20-4 Investigating Retrofits to Improve Water Quality and Reduce Streambank Erosion in Aging Flood Control Basins in Northeast Ohio

Abstract:

Streambed and streambank erosion, a particularly vexing problem in urban watersheds, occurs when the stream’s critical discharge (Qcritical) is exceeded and bed and bank material are mobilized. In many Ohio watersheds, especially those developed between 1980 and 2003, urban runoff is routed to stormwater basins designed primarily for flood control (i.e., shaving the peak discharge without meeting other stream protection goals). As a result, many older stormwater basins fail to retain and slowly release runoff during frequently occurring small storm events, providing little to no water quality benefits nor mitigating (in fact, often exacerbating) erosion in receiving streams. Simple retrofits these basins (e.g., adding orifices, weirs, and other flow mitigation features to existing outlet structures) could improve their water quality benefits while reducing the occurrence of downstream erosion and maintaining flood control capacity. An extensive pilot-scale modeling effort was undertaken to evaluate the benefits of retrofits to flood control basins in Aurora and Mayfield Heights, Ohio. Retrofit strategies were designed to meet current regulations (i.e., slowly release runoff from a 0.9-in storm event over a 48hr period) while reducing opportunities for exceedance of Qcritical. In Aurora, targeted retrofits, especially to the largest, most downstream basin, achieved significant reductions in peak discharge and met all design objectives, including reducing instances of Qcritical to below pre-development levels, safely passing runoff from the 100yr-24hr event, and meeting current water quality drawdown requirements without changing existing normal pool elevations. Conversely, modeling for a suite of stormwater retention ponds in a commercial area in Mayfield Heights, Ohio underscores the limitations of basin retrofits in highly developed watersheds with large outlet structures which provide little hydraulic restriction to flow. Even extensive structural modifications failed to meet design objectives, suggesting that retrofits alone may not be sufficient in such contexts. These outcomes emphasize the need for a watershed-specific approach that considers outlet design, drainage area size, and development intensity when evaluating basins for potential retrofits. Findings from this research can inform similar retrofit efforts targeting flood control basins to bolster their water quality benefits and mitigate downstream erosion caused by aging stormwater infrastructure designed under outdated regulations.

© Copyright 2026 The Geological Society of America (GSA), all rights reserved.