to 35°N latitude) to account for the judgment involved in the process, to allow consideration of general topographic similarities to regions around 35°N latitude, and to produce more spatially consistent PMP depths between where this storm was transposed and where it wasn't transposed . For this storm , it was determined that the combination of available moisture and storm dynamics could not occur further north during the March timeframe without changing the storm dynamics significantly.
. * - li. , ........- ~ .
. , CT
...* * . . . .I
\ -- . ~ ...... -... __ (.~
- 1. ,
.' .f-- ---.. I
; 0 I - - - - - - ...
" * * - ~ ---11
* . J ' - :
. -~*::..-:..- -
,l * - - - - - ..
TVA Figure 20. NWS transposition map for the Elba, AL March 1929 storm
- The Americus , GA and Larto Lake , LA storms were both tropical events. As mentioned previously, each storm 's individual synoptic meteorological environment, interaction of moisture source and topography, and previous transposition limits were investigated in relation to the overall TVA basin . This demonstrated that these two storms should not be transpositioned any further north than currently applied . This is because direct land falling tropical systems do not affect most of the TVA basin without significant degradation and changes to the structure. This results from interactions with topography and distance from the moisture source.
Page 42 of 71
- For the Big Rapids storm , similar AWA discussions with the TVA Review Board and PMP study team took place regarding moisture source, topograph ic interactions and time of the year when the storm occurred. Explicit limits on the Big Rapids storm were applied because it was a controlling event and therefore required further evaluation to ensure spatial continuity in the PMP depths between the regions where it was used and the regions that bordered that area .
In this case , a PMP-type general storm occurring in September that included the required storm dynamics related to significant thermodynamic contrast would not be able to occur in the same fash ion further south . In th is case , the southern limit was judged to be 36.5° N latitude. Th is southern limit also considered the latitudinal extent of the storm and constraints following HMR guidance of applying a 5-6° latitude constraint (HMR 57 , page 69). Analog storm events with explicit NWS transposition limits maps were consulted as well as an analog data source to corroborate the transposition limits applied . Storms of similar type and season in the NWS transposition library of storms in Michigan and Wiscons in were not transpositioned south of 37° N latitude. TVA Figures 21-23 explicitly show NWS transposition limits maps and show that none were used further south than 37N latitude. Given these considerations , the application of this storm to 36 .5° N latitude was a conservative application compared to prior NWS guidance .
* ; 1- Sep t .1 . 191 ... Co oper. l.
8 ** 250 s-;i ** f~ 11;ss~ .- rtarth. to : . Or <1cr zu,~ t o: a t e- ~ba. t ta.noo g 11.ri Sou t b to~ 37 e* to : 99
. (. .' ~- -
... *-~......; ,. .
j \ j 1---=-
* .* *l -;
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I
/
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TVA Figure 21. NWS transposition map for the Cooper, Ml September 1914 storm Page 43 of 71
.... . 'l l- =- .. ~ .. ... *.
*I * *
- o. _-;. .
~ .
l *
.* I I
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/
..,I TVA Figure 22. NWS transposition map for the Hayward, WI August 1941 storm Page 44 of 71
*-v * * ., '"; * *"
_:_ 7..
~
.*. . . I I'
j
/
/
TVA Figure 23. NWS transposition map for the Merrill, WI July 1912 storm TVA Response - 10(b): TVA Figures 24-29 display the transposition limits appl ied to storms by the NWS , which occurred in similar regions as Larto Lake, LA and Americus , GA events . Each of these figures clearly demonstrates that the NWS did not consider storms in these regions transpositionable to the Tennessee Valley and specifically to the interior regions of the Tennessee River basin . In fact, the custom limits applied by TVA (TVA Figures 30 and 31)are significantly more conservative than the NWS transposition limits. TVA allowed these storms to affect the southern portions of the TVA region to ensure appropriate spatial continuity in PMP values between these regions and to account for the uncertainty of where the exact boundary would occur. However, the rainfall that occurred with the storms would not occur in the same meteorological and topographical setting existing over the interior TVA regions . Note, this does not mean that remnant tropical moisture doesn't produce rainfall in that region. Instead the magnitude of those rainfall events is significantly reduced because of the different interactions of topography and meteorology, thereby violating the definition of transpositionability . This requires that remnant tropical storms that occurred in similar meteorological and topographical settings be used in that region . This excludes the Larto Lake, LA and Americus , GA storms from consideration . Page 45 of 71
~*1 ....._"' * . : .....:.e l.r-0 . :. ~ * . ...i.:e.L::.!:.:.: .!.
- ~- i>r. :-*!'i 750 3ti:-.) .. *os .
- t.o 7e . :
.;..iS t. -;o : d:
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TVA Figure 24. NWS transposition map for the Alexandria, LA June 1886 storm Page 46 of 71
\,
TVA Figure 25. NWS transposition map for the Simmesport, LA May 1935 storm Page 47 of 71
_ .* (..- . . ;-. ....~ r. : ; - l o. : ; ~ *
- 1Uta~ * .
__- .r . ~ ~..,. CO ~ ':..."". ) * * - .:. :35 ,i . * ,:. C ~
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.... - -- - .t.1-....:---.J TVA Figure 26. NWS transposition map for the Eutaw, LA April 1900 storm Page 48 of 71
r* -~, . - .~ _,_.
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' I ,; / .,' ii.,_*" I"';.' :* I.I.,: L.. . ' '- c* ' .> ..., _,,, rr==~""""="" ~ ~~~====,,,,;,,...-=-::~
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( TVA Figure 27. NWS transposition map for the Merryville, LA March 1914 storm Page 49 of 71
L/1JI 3 . , . . , 7 t~::211 ) 1L-:.- , 2 1:::,,,/}':lJf'4 12 J...r r i :-{ / 3 ( / "ft ) .? SE, "S ~ J o,.c_ ;:: 11e 5 C tt.-'
-- ..--.. ' i
*--***-~ ~ * -- - -.... ,
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.i I TVA Figure 28. NWS transposition map for the Lakeside, LA November 1922 storm Page 50 of 71
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--- r . . -- .
... ':'A
- ..., ,:: * "
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- t;O '; t _; ... :if j-
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TVA Figure 29. NWS transposition map for the Elba, AL March 1929 storm Page 51 of 71
Larto Lake, LA , Sep. 2008 (SPAS 11 82) Transposition Limits TVA PMP Study ttrw 86'"11 <<."W 6.l"W P/J"W 34' I ... At,~1 l ......... .
, .\1tl* II
- _... ;\4'N
. ~.
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~s,,,:wn USA~s...,.,, EC,.-AruQnic DP.Im HOlth NMK".all 1913 Mijes 0 50 100 150 200 TVA Figure 30. TVA transposition map for the Larto Lake, LA September 2008 storm Page 52 of 71
Americus, GA, July, 1994 (SPAS I 317) Transposition Limits TVA PMP Study VW 86"W 15'W &l 'W 83"W rr,-w JT"N
,.. ,, ~k,l*u L**u,o.. O'l ~le A~ffi ll!'llt*~
Alt,ml.
. ."W rrw ,,,-w . ."W ...w .,.., lfZ'W
*t
- COOrcttn*S,-s:n USACorltlguws~EQ,lalArwConlc Pt-..onM>trs o.tffl NDrtti AtnwfQn 19e3 Mi es 0 50 100 150 200 TVA Figure 31. TVA transposition map for the Americus, GA July 1994 storm After further review and discussion with the NRC staff in regard to this question , TVA has agreed to remove reference to use of the TSR L-Cv 0.24 contour as one of the reasons for defining the updated transposition limits of these two storms. Instead, discussions specifically related to the differences of the meteorological and topographical environments are used to define and justify the transposition limits . These updates to th is data set will affect Section 5.3 of Calculation No. CDQ0000002016000041 and will be subm itted as Revision 1 to Topical Report TVA-NPG-AWA16 (TVA Calcu lation CDQ0000002016000041 ).
Page 53 of 71
RAI #11: Storm Representative Dew Point Selection: Timeframe and Location Technical Deficiency: Staff's review of the licensee 's storm representative dew point data for short list storms resulted in the identification of several storms for which questionable timeframe and/or location data may have been used when selecting the storm representative dew point. This issue can significantly impact PMP values for controlling storms As a part of its assessment, staff reviewed the rainfall mass curves, HYSPLIT trajectories, and storm representative dew point information that the licensee provided in response to RA/ #1 and RA/ #2. Staff also independently evaluated this information to assess the reasonableness of the data application. Staff's review of the above information revealed that the licensee 's storm representative dew point selection used dew point data which were observed at locations far upwind of the storm center and during timeframes in which significant rainfall had already occurred. Conducting the analysis in this way could inadequately represent the storm characteristics and (in these cases) result in PMP underestimation since the relatively higher moisture observed could not induced the observed rainfall. Staff believes the storm representative dew point methodology regarding HYSPLIT trajectories and/or dew point timeframes may be flawed for the following storms. A comparison of the TVA and NRG storm representative dew point temperatures these storms is provided in NRG Table 3.
- 1. General Storm, SPAS 1206 (Big Rapids, Ml) - see NRG Figure 7
- a. The licensee 's dew point temperature observations correspond to a period after significant rainfall had already occurred. The representative dew point location is approximately 230 miles SW of the storm center location.
- 2. General Storm, SPAS 1208 (Warner Park, TN) - see NRG Figure 8
- a. The licensee 's dew point temperature observations correspond to a period when the most intense rainfall occurred. The representative dew point location is approximately 360 miles SSW of the storm center location.
- 3. Tropical Storm, SPAS 1276 (Wellsville, NY) - see NRG Figure 9
- a. The licensee 's dew point temperature observations correspond to a period when the most intense rainfall occurred. The representative dew point location is approximately 385 miles SSW of the storm center location.
- b. By adjusting the HYSPLIT backward trajectory timing to more closely align with the onset of rainfall, staff identified a moisture inflow direction of SE rather than SSW
- 4. Tropical Storm, SPAS 1317 (Americus, GA) - see NRG Figure 10
- a. By adjusting the HYSPLIT backward trajectory timing to more closely align with the onset of rainfall, staff identified a moisture inflow direction of SE-to-S rather than WSW Page 54 of 71
- 5. Additional storms which exhibit timeframe issues but do not control PMP
- a. General Storm, SPAS 1218 (Douglasville, GA & LaFayette, GA) - see NRG Figure 11
- b. Local Storm, SPAS 1226 (College Hill, OH) - see NRG Figure 12
- c. Local Storm, SPAS 1209 (Wooster, OH) - see NRG Figure 13
- d. Tropical Storm, SPAS 1182 (Larto Lake, LA) - see NRG Figure 14 NRC Table 3. Comparison of TVA vs NRC storm representative dew point temperature for storms W I'th po ten f 1a I HYSPLIT or f 1mmg
. .issues Storm Storm Rep. Td Difference Number Storm Name SPAS Number Type (deg F) (TVA-NRC)
TVA Td NRC Td 1 Big Rapids , Ml SPAS 1206 General 70.5 68.5 +2 2 Warner Park, TN SPAS 1208 General 75 74 +1 3 Wellsville , NY SPAS 1276 Tropical 72.5 70.5 +2 4 Americus, GA SPAS 1317 Tropical 76 74 .5 +1 .5 Sa Douglasville, GA SPAS 1218_1 General 76 75 +1 Sa LaFayette, GA SPAS 1218_2 General 76 75 +1 Sb College Hill , OH SPAS 1226 Local 68.5 66 .5 +2 Sc Wooster, OH SPAS 1209 Local 76 72 +4 5d Larto Lake , LA SPAS 1182 Tropical 76 73 +3 Request: Provide justification for the selection of storm representative dew point values for the above storms with respect to timeframe and location selected, especially considering the timeframe of when rainfall occurs at the storm center. If corrections are warranted, provide an updated analysis as it may affect TVA 's 3 NPP sites. Page 55 of 71
TVA Mass Curve - TVA Dew Point Selection SPAS 1206 Storm Center Mass Curve: Zone 1 September9 (0600 UTC) to September 13 (0500 UTC), 1986 Lal 43 61 25 Lon -85 3125 travel time offset ,. based on HYSPLIT 1.5 ,, 20 60 60 Index Hour SPAS 1206 Big Rapids. Ml Storm An alysis September9-13 1986 NRC Figure 7. General Storm, SPAS 1206 (Big Rapids, Ml) rainfall mass curve (top) and dew point analysis (bottom) Page 56 of 71
TVA Mass Curve - TVA Dew Point Selection SPAS 1208 Storm C*nterMassCurve: ZoM 1 May 1 (0100 UTC) to June 3 (1200 UTC), 2010 Lat 36 11 Lon -S8 05 2.5 Perlodof ralnfllll reprwww._.ve ofAWAdew polntseledlon 10 2l) 30 50 fD lndeX Hour SPAS 1209 - Dew Point Temper*ture (F) April 29
- May 2. 2010 NRC Figure 8. General Storm, SPAS 1208 (Warner Park, TN) rainfall mass curve (top) and dew point analysis (bottom)
Page 57 of 71
TVA Mass Curve - TVA Dew Point Selection SPAS 1276 Storm C*nt*r Mass Curve: Zone 1 Jun* 18 (0700 UTC)-.June 25 (0600 UTC), 1972 Lat *2 0375 Lon -78 0708 tra vel time offset based on HYSPL/T 2.0 0.5 o.o - - ~ 10 - 20
~ -lO- -- - ~ 70 90 100 110 120 130 ,,o 150 160
" 50 60 60 Index Hour SPAS 127& Scorm An,1fy ais June 1~ 22. 19n TVA HYSPLIT NRC HYSPLIT Backward trajectories ending at 0000 UTC 20 J un 72 NOM HYSPLIT MODEL Bad<Ward trajectories ending at 0000 UTC 21 Jun 72 CDC 1 Meteorolog ical Data CDC1 Meteorologlcal Dala
.-< -./
~
<O
~
z {."' ;!; N j V
." r' l!
s J.o-~ r NRC Figure 9. Tropical Storm, SPAS 1276 (Wellsville, NY) rainfall mass curve, dew point analysis, TVA HYSPLIT, and NRC HYSPLIT (from top, middle and bottom) Page 58 of 71
TVA Mass Curve - TVA Dew Point Selection SPAS 1317 Storm Center Mass Curve: Zone 1 June 30 (0700 UTC ). July 9 (0600 UTC), 1994 Lat 32 0958 Lon -114 22!12 travel time offset based on HYSPL/T, 50 Index Hour 1SO 200 SPAS 1317 Alberto, GA Storm Analysis
!'!,lf 5-6, 1~
TVA HYSPLIT NRC HYSPLIT NOAA HYSPLIT MODEL NOAA HYSPLIT MODEL Backward trajeciories ending at 0600 lJTC 06 Jul 94 Backwa rd trajectories ending at 0000 UTC 04 Jul 94 CDC 1 Meteorological Data CDC1 Meteorological Data 1i j
~
.:moe II 12 M 00 18 01.os ll ~~
01;o. II 12 700 750 rs& 900 950 1000
~
JcblOlUl:538 J.IS&.1* 5-1 2822.23 45U1C:.01l
~ 111'1
- 3'l090000 Ian 4"1.2!l0000 11g9* 0 1.a2D, !JO!iO,nAQ.
~~~~~\l~VlloUr
~OOCXlZ 1..ltl2'>><1---1
- NRC Figure 10. Tropical Storm, SPAS 1317 (Americus, GA) backwards HYSPLIT trajectory from TVA (left) and NRC (right)
Page 59 of 71
TVA Mass Curve - TVA Dew Point Selection SPAS 1218 Sto rm Center Mass Curve: Zone 1 September 19 (1300 UTC) to September 22 (1 2.00 U TC), 2009 Lal 33.87 Lon. -84 76 3.01~= .. 1 2.S m
~
12 " .0
~ 1.5 Q. "cE 1.0 ~
o.s 7D SPAS 1218 . Dew Po in t Te mperat ur e (F) September 18-22. 2009
-* hrll,c,e * !eiDl'Jl :l
- JOCnO NRC Figure 11. General Storm, SPAS 1218 (Douglasville, GA
[shown] & LaFayette, GA) rainfall mass curve (top) and dew point analysis (bottom) Page 60 of 71
TVA Mass Curve - TVA Dew Point Selection SPAS 1226 Storm C*nt*r Mus Curv*: Zon. 1 Jun* 4 (0600 UTC) to Jun* 6 (0600 UTC), 1963 Lat 40 0854 Lon -S1 6479 travel time offset 3.S I~=:~ I 20 3.0 I
~2.S AWA dew
- C
- ~i sellctlon 2.D
~
a. 1i 1 S c E
~ 10 £ o.s 10 *o lndelCHour '° College HIN , OH StOfm Ana.lyals June 1*5. 1963 NRC Figure 12. Local Storm, SPAS 1226 (College Hill, OH) rainfall mass curve (top) and dew point analysis (bottom)
Page 61 of 71
TVA Mass Curve - TVA Dew Point Selection SPAS 1209 Storm Center M~s Curve: Zone 1 July 4 (0600 UTC) to July 7 (0500 UTC), 1969 travel time offset La1 40 915 Lon .a, 973 3.0
----------------t" . ...
2.5 I \S., 2 .0 j
- ~ 1.S Q.
