Effects of geomorphological and geohydrological features on flood hazard in a coastal basin

Anomalous precipitation (more than 360 mm/d) caused severe flood inundation in the Kujukuri plain, Japan, on October 25, 2019. In this contribution, we employed a hydrological and hydraulic model to reproduce this flood and to examine the possible factors affecting the disaster. The simulated results showed consistent areal extents of inundation compared with the field investigation, suggesting potential application of numerical approaches for assessing the flood hazard. Details of the simulation results obtained from the flood on October 25, 2019, together with those on October 8, 2004 and October 15, 2013, revealed that the local geomorphological and geohydrological features significantly influenced the flood hazard. The low elevation lands located near the confluence zones of the rivers and enclosed by higher elevation terraces were found to be quite vulnerable to floods. Numerical simulation suggested that the study area showed both direct groundwater recharge from the inundated surface and indirect recharge from the stream leakage. Groundwater stored during the flood fed the river after the peaks of the precipitation, causing the delay of river water recession process in the stations situated inside the inundated areas. Our results showed that considering local geomorphological and geohydrological characteristics are crucial in evaluating flood hazard.