NOC-AE-14003085, Second Partial Response to Request for Additional Information Associated with Near-Term Task Force Recommendation 2.1, Flood Hazard Reevaluation

Second Partial Response to Request for Additional Information Associated with Near-Term Task Force Recommendation 2.1, Flood Hazard Reevaluation

ML14056A195
Person / Time
Site:	South Texas
Issue date:	02/13/2014
From:	Powell G T South Texas
To:	Document Control Desk, Office of Nuclear Reactor Regulation
References
NOC-AE-14003085, TAC MF1110, TAC MF1111
Download: ML14056A195 (11)

Text

Nuclear Operating Company South Teas eProe Elecftic Generating Station PO. Box 289 Wadstvorth, Texas 77483-February 13, 2014 NOC-AE-14003085 10 CFR 50.54(f)U.S. Nuclear Regulatory Commission Attention:

Document Control Desk Washington, DC 20555-0001 South Texas Project Units 1 & 2 Docket Nos. STN 50-498, STN 50-499 STPNOC Second Partial Response to Request for Additional Information Associated With Near-Term Task Force Recommendation 2.1, Flood Hazard Reevaluation (TAC Nos. MF1110 and MF1111)

References:

1. Letter from NRC to All Power Reactor Licensees, "Request for Information Pursuant to Title 10 of the Code of Federal Regulations 50.54(f) Regarding Recommendations 2.1, 2.3 and 9.3, of the Near-Term Task Force Review of the Insights from the Fukushima Dai-ichi Accident" March 12, 2012 (ML12056A046).

2. Letter from G.T. Powell, STPNOC, to NRC Document Control Desk,"Response to NRC Request for Information Pursuant to 10 CFR 50.54(f)Regarding Recommendation

2.1 Flooding

of the Near-Term Task Force Review of Insights from the Fukushima Dai-ichi Accident, Enclosure 2, Required Response 2, Flood Hazard Reevaluation Report", March 11, 2013 (ML13079A806)

3. Letter from B. K. Singal, NRC, to D.L. Koehl, STPNOC, "South Texas Project, Units 1 and 2 -Request for Additional Information Regarding Fukushima Lessons Learned -Flooding Hazard Reanalysis Report ", January 14, 2014 (ML13358A065)

4. Letter from G.T. Powell, STPNOC, to NRC Document Control Desk,"STPNOC Partial Response to Request for Additional Information Associated with Near-Term Task Force Recommendation 2.1, Flood Hazard -Reevaluation", January 22, 2014 (NOC-AE-1 4003074)STI: 33824629 NOC-AE-14003085 Page 2 of 3 On March 12, 2012 (Reference 1), the U.S. Nuclear Regulatory Commission (NRC) staff issued a letter requesting additional information per Title 10 of the Code of Federal Regulations, Section 50.54(f) (hereafter called the 50.54(f) letter). The 50.54(f) letter requested that licensees reevaluate the flooding hazards at their sites against present-day regulatory guidance and methodologies.

STPNOC submitted the requested Flood Hazard Reevaluation Report to the NRC on March 11, 2013 (Reference 2).By letter dated January 14, 2014 (Reference 3), the NRC requested additional information (RAI)related to the flooding reevaluations.

Per an agreement with the NRC, STPNOC has responded to the RAls in two parts. The first part of the STPNOC response addressed RAls 1, 2, 7, 8 and 9 (Reference 4). The second part of the requested information addressing RAIs 3, 4, 5 and 6 is included in the attachment and enclosures submitted with this letter.There are no commitments in this letter.If there are any questions regarding this letter, please contact Wendy Brost at (361) 972-8516 or me at (361) 972-7566.I declare under penalty of perjury that the foregoing is true and correct.Executed on ,. .Zo, G.T. Powell Site Vice President web

Attachment:

STPNOC Second Partial Response to Request for Additional Information Related to Fukushima Lessons Learned Flooding Hazard Reevaluation Report

