Comparison of Floodline Delineation Methods and Results: HEC-2, SWMM5/PCSWMM and PCSWMM 2D

Flood plains are employed to allocate lands within a watershed. Flood lines are statistically-based flood extents marked on an official map as computed by high-resolution hydraulic models based on river bathymetry, and protect sensitive areas and manage flood damage by preserving natural storage volumes. Modeling suites for floodline delineation include HEC-RAS, HEC-2 (the predecessor of HEC-RAS) and SWMM5 all of which are approved by the Federal Emergency Management Agency (FEMA). More recently high-resolution hydraulic models have leveraged GIS utilities within flood analysis models; improved computation power has allowed advanced flood line analysis in an open GIS environment, like PCSWMM, whereby transects are directly derived from digital elevation models (DEM) and water surface elevations computed against design storm inputs based on observed, long, continuous time series.

In this study, statutory flood lines were compared to two SWMM5-based floodline delineation methods: Method 1 uses 1-D SWMM5 and DEM-sampled transects to linearly interpolate flood inundation polygons that mark the extent of the flooding. Method 2 is a SWMM5-based integrated 1D-2D approach that estimates the extent and duration of overland flooding. PCSWMM generates a 2D overland flow mesh over the existing 1D major system model, and animates the flooding illustrating the duration and extent. Additionally these analyses allow for incorporation of a flood vulnerable assets layer providing additional information about buildings constructed within the floodplain. Both methods permit the hydrology and hydraulics to be modeled, however in the present study the hydrology component was ignored and design event inflows were entered directly into the hydraulic system to replicate official HEC-2 floodlines. Both new methods are inherently more complex than the original, and locally both match and significantly depart from approved floodlines.