Evaluation of drainage efficiency via street inlets under the influence of terrain slope in the course of pluvial urban flood event

Study region: This study is conducted in the northern urbanized region of Fuzhou City, China. Study focus: This study aims to conduct an investigation into the evolution of drainage efficiency via street inlets over the course of flood events in urbanized areas. Specific emphasis is placed on elucidating the influences imparted by variance in terrain slope, together with the revelation of the underlying mechanisms by which urban terrain slopes influence the dynamics of floodwater in the vicinity of street inlets. New hydrological insights for region: This study views the drainage efficiency in urbanized area as dynamic variable rather than static parameter and quantify the impacts of terrain slope during different stages of rainfall event. Through hydrodynamic modeling, the responses of drainage effects via street inlets to terrain slope changes in urbanized area are examined. Delayed peak responses in drainage efficiency under steeper terrain slope conditions are revealed, attributable to increased flood flow velocities bypassing street inlets. Localized flow fields demonstrate discernible terrain slope influence upon drainage functionality, with gravity-dominated directional flows disrupting inlet drainage above certain slope threshold. Data-driven regression captures distinct correlation of drainage efficiency with terrain slope and rainfall timing, enabling accurate event-scale predictions for specific urbanized area. The analysis provides novel insights into how terrain slope alters the floodwater flow on urban surface, leading to significant influence on functioning of street inlets. • Indicator proposed for evaluating drainage efficiency during urban flood. • Drainage efficiency via street inlets found to drop with steeper terrain slope. • Turning point identified for drainage efficiency change with terrain slope. • Peaks of drainage efficiency delay in floods and advance on steeper slopes. • Temporal evolution of drainage efficiency captured by ensemble models.