Creating Flood-Resilient Communities: A Case Study in Floodplain Management

In the land of 10,000 lakes, Minnesotans love to live near and play in our abundant water systems. But the effects of population growth, land use development, and climate change pose challenges within floodplain areas, contributing to persistent flooding issues.

The City of Owatonna experienced major flood events in 2010, 2014, and 2016. An undersized storm sewer and flat topography created significant flooding during these events. Excessive surface runoff overwhelmed the City’s storm sewer system and caused localized downtown flooding. These issues prompted the City to take action on developing comprehensive flood mitigation strategies and enlist the expertise of ISG’s Water Resources team to determine cost-effective, alternative approaches. Beginning with the development of a complex hydrologic and hydraulic (H/H) model of the City’s watershed, ISG evaluated stormwater runoff and systematically addressed key problem areas over time by implementing permanent Best Management Practices (BMPs)—leading to a more flood-resilient community.

Tackling Urban Water Challenges with Modeling Tools and Green Infrastructure

Using the H/H model, ISG conducted stormwater runoff analysis and recommended flood control strategies in the Downtown Flooding Report. Incorporating feasibility of implementation, necessity or priority of action, and associated costs, the report proposed a phased approach to incorporate BMPs and green infrastructure practices rather than major reconstruction of the City’s stormwater infrastructure. This approach provided cost savings through reduced construction and maintenance while also increasing water quality and flood mitigation benefits. One primary green infrastructure solution was the incorporation of infiltration basins and permeable surfaces within downtown alleyways, capturing stormwater through regional ponding to alleviate flooding. Additionally, three downtown locations were selected for potential improvements based on size and the ability to provide meaningful runoff reduction.

One of the three larger-scale improvement locations addressed surface flooding in the vicinity of the Public Library and US Post Office. ISG designed two infiltration basins adjacent to the Library to offer storage during heavy rainfall events and infiltration to groundwater to reduce flow rates to the storm sewer system. The infiltration basins also improved water quality and allowed the City to realize flood reduction goals while also saving on the construction and maintenance costs of larger storm sewer pipes.

Industrial Park Regional Ponding

Outside of the downtown area, ISG worked with the City to develop regional ponding for a proposed 700-acre industrial park. A stormwater master plan was developed for the entire area to be completed in three phases. Phase One designs and construction included ditch realignment, wetland mitigation, and a multi-cell pond system, with room for expansion in future phases of development.

Combined Effect of Flood Control Strategies

Improved data on flood issues from hydrologic and hydraulic modeling and alternative approaches such as green infrastructure are providing communities like Owatonna with methods to limit flood damage in a timely and cost-efficient manner. Simple, yet effective combinations of flood mitigation strategies, such as master planning, permeable pavement, regional ponds, and infiltration basins, all contribute to fortifying flood resiliency, helping communities bounce back quicker after a flood while maintaining valuable floodplain functions.