Challenges of Developing a Rain-on-Snow Grid-Based Hydrologic Model with HEC-HMS for the Willow Creek Watershed, Oregon

The town of Heppner, Oregon, located at the confluence of four river valleys, has historically been subject to major flooding events causing both property damage and loss of life. This includes a large thunderstorm event in 1903 with nearly 300 fatalities. To provide flood protection, the Willow Creek Lake Dam Project was developed by the U.S. Army Corps of Engineers. Recreation, fish and wildlife, sedimentation, water quality, and irrigation are secondary uses of the storage pool. Under a contract to the Portland District Corps of Engineers, WEST Consultants, Inc. (WEST) developed a hydrologic model of the watersheds surrounding the Willow Creek Lake Dam and the downstream city of Heppner. The model was used to update the Water Control Manual for the project. WEST developed the hydrologic model using the U.S. Army Corps of Engineers Hydrologic Engineering Center's (HEC) next generation (NexGen) software products. This included use of HEC-GeoHMS for GIS delineation of subbasins and HEC-HMS (Hydrologic Modeling System) for rainfall/runoff simulations. DSPM (Distributed Snow Process Model), developed at the U.S. Army Corps of Engineers Cold Regions Research and Engineering Laboratory (CRREL), was applied to simulate snowmelt. The models were constructed on a grid scale in order to incorporate the HEC-HMS model at a later date into the Corps Water Management System (CWMS) for developing forecasts and providing flood warnings. The model development and calibration effort illustrated the utility and versatility of the HEC software. Additionally, important lessons were learned about the effective and efficient application of these analysis tools.