~ E 1.0 i 0.5 10 70 SPAS 1209 Wooster. OH Storm A rudysta
.klly2*5. 11169 NRC Figure 13. Local Storm, SPAS 1209 (Wooster, OH) rainfall mass curve (top) and dew point analysis (bottom)
Page 62 of 71
TVA Mass Curve TVA Dew Point Selection SPAS 11 82 Storm Center Mass Curve: Zone 1 NOAA HYSPUT MODEL Septemti.r 1 (0100 UTC) to September 5 (0000 UTC), 2008 Backward traiectory ending at 0000 UTC 03 Sep 08 COC1 M*t.orologic.'IJ Oa.t:l
.........- - ~- -~ ~- - -~ - - - - -
1000 Index Hour NRC Figure 14. Tropical Storm, SPAS 1182 (Larto Lake, LA) rainfall mass curve (left) and dew point analysis (right) TVA Response: In each of the four cases (NRC Table 3, Storms 1 through 4) noted as potentially controlling the PMP, the air mass evaluated as represented by the TVA storm representative dew point selection was inclusive of the overall air mass advection into the overall storm domain. Although , the exact timing of the 12- or 24-hour period chosen occurs during a later portion of the overall ra infall period at the storm center, it is still representative of the overall air mass that was part of the rainfall event across the entire region. This is the intent of the storm maximization process to represent the overall air mass resulting in the observed event. The final process requires the analyst to calculate a specific value at a specific location at a specific point in time. However, in actuality, the moisture advection and storm development processes change in space, time , and magnitude. One of the more important considerations relative to the intent of the in-place maximization process is that moisture associated with these events is at higher than normal levels. Therefore , the region chosen as the moisture source region and the eventual storm representative dew point should also represent values that are higher than normal. In some cases , data available to analyze and select a value were inadequate and require judgments not fitting the standard processes. For example, there may be a lack of surface dew point observations in a general region where the air mass source region would be expected to originate in either space or time . Further, the storms that are being maximized are extreme rainfall events. In such cases , judgments must be appl ied to allow for selection of storm representative dew point values that can be used in the maximization process and represent a high level of atmospheric moisture. Storm No. 1: Big Rapids, Ml The storm representative dew point selected by TVA does represent a later portion of the storm , which is acceptable, and within the standard process of storm representative dew point selection. This process allows for selection of a 24-hour average value even though the overall storm period may be significantly longer than 24-hours. The value selected by TVA represents a period when the heaviest rainfall occurred during the storm and is Page 63 of 71
representative of the overall air mass, which advected moisture into the overall storm environment over several days. TVA investigated daily weather maps to determine the general location of high and low pressure centers and fronts immediately preceding and during the event. This analysis confirmed the general air mass source region as shown by HYSPLIT and used for the storm representative dew point location . Hourly surface observations were investigated for stations in a large region surrounding the general air mass source region from west through south-southwest as shown . Stations in this general region showed values ranging from the upper 60°'s to 70°F as 24-hour averages. TVA checked various surface observations to find a region that was most appropriate given the data available, the general moisture inflow region, the synoptic environment, and the severity of the resulting rainfall. These investigations showed that a region to the wesUsouthwest was most synoptically relevant and that high values in that region were necessary to have resulted in the record rainfall that occurred. This storm controls PMP at 48- and 72-hours for area sizes greater than 5,000-sqaure miles in many studies in the region. Even after choosing the highest of the available values (70°F at KMMO), the in-place maximization factor (IPMF) was still 1.40. The KMMO wind speed and wind direction in relation to moisture inflow timing at the storm also support the general air mass source and timing as identified in HYSPLIT and daily surface weather maps. This is an example where the available surface dew point data and the general process used to derive a storm representative value isn't necessarily representative of the overall environment. This can occur when there is an intervening frontal boundary and the most critical moisture is along an elevated boundary above the surface and not best reflected by the surface dew point values. However, the data available is still limited and the standard process and choosing the KMMO values was determined the best solution for this storm . Storm No. 2: Warner Park, TN This storm was associated with a large moist air mass flowing from a general south to north direction off the Gulf of Mexico, with a deep moisture tap well into the southern Caribbean. This feed of moisture lasted for several days and is one of the reasons the timing utilized by TVA is appropriate for this IPMF analysis. Recent research has termed these types of events as the "Mayan Express" , with similar characteristics to an Atmospheric River event (Higgins, et al. 2011 ). From https ://www. climate. g ov/news-featu res/event-tracker/maya-express-beh ind-gulf-coast-soa king, (Di Liberto 2016), "Four-day rainfall totals near two feet caused devastating flooding in parts of Louisiana, Texas, and Mississippi in mid-March 2016. To blame was a seemingly never-ending stream of moisture straight out of the tropics ." The surface analysis completed by TVA reflects these synoptic characteristics and shows a very uniform air mass in time and space over much of the regions from Louisiana to north Florida. TVA investigated several regions with dew point observations and chose the area over southern Mississippi because it exhibited high dew points over a large area with consistent values that were within the air mass advection region. Again, this type of rainfall would only be associated with extremely moist air masses, which were reflected by the 74-76°F 12-hour average values over a large region of Louisiana, Mississippi and Alabama . This analysis shows that the timeframe and location used by TVA is appropriate given the overall rainfall accumulation period and the continued moisture advection over time through the region and into the Warner Park storm center location . Page 64 of 71
Storm No. 3: Wellsville, NY The air mass associated with this storm covered a large region of the eastern United States from the Gulf of Mexico through New England over a several day period . This was shown by the synoptic analysis of the storm after landfall as it traveled through the region producing several rainfall centers over a period of several days. As has been explicitly demonstrated, the Wellsville, NY center received moisture that was advected over the Piedmont region of the Carolinas and Virginia , which included very moist air supplied by the Gulf Stream off of the Atlantic. Again , this air mass was present for several days and the values chosen by TVA represent the overall air mass over the several day period. The surface dew points used were representative of the overall region in space and time, with low ?O's through Virginia, North Carolina, and Maryland . Higher values were occurring at the same time along the immediate coast and Outer Banks region (mid ?O's). Given that this storm produced record rainfalls and many floods of record in Pennsylvania, it was appropriate that the dew points chosen represented an extremely moist air mass. Justification could be made that the values along the Outer Banks could be used , which would have resulted in a lower IPMF. The current value of 1.29 is conservative given these factors . Storm No. 4 : Americus, GA The overall air mass for this storm covered a large region extending from the Gulf of Mexico inland through Louisiana, Mississippi, Alabama, and Florida. The storm produced extreme rainfall because it was able to tap into this consistent moisture feed for several days while remaining over the same general region of Alabama and Georgia . The location TVA chose was just outside of the main rain shield and in a region reflective of this type of air mass over southern Louisiana and Mississippi. Values were consistent through a large domain , in the mid ?O's over several days and also consistent with values as far away as north and west Florida . Even with these high dew points, the IPMF was still 1.21, wh ich is conservative given the extreme amounts of rainfall associated with this storm . The following storms, 5a through 5d , do not control PMP. Storm No. 5a : Douglasville/Lafayette, GA Hourly surface observations were investigated for stations in a large region surrounding the general air mass source region . The average wind speed for the 24-hour storm representative dew point was 5.0mph (-9.2 mph maximum) with an average wind direction from the southeast. The time for moisture to travel from the source location to the storm center (-350-miles) would be approximately 72.0-hours (38.0-hours for max wind speed) . The general area wind speed and wind direction in relation moisture inflow timing at the storm also support the general air mass source and timing as identified in HYSPLIT and daily surface weather maps. The region chosen by-TVA best represents the region that supplied the low-level moisture to the storm from the Gulf of Mexico given the general synoptic patterns and movement of the storm and frontal system . Storm No. 5b: College Hill, OH This storm is an example where available surface dew point values did not accurately capture the air mass that contributed to the extreme rainfall. Therefore, meteorological judgment was applied to derive a storm representative value that represented a moisture air mass that would have been required to produce the significant amount of rainfall that Page 65 of 71
occurred . This required the use of surface dew point observations that occurred in a timeframe that was not ideal for the storm environment. The average of 66.SF provided by the NRC in Table 3 is accurate given the exact timeframe. The value chosen by TVA of 68.SF was based on values that generally occurred after the main storm precipitation period , but within the same general air mass, region , and synoptic environment that resulted in the storm . Note that the use of 66.SF or 68.SF has no impact to the TVA PMP values as the in-place maximization factor was already at the upper limit at 1.48 and is capped at 1.50. Storm No. Sc: Wooster, OH Hourly surface observations were investigated for stations in a large region surrounding the general air mass source region . The average wind speed for the 24-hour storm representative dew point was 13.0mph (-19 .5 mph maximum) with an average wind direction from the west-southwest. The time for moisture to travel from the source location to the storm center (-140miles) would be approximately 10.8-hours (7 .2-hours for max wind speed) . The general area wind speed and wind direction in relation to the moisture inflow timing at the storm also support the general air mass source and timing as identified in HYSPLIT and daily surface weather maps. Storm No . 5d : Larto Lake, LA Hourly surface observations were investigated for stations in a large region surrounding the general air mass source region originating from the Gulf of Mexico and crossing the Texas/Louisiana coastal regions . The average wind speed for the 24-hour storm representative dew point was 11 .0mph (-17 .0-20.0 mph maximum) with an average wind direction from the southwest. The time for moisture to travel from the source location to the storm center (-180miles) would be approximately 15.8-hours (10.4-hours for max wind speed). The general area wind speed and wind direction in relation to the moisture inflow timing at the storm also support the general air mass source and timing as identified in HYSPLIT and daily surface weather maps. After extensive discussions and review with the NRC and TVA personnel , TVA has agreed to utilize the more conservative NRC storm representative dew point values for this PMP study for storms 1-4 in NRC Table 3. TVA will update the storm representative dew point values in the storm database and recalculate the PMP with those values implemented. This will affect Sections 6.8, 6 .8.1, 6 .8.4 , and Appendix F of Calculation No. CDQ0000002016000041 and will be submitted as Revision 1 to Topical Report TVA-NPG-AWA16 (TVA Calculation CDQ0000002016000041 ). RAI #12: Staff Independent Analysis of Dew Point Climatology Technical Deficiency: Staff's independent evaluation of dew point climatology reveals that the licensee's values may be non-conservative due to potential data source and processing issues which may impact the estimated PMP values. As a part of its assessment, staff reviewed the dew point climatology data provided by the licensee in response to RA/ #1 and RA/ #4; staff also independently evaluated these data to assess the reasonableness of the climatology data used. Staff has concerns with the dew point climatology data source and processing used by TVA. While TVA used NOAA 's TDL data set, NRG staff used NOAA 's TD3505 data set. Both TD3505 Page 66 of 71
and TDL data sets are officially released by NOAA, but the TDL data set used by TVA is basically a collection of instantaneous weather station observations whereas the TD3505 used by NRG is subjected to additional QC and processing by NOAA. Although both data sets are largely similar, there are some differences in the annual maximum series (AMS) caused by missing/erroneous values originally included in the TDL data set. This leads to different AMS and 100 y dew point estimates because of the existence and treatment of missing observations. Such differences result in systematic biases which could affect moisture maximization factors and transposition factors for all storms. To assess the impacts of using different data (and some minor differences in processing), NRG staff conducted independent evaluation of dew point climatology for all short list storms, and it yielded a number of differences from TVA 's evaluation. In general, NRC's independent evaluation resulted in higher dew point climatology values, with variation both temporally and spatially. For all else being equal, an increase in dew point climatology values will result in higher PMP estimates since historical storms would be subject to higher levels of moisture maximization. NRG Figure 15 shows the difference in the NRG and TVA dew point climatology values for each comparable station for all short list storms. The stations selected represent the stations which would have most influenced the dew point climatology at the transpositioned moisture source location and for which climatology values were available from both the TVA and NRG data sets. Positive values indicate that NRC's evaluation resulted in higher dew point climatology values than TVA, while negative values indicate that NRC 's evaluation resulted in lower dew point climatology values than TVA. On average, the difference for General, Local, and Tropical storms is +O. 69 F, +O. 61 F, and +O. 52 F, respectively, with an overall average station difference of +0.65 F. Individual station differences range from -1 .44 F to +3.67 F. D iffere nce i n D e w P oint Cli m ato logy for Short List Storms 4 G:"
- 0 0
3 0
- s. 0 0
0 0 0 8 C: 2 0 0 0 .. O 0 g C) 0 8 0 : 0 0 0 le: 1 0
<(
i....'.J 0 z a::
-1 ... 0 0
-2
~ne ra f Sto mi Avg Oi e rence l oca l S torm Avg Diffe rence rop,cal Stonn Avg Oi erence NRC Figure 15. Difference in dew point climatology values between NRC (ORNL) evaluation and TVA (AWA) evaluation for all short list storms Each column of data points corresponds to one short list storm. Black-outlined diamonds represent station data (one diamond corresponds to the NRG-TVA difference for a single Page 67 of 71
_J
station; for most storms, multiple stations were available for comparison) which influenced the dew point climatology at the transpositioned moisture source location and for which a direct comparison could be made. Colored squared represent the average difference in station data for each storm. The deviations in climatology values resulting from the two analyses indicates a systematic bias in the overall values, with NRC's values typically 0. 5 to 1.O degree F higher than TVA 's values. Request: Given the significant impacts noted above please update the dew point climatology using TD3505 dew point data and revising both the LIP and basin-wide PMP values accordingly or provide a justification for not updating it. TVA Response: (Note : The NRC question references RAI #1 and RAI #4. These RAI references actually refer to informal information provided in response to NRC's audit Information Need #1 and Information Need #4) After further review and discussion with the NRC staff in regard to the most appropriate dew point climatology database, TVA has agreed to conservatively revise the dew point climatology applied in Calculation No. CDQ0000002016000041 and to utilize the NCEI TD3505 hourly dew point database. This will extend the period of record and provide additional dew point observational data for use in developing updated dew point climatology. The updated climatology will replace the previously used GIS layers. The updated storm adjustments will be processed and applied to each storm used for the PMP development. Updates to this data set are anticipated to affect Sections 5.1.1, 6.1.1, 6.5.1, 6.8.1, 6.8.4 , and Appendix C of Calculation No. CDQ0000002016000041 and will be submitted as Revision 1 to Topical Report TVA-NPG-AWA16 (TVA Calculation CDQ0000002016000041). RAI #13: Warner Park, TN Dew Point Duration Clarification Technical Deficiency: Staff's review of the licensee's documentation and files related to the Warner Park, TN (SPAS 1208) storm representative dew point and dew point climatology data appears to indicate inconsistent use of dew point duration. As a part of its assessment, staff reviewed the text and digital information related to the Warner Park storm representative dew point and dew point climatology provided by the licensee. Figure 404 in the Topical Report shows that a 12-h duration was used to analyze the Warner Park storm representative dew point; however, upon further review, staff believe that a 24-h duration was used. Figure 415 in the Topical Report shows maximum average dew point data for several stations. Comparison with the "surface_summary" worksheet in the "SPAS_ 1208_ 0bs_data.xlsx" file reveals that the data plotted in Figure 415 correspond to the 24-h maximum average dew point. Also, staff confirmed that the licensee used a 12-h duration for the Warner Park dew point climatology. Therefore, it appears that the dew point duration was used inconsistently. Staff understands that if this is the case, then the licensee 's application could be slightly overly conservative; however, since it appears that a 12-h duration was intended, only the storm Page 68 of 71
representative dew point would change. The 24-h value used by the licensee is 74. 8 F based on the average of 4 stations (KHBG, KASD, KJAN, and KMCB); this value was rounded to 75. 0 F by the licensee. The 12-h value computed by the licensee is 75. 1 F based on the average of the same 4 stations and would be rounded to 75. 0 F. Therefore, it appears that changing the storm representative dew point would not change the results of the Warner Park analysis. Request: Provide confirmation of whether this dew point duration discrepancy exists, what the intended dew point duration is, and what (if any) changes are needed. TVA Response: The 12-hour duration was used in all calculations and is the appropriate duration to use. The image included in the report documentation mistakenly plotted the 24-hour average dew point data. The correct image is provided below (TVA Figure 32) . As noted by the NRC, use of either the 12-hour or 24-hour duration results in the same storm representative value , 75.0 F. A corrected Figure 415 will be provided in Appendix F of Calculation No . CDQ0000002016000041 and will be submitted as Revision 1 to Topical Report TVA-NPG-AWA 16 (TVA Calculation CDQ0000002016000041 ). SPAS 1208 - Dew Point Temperature (F) April 29 - May 2 , 2010 38"N