Enclosure:

Supplemental Information for RAI 5 (CD labeled NOC-AE-14003085 Disc 1)

NOC-AE-14003085 Page 3 of 3 cc: (paper copy)(electronic copy)Regional Administrator, Region IV U. S. Nuclear Regulatory Commission 1600 East Lamar Boulevard Arlington, TX 76011-4511 Balwant K. Singal *Senior Project Manager U.S. Nuclear Regulatory Commission One White Flint North (MS 8 B1)11555 Rockville Pike Rockville, MD 20852 NRC Resident Inspector U. S. Nuclear Regulatory Commission P. 0. Box 289, Mail Code: MN1 16 Wadsworth, TX 77483 Jim Collins City of Austin Electric Utility Department 721 Barton Springs Road Austin, TX 78704 A. H. Gutterman, Esquire Morgan, Lewis & Bockius LLP Balwant K. Singal U. S. Nuclear Regulatory Commission John Ragan Chris O'Hara Jim von Suskil NRG South Texas LP Kevin Polio Richard Pefia L.D. Blaylock City Public Service Peter Nemeth Crain Caton & James, P.C.C. Mele City of Austin Richard A. Ratliff Robert Free Texas Department of State Health Services* Digital media enclosure is only being transmitted to the NRC Document Control Desk and to STPNOC's NRC Project Manager, Balwant K. Singal, for distribution to the NRC staff reviewers Attachment 1 NOC-AE-14003085 Page 1 of 8 STPNOC Second Partial Response to Request for Additional Information Related to Fukushima Lessons Learned Flooding Hazard Reevaluation Report Note: References described in this Attachment are found at the end of the Attachment.

RAI 1: Local Intense Precipitation and Associated Site Drainage Please provide electronic versions of the input files used for HEC-HMS analysis in the flood hazard reevaluation report (FHRR) related to the local intense precipitation analyses.STPNOC Response: The requested information was provided in the first partial RAI response (Reference 1).RAI 2: Local Intense Precipitation and Associated Site Drainage Please provide electronic versions of the input files used for HEC-RAS analysis in the FHRR related to local intense precipitation analyses.STPNOC Response: The requested information was provided in the first partial RAI response (Reference 1).RAI 3: Local Intense Precipitation and Associated Site Drainage Please provide descriptions of the sources of elevation data, the methods used to incorporate elevation measurements into local intense precipitation flood analysis, and the likely magnitude of the errors associated with these elevations.

STPNOC Response: Ground topography within the South Texas Project (STP) property boundary was obtained from an aerial photogrammetric survey. At least 90% of all elevation data obtained from elevation contours was required to have an accuracy of +/-0. 5ft according to the survey specification and the remaining 10% of the data was required to have an accuracy of +/- 1. Oft. Additionally, a ground survey was performed to compare field measurements with aerial survey elevations at selected locations.

The root-mean-square error for the difference in elevation between aerial survey and ground survey data was found to be approximately

0. 196ft, demonstrating that the elevations used in the local intense precipitation evaluation are of reasonable accuracy.

Of the 311 comparison points, there are only two locations where the difference was above 0.5ft (one at 0.538ft and one at 1.043ft).

For both these instances, elevations from the aerial survey were higher than the elevations from field measurements.

Outside of the STP property boundary ground elevation data were obtained from the United States Geological Survey (USGS) topographic map for the Blessings SE Quadrangle.

The aerial survey and USGS topographic data were used to develop the HEC-RAS cross sections as shown on Figure 2.1-7 of the Flooding Hazard Reevaluation Report (FHRR).

Attachment 1 NOC-AE-1 4003085 Page 2 of 8 In addition to using the accurate ground elevation data to develop the local intense precipitation flood model, the HEC-HMS and HEC-RAS models incorporate highly conservative assumptions.

