Utilizing GIS, HEC–GeoHMS, HEC–GeoRAS, and ArcHydro Interfacing Tools

This paper describes how the interfacing tools between the geographic information system (GIS), U.S. Army Corps of Engineers Hydrologic Engineering Center's Hydrologic Modeling System (HEC-HMS), River Analysis System (HEC-RAS), and the ArcHydro data model (ESRI) were used to solve flooding problems within the town of Greenwich, Connecticut. CDM was contracted by the town to complete drainage evaluations for nine watersheds that drain into Long Island Sound, covering approximately 27,800 acres. Detailed hydrologic and hydraulic models were developed for each watershed, to understand current flooding problems and identify improvements to alleviate flooding.The key to the success of this project was utilizing current technological advances in hydrologic and hydraulic river modeling to effectively and efficiently develop accurate models of the various watersheds. Three key aspects of the interfacing tools used for this project were:The geospatial extensions, HEC-GeoHMS and HEC-GeoRAS, were instrumental in the development of the models. These extensions to the models were used to extract watershed parameters, create river cross sections, and map simulated flooding extents.ArcHydro was used to delineate watersheds and determine longest flow paths, reducing the time required to perform these tasks.GIS not only facilitated model development and analysis, but it was also critical in helping local residents and municipal engineers understand the causes and extents of flooding, as well as the potential reductions in flooding extents as a result of proposed improvements.The model development tools contributed to the success of the project and increased the efficiency in model development. With the new interfacing capabilities, these tools were used together to better represent the watershed, improving simulation results. These tools also played a significant role in disseminating information to town officials and residents.One key aspect relative to coordinating information with the public was the generation of easy-to-understand maps that showed the flooding extents for a 25-year storm event. This event correlated to an actual flood event that occurred in April 2007, based on statistical rainfall data. Because of this correlation, actual accounts by residents and town personnel were used to calibrate the model. Figures were generated using HEC-GeoRAS and GIS interfacing tools to show simulated flood extents. These flood inundation areas overlaid on the GIS base mapping were used to graphically represent flooding limits to residents for validation of actual flooding that they experienced during the April 2007 storm event.The HEC-GeoHMS, HEC-GeoRAS, and GIS interfacing tools contributed greatly to the projects' success, allowing the project team to spatially relate and combine the GIS data and modeling results. These tools facilitated model development and analysis. They were 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 reductions in flooding extents as a result of improvements.