'7"N 30"N 38"N 38"N 3'"N 33"N 33"N 37'N 31'N 31"N
- ,O"N :,O" N 29'N -t- - '9"N H7splrt
- Surface
- 850mb
- 700mb -----======---------**,. 290 "80 TV A Figure 32. Updated storm representative dew point map using the 12-hour average dew point values Page 69 of 71
RAI #14: Scope of NRC's Review Regulatory Deficiency: This topical report describes the work performed to calculate the Probable Maximum Precipitation for any location within the overall TVA basin and Local Intense Precipitation (LIP) at the BFN, SQN, and WBN sites. The Summary and Conclusions section of the Topical Report states that the precipitation values in the report replace those in HMRs 41, 45, 47, and 56 (which provide PMP estimates for the Tennessee River Basin, including LIP), as well as HMRs 51 and 52 (which provide PMP estimates for the eastern half of the continental US) . NRC's regulatory authority limits its approval of the precipitation values contained in the Topical Report to only those values that could potentially result in flooding at TVA 's nuclear plant sites. Request: Please clarify that the scope of the NRC's requested review is concerned with potential SSPMP impacts at the 3 TVA nuclear power plant sites and does not necessarily reflect positions with respect to the entire Tennessee River watershed except as it impacts river flooding effects and local rainfall effects at the sites. TVA Response The scope of the NRC's requested review addresses only the potential SSPMP impacts at the Browns Ferry, Sequoyah and Watts Bar nuclear power plant sites and does not necessarily reflect NRC positions with respect to the entire Tennessee River watershed , except as it impacts river flooding effects and local rainfall effects at the nuclear plant sites. Section 7.0 of Calculation No. CDQ0000002016000041 will be revised to state that NRC review and approval of the calculation results is applicable only to the assessment of river flooding effects and local rainfall effects at the Browns Ferry, Sequoyah and Watts Bar nuclear plant sites and will be submitted as Revision 1 to Topical Report TVA-NPG-AWA16 (TVA Calculation CDQ0000002016000041 ). Page 70 of 71
- Referenced Data Files
- 1. RAI 1 - Complete Storm Analysis Information for All Short List Storms
- 2. RAl2 - TVA Observed Hourly Dew Point Data Sheet for All Short List Storms
- 3. RAl3 - TVA Storm Adjustment Factor Feature Class Table for All Short List Storms
- 4. RAl4 - TVA Dew Point Climatology Data and GIS Layers
- 5. RAIS - TVA Probable Maximum Precipitation Data and GIS Layers Page 71 of 71
Tennessee Valley Authority , 1101 Market Street, Chattanooga, Tennessee 37402 CNL-18-044 April 19, 2018 10 CFR 50.4 ATTN : Document Control Desk U.S. Nuclear Regulatory Commission Washington , D.C. 20555-0001 Browns Ferry Nuclear Plant, Units 1, 2, and 3 Renewed Facility Operating License Nos. DPR-33, DPR-52, and DPR-68 NRC Docket Nos. 50-259, 50-260 , and 50-296 Sequoyah Nuclear Plant, Units 1 and 2 Renewed Facility Operating License Nos. DPR-77 and DPR-79 NRC Docket Nos. 50-327 and 50-328 Watts Bar Nuclear Plant, Units 1 and 2 Facility Operating License Nos. NPF-90 and NPF-96 NRC Docket Nos. 50-390 and 50-391
Subject:
Tennessee Valley Authority Response to NRC Request for Additional Information Related to Topical Report TVA-NPG-AWA16, "TVA Overall Basin Probable Maximum Precipitation and Local Intense Precipitation Analysis, Calculation CDQ0000002016000041" References : 1. TVA Letter to NRC , "Request for Review and Approval of Topical Report TVA-NPG-AWA16, 'TVA Overall Basin Probable Maximum Precipitation and Local Intense Precipitation Analysis , Calculation CDQ0000002016000041 '," dated September 20, 2016 (ML16264A454)
- 2. Electronic Mail from NRC to TVA, "Request For Additional Information Related to TVA Fleet Topical Report TVA-NPGAWA16 (EPIC : L-2016-TOP-0011 )," dated February 23, 2018 (ML18057A637)
By letter dated September 20, 2016 (Reference 1), Tennessee Valley Authority (TVA) submitted topical report TVA-NPG-AWA16, "TVA Overall Basin Probable Maximum Precipitation and Local Intense Precipitation Analysis , Calculation CDQ0000002016000041 " for review and approval. Approval of this Topical Report was requested to support Probable Maximum Flood D calculatio_ns and associated with planned License Am~ndment Requests for S~qu~yah Nuclear -(1>/ Plant, Units 1 and 2, and Watts Bar Nuclear Plant, Units 1 and 2, and a potential License Amendment Request for Browns Ferry Nuclear Plant, Units 1, 2, and 3. l) D 30
U.S. Nuclear Regulatory Commission CNL-18-044 Page 2 April 19, 2018 In Reference 2, the NRC transmitted a Request for Additional Information (RAI) related to the TVA Topical Report. As described in the Reference 2 email, TVA agreed to provide responses to the RAls by April 20 , 2018. The enclosure to this letter contains TVA's response to the RAIS . As discussed in the Enclosure, a revision of the Topical Report will be submitted to incorporate the required changes needed as a result of these RAls . There are no regulatory commitments associated with this submittal. Please address any questions regarding this request to Russell Thompson at 423-751-2567 . Respectfully , J. W . Shea Vice President, Nuclear Regulatory Affairs & Support Services
Enclosure:
Response to NRC Request for Additional Information Related to Topical Report TVA-NPG-AWA16, "TVA Overall Basin Probable Maximum Precipitation and Local Intense Precipitation Analysis , Calculation CDQ0000002016000041" cc (Enclosure) : NRC Regional Administrator - Region II NRC Senior Resident Inspector - Browns Ferry Nuclear Plant NRR Project Manager - Browns Ferry Nuclear Plant NRC Senior Resident Inspector - Sequoyah Nuclear Plant NRR Project Manager - Sequoyah Nuclear Plant NRC Senior Resident Inspector - Watts Bar Nuclear Plant NRR Project Manager - Watts Bar Nuclear Plant
Enclosure Response to NRC Request for Additional Information Related to Topical Report TVA-NPG-AWA16, "TVA Overall Basin Probable Maximum Precipitation and Local Intense Precipitation Analysis, Calculation CDQ0000002016000041" By a letter dated September 20, 2016 (Agencywide Documents Access and Management System (ADAMS) Accession Number ML16264A454), the Tennessee Valley Authority (TVA) submitted the a fleet topical report (TR) TVA-NPG-AWA 16 "Overall Basin probable Maximum Precipitation and Local Intense Precipitation Analysis. " This TR will be used for future licensing actions for Browns Ferry Units 1, 2 and 3, Sequoyah Units 1 and 2 and Watts Bar Units 1 and 2. The U.S. Nuclear Regulatory Commission (NRG) staff is reviewing your submittal and has determined that additional information is required to complete the review. The specific information requested is attached to this email. The proposed questions were emailed in draft form and a clarification call was held on January 22, 2018. Your staff confirmed that these draft questions did not include proprietary or security-related information and agreed to provide a response April 20, 2018 to this request for additional information (RA/) . The NRG staff considers that timely responses to RA ls help ensure sufficient time is available for staff review and contribute toward the NRC's goal of efficient and effective use of staff resources. Please note that if you do not respond to this request by the agreed-upon date or provide an acceptable alternate date, we may deny your application for amendment under the provisions of Title 10 of the Code of Federal Regulations, Section 2. 108. If circumstances result in the need to revise the agreed upon response date, please contact me at (301) 415-8480 or via e-mail Andrew.Hon@nrc.gov. Regulatory Basis 10 CFR Part 50, Appendix A, General Design Criterion (GDC) 2, states, in part, that structures, systems, and components important to safety shall be designed to withstand the effects of natural phenomena such as floods without loss of capability to perform their safety functions. The design bases for these structures, systems, and components shall reflect appropriate consideration of the most severe of the natural phenomena that have been historically reported for the site and surrounding area, with sufficient margin for the limited accuracy, quantity, and period of time in which the historical data have been accumulated. NUREG-0800, "Standard Review Plan for the Review of Safety Analysis Reports for Nuclear Power Plants: LWR Edition, Section 2.4.3, "Probable Maximum Flood (PMF) On Streams and Rivers," states that to meet the requirements of GDC 2 with regards to design bases for flooding in streams and rivers, the probable maximum precipitation (PMP) on the drainage area that contributes to runoff on the stream network adjacent to the plant site should be determined. Similarly, NUREG-0800 Section 2.4.2, "Floods," states that estimates of potential local flooding on the site and drainage design should be based on estimates of local intense precipitation or local PMP. Page 1 of 71
RAI #1: Complete Storm Analysis Information for All Short List Storms Technical Deficiency: Information in the Topical Report is incomplete; additional information is necessary for the staff to make its regulatory finding. Request: Provide the analysis information for all short list storms that were used for PMP calculation. The detailed storm analysis information should include:
- Storm calculation spreadsheet
- Depth-area-duration values and chart
- Storm cumulative mass curve chart
- Total storm isohyetal analysis map
- HYSPL/T moisture trajectory map
- In-place storm representative dew point (or sea surface temperature) analysis map TVA Response:
The detailed storm analysis information requested above for the short list storms used in the PMP Calculation CDQ0000002016000041 is provided in Attachment 1 - Folder RAI 1. The information is located in the Storm Precipitation Analysis System (SPAS) folder for each storm . Note, no Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model output is available for storms that occurred prior to 1948. Also , for the SPAS 1299 Zone 1 (Alta Pass, NC-July 1916) storm , the storm representative dew point analysis data provided in Tennessee Valley Authority Floods and Flood Control document, Figure 47 (Tennessee Valley Authority, Technical Report 26, 1961) are utilized and included within the RAl1 folder. RAI #2: TVA Observed Hourly Dew Point Data Sheet for All Short List Storms Technical Deficiency: Information in the Topical Report is incomplete; additional information is necessary for the staff to make its regulatory finding. Request: For each short list storm, provide an individual spreadsheet documenting the hourly dew point data that were used for storm representative dew point selection. If publicly-accessible dew point data were used (e.g., NGOC ISO), the unique station identifier (e.g., USAF, WBAN, and/or /CAO) and the starting/ending dew point date and hour (used for the calculation of average 6-, 12-, or 24-hour dew points) should be clearly specified. Provide detailed meteorological reasoning if the selection of storm representative dew point location deviated significantly from the HYSPLIT trajectories. If sea surface temperature is used as a surrogate of surface dew point observation, the sea surface temperature observation should be provided. Provide the relevant data or source information used to determine the storm representative dew point for short list storms for which hourly dew point data were unavailable or not used. Page 2 of 71
TVA Response: Individual spreadsheets containing the hourly dew point or sea surface temperature (SST) data used in the selection of the storm representative dew point for short list storms used in PMP Calculation CDQ0000002016000041 are provided in Attachment 1 - Folder RAl2 . The publicly-accessible dew point station identifier information and a table cross-referencing the starting/ending dew point timeframe for each duration investigated (i.e., 6-, 12-, and 24-hours) are provided within each spreadsheet. In each of the storms evaluated, the region used as the storm representative dew point or SST location is within the general region suggested by HYSPLIT, if available. Therefore, no significant deviations from HYSPLIT occurred . As a result, detailed meteorological reasoning is not provided . In situations where SSTs were used as a surrogate for dew point observations, that data is provided in daily format with the data source identified. Note, many previous United States Army Corps of Engineers (USACE) / National Weather Service (NWS) storm representative values can be found in hydrometeorological report (HMR) 25A (storms prior to 1948), HMR 51 , HMR 52, HMR 53, TVA Technical Report 26, and/or the USACE Storm Studies documentation . These sources are identified when they were used to derive storm representative dew point information where updated hourly and/or SST data was not available. RAI #3: TVA Storm Adjustment Factor Feature Class Table for All Short List Storms Technical Deficiency: Information in the Topical Report is incomplete; additional information is necessary for the staff to make its regulatory finding. Request: For each short list storm, provide the storm adjustment factor feature class table developed for the TVA PMP study (as documented in Figure 24 of Calculation CDQ0000002016000041 ). The data layers should be in a common GIS data format that can be processed by ESRI ArcGIS, and should cover all of the information shown in Figure 24 including STORM, LON, LAT, ZONE_, ELEV, IPMF, MTF, OTF, TAF, and TRANS. TVA Response: Geographic Information System (GIS) database files with all the storm adjustment factor feature classes developed for the TVA PMP study are provided in Attachment 1 - Folder RAl3 . The data layers are in a common GIS data format that can be processed by ESRI ArcGIS and cover all of the information requested including STORM , LON , LAT, ZONE_, ELEV, IPMF , MTF, OTF, TAF , and TRANS. Page 3 of 71
RAI #4: TVA Dew Point Climatology Data and GIS Layers Technical Deficiency: Information in the Topical Report is incomplete; additional information is necessary for the staff to make its regulatory finding. Request: Provide the digital dew point climatology GIS data layers used for PMP development. The digital dew point climatology GIS data layers should be provided for the monthly 6-, 12-, and 24-hour, 100-year recurrence interval dew point maps provided in Appendix C of Calculation CDQ0000002016000041. In addition, provide the corresponding monthly dew point climatology values at each gauge that was used to develop the maps provided in Appendix C of Calculation CDQ0000002016000041. TVA Response: The GIS data used for the digital dew point climatology GIS data layers applied in the TVA PMP development are provided in Attachment 1 - Folder RAl4 . The digital dew point climatology GIS data layers provide the monthly 6-, 12-, and 24-hour, 100-year recurrence interval dew point maps shown in Appendix C of Calculation CDQ0000002016000041 . The data also includes the monthly dew point climatology values at each gauge used to develop the maps provided in Appendix C of Calculation CDQ0000002016000041. After further review and discussion with the NRC staff in regard to the most appropriate dew point climatology database, TVA has agreed to conservatively revise the dew point climatology applied in Calculation No. CDQ0000002016000041 and to utilize the National Center for Environmental Information (NCEI) TD3505 hourly dew point database. This will extend the period of record and provide additional dew point observational data for use in developing updated dew point cl imatology. The updated climatology will replace the previously used GIS layers. The updated storm adjustments will be processed and applied to each storm used for the TVA PMP development. Updates to this data set are anticipated to affect Sections 5.1.1 , 6.1.1 , 6.5.1, 6.8.1, 6.8.4, and Appendix C of Calculation No. CDQ0000002016000041 and will be submitted as Revision 1 to Topical Report TVA-NPG-AWA16 (TVA Calculation CDQ0000002016000041 ). Page 4 of 71
RAI #5: TVA Probable Maximum Precipitation Data and GIS Layers Technical Deficiency: Information in the Topical Report is incomplete; additional information is necessary for the staff to make its regulatory finding. Request: Provide the final digital PMP GIS data layers (across all durations and areas) developed for the TVA Overall Basin Probable Maximum Precipitation and Local Intense Precipitation Analysis. The digital PMP GIS data layer should cover the full TVA Basin for which PMP values have been determined. TVA Response: - Folder RAl5 contains 3 folders , one for each storm type (i.e ., general , local , and tropical) used in the TVA PMP calculation for the domain at a spatial resolution of 90 arc-seconds or .025 x .025 decimal degrees. GIS raster files are included for the entire domain and all durations. After further review and discussion with the NRC staff in regard to the most appropriate dew point climatology database and storm representative dew points for some storms , TVA has agreed to conservaHvely revise Calculation No. CDQ0000002016000041 as further discussed in the TVA responses to RAls #11 and #12 . Those changes are anticipated to affect Section 2.5, Section 5.1, Section 5.7 and Appendices A, C and F of Calculation No. CDQ0000002016000041 and will be submitted as Revision 1 to Topical Report TVA-NPG-AWA16 (TVA Calculation No.CDQ0000002016000041). RAI #6: Reasonableness of OTF Values Technical Deficiency: Staff's review of Orographic Transposition Factor (OTF) application examples demonstrates discrepancies between expected OTF values and OTF values used by the licensee for selected storms. The OTF is applied after an observed precipitation event is 1) moisture maximized using the in-place maximization factor (IPMF) and 2) geographically transpositioned (on a Lat-Lon plane) using a moisture transposition factor (MTF) . The MTF captures geographic differences in moisture availability through comparison of dew point climatology. While it captures spatial variation in moisture, the MTF may not adequately capture the effects of terrain, hence the need for a terrain adjustment (e.g. , the OTF, Barrier Adjustment Factor, or Storm Separation Method). Staff interprets from the licensee 's descriptions in the Technical Report that the OTF is intended to capture the impact that terrain will have on rainfall depths when transpositioning a storm from the original location to a new location. Therefore, staff believes that the OTF should be independent of geographical moisture influence (which is captured through use of the MTF) . Consequently, staff believes that the OTF should not be significant when moving storms between regions with similar orographic characteristics (i.e., such regions should have a calculated OTF close to 1.00). To assess whether the OTF calculation process produces OTF values close to 1.00 in cases where the original and transpositioned storm paths have similar orographic characteristics, staff evaluated the OTF for a series of case studies using data supplied by the licensee for TVA short Page 5 of 71
list storms. NRG Table 1 includes a summary of the rationale for evaluating these case studies and the subsequent observations. NRC T a bl e 1 OTF cases tud"1es eva ua tdb e ,y stffa Storm Rationale Observations Warner, OK The storm center location and TVA Zone 1 Avg . Zone 1 OTF: 0.80 (Example 1) share similar orographic characteristics (e.g ., both locations exh ibit minimal barriers, are of similar elevation , and are located at a sim ilar distance-to-coast). Fall River, KS The storm center location and TVA Zone 1 Avg . Zone 1 OTF : 0.75 (Example 2) share similar orographic characteristics (e.g., both locations exhibit minimal barriers, are of similar elevation , and are located at a similar distance-to-coast) . Smethport, PA The storm center location and TVA Zone 4 Avg . Zone 4 OTF*: 0.66 (Example 3a) share sim ilar orographic characteristics (e.g., both locations exh ibit orographic influence and are a similar distance-to-coast) . TVA Zone 4 has higher overall orographic influence than the storm center due to higher terrain elevation and complexity. Smethport, PA The storm center location and TVA Zone 2 Avg. Zone 2 OTF*: 0.59 (Example 3b) share sim ilar orograph ic characteristics (e.g. , both locations exh ibit orographic influence, are of similar elevation , and are a sim ilar distance-to-coast) .
- Note: the OTF for the Smethport, PA storm was manually adjusted by the licensee to include rescaling to a maximum value of 1. 00. An additional question related to the Smethport, PA storm is included in RA/ #7.
