1. Development of a Sediment Transport Component for DHSVM Colleen O. Doten a, Laura C. Bowling b, Edwin P. Maurer c, Nathalie Voisin a, and Dennis P. Lettenmaier a a. University of Washington, Department of Civil and Environmental Engineering, Box 352700, Seattle, WA 98195 b. Purdue University, Department of Agronomy, Lilly Hall of Life Sciences, 915 West State Street, West Lafayette, IN 47907 c. Santa Clara University, Department of Civil Engineering, 500 El Camino Real, Santa Clara, CA 95053 Abstract The effect of forest management and disturbance on aquatic resources is a problem of considerable, contemporary, scientific and public concern in the West. Sediment generation is one of the factors linking land surface conditions with aquatic systems, with implications for fisheries protection and enhancement. Better predictive techniques that allow assessment of the effects of fire and logging, in particular, on sediment transport could help to provide a more scientific basis for the management of forests in the West. We describe the development of a sediment transport component for the Distributed Hydrology Soil Vegetation Model (DHSVM), a spatially distributed hydrologic model that was developed specifically for assessment of the hydrologic consequences of forest management. The sediment transport module extends the hydrologic dynamics of DHSVM to predict sediment generation in response to dynamic meteorological inputs and hydrologic conditions via mass wasting and surface erosion from forest roads and hillslopes. The mass wasting component builds on existing stochastic slope stability models, by incorporating distributed basin hydrology (from DHSVM), and post-failure, rule-based redistribution of sediment downslope. The stochastic nature of the mass wasting component allows specification of probability distributions that describe the spatial variability of soil and vegetation characteristics used in the infinite slope model. The forest roads and hillslope surface erosion algorithms account for erosion from rain drop impact and overland erosion. A simple routing scheme is used to transport eroded sediment from mass wasting and forest roads surface erosion that reaches the channel system to the basin outlet. A sensitivity analysis of the model input parameters and forest cover conditions is described for the Little Wenatchee River basin in the northeastern Washington Cascades. Model Description – Channel Routing andErosion 5 MASS WASTING SURFACE EROSION Motivation 1 2 Model Description - General model that solves the energy and water balance at each grid cell at each time step. The Sediment Transport Component of DHSVM predicts the range and variability of catchment sediment yield in response to dynamic meteorological and hydrologic conditions. It builds on existing models such as LISA (Hammond et al., 1992) and SHETRAN (Burton and Bathurst, 1998 and Wicks and Bathurst, 1996), by including stochastic mass failure predictions and forest road erosion. It also includes concepts from the KINEROS (Woolhiser et al., 1990 and Smith et al., 1995) and EUROSEM (Morgan et al., 1998) models. The model consists of three components: mass wasting, surface erosion (including forest roads), and channel erosion and routing. DHSVM provides hydrological inputs to all three components (see figure above). Failure occurs one pixel at a time. Therefore, a second, smaller resolution digital elevation map (DEM) is used to allow for a smaller minimum failure area. Failure depth is equal to the failed grid cell soil depth. The failed material travels down the slope of steepest decent. The failed area can increase in response to the initial failure due to changes in slope and soil loading, which are inputs in the slope stability model. Landslides stop at an empirically determined runout distance (Burton and Bathurst, 1998). The failed volume is evenly distributed among all downslope pixels. Landslides entering a channel segment can continue as debris flows depending on the junction angle (Benda and Dunne, 1997). Sediment enters the channel system as the result of debris flows originating from mass wasting and/or as lateral inflows from the hillslope and forest roads. Sediment particles are binned into a user-specified number of grain size classes as a function of the d 50 and d 90 (user-specified). Transport depends on the available sediment in each class and the transport capacity of each class. Sediment is routed through the channel network using a four-point finite difference solution of the two-dimensional conservation of mass similar to Wicks and Bathurst, 1996. Soil Moisture Content CHANNEL EROSION & ROUTING Channel Flow Precipitation Leaf Drip Infiltration and Saturation Excess Runoff DHSVM Q Q sed MODEL OUTPUT The Little Wenatchee Basin is located in the Wenatchee National Forest on the eastern side of the Cascade Mountains. It has a drainage area of approximately 260 km 2. Rainy Creek is a tributary to the Little Wenatchee and has a basin area of approximately 44 km 2. Both are snow melt dominated basins with mean annual precipitation ranging from 400 cm at higher elevations to 125 cm in the lower elevations. Rainy Creek elevation ranges from 630 to 2100 meters, with 300 to 600 meters of relief from ridge tops to valley bottoms. Upper slopes are steep (30 to 45 degrees) with excessive rock outcrop. Lower slope gradients range from 10 to 30 degrees and are mantled with glacial drift and colluvium. The dominant types of bedrock are igneous and metamorphic, which have been carved out by alpine glaciation. The Vegetation management, forest road construction and forest fire impact basin sediment yield, by increasing the amount of sediment available for transport and the amount of surface water available to transport it. Increased sediment yield can cause: sedimentation of water supplies and salmon habitat, damage to hydraulic structures, and exceedances of Total Maximum Daily Loads. Predictive tools would allow land managers to evaluate management options prior to implementation. Because mass wasting is considered to be the dominant erosion mechanism in many Pacific Northwest forested catchments, our sensitivity analyses was focused