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Erosion Analysis for Los Alamos National Laboratory Material Disposal Area G
LA-UR-16-XXXXX
Phil StaufferAdam Atchley
Kay BirdsellKelly Crowell
Area G is an onsite active low-level radioactive wasterepository facility that is required to comply with DOEM 435.1 performanceObjectives for 1,000 years
Potential erosion of the Area G cover and mesa tops present considerable risk to 1,000 year compliance and greater long-term performance (10,000 year).
Goals:Approximate 1,000 year erosion
Approximate 10,000 year erosionSuggestion cover designs to ‘keep waste in place’
The long-term integrity of the cover will depend on the ability to withstand surface erosion
Surface erosion may expose waste buried in disposal pits and shafts thus initiate release of radionuclides
SIBERIA Landscape evolution Model
SIBERIA (Willgoose and Riley, 1998) predicts sediment transport derived from shallow sheet and rill processes for a range of soil, runoff, vegetation cover and hillslope properties.
SIBERIA predicts the spatial distribution of deformation across complex, 3-D topography over thousands of years.
This includes the lowering of ridges, the incision or infilling of valleys and hollows, and the development of gullies and fans.
The erosion model predicts steady-state erosion and sediment transport across a landscape that is represented as elevations in a gridded digital elevation model (DEM).
Governing EquationWhere:
Qs = The annual sediment flux through a grid cell.
B = Coefficient that represents all grid cell runoff-driven erosion, except slope and runoff.AmSn = Relationship between contributing area (A), slope (S), and sediment yield. Slope Area = SIBERIA
Dz= Diffusion coefficient.
S = The terrain gradient (slope) (%)
Limitations of Siberia
Geomorphically Effective Event Assumption
In order to get long time projections SIBERIA assumes a singe steady runoff coefficient is presumed to be equivalent in terms of
geomorphic effectiveness to a natural series of runoff events –Willgoose et al., 1989
In addition, the sediment yield, Qs, when applied to each time step over long periods of time, is equivalent to the average annual sediment that would result from large and small events of all return periods.
Calibration and Validationof SIBERIA for Area G use
If data is available, SIBERIA can be parameterized directly using long-term rainfall, runoff, and sediment yield data This is not the case for Area G.
Extensive data collection and Siberia validation undertaken
Multiple surrogate sites and data was collected to provide valid parameter rangers for SIBERIA
Long term event data at surrogate sites
Photo of where or erosion plots here
Experimental waste covers at LANL
Parameterization for Area G SIBERIA modelParameterization, consisted of six major steps: 1. Evaluate precipitation, runoff, and sediment-yield data for
Mesita del Buey, used to parameterize surrogate models rainfall-runoff (ISR9) and runoff-erosion (HEM).
2. Evaluate runoff and sediment-yield datasets from analog site, (Santa Rita Experimental Range, Arizona) to estimate landscape-forming event intervals.
3. Develop rainfall-runoff relationships (ISR9) for MDA G, soils, vegetation and precipitation using landscape-forming event intervals.
4. Use runoff values predicted by ISR9 as input for HEM runoff-erosion modeling to predict sediment yield for a range of slopes and areas.
5. Calibrate SIBERIA B, m and n parameters to match sediment yields predicted by HEM for range of slopes and areas.
6. Estimate Dz by matching SIBERIA results to present-day topography.
Example of vegetation cover
Validation of SIBERIA for Area G useModel simulated sediment yield within range of yield recorded for plots and near by watersheds
Area G Site Description and Model Parameters
Model Parameters Low Moderate High
Hillslope Erosion Parameters for IRS & HEMSoil Texture Sandy Loam Sandy Loam Loam
Canopy Cover / Ground Cover (%) 70 / 70 30 / 70 30 / 30
Landscape-Forming return interval [yr] 2 5 5
Excess Runoff [mm] 2.6 7 1.2
SIBERIA Model ParametersB = grid cell runoff-driven erosion [-] 9.4e-6 4.2e-5 6.8e4
M = Area sediment yield parameter [-] 1.6 1.6 1.3
N = Slope sediment yield parameter [-] 0.86 0.87 0.86
Dz = diffusion coefficient [kg/m] 0.003 0.0025 0.0023
Three sets of parameters where simulated to capture uncertain model parameterization
Area G Bedrock and Armor
Bedrock elevation Riprap armor around the mesa tops
Show BR8Z.DEMGIS mapPit locations
Show C10_RIPRAPUD.RGNGIS mapPit locations
A Bedrock layer is included starting at 1,900 m up to an estimated bedrock ceiling
Riprap Armor is placed around the mesa tops.
Bedrock and Riprap have the same erosive properties, much less then the overlaying cap.
Area G Cap
Engineered cap is placed over pits to the goal of reducing infiltration transport of
radionuclides into the groundwater.
Reducing infiltration may increase runoff and therefore the erosive force to from gulling.
Elevation [m]
2000
2030
2060
1,000 Year Elevation Change
Low Erosion Moderate Erosion High Erosion
-10 100-5 5
In all cases, litter erosion occurred over 1,000 years, and the site performance passes.
10,000 Year Elevation Change
Moderate and high parameters show extensive erosion in 10,000 Years.
-10 100-5 5
Low Erosion Moderate Erosion High Erosion
Higher erosion parameters, especially diffusion (Dz) causes general cover erosions, whereas lower erosion rates causes deeper gulling.
10,000 Year Elevation Change
Moderate Erosion High Erosion
However, riprap is working
No change in elevation around the rim of mesa
-10 100-5 5
10,000 Year Elevation Change
Unrealistic build up of sediment in canyon bottoms over 10,000 years. steady-state erosion or geomorphically effective event assumption may be
preventing sediment from washing out of canyons.
-10 100-5 5
Moderate Erosion High Erosion Unrealistic sediment depositionfrom missing discrete events
Uncertain model parameterization results in long–term uncertainty in Area G erosion.
Low Erosion
High Erosion