For each of the examples included in NRG Table 1, staff believes that the orographic adjustments should be minimal and the OTF should be close to 1.00; instead, the licensee 's analysis results in OTF values significantly less than 1.00 and large reductions in the adjusted rainfall depths. Request: Provide a justification for the departure of O TF from 1. 00 when transpositioning storms across orographically similar zones (examples provided in NRG Table 1), and discuss whether the reductions in OTF are reasonable. Provide a justification for applying the OTF to the transposition of all storms throughout the TVA Basin, given the example results provided in NRG Table 1. Page 6 of 71
TVA Response: Orographic Transposition Factor (OTF) Background The OTF adjustment process is being used to not only capture the difference in terrain effects between two locations but also to capture all processes that result in precipitation reaching the ground at one location versus another location . The OTF is a mathematical representation of the ratio of the precipitation frequency climatology at one location versus another location . The precipitation frequency climatology is derived from actual observed precipitation events. The largest of these storm events each year is then used to define the Annual Maximum Series (AMS) at a given station. These actual precipitation events and their observed precipitation are a result of all precipitation producing processes that occurred during a given storm event. In HMR terms , the resulting observed precipitation represents both the convergence-only component and the orographic component. The gridded precipitation frequency climatology was produced using gridded mean annual maxima (MAM) grids developed with the Parameter-elevation Regressions on Independent Slopes Model (PRISM) . PRISM utilizes geographic information such as elevation, slope, aspect, distance from coast, and terrain weighting for weighting station data at each grid location . Therefore , the use of the precipitation frequency climatology grids is reflective of all precipitation producing processes. The use of the gridded precipitation climatology represents an optimal combination of factors, including representing extreme precipitation events equivalent to the level of rainfall utilized in TVA's storm selection process , and providing the most robust statistics given the period of record used in the development of the precipitation frequency climatologies . Differences in these values between regions of similar topography reflects the variances in other precipitation producing processes, such as access to moisture, seasonality, general synoptic conditions that produce rainfall , and other meteorological parameters, as well as variances in statistical interpolation . The variability between similar topographic regions provided in NRC Table 1 reflects these differences. National Oceanic and Atmospheric Administration (NOAA) Atlas 14 precipitation frequency cl imatology is the best available data set to utilize in quantifying the differences in precipitation processes between two locations within the same transposition region. It provides a reproducible and explicitly quantifiable data set. The limitations of the data are known . The processes used to develop the data are known and have been reviewed and accepted . The assumptions involved in applying the data are known and can be quantified . This includes use in highly orographic and non-orographic regions . Non-TVA examples of this process include use in the entire region covered by the Virginia statewide PMP study (Kappel , et al. , 2015), the Texas statewide PMP study (Kappel, et al. , 2016) , and the entire region covered by the Colorado-New Mexico Regional PMP (in progress). Many of the regions are similar to less orographic regions of TVA zones 1-3 and to the regions discussed in NRC Table 1. The review process of previous and ongoing AWA PMP studies includes representation from the National Weather Service, Corps of Engineers, Bureau of Reclamation, Federal Energy Regulatory Commission, Natural Resource Conservation Service and United States Geological Society, as well as meteorologists on faculty at major universities, private industry meteorologists, and state dam safety regulators . The review boards of these studies have concurred that the OTF process as utilized for TVA is reasonable and acceptable for PMP calculation purposes. Similarly, the Storm Separation Method (SSM) applied in HMRs 55A, 57, and 59 utilized the 100 year 24-hour precipitation frequency climatology from NOAA Atlas 2 in a similar fashion Page 7 of 71
to TVA's appl ication . Like TVA, these HMRs applied the process to all locations including non-orographic regions of those studies (e.g. , HMR 59 Figure 6.4). Specific examples of the use of NOAA Atlas 2 precipitation frequency data to all regions for use in calculating the HMR orographic factor (K factor) include the following :
- HMR 59 Section 6.6.1 states: "The K-factor is derived from two relationships: 1) The first involves the one-percent chance ( 100-year return period) precipitation amount in proximate areas of large and small topographic variation . Th is relationship is represented by TIC where Tis the 100-year, 24-hour return-frequency precipitation ."
- HMR 57 applied the NOAA Atlas 2 100-year, 24-hour precipitation to address orographics and determined it was important to maintain a close spatial correlation of maximum index values of total PMP and maximum values of 100-year, 24-hour precipitation (HMR 57 - pg 88).
- In HMR 57 , a TIC (total 100 yr precipitationl100 yr convergence component) value less than 1 resulted in some areas. In the Snake River plain , where physiographic features could likely account for the low TIC values , the values were accepted .
Values as low as 0.84 to the lee of the Olympic Mountains of Washington where the mountains were believed to disrupt the resupply of boundary-layer moisture were also accepted (HMR 57 - pg 76) .
- HMR 56 Section 2.2.3 states: "Topography is known to play an important role in rainfall in the Tennessee River watershed ."
- HMR 56 Sections 3.5.2 and 3.5.3 discuss the application of the terrain stimulation effect in smooth and intermediate regions and an additional broad scale orographic factor in the mountainous eastern region . These are some of the additional factors NOAA Atlas 14 climatologies captured as part of the OTF process. The most important aspect is defining appropriate and reasonable transposition limits to place storms within similar regions when considering topographical and meteorological interactions. The assumption is the NOAA Atlas 14 climatologies capture the effects of terrain , upwind barriers, access to moisture, preferred moisture inflow directions, seasonal variation of synoptic meteorological environments, etc. These factors are explicitly captured because the NOAA Atlas 14 climatologies are built from observed precipitation events, which inherently included these factors .
- HMR 56 Figures 67 and 68 show topographically significant terrain throughout TVA zone 1, 2, and 3.
These discussions and examples demonstrate that the OTF represents more than the difference in topographic effects between two locations. The OTF represents the difference in all precipitation processes between two locations, such as access to moisture, seasonality, and synoptic conditions, in addition to topographic effects. Page 8 of 71
General Orographic Discussion The orographic component of the topographic effects refers to the influence that terrain has on precipitation production and accumulation , both in-place and upwind/downwind . Orographic effects include many processes; some of the important ones include the following :
- Terrain can help release atmospheric instability by initiating lift (releasing conditional instability through forced ascent), providing extra lift to already rising motions, or producing the opposite effects through descending air. Lifting processes can be triggered by a rise in terrain or upstream blocking terrain (causing downstream convergence) . These forced ascent processes result in rising motions, cooling of the air mass, increased saturation , and enhanced precipitation. Forced descent has the opposite effect, attenuating the precipitation producing processes by warming and drying the atmosphere , resulting in a more stable atmosphere and less precipitation .
- Higher terrain receives more of the precipitation than adjacent lower elevations because there is a better chance the precipitation will reach the ground at higher elevations or experience less evaporation before reaching the ground than adjacent lower elevations.
- Orographic effects depend on many factors such as slope, aspect, angle of interaction, width of barrier, height of barrier, moisture advection duration, moisture depth in the atmosphere, atmospheric profile (e.g ., the amount of instability) , wind speed/direction , the size of the storm , and the combination of these and other factors through space and time.
- For any given storm event, the barrier and upwind topography are constant with changing atmospheric parameters.
There are many orographic processes and interactions related to terrain interactions that are not well understood or quantified . Therefore, observed data (precipitation accumulations) are used as a proxy, where it is assumed that the observed precipitation represents all the precipitation processes associated with a storm event. Given this , it is logical that observed precipitation at a given location represents a combination of all factors that produced the precipitation, including what would have occurred without any terrain influence and what actually occurred because of the terrain influence (if any). The best proxy would be to have thousands of observed events of the storm type being analyzed at any given location and then be able to compare those storm events to a similar number of storm events at another location where the topography and meteorology between the two locations is the same. However, that extensive data set of observed storm events does not exist. Therefore, judgments are made regarding regions that are considered as having the same meteorology and topography and then utilize statistical analyses provided in NOAA Atlas 14 as the comparable data set. As part of the OTF process, the following is assumed :
- NOAA Atlas 14 precipitation frequency climatology represents all precipitation producing factors that have occurred at a location . This is based on the fact that the NOAA Atlas 14 data is derived from AMS values at individual stations that were the result of an actual storm event. That actual storm event included both the amount of precipitation that would have occurred without topography and the amount of precipitation that occurred because of topography (if any) .
- Comparing the precipitation frequency climatology at one point to another will produce a ratio that shows how much more or less efficient the precipitation producing processes are between the two locations.
Page 9 of 71
If there is no orographic influence at either location being compared or between the two locations, then the differences should be a function of ( 1) storm precipitation producing processes in the absence of topography (thermodynamic and dynamic) , (2) how much more or less moisture is available from a climatological perspective, and (3) elevation differences at the location or intervening barriers. Discussion Related to Departure of OTF from 1.00 in Orographically Similar Zones It would be reasonable to expect a near constant 1.00 over orographically similar zones if the OTF only represented the orographic effect. However, as discussed previously, there are other atmospheric components inherent in the precipitation frequency outputs that are carried th rough into the OTF. Example 1 and 2 in NRC Table 1 address the OTF reduction exhibited in storms transposed from eastern Kansas and Oklahoma to TVA transposition zone 1, a region of similar orographic characteristics and elevation . The average OTFs are 0.80 and 0.75, respectively. TVA Figure 1 illustrates the spatial pattern of the NOAA Atlas 14 1,000-year 24-hour rainfall over the region. The climatological precipitation over TVA transposition zone 1 is significantly lower than the storm centers west of the Mississippi River. The meteorological reason for the variation is primarily because the Warner, OK and Fall River, KS storm centers receive their moisture directly from the Gulf of Mexico at a location where air masses originating from the High Plains to the west interact preferably with the low-level jet that is common in the region . These factors combine in this region to produce more efficient thermodynamic contrast , higher instability, and more frequent high-intensity rainfall. In contrast, the frequency of occurrence of the low-level jet is much lower over TVA transposition zone 1 and zone 2 and the thermodynamic contrast in the TVA region is therefore not as extreme from a climatological perspective. Similar types of storms can occur over the Ohio River/Tennessee River Valleys, but are less common and less intense, especially from a frequency of occurrence perspective. The horizontal (reduced to sea level) climatological difference in moisture availability is addressed with the moisture transposition factor (MTF) by comparing the moisture levels associated with the 100-year recurrence interval dew point values . These values are only sl ightly lower over the TVA location than the storm center locations. The more significant reduction exhibited by the OTF reflects individual storm precipitation producing processes inherent in the precipitation climatology, as opposed to the climatological maximum moisture differences used in the MTF. Examples 3 and 4 in NRC Table 1 exhibit special cases as described in the TVA PMP report. This issue is discussed in depth in the response to RAI #7 below. As noted in the above discussions, the OTF captures all the precipitation producing processes, including the effect of topography (orographic effect), if any. The use of this process is relevant in both orographic and non-orographic regions. Therefore, the use of the OTF process is applicable for all locations within the TVA domain and for all storms used in the TVA PMP analysis (except as noted in the RAI #7 response) . Page 10 of 71
1,000..year 24-hr Pr<cfpltation (in)
* ** D *-* * ., ... . ... ,
. . ~.C . . t-10 1111 11 H If I 1111
- tllll to 11 . . 14 HCJ*1t
'* llll n ,~ 1111 ,'!> 1f TVA Figure 1 - Fall River, KS and Warner, OK storm centers over NOAA Atlas 14 1,000-year 24-hour precipitation RAI #7: OTF Reduction for Smethport, PA and Simpson, KY Technical Deficiency: OTF values for two local storms which control PMP estimates were manually rescaled to a maximum of 1.00 (i.e., all original OTF values were divided by the maximum calculated OTF, resulting in widespread reductions and a maximum value of 1.00).
This rescaling greatly reduces the Local Storm PMP. As described in Section 6. 1. 1.5 of the Topical Report, the OTF values for two local storms (Smethport, PA and Simpson, KY) were rescaled to a maximum of 1. 00. Following discussions with the Review Board and the licensee, "it was determined that the factors leading to extreme levels of moisture and instability combined with terrain influences" which produced extreme rainfall at Smethport and Simpson "were similar to what could occur over the eastern foothills and mountainous terrain in the TVA basin. " As a result, the licensee decided it was "unreasonable to further adjust the events upward based on the OTF", and "the OTF factors for these events were normalized to a maximum of 1.00." Page 11 of 71
Staff's review of the data provided by the licensee suggests that the maximum original (i.e. , unadjusted) 0 TF values for the Smethport and Simpson events are 2. 15 and 2. 09, respectively. In comparison, the average Zone 4 original OTF is 1.39 for Smethport and 1.35 for Simpson. After rescaling the original OTF, the average Zone 4 OTF is reduced to 0. 66 for Smethport and
- 0. 65 for Simpson - approximately a 50% reduction. These modifications to the OTF result in a significant reduction in the adjusted DAD values for these storms. In addition, since these storms control PMP estimates, the resultant PMP values are significantly reduced.
NRG Figure 1 provides a comparison of TVA 's rescaled OTF values and the original (i.e. , unadjusted) OTF values for the Smethport storm. Similarly, NRG Figure 2 provides a comparison of TVA 's rescaled O TF values and the original O TF values for the Simpson storm. NRC Figure 1. Comparison of Smethport OTF using TVA's rescaling approach (left) and original approach (right) for TRANS=1 grids (i.e., transpositionable zone) NRC Figure 2. Comparison of Simpson OTF using TVA's rescaling approach (left) and original approach (right) for TRANS=1 grids (i.e., transpositionable zone) Request: Provide justification for adjusting the Smethport, PA and Simpson, KY OTF values to a maximum of 1.00, and for using significantly reduced OTF values throughout the transpositionable zone. Page 12 of 71
TVA Response: OTF Re-scaling Background Discussion The OTF re-scaling is to preserve the spatial distribution of the adjusted rainfall for these storms over the transposed areas without unreasonably inflating the rainfall beyond the maximized in-place depths. These storm's point rainfall depths were at, or near, the world record curve for their respective critical durations. The intent of the original OTF cap of 1.0 (instead of 1.5) was to prevent exceedance of the world record depths (TVA Figure 2) after storm transposition . The cap of 1.50 was applied to these storms initially, but the resulting precipitation was far greater than the world record rainfall amounts. The intent of the normalization process was to preserve the maximum orographic adjustment for the location(s) with the most orographic impact (with a value of 1.0) while decreasing the values elsewhere so that the spatial pattern of the 100-year Atlas 14 6-hour precipitation climatology (TVA Figure 7) was maintained . Further, rainfall amounts associated with both of these events were highly questionable, because no hourly rainfall accumulation data were recorded at or near the storm center locations. Therefore, hourly incremental rainfall data provided were not based on observed data and instead were derived from surrounding stations with significantly lower total precipitation amounts or inferred from depth-duration curves . This results in having low confidence in the incremental hourly rainfall amounts. In fact, the Simpson, KY July 1939 storm is not used in HMR 51 (no working papers or notes are available as to why this storm was left out of that document) . Given these considerations, extensive discussions with the TVA Review Board and AWA evaluations took place regarding the use of both storms . A lthough it is possible that the Smethport, PA storm is not transpositionable to any part of the TVA basin , no conclusive data existed to eliminate the storm . However, several data adjustments suggested this possibility. This included anomalously high OTF values and high MTF values . In these types of situations where there is no clear argument for inclusion or exclusion, the TVA choice is made to include the storm. In doing so, adjustment factors and fit with other adjusted storms must be considered, similar to the discussions in HMR 51 Section 3.2.2 and Section 3.2.4. Use of the storm and the storm adjustment values as calculated, resulted in unreasonably high rainfall depths when transposing the storm from north central Pennsylvania to the TVA basin. An MTF value greater than 1.00 results from the fact that the target grid locations in the TVA basin are closer to the moisture source region than the area associated with the storm center in north central Pennsylvania. More significantly, much of the target region has greater precipitation frequency depths due to more frequent extreme rainfalls and enhanced orographic influence. This results in calculated total adjustment factors at the target grid locations that are greatly inflated where the OTF values can exceed 2.00. Generally, when OTF values are > 1.50, the storm transposition limits are re-evaluated . This is because OTF values greater than 1.50 or less than 0.50 are an indication that the storm may not be transposable to that location since the physical characteristics may be too different from the source location . However, the storm must still be transposed to these locations. In these cases, the practice is to cap all OTF values at a maximum of 1.50. The Smethport event was a world record at its critical durations. Increasing rainfall by an areal-average MTF of 1.12 and applying an OTF in the traditional manner (much of the target cells would be capped at 1.50) would result in an adjusted rainfall over TVA transposition zone 4 that was much too high and not physically possible . In addition, the 1.50 OTF cap would result in constant spatial fields of PMP depth because almost the entire region is greater than the 1.50 OTF. TVA Figure 2 illustrates the adjusted rainfall depths in relation to the world record rainfall depth-duration curve . In summary, typical application of the OTF and MTF to the Smethport, PA storm over transposition zone 4 created unreasonably high rainfall depths without an appropriate spatial distribution . The Simpson, KY storm transposition resulted in a similar problem, although not to the magnitude of Smethport. Page 13 of 71
The two most reasonable approaches to these problems were to either remove the storms from the database as not transposable (this was evaluated and discussed internally and in review board meetings) or to adjust the transposition factors so that the resulting rainfall levels were at a more reasonable level. In reality , the transposition limits for these storms should be very limited , but the project decision was to keep the storms and adjust the OTF. This is a more conservative application . For these storms, the OTF is useful to determine the spatial distribution of gridded PMP over orographic target regions, but cannot be used to provide reasonable adjusted rainfall magnitudes. Thus , it was determined that the OTF should not further increase rainfall beyond the in-place depth . The OTF was re-scaled to a maximum of 1.00, rather than capped at 1.00, to allow the spatial distribution of the precipitation climatology to transfer through the OTF. As a result, the areal-average total adjustment factor (TAF) for Smethport, PA was 0.74, with a maximum of 1.13. For the Simpson, KY event, the areal-average TAF was 0.72 , with a maximum of 1.21. Limiting the adjustment for these storms to an increase of 13% and 21 %, respectively, was determined to be a reasonable constraint, keeping the adjusted depths to a minimum above the world record curve . The adjusted transposed rainfall for Smethport was very similar to the transposed rainfall depths fo r the Johnson City, TN event, a storm that also occurred over the target region (TVA Figures 3 and 4) . This storm provided additional confirmation that the Smethport constraints were reasonable . The maximum 6-hour 10-square mile rainfall is 29.3", which is in line with the world point maximum-recorded rainfall curve . Again , assuming the world-record rainfall curve demonstrates a physical upper limit to rainfall accumulation over time , this suggests the adjusted rainfall is reasonable , if not conservative . Furthermore, the ratio of 24-hour 10-square mile PMP to the 100-year 24-hour Atlas 14 precipitation depth was 3.4 for transposition zone 3 and 3.1 for transposition zone 4. These ratios are consistent with previous AWA and HMR PMP studies for orographic regions and provide further evidence that the constraints are reasonable . However, after further review and discussion with the NRC staff in regard to this question , TVA has agreed to add further conservatism and revise the OTF determination methodology used in LI P calculations . This methodology change ensures the OTF used in transpositioning local storms to the SQN, BFN and WBN plant sites is not less than 1.0. Note, this change only affects the Simpson , KY storm because the Smethport, PA storm was not transpositioned to the plant sites. Updates to this data set will affect Sections 6.1.1.5 and 6.4.4 of Calculation No . CDQ0000002016000041 and will be submitted as Revision 1 to Topical Report TVA-NPG-AWA16 (TVA Calculation CDQ0000002016000041) . Page 14 of 71
Maximum observed point rai1nfall as a functilon of duration 10 000 , - - - - - - - - -
* ~ ortd ,lax,mum US .iaximu 1 000 S1mp.son I ad sted)
Holt
- c i:.
s: i
- Holt (adjusted)
Q 00 Smatl'1C ~ J" * . ... 0 **
- minutes ..---+,o---,--
ears 1 0 1000000 10,000 000 Dura on (min) TVA Figure 2. Smethport, PA and Simpson, KY adjusted rainfall plotted on the world record depth-duration curve Page 15 of 71
Controlling torms hour Loca l Stonn PMP ( 10 111?)
