Development of a Sediment Transport Component for DHSVM Colleen O. Doten, University of Washington Laura C. Bowling, Purdue University Edwin D. Mauer, Santa Clara University Nathalie Voisin, University of Washington Dennis P. Lettenmaier, University of Washington Outline • Motivation • Distributed Hydrology-Soil-Vegetation Model (DHSVM) • Sediment Transport Component • Testing and Evaluation • Scenario Analyses Motivation Forest Roads www.homefirefightingsystems.com Forest Fire Timber Harvest Distributed Hydrology-SoilVegetation Model (DHSVM) 1-D Vertical Water Balance DHSVM Model Representation • Physically based hydrologic model that simulates the effects of spatially variable – topography – soil – vegetation • Solves energy and water balance at each grid cell (pixel), at each time step Surface/Subsurface Flow Redistribution to/from Neighboring Pixels Sediment Transport Component DHSVM SURFACE EROSION Qsed CHANNEL EROSION & ROUTING Provides Inputs for all Three Components MASS WASTING Watershed Sediment Module Q OUTPUT DHSVM Inputs to Sediment Model DHSVM Soil Moisture Content Precipitation Leaf Drip Infiltration and Saturation MASS WASTING Excess Runoff SURFACE EROSION Channel Flow CHANNEL EROSION & ROUTING Mass Wasting http://www.for.gov.bc.ca/research/becweb/zone-MH/mh-photos/ • Dynamic soil saturation predicted by DHSVM • Finer resolution grid (10 m) for failure computation L. Bowling, C. Doten Icicle Creek, WA 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) FS = resisting forces driving forces • Slope instability is indicated by a FS < 1. L. Bowling, C. Doten 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 Simulated Probability of Slope Failure Rainy Creek, WA November 28, 1995 Probability Storm of failure Event L. Bowling MWM - Mass Redistribution L. Bowling • Failure depth is equal to the failed pixel soil depth. • Failed material travels down the slope of steepest descent. • Failed area can increase in response to the initial failure. • Landslide stops at an empirically determined runout distance. The failed volume is evenly distributed among all downslope pixels. • Landslides entering channels system continue as debris flows depending on the junction angle. Surface Erosion & Routing http:www.geo.uni-bonn.de/cgi-bin/geodynamik_main?Rubrik=research&Punkt=geomorphology 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 Runoff Generation – Infiltration Excess • Dominant form of runoff generation on unpaved roads and post burn land surfaces • 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. N. Voisin 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 Surface Erosion raindrop impact leaf drip impact • Transportable sediment is the sum of particles detached by three mechanisms • Erosion is limited by overland flow transport capacity shearing by overland flow Mechanisms of Soil Particle Detachment L. Bowling, N. Voisin 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 a channel (including road side ditches), all sediment and water enters the channel segment. L. Bowling sediment and water Forest Road Erosion raindrop impact shearing by overland flow surface 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 C. Doten Channel Erosion & Routing www.eas.purdue.edu/geomorph/ envben.html Channel Erosion & 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 Channel Erosion & 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 Testing and Evaluation http://access.wa.gov/images/government/waco_2.gif Testing and Evaluation • Mass wasting – Land slide mapping derived from aerial photography • Surface erosion – Observed local and regional land and road surface erosion rates – Run model in location where road data are available • Channel routing – Observed stream sediment concentrations C. Doten Scenarios Analyses I: Forest Roads Road Proximity to Streams Road Location in the Hillslope & Hillslope Curvature C. Doten Coffee Creek, British Columbia Scenarios 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 Questions/Comments Data Input Needed for Sediment Model • Smaller resolution (10m) DEM • Debris Flow Material d50 and d90 • Soils: Bulk Density, Saturated 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) DHSVM Channel Routing • Flow is routed using a cascade of linear reservoirs. • Each channel segment is treated as reservoir of constant width. Discharge is linearly related to storage. This implies a constant flow velocity. • Storage at the next timestep can be calculated knowing the inflow and velocity (determined using Manning’s equation.) • The average outflow from a reach is obtained through a mass balance. References • • • • • • • • • • • Bagnold, R.A., 1966, An approach of sediment transport model from general physics. US Geol. 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Bathurst, 1996, SHESED: a physically based, distributed erosion and sediment yield component for the SHE hydrological modeling system, Journal of Hydrology, 175, 213-238. Wigmosta, M.S., and D.P. Lettenmaier, 1999, A Comparison of Simplified Methods for Routing Topographically-Driven Subsurface Flow, Wat. Resour. Res., 35, 255-264. Wolohiser, D.A., R.E. Smith and D.C. Goodrich, 1990, KINEROS, A kinematic runoff and erosion model: documentation and user manual, USDA-Agricultural Research Service, ARS-77, 130 pp.