Several factors influence the compatibility of results generated by HEC-RAS to those computed using SWMM engines and built from imported HEC-RAS data. In HEC-RAS conveyances are discretized as rectilinear and linearly varying between transects, whereas in SWMM conveyances are prismoidal between junctions, with abrupt changes in geometry. In SWMM the momentum equation is solved over the length of the conveyance, while in HEC-RAS the momentum equation is solved at a point (the transect).
Further, energy gains and losses at hydraulic structures are calculated differently in HEC RAS than in SWMM, and it is important when importing HEC-RAS data to adjust culvert and bridge hydraulic parameters accordingly. Key parameters include high chords, bank stations, Manning’s roughness coefficients and channel transects.
An automatic method of bridge and culvert importation, node placement and culvert placement has been developed for importing HEC-RAS bridge cross sections, and developed as a tool in PCSWMM.
This presentation details the automatic methodology, and compares computed results for several different applications.