.... I'"-', "\\
T A tudy Area IS"A M'W ll'W aw
.t*
- r.**
Contrtbudn9 Storms
- Johnson City. TN 1924
- Smeth port. PA 1042 flb"N ..., '"'
llW Qoc,oir,cci,~ 12"W t'9,A~~l:oi*"'.. ec-- P.,qett0" .Ahn
- r:=:aaa:==--======------======M*'
0 50 100 150 200 es No,!:l'~i,,tsJ TVA Figure 3. Controlling storms for the 6-hour 10-sqmi PMP. Smethport, PA controls a portion over southern Zone 4 Page 16 of 71
6-hour Local Stonn P IP ( LO mi:) T VA Study Arca l<'W Averoge Depth : 23.16" Maximum Depth: 29.29" ,,.. Minumum Depth : 14 .31*
,.., PMI' O.pth
{lo.chH,
-
- 2 12
- 14 26
- 2 -4 - 14 6 - 26 - 26
- * -8 1 8 30 6-8 20 32 8 - 10
- 20 - 22 32 - 34 10 22 - 2* CJ> 34
-~ - -==--==-=====
0 50 100
- - - --=====M***
150 200 TVA Figure 4. 6-hour 10-square mile PMP resulting from the Johnson City, TN and Smethport, PA adjusted rainfall RAI #8: OTF Calculation using NWS Atlas 14 Technical Deficiency: The OTF is intended to accurately capture localized spatial variation in orography. However, the NWS Atlas 14 data used to calculate OTF are inherently regionalized, which poses a concern whether the original intention of developing an OTF was fully captured. Section 4.5 of the Topical Report states that, in comparison to the topographic adjustments used in the TVA HMR, "the OTF, along with hourly gridded rainfall data from SPAS analyses, is able to evaluate and quantify ... variations over a much more refined scale both spatially and temporally. " Localized refinement is achieved through use of Atlas 14 precipitation frequency (PF) data, which were developed using L-moment regional frequency analysis. However, during the development of the PF data, Atlas 14 identified homogeneity zones (i.e., regional groups) for data pooling. Based on Section 4. 2. 2 of Atlas 14, Volume 2, the regional application of L-moments derives "the shape parameters from all stations in a homogeneous region rather than from each station individually. " From Section 4.4 of Atlas 14, Volume 2: "effort was made during the subdivision process to mitigate discrepancies that could be caused by (1) sampling error due to small sample sizes, or (2) regionalization that does not reflect a local situation." NRC Figure 3 shows the 24-h through 60-day regional groupings identified in Atlas 14, Volume 2. Page 17 of 71
Generally speaking, all precipitation data within a homogeneity zone were first locally normalized, and then pooled together for probabilistic density function fitting. Therefore, it is important to understand that the NWS Atlas 14 values do not only capture the local precipitation features. It is jointly influenced by the local mean (of annual maximum series at each gauge}, regional probability density distribution, and final interpolation by PRISM. X"I', 24-HOUR THROUGH 60-0AY L..MOMENT REGIONS NRC Figure 3. Regional groupings for daily data used to prepare NOAA Atlas 14 Volume 2 Section 3. 1. 4 of the WMO-No. 1045 Manual on Estimation of Probable Maximum Precipitation states: "Precipitation-frequency values represent an equal probability level of rainfall. The values for the rarer recurrence intervals, for example the 50-year or 100-year recurrence interval, are associated with severe weather systems. Therefore, they are better indicators of the geographic variation of PMP than mean seasonal or annual precipitation maps. " Thus, staff believes that the specific features of Atlas 14 are important artifacts influencing the OTF and are worth considering. Given that the Atlas 14 method scales station PF data by the mean of the annual maximum series and uses PRISM for base map smoothing, the final rainfall estimates would induce spatial smoothing based on averages rather than rarer recurrence intervals associated with severe weather systems. Page 18 of 71
OTF Best Fit Linear Trend Method The licensee used a 6-h precipitation frequency climatology to compute local storm OTF and a 24-h precipitation frequency climatology to compute general and tropical storm OTF. For each short list storm, the OTF calculation approach used for the TVA Topical Report used linear regression to estimate the ratio between precipitation frequency depths for the recurrence interval associated with the storm's maximum point rainfall at either 6-h or 24-h. OTF 100-year Ratio Method Other AWA PMP studies have calculated the OTF using the 100-y precipitation frequency ratio rather than the linear regression approach. Since longer recurrence interval estimates may be more representative of PMP-type storms but may lack reliable estimates, AWA has used the 100-year precipitation frequency ratio to compute OTF in other studies (e.g., the PMP study for Texas) . NRG staff has also conducted limited sensitivity analysis and finds that the 100-year ratio is more stable than the regression approach. For example, precipitation frequency data provide higher precipitation depths at BFN than at WBN and SQN; however, the linear regression method can result in lower OTF values at BFN than at WBN and SQN. Request: a) Considering Atlas 14 's regional features, provide a justification regarding whether the Atlas 14 PF data represent reasonable spatial variation representative of orographic PMP effects or PMP in general. b) Provide a justification for using the best fit linear trend method in lieu of the 100-y ratio method for determining LIP and basin-wide PMP values. TVA Response - 8(a}: NOAA Atlas 14 is based on several layers, decisions and assumptions that result in the final PF estimates ultimately used for OTF calculations . A high level of review on regional PF analysis is provided to ensure all aspects of the NOAA Atlas 14 PF estimates are completely understood . A regional frequency analysis approach utilizes L-moments, decreases the uncertainty of rainfall frequency estimates for more rare events and dampens the influence of outlier precipitation amounts from extreme storms as compared to site-specific station analysis. The basis of a regional frequency analysis is that data from sites with in a homogeneous region can be pooled to improve the rel iability of the magnitude-frequency estimates for all sites (especially the upper tail of the distribution) . A homogeneous region may be a geographic area delineated by meteorological climatologies or may be a collection of sites having similar characteristics pertinent to the phenomenon being investigated. The definition of a homogeneous region is the condition that all sites can be described by one probability distribution having common distribution parameters after the site data are rescaled by their at-site mean. Thus, all sites within a homogeneous region have a common regional magnitude-frequency curve , termed a regional growth curve , that becomes site-specific after scaling by the at-site mean of the data . Quantile estimates at a site are estimated by: Equation 1 where Qi(F) is the at-site inverse Cumulative Distribution Function (CDF), Ui is the estimate of the at-site mean, and q(F) is the reg ional growth curve , regional inverse CDF. Page 19 of 71
NOAA performed measurements of heterogeneity and station discordancy tests (Hosking and Wallis , 1997) to ensure all sites meet the criteria of a "homogenous region", meaning one probability distribution having common distribution parameters after the site data are rescaled by their at-site mean . Regionalization is captured through the regional growth curve for the specified homogenous region , which is then localized by the at-site scaling factor "MAM". Note, that NOAA HDSC group utilized different reg ionalization approaches for the various Atlas 14 volumes. The PRISM group utilized the PRISM model to derive MAM grids based on the MAM of station data. The PRISM group used similar methods to derive 30-year climatologies when creating the MAM grids; predictor variables are listed in TVA Table 1 (NOAA Atlas Table 2) . The resulting MAM PRISM grids served as the basis for deriving precipitation frequency estimates at different recurrence intervals using a spatial interpolation procedure called the Cascade, Residual Add- Back (CRAB) derivation procedure. The level of smoothing applied in orographic areas in NOAA Atlas 2 was "LIGHT" as described below. Additional text and TVA Figure 5 from NOAA Atlas 14 are below and provide details on the amount of smoothing applied to the final PF estimates. Page 20 of 71
TVA Table 1. PRISM redictors used to derive MAM rids From NOAA Atlas 14 v2 Table __ Values of rele :-ant PRISM pmnme '"or mo
- fuig of 1- and ""-1_Jho
- m~x flood <itatistics foi- the ORB (Om Ri~~ Ba :in). See aly e al. 200_) fonietails oo. PRIS I pai-an:ieteP.>.
D . rion 1-ho hour\ alues Ra : lS f i.n:lluence 60170 km* s l\,finimum mm r f on-face "!.I _ s atioo.s* s, 2 _osta: "oos* desired in re~ession P1 I\ifiniml.ID.li va *d re~s ion slope 0.6/ __+
/3111 Maximum** *dreg,* si slope 30.0t Jo.o-P1a Ikfault :alid regression :;lope ~ .5.1 5.9-Distance Weiehtin~
Di.sranc:r weighting exponent 2.0CO Impomw.c:e fa or distance o.: lo.5 1.'i'"eighfmg l\.fini.mum all wab e dis ce 50l50 km MAP wei~;hting expooent .0/1 .0 Importance fa ,o r or :MAP o.: lo.5 W\:i gl!rtmg Mininnun station-grid. oell MAP 50}50 ,o di.fJerenre below which M..\P
,.,reigbting
- maxinnun
- Maxim.um station-grid c-e* l\iLA..P 500/500%
differenc-.e abo-\.-e which :M:...i\P ,,.~ *gbt ts zero Facet Meimltine: C face weighting exponent 0.5/0.S: CT Minimum mter-ce ele , *on / 01/ re grad! belo\.V which a cell is
- t Mrurimum DEl\.f filtetma 80/80 bn waveletlt:,oth oc topographic facet amioation Coasrai Proximitv eimtin2
- 1. .a.=
- Optimized .l. *
- cross- *alida
- s * *cs see Ta e 4).
- S .o pes are exp1'essed mtlmi.s are normalized b, the a,..-erage o red value of the pl'ecipim *on in the l'egression da a se ii the g cell. :ni
- here are l scp: ,LAP mm))* 000]-
** Nonnal.1, referred o a: elei.*.ati ,.:.reigh.ting
- ~ la.,wmim value: .actu ..11 v ne raried dynanlically ti*
- ilie ni.ode .
Page 21 of 71
go*w 85"W OO"W 75"W 40"N 40"N 85°N 85"N
- Heavy smooth ing D Moderate smoothing
- Light smoothing 85"W OO"W 75"W 0 55 110 220 130 Coastal Proximity and Effe cti ve Terrain Height-based Sm oothing Thre sholds Ohio River Basin and Surrounding States TVA Figure 5. A map of areas receiving different degrees of spatial smoothing based on PRISM's effective terrain height and coastal proximity grids from (NOAA Atlas 14 v2 Figure 4.8.4)
- 1. HEAVY: Flat areas were determined if effective terrain height is less than 100 m (328 ft) ,
and then a 17x17 grid cell (approximately 15 miles by 15 miles) , center-weighted filter was used at the longer durations and a 25x25 grid cell (approximately 25 miles by 25 miles) filter at the shorter (<24-hour) durations . The shorter durations were subjected to greater smoothing because the lower stati on density was prone to cause unnatural variabil ity.
- 2. MODE RATE: Moderately complex terrain areas were determined if effective terra in height was greater than 100 meters (328 feet) and less than 200 meters (656 feet), and then a 11 x11 grid cell (approximately 5.5 miles x 5.5 miles), center weighted filter was used for all durations.
- 3. LIGHT: Complex terrain areas and coastl ines were determined if effective terrain height was greater than 200 meters (656 feet) or if the coastal proximity grid (a grid of values indicating distance from coast) was <=5, and then no filter was used at th is stage.
However, light smoothing was conducted during the next stage. Page 22 of 71
The CRAB process used in NOAA Atlas 14 v2 uses the previously derived PF grid to derive the next PF grid in a cascading fashion . From NOAA Atlas 14 v2 , "The technique derives grids along the frequency dimension with quantile estimates for different durations being separately interpolated . Hence, duration-dependent spatial patterns evolve independently of other durations." The initial PRISM MAM grid is used to estimate the 2 year grid , the final 2 year grids are used to estimate the 5 year grid, and this continues through all frequencies . A comparison of the 2-year PF (analogous to the MAM grid) to the 100 year grid are provided for the 6 hour and 24 hour (TVA Figures 6 through 9) . For each frequency and duration , the grids were normalized to the highest PF value; the normalized grids are used to illustrate the variability in the spatial distribution for the 2-year and 100-year frequencies (TVA Figures 10 through 13). These figures illustrate that the variability of a more frequent event (2 year) is not that different from a more rare event (100 year). 2-year 6-hour OAA Alla 14 Precipitation estimate (inchc ) Tcncsscc Valley Authority Drainage Arca
-W *w M*.-.. l:'"Vf tt'W 2..,...- e-nour , r.cip1tat1on (indtes l
< 25 -
2 5* 3 -
*5 - 5 5-55 0 3 - 3 5 - 5 5-6 0 35 6 - 6.5
- * * *5 C] > 65 8'"# *N .,w
~ ! - . f.... 'ffl.oS191WIJ'"Mlcnt1""'
Pr ~Mel{--
-==-ic:::i-====----====Mi 0 50 100 150 200 les OIIUtn WGS ll&I TVA Figure 6. 2-year 6-hour NOAA Atlas 14 spatial pattern Page 23 of 71
IOO-year 6-hottr 1 0 A tla 14 Precipitation - timate (inche ) Tcncsscc Valley Aut hority Drainage Arca
...w ,,*~
100.year 6-hour Precipitation (inches)
. ,o . 35c:J.s - 5 * *.o -o5 C] 7.5-B
- 35., - 50
- 55 - 65-7 CJ ,o .,s . s.s.o - 10 . 15
~~flffl~ll&4l.lTt.tZ~1fM 0
- =- c:::ma:======------=====::::iMlles 50 100 150 200
~ ,, M<:,f#(I' QllllrloWGSt_.
TVA Figure 7. 100-year 6-hour NOAA Atlas 14 spatial pattern Page 24 of 71
_-year 24-hour OAA Alias 14 Prec ipitation st imate (inches) Tcncsscc Valley Authority Drainage Arca w .... .,., 1l"t,1 2-yur 24-hour PrK1P4t1tion (Inches) 0
< 175 175 2 - 225 -
- 275 - 3 3-325 325 - 35 0 225 35 - 3.75 Alabama - 25 - 275 0 > 375
...,. p*w .,.,. IJW ,>-w
~S.-*an WGS191MllfM1r,w,,eN Pr(NQOl'l ! 1.1_..,... M t'ftlllOI' 0
- = - = =--======------=====:::iM1l 50 100 150 200 es ~VWGS'lfll,I TVA Figure 8. 2-year 24-hour NOAA Atlas 14 spatial pattern Page 25 of 71
IOO-year 24-hour NOAA Atla 14 Precipitation Estima tes (inchc ) Tcnc cc Valley rnhority Drainage Arca
, ~ - - ~ WOSI ... IJTMlfM wt
""~t-..u~.,.
- =- =- =====-----=====Mjes 50 100 150 200 Dllll.,,;'I WO$ta81 TVA Figure 9. 100-year 24-hou r NOAA Atlas 14 spatial pattern Page 26 of 71
_-year 6-hour OAA Atlas 14 Precipitation stimates ormalizcd to a Maximum of t .00
..-w ...., M"W IO"W ... w e,*w
""" ,,*w wast lO"N
- < 035 - 065-070
- 035 -0.40 - 0 70 -0 75 040 -0.45 - 0.75 -0 60
,.. CJ 045 - 0 .5 0 - 0.60 - 0.85 C) 050 - 0 55 - 085 - 090 Georg*
- 055 - 0 .60 0 .90 - 0 95
- 060 - 065 0 095 - 100 t'..ocrdlN!M S,.~em wGS 1984 VTM J,:,oe 'l{IH
~
- _c::_c::_c::====-----====:::::iMtles 50 100 150 200 Plqecll(llll1~MeR'..,,
o.anwos,-. TVA Figure 10. 2-year 6-hour NOAA Atlas 14 Normalized spatial pattern Page 27 of 71
100-year 6-hour OAA Atlas 14 Prec.ipitation stimates onnalizcd to a Maximum of 1.00 Nomulized 100-yeu
~oul' Precipitation
- < 0 .35 - 065-070
- 0.35 -0 40 -
0.40 - 0 .45 - 0.70
- 0 75 0.75
- 0 80 r:::J 045 -0.50 - 0.80
- 0 85 C) 050 - 0 55 - 085
- O 90
- 055-060 090-095
- 060 -0650095-100 CrW C-...<<* S~*- WCS tilM UTM llllM 1&N
- c::-c:::a-=:=====------======Mdes 50 100 150 200 Pr~T~~
0...... WGS1'il84 TVA Figure 11. 100-year 6-hour NOAA Atlas 14 Normalized spatial pattern Page 28 of 71
2-year 24-hour , A Arla 14 Precipitation tima tes ormalizcd to a Maximum of 1.00
,rw J&' pj
- < 0 35 - 065-0 70
- 035 - 040 0 70 - 0 75 o*o-o *5
- o 75 - 080 JI 0 045 . o 50-080
- 0.85 c:) 050 - 0 55 - 085
- O 90
- 055 - 0 60 0.90 - 0 95
- 060 - 065 0 095 - 100 C000 ~ " IIGS11'4U'lMl~~
..._~,,~u
- =- =- -======------=====:: :i M 0 50 100 150 200 oles a<<
OIIM',\WOS1Mt TVA Figure 12. 2-year 24-hour NOAA Atlas 14 Normalized spatial pattern Page 29 of 71
100-year 24-bour Oi\A A 1la l4 Precipitation E ti mate* onnalizcd to a Maximum of 1.00
... rw ...w A01Qr. . s,,on WC.Sl ttllU.Ic,,*1flf 1-'t~l~Mflf<C'.9'
..::=ac::-=====-----=====Mlles 0 50 100 150 200 0..,., WG41!1t,1 TVA Figure 13. 100-year 24-hour NOAA Atlas 14 Normalized spatial pattern In summary, the NOAA Atlas 14 PF data provide reasonable spatial variation representative of orographic PMP because:
- 1. A regional approach decreases the uncertainty of rainfall frequency estimates for more rare events (upper end of distribution tail) , trading space for time.
