Advanced Guidance on Use of Steady HEC-RAS

HEC-RAS is a one-dimensional steady or unsteady open channel flow model developed by the U.S. Army Corps Engineers' Hydrologic Engineering Center. It is probably the most widely used one-dimensional program in the world, and is used for a variety of applications, such as flood insurance and flood reduction studies, bridge hydraulics, stream restoration, and dam break analyses. While the HEC-RAS modeling system is well documented and general guidance on model applications and parameter selections is provided, modelers often do not understand the uncertainty or sensitivity associated with some model parameters. In addition, HEC-RAS is sometimes misapplied as modelers often do not understand the applicability of HEC-RAS. This paper presents a series of steady flow model examples to test the sensitivities of three key parameters or processes, including cross section spacing, expansion ratio, and lateral weir discharge coefficient. Using the model test results, practical guidance on model parameterization is provided. In addition, using an example application, cautions of using HEC-RAS results to interpret a two-dimensional floodplain are discussed.