HEC-RAS Rain-on-Grid vs. Traditional Dam Hydrologic Modeling Approaches

The US Army Corps of Engineers’ HEC-RAS has been around for nearly 30 years and modelers around the world have implemented its tools into their hydraulic analyses. Traditionally, drainage basin modelers would have to utilize separate software for their hydrologic calculations. However, for dam engineering analyses with HEC-RAS 2D Rain-On-Grid capabilities, the software now offers a unique hydrology component to do more hydrology calculations than ever before.

This presentation focuses on a case study at a reservoir in western Maryland, and its ability to pass the spillway design flood. The case study highlights the strengths and limitations of the Rain-On-Grid approach. The strengths include curve number generation by combining the soils and landcover raster data and assigning values to each grid cell. As well as Time of Concentration calculations going away with the inclusion of watershed infiltration and routing capabilities. This creates a more realistic approach to runoff catchment with localized losses now being considered. Noteworthy limitations are the reliance upon HEC-HMS to create a frequency storm that can be imported into HEC-RAS and has the ability to generate a variety of storm events from one hydrograph. In closing the inflows generated from Rain-on-Grid modeling will then be compared to previously published results to estimate the feasibility of this new modeling process.