Advances in Physically Based Hydrologic Modeling with CASC2D

CASC2D is a distributed-parameter, physics-based hydrologic model. The original single-event formulation of CASC2D includes two-dimensional diffusive-wave overland flow routing, Green and Ampt infiltration, and one-dimensional diffusive-wave routing in rectangular channels. CASC2D has been extensively enhanced over the past several years. The US Army Corps of Engineers (USACE) is currently using version 1.19. This version has a continuous formulation that includes the effect of soil moisture redistribution on infiltration, calculates evapo-transpiration, and performs soil moisture accounting between rainfall events. Version 1.19 also performs upland erosion based on three sediment sizes, transports the sediment overland to the channel, and routes the sediment through the channel as wash or bed load. Improved overland flow routing routines have been added to increase model stability and decrease run times. Other additions in version 1.19 include a three-layer infiltration algorithm, a variety of channel cross-sections, and integration with the Dept. of Defense Watershed Modeling System (WMS). This version of CASC2D has been rigorously tested, verified, and applied at numerous US Army locations. CASC2D version 1.19 is applicable only in Hortonian watersheds. The model formulation is now being extensively revised to promote its applicability in non-Hortonian or mixed watersheds. The new version includes two-dimensional saturated groundwater flow and one-dimensional vadose zone flow linked to the current overland flow and channel routing routines. Improved dynamic-wave channel routing with hydraulic structures, lakes, wetlands, and detention basins are being incorporated into the model. Future versions of CASC2D will also have a more robust channel erosion and transport scheme, a bank failure algorithm, and the ability to transport sediment through hydraulic structures (i.e., culverts, bridges, etc.). It is anticipated that this new version will be completed in the year 2000. The new model will retain all the features of version 1.19. After testing and verification, the revised model, which is named GEISSHA, will replace CASC2D version 1.19 as the USACE's working distributed, physics-based watershed hydrodynamics model.