Finding Flooding Solutions Using Interfacing Tools to Aid in Model Calibration and Public Collaboration

摘要

This paper describes how geographic information system (GIS) tools contributed to solving flooding problems. GIS not only facilitated model development and analysis, but it was also critical in helping local residents and municipal engineers understand the causes of flooding, the relationship between land use alterations and floodplain impacts, and the potential changes in flooding extents as a result of proposed improvements.CDM was contracted by the town of Greenwich, Connecticut, to complete drainage evaluations for six watersheds that drain into Long Island Sound, covering approximately 26,000 acres. Detailed hydrologic and hydraulic models were developed for each watershed to understand current flooding problems and identify improvements that would alleviate flooding. For each watershed study, CDM participated in meetings with the Town Drainage Task Force, Flood and Erosion Control Board, community residents, and town officials, to understand community concerns and identify flooding problem areas.Based on the data collected, field investigations performed and information from residents, hydrologic and hydraulic models were developed utilizing the Hydrologic Engineering Center's Hydrologic Modeling System (HEC-HMS) and the Hydrologic Engineering Center's River Analysis System (HEC-RAS) computer programs, combined with GIS tools. The Geospatial Hydrologic Modeling Extension, HEC-GeoHMS, was used to extract watershed parameters while the Geospatial Hydraulic Modeling Extension, HEC-GeoRAS, utilized the town's GIS mapping to create river cross sections and map simulated flooding extents. ArcHydro was also used to delineate watersheds and determine longest flow paths.Calibration of the existing conditions model was based on an actual flood event in April 2007. Based on statistical rainfall data, this event correlated to a 25-year storm and was used to calibrate the model. Figures generated using HEC-GeoRAS showing simulated flood extents were presented to residents for validation of actual flooding experienced during the April 2007 storm event.CDM evaluated different alternatives and recommended improvements to alleviate flooding. These improvements were compared using capital improvements ranking criteria developed for the project to prioritize 33 stormwater improvements.The GIS tools were instrumental throughout the project. The GIS database aided in model development and analysis. Also, spatially relating and combining the GIS data and modeling results was critical in helping local residents and municipal engineers understand the causes of flooding, the relationship between land use development and floodplain impacts, and the potential changes in flooding extents anticipated as a result of recommended improvements.