These include conservative Manning's roughness coefficients, the entire site considered impervious and modeled with a curve number of 98, reduced time of concentration values, and all storm drains considered to be non-operational.

The combined effect of these conservative assumptions more than adequately compensates for the small uncertainty in the ground elevations such that estimated flood levels can be considered as the maximum possible flood levels due to the local intense precipitation over the site. Moreover, as indicated in the FHRR, the maximum local intense precipitation flood elevation (33ft MSL) would not adversely impact the safety functions of the plant as this elevation is much lower than the design basis flood elevations associated with the Main Cooling Reservoir embankment breach, which range between 44.5ft and 50. 8ft MSL at the power block structures and 40. 8ft MSL at the emergency cooling water intake structure.

This conclusion remains accurate even with the consideration of the small uncertainty in the elevation measurements.

No FHRR revision is required as a result of this RAI response.RAI 4: Local Intense Precipitation and Associated Site Drainage Please provide a description of the basis used to classify Probable Maximum Flood (PMF) flow as shallow concentrated flow used in the Natural Resources Conservation Service (NRCS) TR-55 methodology.

STPNOC Response: The NRCS TR-55 methodology (Reference

2) was used to estimate the time of concentration for each sub-basin.

Time of concentration, which is defined as, "the time for runoff to travel from the hydraulically most distant point of the watershed to a point of interest within the watershed" (Reference 2), is the sum of flow travel times from consecutive flow segments of the drainage conveyance system. According to Reference 2, the time of concentration flow paths are divided into three segments:

sheet flow, shallow concentrated flow, and ditch flow. The equations used to determine the travel time for each segment are described in Reference 2.The three flow segments delineated for the flow path in each sub-basin are summarized below. The sub-basin areas and corresponding flow paths are shown on Figure 2.1-3 of the FHRR.Sheet Flow: The sheet flow occurs over plane surfaces at the headwater of streams (Reference

2) and represents the upstream-most segment of the flow path. The travel time in this segment is affected by the flow path length (ft), overland flow roughness coefficient, surface slope (ft/ft) and 2-year 24-hour rainfall intensity (inch). The sheet flow segments for sub-basins STPI, STP3b, STP3c, STP4a, and STP5a are over short grass-covered areas; therefore, a roughness coefficient of 0. 15 representing this condition was selected per Reference 2 (also see FHRR Figure 2.1-3 for sub-basin locations).

A roughness coefficient of 0. 011 (Reference

2) was selected for the sheet flow segments for sub-basins STP2b, STP4b, STP5b, and SW to represent asphalt and gravel covered area conditions.

A roughness coefficient of 0. 06 (Reference

2) was selected for the sheet flow segments for sub-basins STP2a, STP3a, North B, North I and North 2 to represent cultivated soil covered area conditions.

Slopes along the sheet flow paths were determined from the topographic data. McCuen and Spiess (Reference

3)

Attachment 1 NOC-AE-1 4003085 Page 3 of 8 argued that to use a specific fixed length (e.g. 300ft) for all sub-basins could lead to inaccuracies, and proposed the following relationship based on empirical results: L= (I 0N/S)/n, where L is the flow path length (ft), S is the flow path slope (ft/ft), and n is the overland flow roughness coefficient as defined in Reference

2. McCuen and Spiess (Reference

3) relationship was used to verify sheet flow lengths in the FHRR. In general, the maximum sheet flow length value used was 5Oft, with the exception of sub-basins North 1 and North 2, where a sheet flow length of lOOft was used.Shallow Concentrated Flow: Beyond the sheet flow length, flows convert to shallow concentrated flow where the travel times are defined based on empirical velocity (ft's) relationships for unpaved and paved surfaces and flow path lengths (ft) (Reference 2). In the FHRR, the velocity equation for unpaved surfaces was used for all sub-basins, except for sub-basins STP3c, STP4b and STP5b. The paved surface equation was used for the latter sub-basins because the shallow concentrated flow segments at those locations are characterized by paved and gravelly surfaces due to the development at the S TP I & 2 power block and surroundings.

