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IGS Research Supports Flood Control Efforts in NE Iowa

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Selected Projects

Selected Projects

By Ryan Clark

WHEN THE UPPER IOWA RIVER (UIR) Watershed Management Authority (WMA) formed in 2015, its goal was to mitigate the destructive effects of flash flooding during intense rain events. The landscape of the UIRW is unique compared to other Iowa watersheds in its extreme topography and the prevalence of karst geology.

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The UIRWMA, along with engineering consultant Shive-Hattery Inc., enlisted the services of the Iowa Geological Survey (IGS) to evaluate the viability of three sites in karst-prone areas for stormwater retention structures. Two sites were in the Canoe Creek subwatershed, and the third was in the Coon Creek subwatershed. IGS staff conducted field reconnaissance, performed electrical resistivity (ER) surveys, and coordinated subsurface drilling to assess the geologic conditions at each site.

All three sites were characterized as having less than 20 feet of unconsolidated materials overlying limestone bedrock. Bedrock exposures were observed at two of the sites. Based on the ER surveys, the team identified several potential anomalies as potential karst-forming areas. The team targeted those areas for drilling and determined that they were underlain by competent bedrock.

Ultimately, all three sites were deemed viable due to the subsurface conditions observed. IGS researchers determined that water retention would be temporary, thus reducing the risk of infiltration into the karst bedrock below.

DIGITAL ELEVATION MODEL (DEM) created from LiDAR coverage of the Upper Iowa River Watershed illustrating the extreme topography in the study area. Sinkholes and springs are noted based on field verification from a previous survey of the watershed. Inset map shows the UIRW (blue), Canoe Creek (green), and Coon Creek (red) subwatershed boundaries.

SOUTH LINE 1

RESULTS OF the electrical resistivity survey for one of the four lines deployed at the southern site in the Canoe Creek subwatershed. Black dashed line illustrates the estimated bedrock surface. Black dots on the surface are electrodes. Boreholes drilled are labeled. Warm colors are resistive (limestone bedrock, dry zones) and cool colors are conductive (clayey material, void spaces, or moisture-rich zones).

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