One of the first steps in a dam rehabilitation is calculating the amount of runoff that enters the reservoir during its specific design storm. The hydrologic analysis of a dam includes assessing upstream sources of flow such as those from a single drainage area or discharge from an upstream dam. More complex drainage areas can be difficult to model in traditional hydrology programs such as USACE’s HEC-HMS or NRCS’s SITES as these rely on routing methods such as Muskingum-Cunge, Lag time, or Normal Depth flow. These reach routing methods work well for smaller frequency storms; however, large design storms such as the Probable Maximum Precipitation (PMP) are often poorly represented by these traditional methods when atypical watershed topography is present. Dams with complex flow conditions upstream can utilize hydraulic routing software such as  to model the reach routing of the design inflow hydrographs. These complex flow conditions include:

• Run-of-the-River Dams
• Dams with multiple dams upstream
• Dams with atypical watershed topography

USACE’s Hydrologic Engineering Center’s River Analysis System (HEC-RAS) can be used to model complex reaches in two-dimensional, unsteady flow. This presentation will highlight reach modeling techniques in HEC-RAS, identify examples of how HEC-RAS can be applied to unique or challenging inflow conditions, and an example of how these techniques can be used in dam design by showcasing the unique flow characteristics from South River Dam #10A in Augusta County, Virginia.