ML20205K340
| ML20205K340 | |
| Person / Time | |
|---|---|
| Issue date: | 03/16/1999 |
| From: | Jack Parrott NRC OFFICE OF NUCLEAR MATERIAL SAFETY & SAFEGUARDS (NMSS) |
| To: | NRC OFFICE OF NUCLEAR MATERIAL SAFETY & SAFEGUARDS (NMSS) |
| References | |
| REF-PROJ-M-32 NUDOCS 9904130114 | |
| Download: ML20205K340 (48) | |
Text
F-M-31 Meeting Report West Valley Erosion Model March 16,1999 By Jack D. Parrott, Project Scientist NRC/NMSS/DWM This meeting was called at the request of the U.S. Department of Energy (DOE) and the New York State Energy Research and Development Authority (NYSERDA) to discuss, with the U.S.
Nuclear Regulatory Commission (NRC), the erosion modeling that has been done to support the Environmental Impact Statement (EIS) for the completion of the West Valley Demonstration Project and the closure or long-term management of the West Valley site. The presentation materials and an attendance list are attached. The meeting was held at the NRC headquarters building in Rockville, Maryland.
The model is being used to help DOE and NYSERDA determine what the possible impact of erosion would be on the site facilities if they were left in place over the long term. This information will be factored into the decision process for the preferred alternative in the EIS.
The purpose of the discussion was to determine the adequacy of the model for its intended purpose. The details of the erosion modeling were presented. The erosion model uses the SIBERIA code developed by Dr. Garry R. Willgoose. The code is being implemented and modified by Science Applications International Corporation (SAIC). This erosion modeling is considered by SAIC to be more reasonable than what they consider to be a more simplistic and conservative analysis of erosion done in the draft EIS.
A description of the sitef modeled area, previous erosion studies, and code selection process was given. The model concept and elements, input, verification, and calibration were described.
The results were presented in graphical form for 1,000 and 10,000 year modeling periods.
Sensitivity cases and conclusions were presented. The conclusions from the modeling were that the SIBERIA code provides a reasonable representation of long-term erosion impacts, that unmitigated erosion occurs slowly, but that over long periods of time unmitigated erosion will affect site facilities. The management implications of this erosion modeling were that erosion will have to be managed if waste remains on site.
Questions and comments from the attendees focused on:
how the model could factor in peak flows and extreme events Q
additional ways that the model could be calibrated to site specific conditions i
how the model could be modified to consider site specific conditions how the model results could be used to determine mitigstion strategies or environmental impact statement alternatives can the model consider the probability of discreet events
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9904130114 990316 PDR PROJ M-32 PDR 10 U U l d_
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what was the source of the data for input to the model how conservative is the model i
was a sloughing mechanism considered in the model how was the steepness of the slopes considered in the model were changes in the watershed over long periods of time considered
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what node size and the time increments were used what is the availability of the code The meeting was concluded by agreeing that a site tour should be arranged for the meeting participants that had not been to the site. After the site tour, a review schedule will be arranged.
The participants list, a copy of the viewgraphs presented, and a copy of the draft erosion report dated March 8,1999, are attached.