- 2. At-site mean or MAM is used to capture local rainfall influence, while utilizing regional distribution to attain better estimates at more rare frequencies .
- 3. PRISM MAM development, spatial interpolation , and smoothing provide realistic representation of spatial precipitation patterns. MAM grid is based on Mean Annual Precipitation (MAP) and other climate parameters.
- 4. The concern of "spatial smoothing of averages rather than rarer events" is not an issue as NOAA Atlas 14 states "duration-dependent spatial patterns evolve independently of other durations" which is evident by looking at TVA Figures 6 through 9.
- 5. As compared to NOAA Technical Papers 40 and 49, the regional approach (vs . site- specific) and spatial interpolation methods (PRISM and CRAB method vs . iso-contours) , the NOAA Atlas 14 datasets provide a more realistic representation of orographic precipitation and the spatial distribution .
Page 30 of 71
TVA Response - 8(b): After further review and discussion with the NRC staff in regard to this question , TVA has agreed to conservatively revise OTF determination methodology used in Calculation No. CDQ0000002016000041 LIP calculations . For this aspect, TVA will utilize the 6-hour 100-year precipitation frequency climatology from NOAA Atlas 14 to adjust storms during the transposition process. This approach will be applied to the storms moved to the SQN , BFN and WBN plant sites. This update replaces the use of the linear fit method of the NOAA Atlas 14 precipitation frequency climatology . This change does not affect the Simpson , KY storm or the Smethport, PA storm as those OTF values were held to 1.00 as discussed in response to RAI #7 . Updates to this data set will affect Sections 6.4 of Calculation No. CDQ0000002016000041 and will be submitted as Revision 1 to Topical Report TVA-NPG-AWA16 (TVA Calculation CDQ0000002016000041) . Note, that TVA also performed sensitivity of using the 100-year only data versus the linear fit method in all other regions of the TVA basin . Differences between using the 100-year only values and the best fit linear trend are minimal for the TVA basin and within the margin of error associated with the uncertainty in the overall PMP development. Therefore , implementation as currently applied is acceptable with the exception noted above. RAI #9: OTF Calculation Issues Technical Deficiency: Potential issues with the OTF calculations in certain regions were identified by staff and require clarification. Staff's review of the Total Adjustment Factor (TAF) Excel files provided in response to RA/ #1 revealed some anomalies in how the OTF was calculated. For a select set of grid cells, the OTF was calculated using an absolute cell value in the Excel spreadsheet rather than using the OTF regression-based formula used in other cells. Visualization of the areas using the absolute cell reference value is provided in NRG Figure 4 (for general and tropical storms) and NRG Figure 5 (for local storms). NRC Figure 4: Grid cells for which OTF calculation used an absolute cell reference value for General & Tropical storms (the red grid cell indicates the location of the grid cell used for assigning an OTF value for all yellow colored grid cells) Page 31 of 71
NRC Figure 5: Grid cells (in yellow) for which OTF calculation used an absolute cell reference value for Local storms (the red grid cell indicates the location of the grid cell used for assigning an OTF value for all yellow colored grid cells) In addition, staff compared the Excel-based OTF values from the RA/ #1 response and the GIS-based OTF values from RA/ #3. The comparison revealed a discrepancy in calculated OTF values for local storms which was confined to a region of the southern Appalachians. Visualization of the areas affected by this discrepancy is provided in NRG Figure 6 . NRC Figure 6: Grid cells (in red) for which OTF differs between RAI #1 & RAI #3 for Local storms Page 32 of 71
Request: a) Provide an explanation for why the OTF was calculated using an absolute cell reference value for the grid cells identified in NRG Figure 4 and NRG Figure 5 rather than using the OTF regression-based formula used for the other cells. b) Provide an explanation for why the OTF values provided in RAJ #1 and RAJ #3 differ, as illustrated in NRG Figure 6. TVA Response - 9(a): The primary reason for applying a constant OTF based on an absolute cell reference rather than using the OTF regression-based formula over the regions highlighted in TVA Figures 14 and 15 was to address a significant discontinuity in OTF values for 35° N latitude. This discontinuity is a direct result of boundary discrepancies between NOAA Atlas 14 Volume 2, which covers the project area north of 35° latitude, and Volume 9, which covers the area south of 35° latitude . The boundary issues are briefly acknowledged in Atlas 14 Volume 9: "Precipitation frequency estimates for each volume of NOAA Atlas 14 were computed independently using all available data at the time. Some discrepancies between volumes at project boundaries are inevitable and they will generally be more pronounced for rarer frequencies" (Perica, et al., 2013, pg . 4). A secondary reason for applying the constant OTF was to smooth out "bubbles" that occurred over the non-orographic western portion of the project area, primarily south of 35° N, that do not necessarily reflect orography, terrain features , or elevation . General storm and tropical storm PMP utilize the 24-hour Atlas 14 precipitation frequency grids for OTF calculations . At the 24-hour duration, the boundary issue is most prevalent over northeast Mississippi/northwest Alabama and northeast Alabama/northwest Georgia , as shown in TVA Figure 14. For general and tropical storm PMP, the discontinuity is less of a concern than local storm PMP as general and tropical PMP tends to control the PMF for larger basins. The discontinuity tends to dissolve through the basin average over very large basins. Page 33 of 71
JOO-year 24-hour NOAA Atlas 14 Precipitation Estimates (inches) Teoessee Valley Authority Drainage Area 37'N 3N
'8'W 1/T'W f!lj"VJ 8','W ... w .,.... srw CoolQlnllle SySl8TI WGS 1i64 VTM lane 16N Prq<<11e1n Tranwe,w Meru:ot Mnes on.,n, WGS198.t 0 50 100 150 200 TVA Figure 14. 24-hour 100-year precipitation illustrating discontinuity between volumes for 35° N Page 34 of 71
Local storm PMP utilizes the 6-hour Atlas 14 precipitation frequency grids for OTF calculations . At the 6-hour duration , the boundary issue exists along the 35° N state boundaries similarly to the 24-hour duration . Furthermore, there is also a significant discontinuity over the Tennessee/North Carolina/Central Georgia state border. The depth of rainfall over the Hiwassee River drainage basin south of 35°N (Atlas 14 Volume 9) is significantly less than north of 35°N (Volume 2) , as shown in TVA Figure 15. 100-year &-hour OA A Atla 14 Precipitation Estim ates (inches) Tenessee Valley Authority Drainage Area
~*w vw "W 16-W 64 ' W .,.w a,*w 81 ' W JI'
- ,;*N 3N E4'W . .,, ,_,,W lO'W CclcrOr\a Systan WGS 1~ VTM 2ai.1GN
~qec>>onlrl 'ISW'WW~
-==--==-=== ==-----=====Mies 0 50 100 150 200 Dnm WGS 198A TVA Figure 15. 6-hour 100-year precipitation illustrating discontinuity between volumes for 35° N The western portion of the project area is non-orographic and predominantly lacking terrain features that could influence rainfall. Ideally there would be very little variation in the precipitation frequency estimates used to determine the OTF in this region . To correct for the significant variation occurring on either side of 35°N, a decision was made to recalculate the OTF over the 35°N region using a constant OTF from a representative point chosen in western Tennessee. This process was employed to remove the most significant portion of the OTF discontinuity resulting from the Atlas 14 boundary issues . Secondarily, the recalculation would smooth out any variations in the OTF over this non-orographic region that might occur from variations in the underlying Atlas 14 datasets.
Page 35 of 71
There are two areas of subjectivity involved in the recalculation process; defining the region to be recalculated to a constant OTF, and selecting an absolute cell reference as a representative location from which to take the OTF to assign to the recalculation area rather than using normal OTF regression-based formula . For general and tropical storm PMP, the recalculated region is shown in yellow in NRC Figure 4. The region was manually delineated in a manner that encompassed the problem area and followed the isopleth of the OTF spatial pattern consistent with the representative point chosen . The point location of 35.2° N, 87 .3° W was chosen to represent the non-orographic general and tropical storm recalculation region. For each storm , the OTF at each grid point inside the recalculation area was set to match the OTF value at the representative location . For local storm PMP, the recalculated region is shown in yellow in NRC Figure 5. The general process that was applied to the general and tropical storm OTF recalculations was applied to the local storm OTF. The local storm recalculation area covers a somewhat different area and different shape than the general/tropical storm area due to the spatial pattern of the 6-hour precipitation frequency differing from the 24-hour patterns. For local storm OTF recalculation, the point of representation is located at 35.475° N, -88.175° W. In addition, the area in red shown in NRC Figure 6 was reevaluated due to the significant disparity between the Atlas 14 volumes over the Hiwassee drainage area. The OTF for this area was recalculated in a similar way to the process described above for the western region of the project area where a representative location was chosen and each grid point within the area of interest (AOI) was assigned the OTF from the representative location . For this area , the absolute cell reference representative location was chosen as a point within the basin near the outlet of the basin at 35.15° N, -84.45° W. Due to the highly orographic nature of the AOI, a constant OTF reassignment alone is not sufficient; therefore an elevation adjustment factor was applied to the constant OTF to estimate the orographic effect over the AOI. The elevation adjustment factor was determined as the ratio of elevation at the target location to the elevation at the representative location , which is 1,624'. A sixth root is applied to sufficiently mute the ratio to be consistent with the surrounding OTF values outside the AOI. 6 htarget ElevationAdjustmentFactor = -h-- rep.
- where, htarget = elevation at the target location hrep = elevation at the representative location (1 ,624')
An example map of general storm PMP before the constant OTF adjustments over the western portion of the project area is provided in TVA Figure 16. TVA Figure 17 shows the same PMP map after the general/tropical storm OTF adjustments are implemented . Page 36 of 71
72-hour General Stonn PMP ( L0,000 mi") - Before OTF Adju tme nt TVA Study Area Ol'W erw eft"W 115-W (WW ID'W rrrw J7'N l7'N ,.. l'>'N
- M' N PM P Depth 3N (In ches)
- <2 1111 12 24 - 26
- 2 14 26 - 28 4-6
- 16-18
- 26-30 6 18 30 - 32 o s - 10
- 20-22 0 32 . 34 0 10 22-24 0 > 34
..... ,,,... a.:rw &rw Coord11'1Mi1S1'Stf11'1 USA~Abl!t1~1Ar.. eor.c:
llli(t(ICll(ln~
-c:=--==--======------======M 0 50 100 150 200 iles OIi.im NOnh A.Intl 1CM1 t983 TVA Figure 16. Example of general storm PMP before constant OTF adjustment Page 37 of 7 1
72-hour General Storm PMP ( I0,000 m?) - After OTF Adjustments TVA Stud y Area
~ fJIJ"W ~"W !W'W a:rw ,rrw J7"N 34* PMPOepth ,. ..
(lncM s)
- <2 14 26
- 2 14-16-26-28
- 4 16-1 8 - 28 - 30 6 18 30 - 32 6 20- 22 32 - 34 0 10 ..-w 22-24 0 > 34 vw .
-::::11-==--=== =::::11----i:====:::iM 0 50 100 150
..-w 200 iles
~ S , s H m USA~~E~mt!Coni< _
OIIUffl North "menc*n1983 TVA Figure 17. Example of general storm PMP after constant OTF adjustment An example map of local storm PMP before the constant OTF adjustments over the western portion of the project area and the elevation-based OTF adjustments over northern Georgia is provided in TVA Figure 18. TVA Figure 19 shows the same PMP map after the local storm OTF adjustments are implemented . Page 38 of 71
6-hour Local Stonn PM P ( IO mi2) - Before OTF Adjustment TVA tudy Arca IO'W erw arw ~-w M-w m*w ,,.., V'N .6'N ,.. PIIP Depth ,.... (Inches)
- <2 - 12 24 - 26
- 2 14-16 28
- 4 16 28 - 30 6 18 30 - 32 8 20 - 22 32 . 34 0 10 .....
22-24 0 > 34 _c:::. . 0 ic:=-a:::::=====------======Miles 50 100 150 200
... w 12",\i C.OO,....,S.514'1'1 USA~,Atlft1Eo,i,AIMCoril< _..
OP.,rrl No11'1¥Wftt.M 19') TVA Figure 18. Example of local storm PMP before constant and elevation-dependent OTF adjustments Pag e 39 of 71
6-hour Local Stom1 PMP ( 10 mi~) - Afl er OTF AcU u tment T A tudy Arca
**w arw 81'J"W ar.*w &a'W a:,-w 3TN ,, .
~*N
.M' PUP Depth pnches)
- <2 - 12 24 - 26
- 2-4 - 14 26-28
- 4-6 - 16 28 - 30 6-8 - 18 30 - 32 8-10 20 - 22 32-34 10* 12 - 22-24 0 > 34 erw
*+
M"W MW 111a:::::::a-===-a:==:==:==::=::a--------===:==:==:::::i Moos 0 50 100 150 200 TVA Figure 19. Example of local storm PMP after constant and elevation-dependent OTF adjustments TVA Response - 9(b): (Note: NRC 9(b) question references RAI #1 and RAI #3 . These RAI references actually refer to informal information provided in response to NRC's audit Information Need #1 and #3 , respectively .) TVA inadvertently provided an older version of the local storm OTF values for Information Need
- 1 . The Total Adjustment Factor spreadsheets provided for Information Need #1 included the constant OTF adjustments made over the western portion of the project area , but did not yet include the elevation-based adjustments that were applied over the Hiwassee drainage as described above. The GIS files provided in Information Need #3 were the final version of the local storm OTF and included all adjustments and therefore were different than the values provided for Information Need #1 for the grid points highlighted in NRC Figure 6.
Page 40 of 71
RAI #10: Custom Transposition Limits Technical Deficiency: Based on staff's review of information provided in response to RA/ #1 , the majority of storms included transposition limits that conform to the TVA Zone boundaries. However, at least four storms appeared to contain custom transposition limits, as listed in NRG Table 2 that don 't conform to the TVA Zone boundaries. NRC T a bl e 2 S ummary of storms su b"11ec ted t o cus t om t ranspos1T10n I"1m1*ts Storm SPAS No. Storm Type Transposition Limits ' Elba , AL 1305 General South of 35 deg N (exclusive of Zone 4) Americus , GA 1317 Tropical Based on TSR L-Cv 0.24 contour* Larto Lake, LA 1182 Tropical Based on TSR L-Cv 0.24 contour* Big Rapids , Ml 1206 General North of 36.5 deg N (exclusive of Zone 4)
- Note: information from TAF Excel file, OTF sheet Request:
a) Provide a justification as to why each of the storms listed in NRG Table 2 was subjected to custom transposition limits. b) Provide a justification for the use of custom transposition limits for the Americus, GA and Larto Lake, LA storm using TSR L-Gv 0.24 contour. Provide the physical basis used to justify this custom approach. TVA Response - 10(a): (Note: The NRC question references RAI #1 . This RAI reference actually refers to informal information provided in response to NRC's aud it Information Need #1 .) Each storm on the final storm short list was evaluated for explicit transposition limits . TVA transposition Zones (1-4) were used as initial reference for all storms. Further refinements between and within zones took place only as required based on unique individual storm characteristics and/or maintaining spatial continuity of adjustment factors and PMP depths. Discussions took place between AWA and other TVA study participants during the Review Board meetings to evaluate specific storm transposition limits. Extensive discussions regarding transposition limits and specifically refined boundaries or constraints were requ ired because of the meteorological judgment that is applied in developing and assigning the transposition limits for a given storm . Specific to the four storms listed in the request, the following response is provided :
- The Elba , AL storm was limited to areas south of 35°N latitude based on the synoptic meteorology associated with the storm , including direct access to Gulf of Mexico moisture without any intervening topography , the combination of synoptic meteorological factors that led to the storm versus what could occur over the TVA basin , and previous transposition limits applied by the NWS (TVA Figure 20) . The synoptic meteorology associated with this event was directly related to the moisture Page 41 of 71
and thermal environment associated with the interaction of the front moving through the region and the relatively warm waters of the Gulf of Mexico. This combination would not occur further north during this season of occurrence. Note that the NWS transposition limits map explicitly shows that the storm is only transpositionable to 34°N latitude. It is assumed the NWS considered the synoptic meteorological environment to be a limiting factor of not moving the storm further north . For final application , TVA applied a more conservative transposition limit to this storm (to 35°N latitude) to account for the judgment involved in the process, to allow consideration of general topographic similarities to regions around 35°N latitude, and to produce more spatially consistent PMP depths between where this storm was transposed and where it wasn't transposed . For this storm , it was determined that the combination of available moisture and storm dynamics could not occur further north during the March timeframe without changing the storm dynamics significantly.
. * - li. , ........- ~ .
. , CT
...* * . . . .I
\ -- . ~ ...... -... __ (.~
- 1. ,
.' .f-- ---.. I
; 0 I - - - - - - ...