on this model component and the factor of safety calculation. Similar to Wu and Sidle (1995), we found that of the input parameters tested, the model is most sensitive to soil cohesion, root cohesion (for one vegetation type) and soil depth. For the other vegetation type, the range of root cohesion tested was high enough that the number of failures was not affected. The mass wasting model is less sensitive to changes in effective angle of internal friction, and not very sensitive to soil bulk density and vegetation surcharge. Upon completion of model development and testing, a beta version of the code will be available on http://www.hydro.washington.edu/Lettenmaier/Models/DHSVM/index.htm. Those who have downloaded DHSVM since December 2002 will receive an email notice when the code becomes available. The authors gratefully acknowledge Ted Bohn and Jordan Lanini, University of Washington for their efforts in testing and improving the code. Please see the author for a list of references. Concluding Remarks The Distributed Hydrology Soil Vegetation Model (DHSVM) was developed at the University of Washington and Pacific Northwest National Laboratory for assessment of the hydrologic consequences of forest management (Wigmosta, 1994). It is a physically based model that simulates the effects of spatially variable topograpy, soil and vegetation. It is a grid based Model Description – Mass Wasting 6 7 Sensitivity Analysis = effective soil cohesion = root cohesion = effective angle of internal friction of soil on impermeable layer = soil depth above failure plane = slope angle = vegetative and snow surcharge per unit plan area = weight density of saturated soil = weight density of soil at field moisture content = weight density of water = relative saturated depth where: Study Area 6 Runoff for sediment transport can be generated by saturation excess or infiltration excess (Wigmosta, 1994). Infiltration excess can be determined based on a dynamic infiltration capacity (new method) (Woolhiser et al., 1990 and Smith et al., 1995), or a static infiltration capacity (existing method). Generated runoff is routed pixel to pixel using an explicit finite difference solution of the kinematic wave approximation to the Saint- Venant equation (Chow et al., 1988). Transportable sediment is the sum of particles detached by three mechanism (see adjacent figure). Erosion is limited by the runoff transport capacity. Hillslope sediment is routed using a four-point finite difference solution of the two- dimensional conservation of mass equation (Wicks and Bathurst, 1996). If a pixel contains a channel, all sediment and water enters the channel segment. shearing by overland flow leaf drip impact raindrop impact MECHANISMS FOR SOIL PARTICLE DETACHMENT Model Description – Surface Erosion 4 Slope failure is determined by the well-known infinite slope stability model, using a factor of safety (Ward et al., 1981, Burton and Bathurst, 1998). The factor of safety is a ratio of resisting forces over driving forces (see section 7). When the soil moisture increases to the point that the driving forces exceed the resisting forces, a failure occurs. Some of the uncertainty in failure prediction is due to the spatial variability of soil and vegetation parameter inputs to the slope stability model. Therefore, two soil parameters (cohesion and effective angle of internal friction) and two vegetation parameters (root cohesion and vegetation surcharge) are input as probability distributions (similar to other slope stability models, e.g. Hammond et al., 1992). The figures below show the result for a 12/31/1996 simulation in the Rainy Creek Basin. The figure on the left shows the pixels that failed at least once in 1,000 iterations. The figure on the right shows the probability of failure for each of those pixels. 3 after Burton and Bathurst (1998) The analysis consisted of running the model in deterministic mode for a two-year period. Hydrological data from 1996 and 1997 were used to force the model. These data indicate that 1996 was the wettest year in the 1990s and 1997 was a drier year (less then the annual mean for the decade). A baseline set of parameters (shown in red in the adjacent figures and noted in the adjacent table) were selected and a single input parameter for the factor of safety was changed for each model run. Parameters were changed as shown in the table based on reported values in the references noted. Results of model runs with changes made to the parameters ranked high or medium sensitive are shown in the adjacent figures. For all model runs the total number of failed pixels for the entire 2 year run are shown. The Rainy Creek model has 437,400 pixels that are 10 meters by 10 meters in area. It has a single soil type, sandy loam, and four vegetation types: closed shrub (~5% of the basin), mesic conifer forest (~90%), and subalpine conifer forest (~1%). Results From Baseline Run Input ParameterBaselineMinMaxSensitivityReferences) effective soil cohesion, kPa30.25100highHammond et al. (1992) Lindeburg (2001) effective angle of internal friction, degrees 312834highHammond et al. (1992) Holtz and Kovacs (1981) Lindeburg (2001) root cohesion, kPa (5% of basin) (90% of basin) 3 15 2 12 6 23 high low Hammond et al. (1992) vegetation surcharge, kg/m 2 (5% of basin) (90% of basin) 12.5 122 0 49 25 1995 low Sidle (1992) a Hammond et al. (1992) soil bulk density (kg/m 3 )156914001600mediumSTATSGO (USDA, 1994) soil depth (m)0.76- 1.278 Baseline1.76- 2.278 highb Notes: a. The weight of the understory or brush vegetation can be assumed negligible (Sidle, 1992). b. The soil depth file was created based on local slope (from the DEM), upstream source area and elevation. The sensitivity analysis consisted on increasing all depths by 0.5 m and 1.0 m. 0.25 3 100 2 4 6 28 31 34 +0 +0.5 +11400 1500 1569 1600 major vegetative type is coniferous forest influenced by maritime climatic conditions. The basin is heavily forest roaded (road density = 1.5 mi/mi 2 ). Major debris slides occurred in Rainy Creek in 1990 and 1995/1996 during flood events (information based on work by Carl Davis, Claudia Narcisco, and Terry Lillybridge, Wenatchee National Forest).