4 West Valley Erosion Modeling Meeting March 16,1999 Particioants List:
J Steve Abt, Colorado State University Paul Bembia, New York State Energy Research and Development Authority Sidney Crawford, member of the public Patrick Deliman, U.S. Army Corps of Engineers Diane D'Arrigo, Nuclear Information and Resource Service Sandi Doty, Science Applications Intemational Corporation l
Colleen Gerwitz, New York State Energy Research and Development Authority Jim Hammelman, Science Applications Intemational Corporation Lisa Hubbard, U.S. Army Corps of Engineers Kent Johnson, New York State Department of Environmental Conservation Ted Johnson, U.S. Nuclear Regulatory Commission
' Tim Johnson, U.S. Nuclear Regulatory Commission Jack Kadlecek, New York State Department of Environmental Conservation Jack Krajewski, New York State Department of Environmental Conservation Jack Parrott, U.S. Nuclear Regulatory Commission Ray Pilon, U.S. Army Corps of Engineers Joe Price, Science Applications International Corporation Tim Rice, New York State Department of Environmental Conservation Dan Sullivan, U.S. Department of Energy Patti Swain, Science Applications intemational Corporation Dan Westcott, West Valley Nuclear Services Rao Yalamanchili, U.S. Army Corps of Engineers By Phone:
David Farrell, Center for Nuclear Waste Regulatory Analysis Pat Mackin, Center for Nuclear Waste Regulatory Analysis Pat Primi, New York State Office of the Attorney General Eric Wohlers, West Valley Citizen Task Force Ray Vaughan, Coalition on West Valley Nuclear Wastes and West Valley Citizen Task Force l
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DRAFT LANDSCAPE EVOLUTION MODELING OF THE WESTERN NEW YORK NUCLEAR SERVICE CENTER Prepared for: U. S. Department of Energy New York State Energy Research and Development Authority West Valley, New York Prepared by: Science Applications International Corporation Reston, Virginia March 8,1999 Rev 5
TABLE OF CONTENTS 1. INTRODUCTION...................... .1 Obj ec ti ve s........................................................... ...... 1 B ac k grou nd........................................................... .. 1 Code Selection Process................................ ... 1 II. DESCRIPTION OF THE MODEL............................. 2 Code De script ion............................................................................................... 2 Code Structure................................ ....................................................2 j Boundary Locations....................................................................................... 3 Input Paramete rs.......................................................................................... 3 III. VERIFICATION................ ............................................................................6 IV. CALIB RATION...................................................................................... 7 V. RESULTS..................................................................................................................9 VI. SENS ITIVITY STUDIES....................................................................................... 13 VII. CONCLUS ION S................................................................................................. 14 VIII. REFERENCES.........................................................................................................15 LIST OF FIGURES Figure 1.. Map of the Buttermilk Creek Drainage Basin........................................... ...... 4 Figure 2. Map of the Western New York Nuclear Services Center......................................... 5 Figure 3. Base Case Erosion Impacts for the 1,000-Year Period............................................10 Figure 4. Base Case Erosion Impacts for the 10,000-Year Period. ................................11 Figure 5. Base Case Impacts for the Project Premises and the SDA for the l 10,000-Year Period............................................................................................ 12 LIST OF TABLES Table 1. Comparison of SIBERIA Sediment Transport Rate Correlation Predictions and Field Data........................................... ..................................................8 Table 2. Comparison of SIBERIA Sediment Transport Correlation and SEDIMOT II Predictions............................................................................................................9 Table 3. Parameter Values tur the Sensitivity Analyses...............................................13
g l l l. -INTRODUCTION. l This report presents the results of a landscape evolution modeling study for the Western New York Nuclear Service Center (Center) in West Valley, New York. The study assesses the long-term erosive impacts caused by the mnoff, sheet / rill erosion, channel incision, landsliding, and gullying processes. The results will be used in future Environmental Impact Statement (EIS) analyses.