Floodplain Delineation

Floodplain Delineation

Bohler Engineering, P.C. contracted Princeton Hydro, LLC to delineate the 100-year floodplain on sites in various municipalities in New Jersey.  When proposed construction is near a non-delineated watercourse in New Jersey, an analysis is required to establish the flood plain limits.  Stream flows were determined, in part by the Soil Conservation Service, TR-55 method, utilizing soil information from the County soil surveys, zoning designations to determine the possible future impervious coverage (as required by New Jersey regulations), and offsite drainage area from the USGS quads.  Roughness coefficients for the channel and overbank were estimated for the natural channels.  Ground surveys were supplemented with aerial topography, when necessary, to create a composite topographic map.

 A triangular irregular network (TIN) was created within the ArcVIEW GIS platform.  Cross sections were defined and pre-processed into the HEC-RAS input file.  Culverts and specific hydraulic data were entered directly into the HEC-RAS program.  Several projects required the additional delineation of the floodway and floodway fringe.  The floodway was obtained by equal conveyance reduction calculations to raise the 100-year flood elevation by less than 0.2 feet. Post-processing was reversed from HEC-RAS through HEC-GeoRAS and ArcVIEW to create the Floodplain limit with agreement with the topographic surface. 

 Princeton Hydro’s proficiency in CAD, GIS, and hydraulic modeling provided expedient Floodplain and Floodway limits to guide the designs of the site engineer, fulfill the requirements of NJDEP permitting, and protect critical water resources.