" * * - ~ ---11
* . J ' - :
. -~*::..-:..- -
,l * - - - - - ..
TVA Figure 20. NWS transposition map for the Elba, AL March 1929 storm
- The Americus , GA and Larto Lake , LA storms were both tropical events. As mentioned previously, each storm 's individual synoptic meteorological environment, interaction of moisture source and topography, and previous transposition limits were investigated in relation to the overall TVA basin . This demonstrated that these two storms should not be transpositioned any further north than currently applied . This is because direct land falling tropical systems do not affect most of the TVA basin without significant degradation and changes to the structure. This results from interactions with topography and distance from the moisture source.
Page 42 of 71
- For the Big Rapids storm , similar AWA discussions with the TVA Review Board and PMP study team took place regarding moisture source, topograph ic interactions and time of the year when the storm occurred. Explicit limits on the Big Rapids storm were applied because it was a controlling event and therefore required further evaluation to ensure spatial continuity in the PMP depths between the regions where it was used and the regions that bordered that area .
In this case , a PMP-type general storm occurring in September that included the required storm dynamics related to significant thermodynamic contrast would not be able to occur in the same fash ion further south . In th is case , the southern limit was judged to be 36.5° N latitude. Th is southern limit also considered the latitudinal extent of the storm and constraints following HMR guidance of applying a 5-6° latitude constraint (HMR 57 , page 69). Analog storm events with explicit NWS transposition limits maps were consulted as well as an analog data source to corroborate the transposition limits applied . Storms of similar type and season in the NWS transposition library of storms in Michigan and Wiscons in were not transpositioned south of 37° N latitude. TVA Figures 21-23 explicitly show NWS transposition limits maps and show that none were used further south than 37N latitude. Given these considerations , the application of this storm to 36 .5° N latitude was a conservative application compared to prior NWS guidance .
* ; 1- Sep t .1 . 191 ... Co oper. l.
8 ** 250 s-;i ** f~ 11;ss~ .- rtarth. to : . Or <1cr zu,~ t o: a t e- ~ba. t ta.noo g 11.ri Sou t b to~ 37 e* to : 99
. (. .' ~- -
... *-~......; ,. .
j \ j 1---=-
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I
/
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TVA Figure 21. NWS transposition map for the Cooper, Ml September 1914 storm Page 43 of 71
.... . 'l l- =- .. ~ .. ... *.
*I * *
- o. _-;. .
~ .
l *
.* I I
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/
..,I TVA Figure 22. NWS transposition map for the Hayward, WI August 1941 storm Page 44 of 71
*-v * * ., '"; * *"
_:_ 7..
~
.*. . . I I'
j
/
/
TVA Figure 23. NWS transposition map for the Merrill, WI July 1912 storm TVA Response - 10(b): TVA Figures 24-29 display the transposition limits appl ied to storms by the NWS , which occurred in similar regions as Larto Lake, LA and Americus , GA events . Each of these figures clearly demonstrates that the NWS did not consider storms in these regions transpositionable to the Tennessee Valley and specifically to the interior regions of the Tennessee River basin . In fact, the custom limits applied by TVA (TVA Figures 30 and 31)are significantly more conservative than the NWS transposition limits. TVA allowed these storms to affect the southern portions of the TVA region to ensure appropriate spatial continuity in PMP values between these regions and to account for the uncertainty of where the exact boundary would occur. However, the rainfall that occurred with the storms would not occur in the same meteorological and topographical setting existing over the interior TVA regions . Note, this does not mean that remnant tropical moisture doesn't produce rainfall in that region. Instead the magnitude of those rainfall events is significantly reduced because of the different interactions of topography and meteorology, thereby violating the definition of transpositionability . This requires that remnant tropical storms that occurred in similar meteorological and topographical settings be used in that region . This excludes the Larto Lake, LA and Americus , GA storms from consideration . Page 45 of 71
~*1 ....._"' * . : .....:.e l.r-0 . :. ~ * . ...i.:e.L::.!:.:.: .!.
- ~- i>r. :-*!'i 750 3ti:-.) .. *os .
- t.o 7e . :
.;..iS t. -;o : d:
.:es. t -;o: 9'7
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TVA Figure 24. NWS transposition map for the Alexandria, LA June 1886 storm Page 46 of 71
\,
TVA Figure 25. NWS transposition map for the Simmesport, LA May 1935 storm Page 47 of 71
_ .* (..- . . ;-. ....~ r. : ; - l o. : ; ~ *
- 1Uta~ * .
__- .r . ~ ~..,. CO ~ ':..."". ) * * - .:. :35 ,i . * ,:. C ~
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.... - -- - .t.1-....:---.J TVA Figure 26. NWS transposition map for the Eutaw, LA April 1900 storm Page 48 of 71
r* -~, . - .~ _,_.
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' I ,; / .,' ii.,_*" I"';.' :* I.I.,: L.. . ' '- c* ' .> ..., _,,, rr==~""""="" ~ ~~~====,,,,;,,...-=-::~
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( TVA Figure 27. NWS transposition map for the Merryville, LA March 1914 storm Page 49 of 71
L/1JI 3 . , . . , 7 t~::211 ) 1L-:.- , 2 1:::,,,/}':lJf'4 12 J...r r i :-{ / 3 ( / "ft ) .? SE, "S ~ J o,.c_ ;:: 11e 5 C tt.-'
-- ..--.. ' i
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.i I TVA Figure 28. NWS transposition map for the Lakeside, LA November 1922 storm Page 50 of 71
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--- r . . -- .
... ':'A
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- t;O '; t _; ... :if j-
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TVA Figure 29. NWS transposition map for the Elba, AL March 1929 storm Page 51 of 71
Larto Lake, LA , Sep. 2008 (SPAS 11 82) Transposition Limits TVA PMP Study ttrw 86'"11 <<."W 6.l"W P/J"W 34' I ... At,~1 l ......... .
, .\1tl* II
- _... ;\4'N
. ~.
~--
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'1'W "3'W 16'W O>'W irrw
~s,,,:wn USA~s...,.,, EC,.-AruQnic DP.Im HOlth NMK".all 1913 Mijes 0 50 100 150 200 TVA Figure 30. TVA transposition map for the Larto Lake, LA September 2008 storm Page 52 of 71
Americus, GA, July, 1994 (SPAS I 317) Transposition Limits TVA PMP Study VW 86"W 15'W &l 'W 83"W rr,-w JT"N
,.. ,, ~k,l*u L**u,o.. O'l ~le A~ffi ll!'llt*~
Alt,ml.
. ."W rrw ,,,-w . ."W ...w .,.., lfZ'W
*t
- COOrcttn*S,-s:n USACorltlguws~EQ,lalArwConlc Pt-..onM>trs o.tffl NDrtti AtnwfQn 19e3 Mi es 0 50 100 150 200 TVA Figure 31. TVA transposition map for the Americus, GA July 1994 storm After further review and discussion with the NRC staff in regard to this question , TVA has agreed to remove reference to use of the TSR L-Cv 0.24 contour as one of the reasons for defining the updated transposition limits of these two storms. Instead, discussions specifically related to the differences of the meteorological and topographical environments are used to define and justify the transposition limits . These updates to th is data set will affect Section 5.3 of Calculation No. CDQ0000002016000041 and will be subm itted as Revision 1 to Topical Report TVA-NPG-AWA16 (TVA Calcu lation CDQ0000002016000041 ).
Page 53 of 71
RAI #11: Storm Representative Dew Point Selection: Timeframe and Location Technical Deficiency: Staff's review of the licensee 's storm representative dew point data for short list storms resulted in the identification of several storms for which questionable timeframe and/or location data may have been used when selecting the storm representative dew point. This issue can significantly impact PMP values for controlling storms As a part of its assessment, staff reviewed the rainfall mass curves, HYSPLIT trajectories, and storm representative dew point information that the licensee provided in response to RA/ #1 and RA/ #2. Staff also independently evaluated this information to assess the reasonableness of the data application. Staff's review of the above information revealed that the licensee 's storm representative dew point selection used dew point data which were observed at locations far upwind of the storm center and during timeframes in which significant rainfall had already occurred. Conducting the analysis in this way could inadequately represent the storm characteristics and (in these cases) result in PMP underestimation since the relatively higher moisture observed could not induced the observed rainfall. Staff believes the storm representative dew point methodology regarding HYSPLIT trajectories and/or dew point timeframes may be flawed for the following storms. A comparison of the TVA and NRG storm representative dew point temperatures these storms is provided in NRG Table 3.
- 1. General Storm, SPAS 1206 (Big Rapids, Ml) - see NRG Figure 7
- a. The licensee 's dew point temperature observations correspond to a period after significant rainfall had already occurred. The representative dew point location is approximately 230 miles SW of the storm center location.
- 2. General Storm, SPAS 1208 (Warner Park, TN) - see NRG Figure 8
- a. The licensee 's dew point temperature observations correspond to a period when the most intense rainfall occurred. The representative dew point location is approximately 360 miles SSW of the storm center location.
- 3. Tropical Storm, SPAS 1276 (Wellsville, NY) - see NRG Figure 9
- a. The licensee 's dew point temperature observations correspond to a period when the most intense rainfall occurred. The representative dew point location is approximately 385 miles SSW of the storm center location.
- b. By adjusting the HYSPLIT backward trajectory timing to more closely align with the onset of rainfall, staff identified a moisture inflow direction of SE rather than SSW
- 4. Tropical Storm, SPAS 1317 (Americus, GA) - see NRG Figure 10
- a. By adjusting the HYSPLIT backward trajectory timing to more closely align with the onset of rainfall, staff identified a moisture inflow direction of SE-to-S rather than WSW Page 54 of 71
- 5. Additional storms which exhibit timeframe issues but do not control PMP
- a. General Storm, SPAS 1218 (Douglasville, GA & LaFayette, GA) - see NRG Figure 11
- b. Local Storm, SPAS 1226 (College Hill, OH) - see NRG Figure 12
- c. Local Storm, SPAS 1209 (Wooster, OH) - see NRG Figure 13
- d. Tropical Storm, SPAS 1182 (Larto Lake, LA) - see NRG Figure 14 NRC Table 3. Comparison of TVA vs NRC storm representative dew point temperature for storms W I'th po ten f 1a I HYSPLIT or f 1mmg
. .issues Storm Storm Rep. Td Difference Number Storm Name SPAS Number Type (deg F) (TVA-NRC)
TVA Td NRC Td 1 Big Rapids , Ml SPAS 1206 General 70.5 68.5 +2 2 Warner Park, TN SPAS 1208 General 75 74 +1 3 Wellsville , NY SPAS 1276 Tropical 72.5 70.5 +2 4 Americus, GA SPAS 1317 Tropical 76 74 .5 +1 .5 Sa Douglasville, GA SPAS 1218_1 General 76 75 +1 Sa LaFayette, GA SPAS 1218_2 General 76 75 +1 Sb College Hill , OH SPAS 1226 Local 68.5 66 .5 +2 Sc Wooster, OH SPAS 1209 Local 76 72 +4 5d Larto Lake , LA SPAS 1182 Tropical 76 73 +3 Request: Provide justification for the selection of storm representative dew point values for the above storms with respect to timeframe and location selected, especially considering the timeframe of when rainfall occurs at the storm center. If corrections are warranted, provide an updated analysis as it may affect TVA 's 3 NPP sites. Page 55 of 71
TVA Mass Curve - TVA Dew Point Selection SPAS 1206 Storm Center Mass Curve: Zone 1 September9 (0600 UTC) to September 13 (0500 UTC), 1986 Lal 43 61 25 Lon -85 3125 travel time offset ,. based on HYSPLIT 1.5 ,, 20 60 60 Index Hour SPAS 1206 Big Rapids. Ml Storm An alysis September9-13 1986 NRC Figure 7. General Storm, SPAS 1206 (Big Rapids, Ml) rainfall mass curve (top) and dew point analysis (bottom) Page 56 of 71
TVA Mass Curve - TVA Dew Point Selection SPAS 1208 Storm C*nterMassCurve: ZoM 1 May 1 (0100 UTC) to June 3 (1200 UTC), 2010 Lat 36 11 Lon -S8 05 2.5 Perlodof ralnfllll reprwww._.ve ofAWAdew polntseledlon 10 2l) 30 50 fD lndeX Hour SPAS 1209 - Dew Point Temper*ture (F) April 29
- May 2. 2010 NRC Figure 8. General Storm, SPAS 1208 (Warner Park, TN) rainfall mass curve (top) and dew point analysis (bottom)
Page 57 of 71
TVA Mass Curve - TVA Dew Point Selection SPAS 1276 Storm C*nt*r Mass Curve: Zone 1 Jun* 18 (0700 UTC)-.June 25 (0600 UTC), 1972 Lat *2 0375 Lon -78 0708 tra vel time offset based on HYSPL/T 2.0 0.5 o.o - - ~ 10 - 20
~ -lO- -- - ~ 70 90 100 110 120 130 ,,o 150 160
" 50 60 60 Index Hour SPAS 127& Scorm An,1fy ais June 1~ 22. 19n TVA HYSPLIT NRC HYSPLIT Backward trajectories ending at 0000 UTC 20 J un 72 NOM HYSPLIT MODEL Bad<Ward trajectories ending at 0000 UTC 21 Jun 72 CDC 1 Meteorolog ical Data CDC1 Meteorologlcal Dala
.-< -./
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z {."' ;!; N j V
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s J.o-~ r NRC Figure 9. Tropical Storm, SPAS 1276 (Wellsville, NY) rainfall mass curve, dew point analysis, TVA HYSPLIT, and NRC HYSPLIT (from top, middle and bottom) Page 58 of 71
TVA Mass Curve - TVA Dew Point Selection SPAS 1317 Storm Center Mass Curve: Zone 1 June 30 (0700 UTC ). July 9 (0600 UTC), 1994 Lat 32 0958 Lon -114 22!12 travel time offset based on HYSPL/T, 50 Index Hour 1SO 200 SPAS 1317 Alberto, GA Storm Analysis
!'!,lf 5-6, 1~
TVA HYSPLIT NRC HYSPLIT NOAA HYSPLIT MODEL NOAA HYSPLIT MODEL Backward trajeciories ending at 0600 lJTC 06 Jul 94 Backwa rd trajectories ending at 0000 UTC 04 Jul 94 CDC 1 Meteorological Data CDC1 Meteorological Data 1i j
~
.:moe II 12 M 00 18 01.os ll ~~
01;o. II 12 700 750 rs& 900 950 1000
~
JcblOlUl:538 J.IS&.1* 5-1 2822.23 45U1C:.01l
~ 111'1
- 3'l090000 Ian 4"1.2!l0000 11g9* 0 1.a2D, !JO!iO,nAQ.
~~~~~\l~VlloUr
~OOCXlZ 1..ltl2'>><1---1
- NRC Figure 10. Tropical Storm, SPAS 1317 (Americus, GA) backwards HYSPLIT trajectory from TVA (left) and NRC (right)
Page 59 of 71
TVA Mass Curve - TVA Dew Point Selection SPAS 1218 Sto rm Center Mass Curve: Zone 1 September 19 (1300 UTC) to September 22 (1 2.00 U TC), 2009 Lal 33.87 Lon. -84 76 3.01~= .. 1 2.S m
~
12 " .0
~ 1.5 Q. "cE 1.0 ~
o.s 7D SPAS 1218 . Dew Po in t Te mperat ur e (F) September 18-22. 2009
-* hrll,c,e * !eiDl'Jl :l
- JOCnO NRC Figure 11. General Storm, SPAS 1218 (Douglasville, GA
[shown] & LaFayette, GA) rainfall mass curve (top) and dew point analysis (bottom) Page 60 of 71
TVA Mass Curve - TVA Dew Point Selection SPAS 1226 Storm C*nt*r Mus Curv*: Zon. 1 Jun* 4 (0600 UTC) to Jun* 6 (0600 UTC), 1963 Lat 40 0854 Lon -S1 6479 travel time offset 3.S I~=:~ I 20 3.0 I
~2.S AWA dew
- C
- ~i sellctlon 2.D
~
a. 1i 1 S c E
~ 10 £ o.s 10 *o lndelCHour '° College HIN , OH StOfm Ana.lyals June 1*5. 1963 NRC Figure 12. Local Storm, SPAS 1226 (College Hill, OH) rainfall mass curve (top) and dew point analysis (bottom)
Page 61 of 71
TVA Mass Curve - TVA Dew Point Selection SPAS 1209 Storm Center M~s Curve: Zone 1 July 4 (0600 UTC) to July 7 (0500 UTC), 1969 travel time offset La1 40 915 Lon .a, 973 3.0
----------------t" . ...
2.5 I \S., 2 .0 j
- ~ 1.S Q.
