Assessing the Efficacy of Stream Restoration and SCM Retrofitting for Channel Stability in Urbanized Catchments

The hydrological benefits of catchment-scale implementation of stormwater control measures (SCMs) in mitigating the adverse effects of urbanization are well established. Nevertheless, recent studies indicate that Maryland's stormwater regulations, mandating the combined use of distributed and end-of-pipe SCMs, fall short in maintaining channel stability, despite their effectiveness in reducing runoff from impervious surfaces. The study objective was to evaluate the incremental impact of SCM retrofits and stream restoration on channel stability in a small, urbanized catchment (0.9 sq. km) in Montgomery County, Maryland, USA. This study employed a refined, well-calibrated, coupled hierarchical modeling approach, integrating a watershed-scale Storm Water Management Model (SWMM) with the Hydrologic Engineering Centers River Analysis System (HEC-RAS). A comprehensive methodology was developed using the calibrated SWMM and HEC-RAS models. The modeling results revealed that only retrofitting SCMs with multi-stage outlet structures designed to maintain the pre-development mobility of bed particles may not effectively reduce channel degradation. Conversely, stream restoration practices, including the removal of legacy sediments from the floodplain, significantly mitigated channel instability. Notably, the combination of SCM retrofitting, aimed at matching the sediment transport capacity of the predevelopment state, and stream restoration practices did not yield better results compared to stream restoration alone. This finding suggests that for streams impacted by legacy sediments, floodplain restoration alone might suffice to achieve channel stability, eliminating the need to retrofit SCMs designed under existing regulations.