USING GSFLOW TO MODEL GROUNDWATER FLOODING RECURRENCE INTERVALS

The Spring Green, Wisconsin area has been susceptible to flooding in the recent past in areas located outside the Federal Emergency Management Agency Digital Flood Insurance Rate Mapped areas for the Wisconsin River. The flooding has not been the result of the Wisconsin River overflowing its banks, but rather has resulted from saturated soils, groundwater inundation, and overland flow runoff during periods of heavy snowpack melt and rainfall. On June 7 and 8, 2008, heavy rains fell on snowmelt-saturated soils in Southern Wisconsin, causing historic flooding that inundated almost 7 square miles of the Spring Green area with standing water for five months and caused contamination to water supply wells, agricultural crop loss, and damage to homes, buildings, and infrastructure.

Surface water-groundwater interactions were modeled using GSFLOW, a coupled model developed by the United States Geological Survey, to recreate the June 2008 groundwater inundation flood event for the purpose of calculating recurrence intervals for this and other historical flood events. The coupled model used historical precipitation, landuse, soils, vegetative canopy, and other climate data to predict surface water runoff and groundwater elevations. Measured climatic data spanning forty years was used to model precipitation, infiltration and runoff using the PRMS portion of GSFLOW. Groundwater was modeled using a modified version of the Wisconsin Geological and Natural History Survey Sauk County MODFLOW model. Modeled groundwater heads at observation points typically predicted the 2008 flood event as the largest magnitude groundwater event during the 40-year model period and the magnitude of head change was similar to regional groundwater observation well records. Predicted areas of shallow groundwater also generally coincided to the field-mapped areas of shallow groundwater indicators. This approach could be used to assess other large areas susceptible to groundwater inundation flooding.