- Obiectives The study objective is to evaluate the long-term impacts of erosion at the Center, particularly the Project Premises and the SDA, in order to support a strategic decision on selection of closure alternatives. An element of this objective is the intent to provide a physically based evaluation that explicitly considers the interaction of hillslope and stream channel processes. Backaround In studies conducted for the Draft EIS (DEIS), various types of erosion rates have been calculated in the vicinity of the site using physically-based models such as HEC-6, CREAMS, USLE, and SEDIMOT II (WVNS,1993). Although these individual studies presented conservative estimates of the erosion rates, they did not show the effects of gully growth or the interaction of the erosive processes in a location-specific manner. In the DEIS analysis, conservative assumptions were applied in order to determine the erosive impacts at specific facilities on the Project Premises. For example, a constant channel incision and rim widening rate estimated using HEC-6 was assumed to occur along the entire length of j Frank's Creek and Erdman Brook. Although the use of a constant rate was a conservative assumption, it did not account for the variation in the stream bank conditions (i.e. reaches along the streams where the banks are currently stable); and therefore, did not represert the georrorphological processes in a location-specific manner. Erosion is caused by slope failures and gully growth that only occur in selected reaches of the stream at discrete time intervals when a specific bank is susceptible to the erosive forces of the water (i.e., it is not a steady-state j process). Therefore, the use of a model that is capable of predicting gully growth and integrating the physical processes to simulate the erosive impacts at specific locations in the vicinity of the Project Premises was needed ) j i Code Selection Process The need to better understand the potential impacts of erosion over long time frames motivated i search for models with more comprehensive capabilities than those used in the DEIS erosion analysis. A literature search was conducted to determine if any codes had been developed to j predict the formation and growth of gullies. The search resulted in the identification of a group i of codes classified as landscape evolution models. These codes were specifically developed to predict gully growth (Willgoose,1989). In addition, these codes are capable of modeling the interactions of the dominant erosive processes occurring at the Center. The dominant erosive l processes represented in the codes are runoff, channel incision, sheet / rill erosion, soil l creep /landsliding, and gullying. The codes model the combined effect of these erosive processes j L on a time-and location-specific basis over long time periods (thousands of years). The evaluation is done on a watershed scale. 1 1
r-The first landscape evolution model (SIBERIA) was developed in 1989 by Garry Willgoose at the Massachusetts Institute of Technology (MIT). Over the next 10 years, additional models were developed (e.g., by Jon Pelletier and Greg Tucker) that ba;ically contain the same features as the original model. l SIBERIA was selected for use in this study because laboratory and field-scale validation studies have been completed on the code (Hancock and Willgoose,1996). Gregory Hancock completed the laboratory-scale validation in 1997. The study compared an experimental model landform to l its SIBERIA-generated analogue. Field-scale studies were completed at three sites to validate the l. model's ability to predict gully and landscape evolution over the short tenn (one wet season), the l medium time scale'(approximately 50 years), and the long time scale (1000's of years). ~ l Va'idation studies have not yet been completed on the landscape evolution models developed by i Pelletier and Tucker. 11. DESCRIPTION OF THE MODEL The SIBERIA' code is described in the following sections along with the boundary conditions and input parameters that were used. Code Description The SIBERIA code is a physically-br.aed model used to simulate the evolution of channel networks and hillslopes exposed to the action of erosion over long time periods. The model interactively evaluates the dominant physical processes within the watershed (e.g., mnoff, sheet / rill erosion, channel incision, landsliding, and gullying). The site-specific role of run-off is quantified in a calibration procedure which uses water budget and erWon rate estimates based on West Valley rainfall and site conditions. Digital terrain maps are usea u the starting point for the determination of drainage areas and'geomorphological features. The landform is adjusted with time in response to the erosion that occurs by applying the physically based equations and failure threshold criteria for the applicable processes at each node within the network. For example, the Fickian diffusion term is applied on the hillslope nodes to predict soil creep and slope failure using a failure threshold criteria of 38 percent (21 degrees) as determined from field studies at the site (WVNS,1993a). All equations used within the model are accepted as standard within the hydrology and agricultural communities as discussed in Willgoose (1989). Code Structure The primary features of SIBERIA are the sediment mass balance and the channelization function imposed at each node in the study area. Both the sediment balance and channelization function ~ are time dependent. A finite difference solution of the discretized eqpations is used to calculate nodal elevations and the configuration of the channel network at each time step and to advance l the calculation through a sequence of time steps. The erosion and channel growth processes interact in a non-linear manner to modify both the topography and the configuration of streams l within the watershed. The sediment balance incorporates terms representing the accumulation or loss of sediment at each node, the rate of transpon of sediment to and from each node in flowing water and the rate of transpon of sediment to and from each node due to soil creep, rainsplash or rocksliding. The rates of sediment transpon are calculated using empirically supported correlations relating sediment transpon to discharge and slope, discharge to areas and slopes, and creep to rate of 2