Ditch Flow: Ditch or open channel flow begins where channels are visible on aerial photographs, where surveyed cross section information is available, or where stream information appears on USGS quadrangle maps (Reference 2). The ditch flow travel time is estimated based on the flow path length (ft) to the end of the sub-basin or a point of interest and average ditch flow velocity.

The flow velocity (ft/s) was initially estimated based on a ditch-full condition, which was verified by comparing with the corresponding average channel velocity obtained from the HEC-RAS model.For the FHRR, a constant velocity of 3 fps was designated for the ditch flow segment, with the exception of sub-basins North 1 and North 2 where a velocity of 6 fps was conservatively used.The ditch flow velocities were found to be higher than those computed in the HEC-RAS model, and, therefore, resulted in a shorter ditch flow travel time and conservatively higher runoff discharge.

In accordance with the US Army Corps of Engineers (Reference 4), for extreme storm runoff analysis the computed time of concentration values should be reduced to account for the non-linear response for extreme rainfall events. Accordingly, the time of concentration values were decreased by 25% for the local intense precipitation event. The numerical rainfall runoff model HEC-HMS requires the 'Lag Time' value instead of the time of concentration value as an input parameter.

Lag Time values are estimated as 0.6 times the time of concentration values (Reference 5). The estimated length for each flow path segment, time of concentration and lag time for all sub-basins are shown in Table 1.No FHRR revision is required as a result of this RAI response.

Attachment 1 NOC-AE-1 4003085 Page 4 of 8 Table 1 -Time of Concentration Estimates Sheet Flow Shallow Concentrated Flow Ditch Flow Ca/c. 25%Sub- (TcI1) (Tc2) (Tc3) Calc. Time ofRed Lag BasinTime of Cadc Basin Length Manning's-Slope Length Slope 2 T2 LengthTL Vel 3 CTin L (ft) n 1 S (ft/ft) L ( V (fps) (hr) (ft) V (fps) (hr) Conc. (hr) (mi)STPI 50 0.15 0.18 0.03 380 0.08 4.6 0.02 7955 3 0.74 0.79 0.59 21 STP2a 46 0.06 0.04 0.03 1947 0.0007 0.4 1.27 5870 3 0.54 1.84 1.38 50 STP2b 47 0.011 0.12 0.004 91 0.024 2.5 0.01 2026 3 0.19 0.20 0.15 5 STP3a 49 0.06 0.0007 0.14 2059 0.003 0.9 0.65 2036 3 0.19 0.97 0.73 26 STP3b 24 0.15 0.0015 0.12 192 0.0015 0.6 0.09 810 3 0.08 0.28 0.21 8 STP3c 4 48 0.15 0.2 0.03 2898 0.004 1.3 0.63 619 3 0.06 0.71 0.53 19 STP4a 27 0.15 0.002 0.12 1937 0.002 0.7 0.75 840 3 0.08 0.94 0.71 25 STP4b 4 49 0.011 0.001 0.03 597 0.00006 0.2 1.05 1471 3 0.14 1.22 0.91 33 STP5a 47 0.15 0.04 0.06 1199 0.0006 0.4 0.84 4566 3 0.42 1.32 0.99 36 STP5b 4 49 0.011 0.0007 0.04 3147 0.0013 0.7 1.19 783 3 0.07 1.30 0.98 35 SW 51 0.011 0.0008 0.04 1339 0.0008 0.5 0.82 4626 3 0.43 1.28 0.96 35 North B 51 0.06 0.001 0.12 1274 0.0006 0.4 0.90 3001 3 0.28 1.30 0.97 35 North 1 100 0.06 0.001 0.21 4760 0.000315 0.3 4.62 10200 6 0.47 5.30 3.98 143 North 2 100 0.06 0.001 0.21 5000 0.0004 0.3 4.30 4060 6 0.19 4.70 3.53 127 1 Reference

2.2 Slopes

for areas North 1, North 2 and North B are estimated from Ift interval contours developed from the USGS 7.5 minute series quad map (Blessing SE Quad); for the rest of the sub-basin areas Ift interval contour from aerial survey data are used.Assumed and compared to HEC-RAS hydraulic model's average velocities.

