Development of a DHSVM Erosion and Sediment Transport Model

1. Development of a DHSVM Erosion and Sediment Transport Model Presented by Jordan S. Lanini, University of Washington Colleen O. Doten, University of WashingtonLaura C. Bowling, Purdue UniversityEdwin D. Mauer, Santa Clara UniversityJordan S. Lanini, University of Washington Nathalie Voisin, University of WashingtonDennis P. Lettenmaier, University of Washington

2. Presentation outline • Motivation for erosion model • Mass wasting component • Surface erosion component • Channel erosion and routing component • Testing and evaluation • Future research directions

3. Motivation for erosion model Forest Roads www.homefirefightingsystems.com Forest Fire Timber Harvest

4. SURFACE EROSION DHSVM Qsed Q OUTPUT MASS WASTING Sediment Model CHANNEL EROSION & ROUTING Provides Inputs for all Three Components Watershed Sediment Module Watershed Sediment Module

5. DHSVM CHANNEL EROSION & ROUTING MASS WASTING SURFACE EROSION DHSVM Inputs to Sediment Model Soil Moisture Content Channel Flow Precipitation Leaf Drip Infiltration and Saturation Excess Runoff

6. Mass Wasting http://www.for.gov.bc.ca/research/becweb/zone-MH/mh-photos/

7. Dynamic soil saturation predicted by DHSVM • Finer resolution grid (10 m) for failure computation Icicle Creek, WA L. Bowling, C. Doten

8. Mass Wasting Module (MWM) • Slope stability is a function of soil moisture, slope, and soil and vegetation characteristics. • Failure is determined by the infinite slope stability model, using a factor of safety (FS) resisting forces driving forces FS = • Slope instability is indicated by a FS < 1. L. Bowling, C. Doten

9. MWM - Stochastic Nature • Four soil and vegetation characteristics: • soil cohesion, • angle of internal friction, • root cohesion, and • vegetation surcharge are input as probability distributions. • They can be assigned to one of three distributions: • uniform, • normal or • triangular. L. Bowling

10. Results of a Stochastic Run Probability of failure Pixels in black failed at least once in 1000 iterations of MWM L. Bowling

11. MWM - Mass Redistribution • Pixels are considered to fail to bedrock. • Failed material travels down the slope of steepest descent. • Downslope pixels can fail in response to the initial failure. • Landslide stops at a critical slope angle. The failed volume is evenly distributed among all downslope pixels. • Landslides entering channels system continue as debris flows depending on the junction angle. L. Bowling

12. Surface Erosion & Routing http:www.geo.uni-bonn.de/cgi-bin/geodynamik_main?Rubrik=research&Punkt=geomorphology

13. Current DHSVM Runoff Generation and Routing Runoff is produced via: • Saturation excess (pixels 6 and 7) • Infiltration excess based on a user-specified static maximum infiltration capacity (pixel 3) Runoff is routed to the downslope neighbors one pixel/time step

14. Runoff Generation – Dynamic Infiltration Excess • Calculation of maximum infiltration capacity: • The first timestep there is surface water on the pixel, all surface water infiltrates. • If there is surface water in the next timestep, the maximum infiltration capacity is calculated based on the amount previously infiltrated. • Dominant form of runoff generation on unpaved roads and post burn land surfaces N. Voisin

15. Kinematic Runoff Routing • Pixel to pixel overland flow routed using an explicit finite difference solution of the kinematic wave approximation to the Saint-Venant equations • Manning’s equation is used to solve for flow area in terms of discharge • Per DHSVM timestep, a new solution sub-timestep is calculated satisfying the Courant condition, which is necessary for solution stability. L. Bowling

16. Surface Erosion • Transportable sediment is the sum of particles detached by three mechanisms • Erosion is limited by overland flow transport capacity raindrop impact leaf drip impact shearing by overland flow Mechanisms of Soil Particle Detachment L. Bowling, J. Lanini, N. Voisin

17. Hillslope Sediment Routing • Sediment is routed using a four-point finite difference solution of the two-dimensional conservation of mass equation. • If the pixel contains achannel (including road side ditches), all sediment and water enters the channelsegment. sediment and water L. Bowling

18. Forest Road Erosion • Transportable sediment consists of particles detached by two mechanisms • Overland flow will be infiltration excess generated. • Routing to include road crown type • insloped • outsloped • crowned surface erosion raindrop impact shearing by overland flow C. Doten

19. Channel Erosion & Routing www.eas.purdue.edu/geomorph/ envben.html

20. Channel Routing • Sediment Supply • channel sediment storage from the MWM • lateral inflow from hillslope and roads • upstream channel segment • Sediment particles • have a constant lognormally distributed grain size which is a function of the user-specified median grain size diameter (d50)and d90 • are binned into a user-specified number of grain size classes E. Maurer

21. Channel Routing • Sediment is routed using a four-point finite difference solution of the two-dimensional conservation of mass equation. • Instantaneous upstream and downstream flow rates are used in the routing. • Transport depends on • available sediment in each grain size class, and • capacity of flow for each grain size calculated using Bagnold’s approach for total sediment load. E. Maurer

22. Testing and Evaluation Little Wenatchee

23. Sensitivity Analysis - Rainy Creek

24. Soil depth typically a hydrologic calibration parameter changes in soil depth will impact mass wasting Soil moisture mass wasting model uses soil moisture in each pixel at a daily time step unrealistic degrees of saturation are going to effect mass wasting MWM Application Challenges C. Doten

25. Model resolution smaller resolutions will result in smaller sub-time step, increasing run time Runoff (Infiltration Excess) Sub-timestep calculated from largest infiltration excess observed for time step unrealistic values will result in smaller sub-time step, increasing run time Surface erosion run time since mass wasting is the predominant form of sediment transport in PNW basin, surface erosion can be limited to user-specified time periods decreasing run time Surface Erosion Application Challenges C. Doten, J. Lanini

26. Mass wasting Land slide mapping of Rainy Creek derived from aerial photography Surface erosion Observed local and regional land and road surface erosion rates Channel routing Observed stream sediment concentrations Testing and Evaluation C. Doten

27. Scenario Analyses I: Forest Roads Forest Road Erosion Road Location in the Hillslope & Hillslope Curvature C. Doten

28. Scenario Analyses II: Timber Harvest and Forest Fire Enhanced Transport Capacity • Decrease in annual evaporation • Increased snow accumulation • Enhanced snow melt • Greater radiation exposure • Increased turbulent energy transfer Enhanced Sediment Supply • Mass wasting (landslides) • Decreased root strength • Enhanced soil moisture • Surface erosion http://www.for.gov.bc.ca/research/becweb/zone-MH/12_Res_Man.htm C. Doten

29. Questions/Comments We would like to acknowledge financial support from the USFS PNW Research Station and Wenatchee Lab for the development of this model

30. Data Input Needed for Sediment Model • Smaller resolution (10m) DEM • Debris Flow Material d50 and d90 • Soils: Bulk Density, Manning n, K index, d50, distributions (mean, stand deviation, minimum value, maximum value) of Cohesion and Angle of Internal Friction • Vegetation: Vegetation Surcharge distribution (minimum value and maximum value) and Root Cohesion distribution (mode, minimum value and maximum value)

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