change of slope with distance. The sediment transport correlations represent the combined effects of the contributing erosional processes without providing a mechanistic representation of these processes. Discharge at a node is calculated as being proportional to the total area contributing to flow at that node. Accumulation of water is not modeled and the water balance is l, not maintained at closed depressions. For study areas not having closed depressions, as is the case at the Center, the nodal representation of flow may be interpreted at a given point in time as a steady-state water balance with no accumulation at nodes, input from rainfall proportional to area and effluent flow from the system at boundary nodes in channels. The site-specific role of run-off is quantified by calibrating SIBERIA estimates of erosion rate using analysis of water budget and erosion rates estimated for West Valley rainfall and site-specific geologic data. The channelization function identifies each node as belonging to a channel or hillslope and transitions hillslope nodes to channel nodes when an activator function of discharge and slope exceeds a critical value. The form of the channelization function leads to development of a dendritic pattern for the channels similar to the Meinhardt leaf model. Channel nodes erode faster than hillslope nodes and growing channel heads repel each other. Boundary Locations The Buttermilk Creek watershed shown in Figure I has a triangular shape with an area of 2 approximately 29.4 mi. The CENTER is located at the apex of the triangle as shown in Figure 2. The model area is a rectangular section of the watershed extending in a southeasterly direction from the confluence of Frank's and Buttermilk Creeks to the southern end of the watershed. The western boundary of the model area is the western boundary of the watershed and the eastern boundary of the model area is located along Buttermilk Creek. j l The locations of the boundaries was determined based on the drainage pattern and the maximum number of nodes allowed by the code. Because the overland flow to the Project Premises is from ) the south and west, it was important to incorporate the total areal extent of the watershed in these directions. The code restriction of a matrix no larger than 200 by 200 nodes made it impossible to incorporate the total areal extent of the watershed in the eastern and northern directions while still maintaining a grid spacing with acceptable resolution. A grid spacing of 70 feet by 70 feet was used in all simulations. Input Parameters Over 70 input parameters are required to determine the starting conditions for the model as discussed in Willgoose,1989. A major input requirement for the model is a digital terrain map that covers the surface area to be simulated. For this study, a digitized map of the Project Premises was generated from aerial photographs and USGS maps. The main physical processes represented by the model are sheet / rill erosion, channel incision, slope creep /landsliding, and gully growth. The input parameters for each of these processes are described as follows. Sheer / Rill Erosion-The sheet / rill erosion rates at each node are correlated in SIBERIA with slope and discharge at that node. The value of parameters used in these correlations was calibrated to West Valley conditions using independent estimates of sheet / rill erosion rate based on site specific conditions. Calibration of the model is described in Section IV of this report. 1 I t 3
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I l l The sheet / rill erosion rate was spatially varied to account for the shallow contact with the bedrock shale in the western and southwestern ponion of the modeling area. To accomplish this, a rock / soil erodibility factor of 0.1 was used. This factor was applied to all areas where the bedrock outcrops, and also, to areas where the beurock is shallow as defined by LaFleur (1979) and gully erosion has progressed to a depth of 5 feet during the course oitbe simulation. ChannelIncision-Specification of the channel incision rate of Buttermilk Creek is a boundary condition for the model area. In this study, the Buttermilk Creek channel incision rate of 0.016 feet per year as determined by Boothroyd !!982) was entered along the Buttermilk Creek drainage. This rate was entered explicitly as a user-defined tectonics module. The channel incision rate for all other streams within the modeling area was determined by the SIBERIA sediment transport and channelization correlations. Slope Creep /lendsliding-The long-tenn average of hillslope soil creep, rainsplash, and landsliding is modeled by use of a spatially constant Fickian diffusion term in the elevation evolution equation. Also, the code requires that a slope angle, which represents the initiation of landsliding, be determined for the site. For this study, a slope angle of 38 percent (21 degrees) was selected based on slope stability