The assumed 3 fps velocity is conservative, as it is greater than the average velocity from the HEC-RAS model.Paved surfaces velocity equation (Reference

2) was used to calculate shallow concentrated flow on sub-basins STP3c, STP4b and STP5b.

Attachment 1 NOC-AE-1 4003085 Page 5 of 8 RAI 5: Failure of Dams and Onsite Water Control/Storacie Structures Please provide details of the ineffective flow areas and levees that were removed from the Halff Associates, Inc. HEC-RAS model while developing the HEC-RAS model used to reevaluate the flood hazard from upstream dam failures at the South Texas Project (STP), Units 1 and 2 site. Also, please provide a justification for removal of these features.STPNOC Response: Enclosed on a CD with this response are: (1) PDF files of the channel cross-sections from Halff's HEC-RAS models (Reference 6), which depict the locations of the ineffective flow areas and levees that were specified for different reaches of Colorado River; and (2) an electronic spreadsheet file that provides details on the Left and Right Stations and Elevations of the ineffective flow areas and levees that were specified in Ha/ff's HEC-RAS models. The PDF files are 13 in total and Table I provides the reaches of Colorado River represented by the PDF files. The tabs in the electronic spreadsheet file, Ineffective-areas-and-Levees.xls, correspond to the reaches represented in the PDF files.Table 2 -Colorado River reaches represented by the PDF files No PDF File Reach Name From To I Inks.pdf Buchanan Dam Inks Dam 2 LBJ.pdf Inks Dam Wirtz Dam 3 Marble.pdf Wirtz Dam Starcke Dam 4 Travis.pdf Starcke Dam Mansfield Dam 5 LakeAustin.pdf Mansfield Dam Tom Miller Dam 6 TownLake.pdf Tom Miller Dam Longhorn Dam 7 Bastrop.pdf Longhorn Dam Bastrop Gauge 8 LaGrange.pdf Bastrop Gauge La Grange Gauge 9 Columbus.pdf La Grange Gauge Columbus Gauge 10 Garwood.pdf Columbus Gauge Garwood Gauge 11 Wharton.pdf Garwood Gauge Wharton Gauge 12 BayCity.pdf Wharton Gauge Bay City Downstream 13 Matagorda.pdf Bay City Boundary The setup of the Halff HEC-RAS model, including the designation of ineffective areas in the channel, was developed to evaluate flow conditions up to the Standard Project Flood (SPF). For the postulated dam break scenario analyzed in the Flooding Hazard Reevaluation Report (FHRR), the peak flood discharges and flood levels are higher than those associated with the SPF condition, inundating floodplains and areas that were previously assigned as ineffective including the areas behind the levees in the Halff model.

Attachment 1 NOC-AE-1 4003085 Page 6 of 8 As a result, these ineffective areas were removed in the HEC-RAS model to more accurately reflect the actual effective flow areas that will be available for the propagation of dam break flood flow. Further, in the FHRR dam break analysis, all bridges in the path of the flood wave were assumed to be washed out to provide more conservative peak flows. As such, the ineffective areas associated with these bridge sections in the Halff model were removed as well to properly represent the washout of the bridges.No FHRR revision is required as a result of this RAI response.RAI 6: Failure of Dams and Onsite Water Control/Storage Structures Please provide details of the intra-basin flows that were allowed to occur in the HEC-RAS model.Also, because allowing intra-basin flows would reduce the discharge at STP, Units 1 and 2 site, the licensee is requested to provide a justification how the flood hazard from upstream dam failures would still be conservative.

