Downtown Stormwater Improvements

Owatonna, MN | Water

The City of Owatonna (City) partnered with ISG to develop cost-effective, innovative green infrastructure solutions to address persistent flooding concerns throughout the City’s downtown.

An undersized storm sewer and flat topography created significant flooding during storm events. Excessive surface runoff was bypassing intakes throughout the City’s watershed, causing localized downtown ponding near the Public Library and Post Office.

ISG investigated root causes of the localized flooding by starting with a combined hydrologic and hydraulic model of the City’s watershed to evaluate stormwater runoff. Using the existing condition model, solutions focused on green infrastructure improvements with an emphasis on practices that would promote infiltration and/or detention of stormwater without having to install a larger storm line, which would lead to costly street reconstruction. ISG developed a condition model that showed reductions in surface flow along curb and gutter systems using recommended solutions. Three primary locations were selected for potential improvements based on size and the ability to provide meaningful runoff reduction.

ISG’s analysis and recommended strategies were outlined in a Downtown Flooding Report for the City to use as part of its flood mitigation strategy. The Report incorporated feasibility, necessity, and costs associated with seven alternative solutions. The City implemented a phase one alternative solution to address the persistent ponding near the Public Library and Post Office. To address the undersized storm sewer system, design enhancements included the regrading of the Library parking lot to redirect stormwater drainage to the east. Curb and gutters were removed and replaced at the new grades with a curb cut on the east edge of the lot, providing a stormwater outlet. Since the Library parking lot no longer needed storm sewer service, the storm sewer running north and south was rerouted further to the east. Two structures were installed to allow the storm sewer system to surcharge into the newly constructed infiltration basins during typical rain events. This prevented stormwater surcharging the catch basin on Broadway Street, a major downtown corridor.

The Report also included cost-effective green infrastructure recommendations, such as planter boxes, vegetated islands, tree wells, modular wetlands, and permeable pavers. Providing an array of alternative solutions, along with simple green infrastructure improvements, the City was armed with the information needed to phase in improvements over time while still seeing immediate, positive results.