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.