field studies that were conducted in the vicinity of the Center (WVNS,1993a). Guay Growth-The rate of gully growth is determined by the channelization equation which is tased on a function developed by Meinhardt in 1982. When the value of the channel initiation function exceeds the channel initiation threshold, that spot in the watershed goes into transition from hillslope to channel; and thus, the gully is advanced. The code used overland flow velocity or sediment transpon shear stress values to establish the channel initiation tnreshold at each node. 111. VERIFICATION Verification of a computer code entails demeastrating that the code provides the correct solution to the equations comprising the model. The three major elements of the SIBERIA model are calculation of flow through the node network using node connectivity relations based on water flow m direction of greatest slope, calculation of sediment balances for each grid block and characterization of nodes as belonging to a hillslope or stream channel. Of these functions, calculation of sediment balances is the most imponant in determining evolution of the landscape because it establishes node elevations that determine slope and influence flow direction and node transitions. SIBERIA sediment balance calculations were verified for a simple test case using both hand calculations and an in-house computer code developed for this purpose. The simple test case involved a linear series of grid blocks for which a single flow direction is detennined by initial ordering of the grid. As in SIBERIA, flow at each node was calculated as proponional to the area draining to that node. Initial slope was constant and changes in flow direction or node character did not occur. In the in-house computer code, the SIBERIA sediment balance relating change in elevation to sediment transport rates was solved using a first-order, explicit integration scheme which allowed independent specification of spatial and temporal integration steps sizes. Sediment transport rates at each node wea correlated with water flow rate and slope using the relation from the SIBERIA model. The hand calculations were performed in a similar manner to the in-house computer code but required a coarse grid and short l time-frame for completion in a reasonable period of time. For this test case, both the hand i 6 l
I 1 calculations and the in-house computer code erosion rates were in agreement with the SIBERIA results for the same conditions. 1 IV. CALIBRATION The SIBERIA model incorporates correlations of water ditcharge and sediment transport rate that may be adjusted to provide the best representation of site-specific conditions. Water discharge at a node is correlated as equal to a constant, designated as beta 3, multiplied by area draining to the node raised to a constant power. Because watershed-scale data (Skinner and Porter,1987) indicate that the exponent of area should be near unity, calibration of discharge involves estimation of the multiplicative constant beta 3. Sediment oansport rate is correlated as equal to e constant, designated as betal, multiplied by discharge and slope each raised to a constant power. Because, prior analyses (Willgoose,1989 and Skinner and Poner,1987) indicates that each of these exponents should be near 2.0, calibration of sediment transport rate is reduced to determination of the multiplicative constant betal. Site-specific studies supporting estimation of SIBERIA water discharge and sediment transport parameters include experimental and modelingefforts. No single study provides the basis for calibration of SIBERIA parameters. Experimental data collected at the site include direct measurement of sheet and rill erosion rates using erosion frames and investigation of mass wasting using surveying and topographic measurements. Results of these experimental studies are of short duration and do not support long-term projections. Other useful data includes measurement of rainfall rates and stream flow rates for the Buttermilk Creek watershed. Data available for the 1962-1968 period indicate that average flow rate of Buttermilk Creek was 46.6 ft'/s (WVNS,1993a) and average rainfall rate was approxime.tely 3.28 ft/yr. Combination of these two measurements provides an upper limit on run-off fraction of 0.55 for this time period. Measurement of rainfall at four stations surrounding the West Valley site have been reported for periods ranging from 30 to 40 years. Average annual rainfall was reponed as 3.66 ftlyr (Lmes and Moore,1992). Combination of the long-term rainfall measurements and the shoner-term run-off fraction estimate provides a run-off rate estimate of 2.0 ftlyr. Site-specific erosion modeling studies have b, en completed using the indiudual storm event model SEDIMOT H and the time-averaging models CREAMS and USLE. The SEDIMOT 11 data included estimates of erosion rate for an ll45-acre ponion of the site for 24-hour storms having return periods of 2,10 and 100 years (WVNS,1993b). This data provides an estimate of the frequency / magnitude relation for erosion rate that may be integrated over storms of all magnitude to obtain an estimete of average erosion rate of 0.11 ton / acre /yr. The CREAMS study considered a 5.5-acre portion of the south plateau that was not bordered or intersected by a stream. The study period was the year 1984 with data for year 1983 used to initialize model calculations. Measured rainfall for 1984 was 3.73 feet and the CREAMS modeling estimated run-off rate as equal to 2.16 ft/yr. The CREAMS estimate of erosion rate was 4.66 ton / acre /yr, equiva'ent to an average elevation change of 0.0023 ft for 1984. The CREAMS analysis, however, did not consider all areas of the site and did consider an area having uncovered soil that is not representative of the balance of the site. The USLE analysis of erosion considered an 1145-acre portion of the site divide I into 22 separate sub-areas. Slopes for the areas ranged from 0.02 to 0.23 ft/ft and flow path lengths ranged from 390 to 3,900 ft. Estimates of erosion rate ranged from 0.003 to 0.236 ton / acre /yr for 7 i
