CS15C - Concurrent Session 15C: Nonlinear Watershed Behavior Using Variable Unit Hydrograph Parameters for California

Watershed nonlinear response to rainfall has been recognized as an important component in hydrologic routing, especially for large synthetic storms that are commonly used within dam safety evaluations. Recent updates to the US Army Corps of Engineers Hydrologic Engineering Center’s Hydrologic Modeling System (HEC-HMS) software includes a new variable Clark unit hydrograph method that expands the classical Clark unit hydrograph by allowing the time of concentration and storage coefficient to vary in response to changes in rainfall intensity through the simulation. This new method allows modelers to efficiently simulate the nonlinear dynamic response to a watershed subject to large excess precipitation rates. HEC partnered with the California Division of Safety of Dams (DSOD) to develop a method to efficiently estimate variable Clark unit hydrograph parameters throughout the state of California to use within dam safety applications. Using 2D diffusion wave equation hydrologic routing, variations to Clark parameters were derived for a range of excess precipitation rates using 16 watersheds in California. The results were then synthetized to develop regression equations that quickly allow estimation of changes in time of concentration and storage coefficient needed for hydrologic routing. Predicted variable Clark parameters derived from these equations were evaluated through a validation process demonstrating that the equations improve predictions compared to classical Clark unit hydrograph theory.