System-scale hydraulic trade-offs of dredging in confined urban rivers: Insights from the Tamsui basin

Study Region: This study focuses on the Tamsui River Basin in northern Taiwan, a densely urbanized river system with limited channel expansion capacity and an extensive levee-floodway network designed for a 200-year flood standard. Study Focus: This study evaluates dredging as a maintenance-based strategy for sustaining fixed flood-protection thresholds in space-constrained urban rivers where flood mitigation depends on coupled main-channel and floodway performance. A system-scale assessment using two-dimensional hydrodynamic and sediment-transport modeling examines how alternative dredging configurations influence hydraulic loading, diversion performance, and sediment adjustment under a representative 50-year flood event, with evaluation focused on a major urban bottleneck, the floodway inlet, and the downstream mainstem outlet. New Hydrological Insights for the Region: The results show that dredging progressively alleviates hydraulic loading at the primary urban bottleneck, with greater relief achieved under more extensive interventions. However, simulations demonstrate that increased main-channel conveyance can reduce floodway diversion by weakening the hydraulic head driving bifurcation flow, confirming a diversion-conveyance trade-off in urban flood-routing systems. Sediment responses are strongly location-dependent, with rapid re-accumulation in confined metropolitan reaches and persistent scouring in the downstream mainstem. Together, these coupled hydraulic and sediment responses indicate that maintaining fixed flood-protection thresholds through maintenance-based dredging becomes increasingly difficult in space-constrained urban rivers.