Local Treatment, System Response: Anticipating Sediment Forebay Performance within a Stormwater Greenway Design Using 2D SWMM

Abstract

Urban channel modification is increasingly employed to integrate green stormwater infrastructure (GSI) with public space, ecological restoration, and climate adaptation objectives. However, the system-scale hydrologic and water quality implications of such interventions—particularly the interaction between local treatment features and altered hydraulic connectivity—remain insufficiently understood. This study uses EPA SWMM to evaluate the performance of sediment forebays (SFBs) integrated into a modified urban “Savannah Channel” under three design storms (1-inch, 1-year, and 10-year), compared to a control scenario representing existing conditions. Results indicate that individual sediment forebays can achieve high local total suspended solids (TSS) removal efficiencies (>95% for through-flow units and near-complete retention for offline units). However, system-scale responses varied by storm magnitude, with channel modification and sediment forebays substantially reducing sediment export during frequent and moderate storms while providing partial mitigation during extreme events. These findings highlight the importance of right-sizing and strategic placement of sediment forebays and demonstrate the value of SWMM modeling as an iterative, pre-design tool that facilitates knowledge transfer between civil engineering and landscape architecture, informing spatial design decisions early in the design process.