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

Complex work flow needed

Now integrated into a single workflow

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

Questions

LA-UR-16-XXXXX

Previous Results

From 2008 PA

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