IDENTIFICATION OF SHALLOW GROUNDWATER INUNDATION FLOOD RISK AREAS, SPRING GREEN AREA, WISCONSIN

The Spring Green, Wisconsin area has been susceptible to groundwater inundation flooding in the recent past in areas located outside the Federal Emergency Management Agency (FEMA) Digital Flood Insurance Rate Mapped (DFIRM) areas for the Wisconsin River. Historic flooding during June 2008 inundated nearly 7 square miles of the Spring Green area with standing water for 5 months and caused contamination to water supply wells, agricultural crop loss, and damage to homes, buildings, and infrastructure. The project objectives were to identify areas in portions of southern Richland and Sauk counties, Wisconsin, that are at risk of groundwater flooding, calculate the frequency of return, and identify mitigation measures that may be feasible. This paper focuses on identifying areas that may be at future risk of groundwater flooding. The approach involved the analysis of existing regional and local data sets and the field mapping of shallow groundwater indicators. Field mapping was conducted June 21 through 25, 2010, following heavy spring rains. Multiple lines of evidence were compiled from historical aerial photographs, terrain model analysis, regional geologic and hydrogeologic setting information, the June 2008 flood extent (mapped by Fred Iausly, Sauk County GIS Analyst), and field-mapped indicators such as standing water, soil types, and wetlands vegetation to identify areas at potential risk of groundwater inundation flooding. The areas identified as having potential inundation flooding risk were ranked into four qualitative risk classes based on frequency potential. The qualitative risk classes are higher frequency, moderately higher frequency, moderately lower frequency, and lower frequency. The risk map that was generated was used to calibrate a GSFLOW model built to calculate the frequency of return and it was used to assist with an evaluation of potential mitigation measures.