Implementation of GIS and Hydrologic/Hydraulic Modelling for Integrated Floodplain/Stormwater Management

Abstract

A very unique stormwater and flooding issue presented itself in Carlisle Borough, Pennsylvania, USA. The Letort Spring Run traversed the town and its watershed had become substantially developed causing excessive runoff, and a series of low capacity bridges caused subsequent flooding. An existing storm sewer conveyance system became submerged during significant rain events, causing a surcharge of the system, which overflowed inlets and flooded local streets, residences and businesses. The Borough also needed to address water quality from stormwater runoff to meet their National Pollutant Discharge Elimination System (NPDES) permit requirements. The analysis was complex due to flat surfaces and storm sewer system profiles, a complex system of multiple open channels with low banks, culverts, bridges, ditches, little documentation of the existing storm sewer system, backwater, and the timing of peak flows that create backwater. The problem required a unique GIS and modelling analysis through development of detailed GIS data, field survey and hydrologic and hydraulic modelling using WMS, HEC-RAS, SWMM, PeakFQ and StreamStats. Data developed in the GIS was exported into the HEC-RAS and SWMM input file formats. Various scenarios were modelled to determine storm frequency and durations that caused the most damage so that various alternative solutions could be determined. The low impact development options were also run in SWMM to evaluate alternative solutions.