Flood redistribution and hydraulic responses to controlled levee breaching in highly Leveed urban rivers

Study region Study region: The upper reach of the Ganjiang River in Ganzhou, Jiangxi Province, China, where rapid urbanization and extensive levee systems have increased exposure to extreme flood hazards. Study focus A coupled 1D-2D hydrodynamic model (GAST) was applied to simulate an overtopping scenario (MY) and three controlled levee-breach scenarios (A, B, C) under 50-, 100-, and 500-year flood events. Breach discharge, downstream hydraulic responses, and spatiotemporal inundation were analysed to quantify how breach location and flood magnitude influence flood redistribution and downstream flood propagation. New hydrological insights for the region Controlled levee breaching modifies flood redistribution between the river channel and adjacent floodplains, with downstream hydraulic responses jointly regulated by breach location and flood magnitude. Upstream breaches produced the greatest downstream peak attenuation, reducing peak discharge by 3.87–10.31% and peak water level by 0.26–0.93 m through earlier redistribution of channel flow. In contrast, peak-delay mechanisms evolved with flood magnitude, as floodplain storage increasingly influenced flood-wave propagation during larger floods. Controlled levee breaching also reshaped the spatial distribution of flood hazards, increasing inundation area by 10.89–39.50% and inundation volume by 28.22–150.33% within receiving areas while reducing downstream flood impacts. These findings advance process-based understanding of flood redistribution in highly leveed urban rivers and provide transferable insights into downstream hydraulic responses in highly leveed river–floodplain systems with comparable channel–floodplain hydraulic controls.