F-- l the 22 sub-areas with an area-weighted average erosion rate of 0.086 tons / acre /yr (WVNS,1993b). The above-described data and studies were reviewed to select conservative values for the SIBERIA water discharge and sediment transpon parameters. For water discharge, a run-off rate or value of beta 3 of 2.16 ft/yr was selected. This is the value predicted by the CREAMS study and is consistent with the upper bound estimate derived from the rainfall s.nd stream flow rate data. A larger value is considered conservative as it will lead to greater conversion of hillslope nodes to channel nodes through the SIBERIA channelization function. Estimation of long-term erosion rate is less sensitive to the selected value of water discharge because water discharge is used both to calibrate the model and to calculate long-term erosion rates. The USLE erosion rate estimates were selected to calculate the SBERIA erosion rate correlation. T'e USLE estimate is a long-term average and through consideration of 22 sub-areas incorporates the range of topography included in the long-term calculations. An iterative calculation procedure was used to identify the value of the SBERIA sediment transpon parameter, betal, which produced the 4 USLE erosion rate for each of the 22 sub-areas. The betal values ranged from 2.6x10 to 1.5x10" (Ib/yr)/(ft'/yr) with an average of 4.2x104 (Ib/yr)/(ft'/yr). In order to provide a conservative assessment of erosion at the site, the largest value of betal, 1.5x10" (ib/yr)/(ft'/yr), was used in the base case simulations. The predictions of the calibrated SIBERIA sediment transport correlation may be compared to limited site sampling data and SEDIMOT 11 predictions. Sediment loadings and flow velocities in Frank's Creek were measured in 1992 following passage of hurricane Andrew (WVNS, 1993a). Flow velocities ranged from approximately 0.12 to 10.13 ft'/s. Selected values of the reported range are presented in Table 1 along with SWERIA predictions for these conditions. SEDIMOT Il predicts time-dependent stream flow rates and sediment loadings for individual storms of variable return period. Peak flow rates and sediment loadings at peak flow predicted by SEDIMOT 11 are summarized in Table 2 along with SBEPJA predictions for these flow rates. The results of comparison of SIBERIA sediment transport correlation predictions with field data and SEDIMOT-Il analyses indicate that the calibrated SIBERIA model provides a consen'ative representation of site conditions. Table 1. Comparison of SIBERIA Sediment Transport Rate Correlation Predictions and Field Data Sediment Transport Rate (ton /d) (ft'/s) Measured Correlation 0.12 0.008 0.028 0.62 0.79 0.53 3.69 8.53 13.1 10.13 53.7 80.8 i 8 L
I Table 2. Comparison of SIBERIA Sediment Transport Correlation and SEDIMOT 11 Predictions Retum Period Peak Flow Rate (yr) (ft'/s) SEDIMOT ll Corre ation 2.0 128.9 2,770 7,870 ) 10 239.0 5,390 23,900 100 399.4 9,610 l 60,300 V. RESULTS i The base case described in this section evaluates long-term erosional impacts for initial conditions of current topography and boundary conditions of current rainfall continuing indefinitely into the future and a Buttermilk Creek incision rate of 0.016 ft/yr. This entails use of the calibrated values of betal and beta 3 discuss:xi in Section IV of this repon. Results are presented in the form of extent of erosion predicted for all locations of the model area in Figures 3 and 4 for times of 1,000 years and 10,000 years, respectively. In these figures, extent of erosion is represented by colorization imposed on the current topography with yellow color indicating more severe erosion than blue color. In Figure 5, extent of erosion for the Project Premises and the SDA at 10,000 years is represented by contour lines with intervals of 5 feet. The areas that are most impacted by erosion in the 1,000-year period are within the glacial till aloi.g Buttermilk Creek and the lower reaches of Frank's Creek and Quarry Creek. Gully growth and channel incision are most pronounced near the headwaters of Buttermilk Creek, which is in the southern i. anion of the watershed. The plateau areas on the Project Premises appear to be relatively stable but erosion is observed along the creek banks and at gully NP-1 extending from Quarry Creek, gully NP-2 extending from Frank's Creek and the equalization pond gully extending from Erdman Brook. This relative stability appears to be due to the fact that the plateaus are in a small drainage basin within the watershed which gets even smaller with the growth of Quarry Creek; and therefore, the discharge quantity is not adequate to cause more extensive erosion. i The results show that the majority of the facilities on the Project Premises and