Using ArcGIS to Link Urban Stormwater Software (SWMM) and Economic Flood Damage Analysis Software (HEC-FDA)
Track: Water Distribution, Wastewater, and Stormwater
Author(s): Thomas Tri
The U.S. Army Corps of Engineers (COE) applies the HEC-FDA model to calculate expected flood damages associated with existing conditions and proposed alternatives. The model requires valuation data for each structure and associated flood elevations at each structure to statistically determine the damages for various flood events. In a highly urban watershed, the gathering and analysis of this data has always been tedious, time-consuming, and costly. The Louisville Southwest Flooding Study provided a special challenge because of the large number of structures in the study area (68,000), the size of the study area (32.0 sq. mi.), and the large amount of data being generated from the urban hydraulics software (SWMM), which modeled a combined sewer area with 4,800 sewer manholes.
GIS proved to be a valuable asset to facilitate the study. The Louisville COE contracted with Skees Engineering to develop GIS applications, which would provide the input for the HEC-FDA model, and to execute the model to obtain the economic impacts for the "without project" conditions. GIS data from the Louisville and Jefferson County Information Consortium (LOJIC) was used to develop input for the FDA model including assessed property value, year of assessment, first floor elevation (derived from TIN points), style of structure, property use classification, parcel identifier, and address.
One particularly challenging aspect of the study was to develop a method for determining anticipated flooding depths at each structure based on the output of the SWMM model. Because the model only provided volumes of surcharge at each node, a method was developed to translate the discharge volumes to a flood elevation for each of the 68,000 structures. Because of the extreme flat nature of the watershed, the discharge volumes spread across multiple sub-basins. A custom ArcGIS/ArcView Spatial Analyst/VBA application was developed to iteratively distribute the volumes of water over the localized subbasins until equilibrium existed across the study area.
Skees Engineering, Inc.
400 Blankenbaker Pkwy.
Louisville , KY 40243