Improved Soil Erosion and Sediment Transport in GSSHA

To describe the new sediment transport formulation in GSSHA and demonstrate the improved capability in predicting storm-total sediment runoff using a research-quality data set. GSSHA simulates overland soil erosion and outputs erosion and deposition for any size class of particles smaller than gravel with specific gravities greater than 1.0. The model first calculates particle detachment by raindrops and surface runoff, and then calculates the transport capacity of surface runoff using one of three user-selected transport equations. The actual sediment transport is determined by comparing the amount of detached soil and the transport capacity of surface runoff. Depending on particle size, soil transported to channels is treated either as wash load or bed load, with sizes less than the user specified value of sand treated as wash load and larger sizes treated as bed load. The original GSSHA sediment transport formulation was hard-coded to simulate only three size classes of sediment (sand, silt, and clay), each with a specific gravity of silicate minerals (S=2.65). The new GSSHA erosion routines have been generalized to allow simulation of arbitrary size classes smaller than gravels, each with unique specific gravity.