SDA (i.e., on the north and south plateau) are not likely to be impacted by erosion during the next 1,000 years. For example, the areas of the high level waste tanks and the Process Building would not be impacted. On the south plateau, the margins of the NDA and SDA are eroded by the growth of Erdman Brook, Lagoon Road Creek and Frank's Creek but the disposal trenches are not affected. Figure 3 shows that the south plateau facilities would likely be impacted by erosion during the next 10,000 years. The centerlines of the creeks exhibit approximately 50 feet of erosion and soil loss in the disposal areas is between 5 and 10 feet. The areas in the vicinity of the process building and high level waste tanks do not experience significant erosion but the areas of the lagoons experience significant erosion. While the banks of both Frank's Creek and Buttermilk Creek exhibit significant erosion, the results do not indicate capture of Frank's Creek by Buttermilk Creek within the 10,000-year period. 9
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[ \\ . These predictions appear to be reasonable based on a comparison with erosion rates predicted in earlier studies. In lower Franks Creek and Quarry Creek, the erosion rate predicted by this study was approximately 30 ft per 1,000 years; and in upper Franks Creek and Erdman Brook, the erosion rate was approximately 12 ft per 1,000 years. These rates are similar to a rate of 16 ft per 1,000 years piedicted by Boothroyd and less than the maximum rate of 199 ft per 1,000 years as predicted by the HEC-6 modeling and longitudinal studies. VI. SENStrlVITY STUDIES l A series of runs were completed to estimate the sensitivity of the predictions to changes in the key input parameters. Three parameters were varied: sheet / rill erosion, channel incision, and soil / rock ratio. The values used for each parameter are shown in Table 3. Tat M % Paramotor Values for the Sensitivity Analyses values Parameters High Medium Low Sheet / rill erosion 0.73 in/100 yr 0.15 in/100 yr (SEDiMOT 11) (USLE) Buttermilk Creek O.16 ft/yr 0.016 ft/yr 0.0049 ftlyr Channelincision (10x Boothmyd) (Boothroyd) (LaFleur) Rock / soil erodibility 1.0 0.1 0.01 Sheet / Rill Erosion Paramete.- Runs - Erosion rate parameters calibrated tr, the USLE and SEDIMOT II studies discussed in Section IV of this report were selec&ed to investigate sensitivity to general erosion rate. The USLE modeling was conducteu on an area that encompassed the Project Premises and the watershed area west of Rock Spriigs Road resulting in a predicted erosion rate ranging from 0.001 to 0.236 tons / acre (0.0005 eo 0.15 inches per 100 years) for 22 sub-areas. The SEDIMOT II model was used to estimate the sheet / rill erosion rate occurring during major storm events using the same area as the USLE model. It predicted an area-weighted average rate of erosion of 0.11 inches per 100 years, which is similar to the results using the USLE model. The largest sub-area erosion rate was estimated as 1.23 ton / acre. For both the USLE and SEDIMOT Il models, the sediment transport parameter corresponding to the largest sub-area erosion rate was use in the sensitivity analysis. j The two sensitivity runs varied only the sheet / rill erosion rate parameter. The channel incision rate used in all of the runs was Boothroyd's value of 0.016 ft/yr. The rock / soil erodibility factor used in all of the runs was 1.0. The results of the sensitivity analyses indicated that increasing the erosion rate parameter by a factor of 5 had a significant impact on the extent of the erosion occurring within the watershed. Results for the USLE case are the same as those reported for the base case. For the larger, SEDIMOT II-calibrated erosion rate parameter value, incision in the creek banks increased to greater than 100 feet within 10.000 years and all facilities on the north an( south plateaus were affected. Channel Incision Rate Parameter Runs-Two of the Buttermilk Creek channel incision rates i vsed in the sensitivity analysis were selected based on results of previous studies. In 1979, Boothroyd and LaFleur independently predicted the long-term channel incision rate along l 13 )
1 Buttermilk Creek based on carbon-14 age dating of one wood sample collected from the highest of 14 terrace levels on the west side of Buttermilk Creek (LaFleur,1979). Boothroyd's analysis resulted in a predicted channel incision rate of 0.005 m/yr (0.016 ft/yr) wnich is higher than the range predicted by LaFleur of 0.0015 to 0.0021 m/yr (0.0049 to 0.0069 ft/yr). For the purpose of the sensitivity analysis, LaFleur' s 0.0049 ft/yr value was used to provide a wider range of values to evaluate on the lower end. In addition to the LaFleur and Boothroyd rates, a value equal to 10 times the Boothroyd rate was modeled to evaluate a wider range of values on the upper end. Of these three values, the Boothroyd rate is considered to be the worst-case value because it is the highest value that is based on scientific evidence. The three sensitivity runs varied only the channel incision rate parameter. The sheet / rill erosion rate used in all of the runs was 0.73 inches per 1,000 years. The rock / soil erodibility factor used in all of the runs was 0.1. The results of the sensitivity analyses indicated that decreasing the channel incision rate by a factor of 3 causes a significant decrease in the extent of the erosion wi.nin both the Buttermilk