Delineation of Floodplains Using GIS-Based Modeling


Flooding due to Hurricane Floyd, September 1999


Flooding problems have been increasing steadily with time due to the increase of impervious areas from urban sprawl. Although Best Management Practices (BMP) have been in many cases successful in mitigating flooding, there is a need for a better quantification of the floodplains with and without BMPs.


Our approach has been to investigate the hydrology and hydraulics of watersheds using the latest GIS (Geographic Information System) and engineering programs. After determining the design 100-year storm for a watershed, we proceed (using the software HEC-HMS) to estimate the peak flows in various reaches in the watershed. Then using GIS and HEC-RAS we determine the water level resulting from the design storm (i.e., the floodplains). The center portion of the floodplains is commonly referred to as floodways.

Landuse information for the Pennypack watershed used to determine the amount of surface runoff from each land parcel. Total length of watershed is about 11 miles.


We applied our approach on the Pennypack watershed (above), which is about 56 mi2 and cover portions of Philadelphia, Montgomery and Bucks counties in South-Eastern Pennsylvania. This region last experienced devastating flooding during the Hurricane Floyd (Sept. 1999) and Hurricane Allison (June 2001) events. The rapid development in these regions over the past two decades had outdated most of the Federal Emergency Management Agency (FEMA) flood maps.