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 In flood mitigation projects, modifying terrain is fundamental, serving as the key to the hydraulic engineer finding solutions to mitigate flooding risks. Two of the most common modifications required are channel widening and the creation of detention ponds. Traditionally, such terrain modifications are carried out using raster analysis tools available in ArcGIS. However, the latest iterations of the Hydrologic Engineering Center’s River Analysis System (HEC-RAS) software, starting from version 6, have introduced a groundbreaking terrain modification tool within the HEC-RAS Mapper. This tool can generate modified terrain layers, significantly simplifying and altering the original landscape to accommodate various hydraulic engineering features.

While channel widening and flat-bottomed ponds without side slopes are well-documented in HEC-RAS training materials, this presentation delves into the modeling of dry bottom detention ponds using the terrain modification tool. It provides an in-depth, step-by-step guide for incorporating side slopes, longitudinal slopes, pilot channels, and inflow weirs into the design of the detention pond. Doing so offers a more efficient and expeditious method for pond modeling within HEC-RAS Mapper, eliminating the need for repetitive raster modifications and terrain regeneration when adjusting pond sizes. Additionally, this approach simplifies the determination of the optimal pond volume, considerably reducing the time of trial-and-error methods.