STPNOC Response: As stated in Subsection 2.3.1.2.1.3 (Channel Geometry) of the Flooding Hazard Reevaluation Report (FHRR), the initial dam break model runs showed that the simulated water level elevations along the drainage divide were higher than the local ground elevations at the divide, indicating the potential for overflow to the adjacent basin. Therefore for the locations where inter-basin spillage was expected to occur, the HEC-RAS cross-sections were extended to better simulate the effect of the inter-basin flow. Without the extension of the cross-sections HEC-RAS would assume by default a vertical wall at both ends of the cross-sections.

This approach would be highly conservative but not realistic, in that the actual flow condition would not be simulated.

Therefore, this approach was not adopted.Based on the USGS topographic data, it was found that inter-basin spillage would occur near Garwood, Texas. The cross-sections, from the Ha/ff HEC-RAS models (Reference

6) downstream of this location were extended on both sides to cover the areas where inter-basin spillage is expected to occur. Extending the HEC-RAS cross-sections, and hence considering inter-basin spillage, provided a more accurate model of the physical hydraulic condition and therefore produced a more representative dam break peak discharge at STP 1 & 2 site.Overall, the modeling results are still conservative in that many conservative assumptions were used in setting up the HEC-RAS model. Some of the conservative assumptions include relatively high Manning's n values for the river channel and flood plains; simultaneous arrival of all upstream dam storage volumes at Lake Buchanan before its failure; initial water level at Buchanan Dam higher than the crest of dam; and removal of all dams and bridges between Buchanan and Mansfield Dams and farther to downstream boundary of the HEC-RAS model.No FHRR revision is required as a result of this RAI response.

Attachment 1 NOC-AE-1 4003085 Page 7 of 8 RAI 7: Failure of Dams and Onsite Water Control/Storage Structures Please provide electronic versions of the input files used for HEC-RAS analysis in the FHRR related to upstream dam failures.STPNOC Response: The requested information was provided in the first partial RAI response (Reference 1).RAI 8: Failure of Dams and Onsite Water Control/Storage Structures Please provide the electronic version of National Weather Service (NWS) BREACH model input files used in the recent Main Cooling Reservoir (MCR) breach analyses of the three postulated breach locations described in FHRR Section 2.3.STPNOC Response: The requested information was provided in the first partial RAI response (Reference 1).RAI 9: Failure of Dams and Onsite Water Control/Storage Structures Please provide a description of model configuration, boundary conditions, and model parameters for the three RMA2 simulations.

Also, Please provide the RMA2 input files, including the computational grids, used for the three simulations.

STPNOC Response: The requested information was provided in the first partial RAI response (Reference 1).

Attachment 1 NOC-AE-1 4003085 Page 8 of 8 References

1. Letter from G.T. Powell, STPNOC, to NRC Document Control Desk, "STPNOC Partial Response to Request for Additional Information Associated with Near-Term Task Force Recommendation 2.1, Flood Hazard Reevaluation" January 22, 2014 (NOC-AE-14003074)

2. U.S. Department of Agriculture, Soil Conservation Services, "Urban Hydrology for Small Watersheds", June 1986 (Technical Release 55)3. R.H. McCuen and J.M. Spiess, American Society of Civil Engineers, "Assessment of Kinematic Wave Time of Concentration", Journal of Hydraulic Engineering, March 1, 1995 4. U.S. Army Corps of Engineers, "Flood-Runoff Analysis', August 31, 1994 (Engineer Manual 1110-2-1417)

5. U.S. Army Corps of Engineers, Hydrologic Engineering Center, HEC-HMS, Hydrologic Modeling System, Technical Reference Manual, August 2010.6. Halff Associates, Inc., "Flood Damage Evaluation Project", Chapter 1-6, Volume I/-C, Volume lI-B, July 2002