RECONNECTING THE FLOODPLAIN TO RESTORE FLOOD STORAGE: A “DIAGNOSE AND TREAT” APPROACH

Many Midwest urban streams have been assessed as degraded, entrenched and disconnected from their historic floodplains with relatively low-valued riparian vegetation dominated by highly invasive species. Floodplain reconnection by excavating lower floodplain benches and/or raising the streambed to engage historic floodplain areas can be a primary restoration strategy to restore acre-feet of flood storage per mile of stream length for a stream. Additionally, the increased hydraulic roughness can reduce downstream flood peaks by metering out runoff more slowly. Hydraulic roughness increases with in-stream habitat structures, channel irregularity (e.g. meanders), bank vegetation, and floodplain vegetation. Story County Conservation, partnering with Iowa State University (ISU) Research Park, set out to restore an unnamed stream and develop an outdoor learning and recreation area, named the Tedesco Environmental Learning Corridor, which occupies 30 acres inside the research park. The site's topography and small to non-existent flooding risk for the surrounding research park made it feasible for most of the project site to achieve floodplain reconnection with a combination of filling in and raising the stream bed and excavating the surrounding historic floodplain while also enhancing and modify existing riffles to improve channel stability. Removing riparian trees to emulate the historic prairie landscape provided the appropriate woody material for in-stream structures that provide channel stability and habitat. Temperature benefits may be realized by incorporating connection to shallow groundwater in the hyporheic zone along the channel or by creating stream channel temperature stratification in backwater pools connected to shallow groundwater seeps. The discussion will focus on floodplain reconnection as a primary restoration strategy, demonstrate the “diagnose and treat” approach, highlight and contrast target plant communities. Multiple objectives, in addition to the flood reduction benefits, include: ecological improvement to stream and riparian habitat, bank stabilization to reduce sediment and nutrient loading, aesthetic and experiential benefits to park users, and increased educational and research opportunities.