Creek and the lower Franks Creek drainages over a 1,000-year time period. The nonh and south plateau ponions of the watershed remain relatively stable for the 1,000-year period regardless of the channel incision rate used. Rock / Soil Ratio Parameter Runs-The three values of the rock / soil erodibility ratio used in the sensitivitv analysis were selected based on scientific judgment. In the modeling area, shale bedrock autcrops in several areas to the west and southwest of the Project Premises. In these same areas, the rock / soil contact is much shallower (5-10 ft) than in the rest of the modeled area of'lne watershed. Based on the physical propenies of the material, it is also known that the areas of the watershed with shallow bedroci contacts will be much less susceptible to erosion. To account for this in the model, the rock areas are delineated and assigned an erodibility factor that is a percentage of the soil erodibility factor. For example, the rock could erode at 10% the rate of . the soil. However, because this percentage is not known for the till versus the shale in the vicinity of the site, determining it's sensitivity is of interest and values of 0.1 (10%),0.01 (1%), and 0.001 (0.1%) were selected for evaluation. The see sensitivity runs varied only the rock / soil ratio erodibility parameter. The sheet / rill erosion rate used in all of the runs was 0.73 inches per 1,000 years. The channel incision rate used in all of the runs was Boothroyd's value of 0.016 ftlyr. The results of the sensitivity analyses indicated that there is not a significant difference between using a value of 0.1 and 0.001. In conclusion, the sensitivity analyses indicated that the modeling results are sensitive to the sheet / rill erosion rate, the channel incision rate, and the use of a rock / soil ratio in the western and southwestern areas of the watershed. Al hough these parameters do vary the quantity of erosion t that occurs within the watershed, they do not significantly change the locations where the erosion is occurring. Thus, all the runs indicated that the north and south plateaus were in an area of the watershed that is relatively stable and not as likely to be impacted by erosion as the other ponions of the watershed. Vll. CONCLUSIONS This study identified and calibrated a landform evolution model useful for long-term erosion modeling of the West Valley site. However, the calibration and application of the model is 14
I dependent upon decontamination and decommissioning criteria that will be specified by the NRC for the site. The modeling results described in section VI indicate that over a 1,000 year time period, unmitigated erosion does not compromise waste isolation at the most sensitive faciF. of the Project Premises and the SDA. On the south plateau, stream incision and rim widening are predicted to occur. The rate of movement is slow enough that the trenches of the NDA and SDA remain intact.' On the north plateau, gully growth along recognized features on the nonhorn margin is the projected dominant process. The location and rate of growth are such that Lagoon I, the process building and the HLW Tanks remain intact. Over a 10,000-year time period, large-scale rim widening along both Erdman Brook and Frank's Creek impacts all facilities of the Project Premises and the SDA. The SIBERIA modeling results and other analyses indicate that crosional processes occurring at a slow rate may have an significant impact on facilities at the West Valley site if mitigation measums are not taken. Over a 1,000-year time period the projected impact are not severe but over longer periods of time the isolation capability of facilities may be affuted. The primary impacts are due to water moviag onto the Project Premises from the south and from action along Frank's Creek, Erdman Break and Quarry Creek. While erosion does occur along Buttermilk Creek, the analyses does not indicate that these changes affect site facilities within a 10,000-year period. Vill. REFERENCES Boothroyd, J.C., B.S. Timson, and L.A. Dunne,1982. "Geomorphic Processes and Evolution of Buttermilk Valley and Selected Tributaries, West Valley, New York", NUREG/CR-2862. Dames and Moore, Inc., Environmental Information Document, Volume III, Air Resources, Part 2, Meteorology, WVDP-EIS-015, Dames and Moore, Inc., Orchard Park, NY,1992 LaFleur, R.G.,1979. " Glacial Geology and Stratigraphy of Western New York Nuclear Service Center and Vicinity, Cattaraugus and Erie Counties, New York", U.S. Geological Survey Open File Report 79-989 Albany, New York. Hancock, G.R. and Willgoose, G.R.,1996. "The Validation of a Catchment Evolution Model". International Geographical Congress, The Hague, August 4-10. Skinner, BJ. and S.C. Poner, Physical Geology, John Wiley and Sons, Inc., New York, New York,1987 West Valley Nuclear Services, Inc., (WVNS,1987), " Application of the CREAMS Computer Model to a Ponion of the West Valley Demonstration Project Site", July 29. West Valley Nuclear Services, Inc. (WVNS,1993a), Environmental Information Document, Volume III Hydrology, Pan 3: Erosion and Mass Wasting, WVDP-EIS-009, West Valley Nuclear Services, Inc., West Valley, New York, January,1993 West. Valley Nuclear Services. Inc. (WVNS,1993b), Environmental Information Document, i Volume III Hydrology, Part 2: Surface Water Hydrology, WVDP-EIS-009, West Valley Nuclear Services, Inc., West Valley, New York, January,1993 15}}