Design and optimization of urban open drainage systems under compound flooding: a rank based sequential SWMM simulation optimization framework

Introduction Urban drainage systems in flood-prone riverine cities are often designed mainly for pluvial flooding, while pluvial-fluvial interactions and river backwater are neglected. This limitation is critical in developing cities where stormwater is conveyed through open-channel and roadside drains whose performance depends on terrain elevation, road elevation, and channel geometry. Existing optimization approaches have largely focused on closed conduit systems or low-impact development placement and can become computationally expensive for large networks. Methods This study develops a rank based sequential SWMM simulation optimization framework for large-scale open drainage design under a specified compound rainfall and river-stage scenario. The framework minimizes total channel excavation volume using a constraint-based rank sequential search. Three strategies are included: (i) hierarchical network ranking from upstream to downstream to enforce non-decreasing capacity and reduce the search space; (ii) hydraulic feasibility constraints keeping peak water depth below design channel depth; and (iii) river backflow constraints for elevated river stages. Results The framework is applied to two watersheds in North Guwahati, Assam, India, affected by Brahmaputra backwater. The optimized design covers a 128.33 km drainage network over a 43.50 km 2 urban area under a scenario-based compound flood boundary comprising a 5-year design storm and a fixed river stage of 49.5 m. Optimization completed in 11.7 h after 3,156 SWMM evaluations. Discussion An independent HEC-RAS 2D rain-on-grid cross-check confirmed the hydraulic consistency of the optimized design under the same compound rainfall-river-stage scenario. The framework provides a computationally tractable, compound-flood-aware approach for open-drain master planning in data-scarce riverine cities.