This FEMA Hazard Mitigation Grant Program project involved analysis of rainfall data and local flooding to complete a cost-benefit analysis justifying the project and leading FEMA and the State of Iowa to fund 85 percent of this $9.9 million project.
After the initial justification analysis, we developed a two-dimensional hydraulic model of the watershed utilizing XPSWMM to analyze and size four regional detention basins, which provided a wide range of flood relief within the watershed.
The final design phase began with wetland delineation, Phase 1 Environmental Site Assessment, subsurface investigations, and identification of protected species habitat. As the lead for this project, we worked closely with the USACE to obtain both nationwide and individual wetland permits and to identify a source for wetland mitigation. Hydraulic modeling was expanded to develop a detailed one-dimensional and two-dimensional models of the existing conditions and proposed improvements using xpswmm software. Modeling allowed for an evaluation of overall flood control benefits due to the interconnectivity of the basins.
Once basin sizes and property impacts were established, work began on property acquisition. Public meetings were held with affected property owners while acquisition and easement plats were prepared. Once all the necessary properties and easement plats were acquired, contract documents were finalized and the project was let for public bidding.
Construction began in fall 2014 and was completed in fall 2015 followed by FEMA closeout in early 2016. Major storm events in 2016 resulted in the first test of the improvements. Unsolicited comments from area residents indicated they witnessed significantly less flooding, if any, compared to similar storms they experienced in the past.