~ E 1.0 i 0.5 10 70 SPAS 1209 Wooster. OH Storm A rudysta
.klly2*5. 11169 NRC Figure 13. Local Storm, SPAS 1209 (Wooster, OH) rainfall mass curve (top) and dew point analysis (bottom)
Page 62 of 71
TVA Mass Curve TVA Dew Point Selection SPAS 11 82 Storm Center Mass Curve: Zone 1 NOAA HYSPUT MODEL Septemti.r 1 (0100 UTC) to September 5 (0000 UTC), 2008 Backward traiectory ending at 0000 UTC 03 Sep 08 COC1 M*t.orologic.'IJ Oa.t:l
.........- - ~- -~ ~- - -~ - - - - -
1000 Index Hour NRC Figure 14. Tropical Storm, SPAS 1182 (Larto Lake, LA) rainfall mass curve (left) and dew point analysis (right) TVA Response: In each of the four cases (NRC Table 3, Storms 1 through 4) noted as potentially controlling the PMP, the air mass evaluated as represented by the TVA storm representative dew point selection was inclusive of the overall air mass advection into the overall storm domain. Although , the exact timing of the 12- or 24-hour period chosen occurs during a later portion of the overall ra infall period at the storm center, it is still representative of the overall air mass that was part of the rainfall event across the entire region. This is the intent of the storm maximization process to represent the overall air mass resulting in the observed event. The final process requires the analyst to calculate a specific value at a specific location at a specific point in time. However, in actuality, the moisture advection and storm development processes change in space, time , and magnitude. One of the more important considerations relative to the intent of the in-place maximization process is that moisture associated with these events is at higher than normal levels. Therefore , the region chosen as the moisture source region and the eventual storm representative dew point should also represent values that are higher than normal. In some cases , data available to analyze and select a value were inadequate and require judgments not fitting the standard processes. For example, there may be a lack of surface dew point observations in a general region where the air mass source region would be expected to originate in either space or time . Further, the storms that are being maximized are extreme rainfall events. In such cases , judgments must be appl ied to allow for selection of storm representative dew point values that can be used in the maximization process and represent a high level of atmospheric moisture. Storm No. 1: Big Rapids, Ml The storm representative dew point selected by TVA does represent a later portion of the storm , which is acceptable, and within the standard process of storm representative dew point selection. This process allows for selection of a 24-hour average value even though the overall storm period may be significantly longer than 24-hours. The value selected by TVA represents a period when the heaviest rainfall occurred during the storm and is Page 63 of 71
representative of the overall air mass, which advected moisture into the overall storm environment over several days. TVA investigated daily weather maps to determine the general location of high and low pressure centers and fronts immediately preceding and during the event. This analysis confirmed the general air mass source region as shown by HYSPLIT and used for the storm representative dew point location . Hourly surface observations were investigated for stations in a large region surrounding the general air mass source region from west through south-southwest as shown . Stations in this general region showed values ranging from the upper 60°'s to 70°F as 24-hour averages. TVA checked various surface observations to find a region that was most appropriate given the data available, the general moisture inflow region, the synoptic environment, and the severity of the resulting rainfall. These investigations showed that a region to the wesUsouthwest was most synoptically relevant and that high values in that region were necessary to have resulted in the record rainfall that occurred. This storm controls PMP at 48- and 72-hours for area sizes greater than 5,000-sqaure miles in many studies in the region. Even after choosing the highest of the available values (70°F at KMMO), the in-place maximization factor (IPMF) was still 1.40. The KMMO wind speed and wind direction in relation to moisture inflow timing at the storm also support the general air mass source and timing as identified in HYSPLIT and daily surface weather maps. This is an example where the available surface dew point data and the general process used to derive a storm representative value isn't necessarily representative of the overall environment. This can occur when there is an intervening frontal boundary and the most critical moisture is along an elevated boundary above the surface and not best reflected by the surface dew point values. However, the data available is still limited and the standard process and choosing the KMMO values was determined the best solution for this storm . Storm No. 2: Warner Park, TN This storm was associated with a large moist air mass flowing from a general south to north direction off the Gulf of Mexico, with a deep moisture tap well into the southern Caribbean. This feed of moisture lasted for several days and is one of the reasons the timing utilized by TVA is appropriate for this IPMF analysis. Recent research has termed these types of events as the "Mayan Express" , with similar characteristics to an Atmospheric River event (Higgins, et al. 2011 ). From https ://www. climate. g ov/news-featu res/event-tracker/maya-express-beh ind-gulf-coast-soa king, (Di Liberto 2016), "Four-day rainfall totals near two feet caused devastating flooding in parts of Louisiana, Texas, and Mississippi in mid-March 2016. To blame was a seemingly never-ending stream of moisture straight out of the tropics ." The surface analysis completed by TVA reflects these synoptic characteristics and shows a very uniform air mass in time and space over much of the regions from Louisiana to north Florida. TVA investigated several regions with dew point observations and chose the area over southern Mississippi because it exhibited high dew points over a large area with consistent values that were within the air mass advection region. Again, this type of rainfall would only be associated with extremely moist air masses, which were reflected by the 74-76°F 12-hour average values over a large region of Louisiana, Mississippi and Alabama . This analysis shows that the timeframe and location used by TVA is appropriate given the overall rainfall accumulation period and the continued moisture advection over time through the region and into the Warner Park storm center location . Page 64 of 71
Storm No. 3: Wellsville, NY The air mass associated with this storm covered a large region of the eastern United States from the Gulf of Mexico through New England over a several day period . This was shown by the synoptic analysis of the storm after landfall as it traveled through the region producing several rainfall centers over a period of several days. As has been explicitly demonstrated, the Wellsville, NY center received moisture that was advected over the Piedmont region of the Carolinas and Virginia , which included very moist air supplied by the Gulf Stream off of the Atlantic. Again , this air mass was present for several days and the values chosen by TVA represent the overall air mass over the several day period. The surface dew points used were representative of the overall region in space and time, with low ?O's through Virginia, North Carolina, and Maryland . Higher values were occurring at the same time along the immediate coast and Outer Banks region (mid ?O's). Given that this storm produced record rainfalls and many floods of record in Pennsylvania, it was appropriate that the dew points chosen represented an extremely moist air mass. Justification could be made that the values along the Outer Banks could be used , which would have resulted in a lower IPMF. The current value of 1.29 is conservative given these factors . Storm No. 4 : Americus, GA The overall air mass for this storm covered a large region extending from the Gulf of Mexico inland through Louisiana, Mississippi, Alabama, and Florida. The storm produced extreme rainfall because it was able to tap into this consistent moisture feed for several days while remaining over the same general region of Alabama and Georgia . The location TVA chose was just outside of the main rain shield and in a region reflective of this type of air mass over southern Louisiana and Mississippi. Values were consistent through a large domain , in the mid ?O's over several days and also consistent with values as far away as north and west Florida . Even with these high dew points, the IPMF was still 1.21, wh ich is conservative given the extreme amounts of rainfall associated with this storm . The following storms, 5a through 5d , do not control PMP. Storm No. 5a : Douglasville/Lafayette, GA Hourly surface observations were investigated for stations in a large region surrounding the general air mass source region . The average wind speed for the 24-hour storm representative dew point was 5.0mph (-9.2 mph maximum) with an average wind direction from the southeast. The time for moisture to travel from the source location to the storm center (-350-miles) would be approximately 72.0-hours (38.0-hours for max wind speed) . The general area wind speed and wind direction in relation moisture inflow timing at the storm also support the general air mass source and timing as identified in HYSPLIT and daily surface weather maps. The region chosen by-TVA best represents the region that supplied the low-level moisture to the storm from the Gulf of Mexico given the general synoptic patterns and movement of the storm and frontal system . Storm No. 5b: College Hill, OH This storm is an example where available surface dew point values did not accurately capture the air mass that contributed to the extreme rainfall. Therefore, meteorological judgment was applied to derive a storm representative value that represented a moisture air mass that would have been required to produce the significant amount of rainfall that Page 65 of 71
occurred . This required the use of surface dew point observations that occurred in a timeframe that was not ideal for the storm environment. The average of 66.SF provided by the NRC in Table 3 is accurate given the exact timeframe. The value chosen by TVA of 68.SF was based on values that generally occurred after the main storm precipitation period , but within the same general air mass, region , and synoptic environment that resulted in the storm . Note that the use of 66.SF or 68.SF has no impact to the TVA PMP values as the in-place maximization factor was already at the upper limit at 1.48 and is capped at 1.50. Storm No. Sc: Wooster, OH Hourly surface observations were investigated for stations in a large region surrounding the general air mass source region . The average wind speed for the 24-hour storm representative dew point was 13.0mph (-19 .5 mph maximum) with an average wind direction from the west-southwest. The time for moisture to travel from the source location to the storm center (-140miles) would be approximately 10.8-hours (7 .2-hours for max wind speed) . The general area wind speed and wind direction in relation to the moisture inflow timing at the storm also support the general air mass source and timing as identified in HYSPLIT and daily surface weather maps. Storm No . 5d : Larto Lake, LA Hourly surface observations were investigated for stations in a large region surrounding the general air mass source region originating from the Gulf of Mexico and crossing the Texas/Louisiana coastal regions . The average wind speed for the 24-hour storm representative dew point was 11 .0mph (-17 .0-20.0 mph maximum) with an average wind direction from the southwest. The time for moisture to travel from the source location to the storm center (-180miles) would be approximately 15.8-hours (10.4-hours for max wind speed). The general area wind speed and wind direction in relation to the moisture inflow timing at the storm also support the general air mass source and timing as identified in HYSPLIT and daily surface weather maps. After extensive discussions and review with the NRC and TVA personnel , TVA has agreed to utilize the more conservative NRC storm representative dew point values for this PMP study for storms 1-4 in NRC Table 3. TVA will update the storm representative dew point values in the storm database and recalculate the PMP with those values implemented. This will affect Sections 6.8, 6 .8.1, 6 .8.4 , and Appendix F of Calculation No. CDQ0000002016000041 and will be submitted as Revision 1 to Topical Report TVA-NPG-AWA16 (TVA Calculation CDQ0000002016000041 ). RAI #12: Staff Independent Analysis of Dew Point Climatology Technical Deficiency: Staff's independent evaluation of dew point climatology reveals that the licensee's values may be non-conservative due to potential data source and processing issues which may impact the estimated PMP values. As a part of its assessment, staff reviewed the dew point climatology data provided by the licensee in response to RA/ #1 and RA/ #4; staff also independently evaluated these data to assess the reasonableness of the climatology data used. Staff has concerns with the dew point climatology data source and processing used by TVA. While TVA used NOAA 's TDL data set, NRG staff used NOAA 's TD3505 data set. Both TD3505 Page 66 of 71
and TDL data sets are officially released by NOAA, but the TDL data set used by TVA is basically a collection of instantaneous weather station observations whereas the TD3505 used by NRG is subjected to additional QC and processing by NOAA. Although both data sets are largely similar, there are some differences in the annual maximum series (AMS) caused by missing/erroneous values originally included in the TDL data set. This leads to different AMS and 100 y dew point estimates because of the existence and treatment of missing observations. Such differences result in systematic biases which could affect moisture maximization factors and transposition factors for all storms. To assess the impacts of using different data (and some minor differences in processing), NRG staff conducted independent evaluation of dew point climatology for all short list storms, and it yielded a number of differences from TVA 's evaluation. In general, NRC's independent evaluation resulted in higher dew point climatology values, with variation both temporally and spatially. For all else being equal, an increase in dew point climatology values will result in higher PMP estimates since historical storms would be subject to higher levels of moisture maximization. NRG Figure 15 shows the difference in the NRG and TVA dew point climatology values for each comparable station for all short list storms. The stations selected represent the stations which would have most influenced the dew point climatology at the transpositioned moisture source location and for which climatology values were available from both the TVA and NRG data sets. Positive values indicate that NRC's evaluation resulted in higher dew point climatology values than TVA, while negative values indicate that NRC 's evaluation resulted in lower dew point climatology values than TVA. On average, the difference for General, Local, and Tropical storms is +O. 69 F, +O. 61 F, and +O. 52 F, respectively, with an overall average station difference of +0.65 F. Individual station differences range from -1 .44 F to +3.67 F. D iffere nce i n D e w P oint Cli m ato logy for Short List Storms 4 G:"
- 0 0
3 0
- s. 0 0
0 0 0 8 C: 2 0 0 0 .. O 0 g C) 0 8 0 : 0 0 0 le: 1 0
<(
i....'.J 0 z a::
-1 ... 0 0
-2
~ne ra f Sto mi Avg Oi e rence l oca l S torm Avg Diffe rence rop,cal Stonn Avg Oi erence NRC Figure 15. Difference in dew point climatology values between NRC (ORNL) evaluation and TVA (AWA) evaluation for all short list storms Each column of data points corresponds to one short list storm. Black-outlined diamonds represent station data (one diamond corresponds to the NRG-TVA difference for a single Page 67 of 71
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station; for most storms, multiple stations were available for comparison) which influenced the dew point climatology at the transpositioned moisture source location and for which a direct comparison could be made. Colored squared represent the average difference in station data for each storm. The deviations in climatology values resulting from the two analyses indicates a systematic bias in the overall values, with NRC's values typically 0. 5 to 1.O degree F higher than TVA 's values. Request: Given the significant impacts noted above please update the dew point climatology using TD3505 dew point data and revising both the LIP and basin-wide PMP values accordingly or provide a justification for not updating it. TVA Response: (Note : The NRC question references RAI #1 and RAI #4. These RAI references actually refer to informal information provided in response to NRC's audit Information Need #1 and Information Need #4) After further review and discussion with the NRC staff in regard to the most appropriate dew point climatology database, TVA has agreed to conservatively revise the dew point climatology applied in Calculation No. CDQ0000002016000041 and to utilize the NCEI TD3505 hourly dew point database. This will extend the period of record and provide additional dew point observational data for use in developing updated dew point climatology. The updated climatology will replace the previously used GIS layers. The updated storm adjustments will be processed and applied to each storm used for the PMP development. Updates to this data set are anticipated to affect Sections 5.1.1, 6.1.1, 6.5.1, 6.8.1, 6.8.4 , and Appendix C of Calculation No. CDQ0000002016000041 and will be submitted as Revision 1 to Topical Report TVA-NPG-AWA16 (TVA Calculation CDQ0000002016000041). RAI #13: Warner Park, TN Dew Point Duration Clarification Technical Deficiency: Staff's review of the licensee's documentation and files related to the Warner Park, TN (SPAS 1208) storm representative dew point and dew point climatology data appears to indicate inconsistent use of dew point duration. As a part of its assessment, staff reviewed the text and digital information related to the Warner Park storm representative dew point and dew point climatology provided by the licensee. Figure 404 in the Topical Report shows that a 12-h duration was used to analyze the Warner Park storm representative dew point; however, upon further review, staff believe that a 24-h duration was used. Figure 415 in the Topical Report shows maximum average dew point data for several stations. Comparison with the "surface_summary" worksheet in the "SPAS_ 1208_ 0bs_data.xlsx" file reveals that the data plotted in Figure 415 correspond to the 24-h maximum average dew point. Also, staff confirmed that the licensee used a 12-h duration for the Warner Park dew point climatology. Therefore, it appears that the dew point duration was used inconsistently. Staff understands that if this is the case, then the licensee 's application could be slightly overly conservative; however, since it appears that a 12-h duration was intended, only the storm Page 68 of 71
representative dew point would change. The 24-h value used by the licensee is 74. 8 F based on the average of 4 stations (KHBG, KASD, KJAN, and KMCB); this value was rounded to 75. 0 F by the licensee. The 12-h value computed by the licensee is 75. 1 F based on the average of the same 4 stations and would be rounded to 75. 0 F. Therefore, it appears that changing the storm representative dew point would not change the results of the Warner Park analysis. Request: Provide confirmation of whether this dew point duration discrepancy exists, what the intended dew point duration is, and what (if any) changes are needed. TVA Response: The 12-hour duration was used in all calculations and is the appropriate duration to use. The image included in the report documentation mistakenly plotted the 24-hour average dew point data. The correct image is provided below (TVA Figure 32) . As noted by the NRC, use of either the 12-hour or 24-hour duration results in the same storm representative value , 75.0 F. A corrected Figure 415 will be provided in Appendix F of Calculation No . CDQ0000002016000041 and will be submitted as Revision 1 to Topical Report TVA-NPG-AWA 16 (TVA Calculation CDQ0000002016000041 ). SPAS 1208 - Dew Point Temperature (F) April 29 - May 2 , 2010 38"N
'7"N 30"N 38"N 38"N 3'"N 33"N 33"N 37'N 31'N 31"N
- ,O"N :,O" N 29'N -t- - '9"N H7splrt
- Surface
- 850mb
- 700mb -----======---------**,. 290 "80 TV A Figure 32. Updated storm representative dew point map using the 12-hour average dew point values Page 69 of 71
RAI #14: Scope of NRC's Review Regulatory Deficiency: This topical report describes the work performed to calculate the Probable Maximum Precipitation for any location within the overall TVA basin and Local Intense Precipitation (LIP) at the BFN, SQN, and WBN sites. The Summary and Conclusions section of the Topical Report states that the precipitation values in the report replace those in HMRs 41, 45, 47, and 56 (which provide PMP estimates for the Tennessee River Basin, including LIP), as well as HMRs 51 and 52 (which provide PMP estimates for the eastern half of the continental US) . NRC's regulatory authority limits its approval of the precipitation values contained in the Topical Report to only those values that could potentially result in flooding at TVA 's nuclear plant sites. Request: Please clarify that the scope of the NRC's requested review is concerned with potential SSPMP impacts at the 3 TVA nuclear power plant sites and does not necessarily reflect positions with respect to the entire Tennessee River watershed except as it impacts river flooding effects and local rainfall effects at the sites. TVA Response The scope of the NRC's requested review addresses only the potential SSPMP impacts at the Browns Ferry, Sequoyah and Watts Bar nuclear power plant sites and does not necessarily reflect NRC positions with respect to the entire Tennessee River watershed , except as it impacts river flooding effects and local rainfall effects at the nuclear plant sites. Section 7.0 of Calculation No. CDQ0000002016000041 will be revised to state that NRC review and approval of the calculation results is applicable only to the assessment of river flooding effects and local rainfall effects at the Browns Ferry, Sequoyah and Watts Bar nuclear plant sites and will be submitted as Revision 1 to Topical Report TVA-NPG-AWA16 (TVA Calculation CDQ0000002016000041 ). Page 70 of 71
- Referenced Data Files
- 1. RAI 1 - Complete Storm Analysis Information for All Short List Storms
- 2. RAl2 - TVA Observed Hourly Dew Point Data Sheet for All Short List Storms
- 3. RAl3 - TVA Storm Adjustment Factor Feature Class Table for All Short List Storms
- 4. RAl4 - TVA Dew Point Climatology Data and GIS Layers
- 5. RAIS - TVA Probable Maximum Precipitation Data and GIS Layers Page 71 of 71}}