Stormwater controls for channel stability: Focusing on bed material transport prevents degradation.

The hydrologic 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 the Unified Stormwater Sizing Criteria (USSC) regulations, mandating the combined use of distributed and storage stormwater controls, do not protect channel stability, despite their effectiveness in reducing runoff from impervious surfaces. The USSC are the basis of SCM design in 11 U.S. states and the District of Columbia. This study employed a calibrated, sequential modeling approach, which integrated a catchment-scale Storm Water Management Model (SWMM) with the Hydrologic Engineering Center River Analysis System (HEC-RAS), to evaluate the effectiveness of two alternative stormwater regulations in preventing channel erosion. A three-step methodology was developed using the calibrated SWMM and HEC-RAS models: (1) establish the pre-development scenario; (2) design SCMs for channel stability under design storm conditions; and, (3) assess regulation effectiveness through continuous simulations. The modeling results revealed that designing stormwater controls using the USSC increased sediment transport for the 1-, 2-, and 5-yr, 24-hr annual recurrence interval (ARI) design storms 2 to 2.7 times the pre-development conditions. SCM designs aimed at matching the sediment transport of the pre-development catchment reduced peak flows 30-70% and prevented knickpoint formation, as compared to designs based on hydrologic targets only. Study results demonstrate that to protect channels from degradation following urban development, the morphology and bed material of the receiving channel must be considered in the design of stormwater controls.