Flood Analysis in the Nuta River Basin During the Western Japan Heavy Rain in July 2018

The heavy rainfall that occurred from 5 to 8 July 2018 in the western Japan have left severe flood damage. While one hour accumulated rainfall was not so large compared to the heavy rainfalls of the previous disasters, 24 hours accumulated rainfall was beyond historical record in many places. The Nuta River located in the east side of Hiroshima prefecture is one of the most damaged places by the disaster, which was induced by overflows, bank erosion, and levee breach. It is important to understand the flood flow characteristics to reduce flood damage. This study aims to reveal the runoff characteristics of the Nuta River basin and the cause of the flooding by using the two calculation models.

Rainfall Runoff Inundation (RRI) model is used to analyze rainfall runoff processes in the Nuta River basin where river data has not measured enough. The flow on the slope grid cells are calculated using 2D diffusive wave model, while the channel flow is calculated by 1D diffusive wave models. The analysis period is from 3 to 8 July 2018. As input data, hourly rainfall data (XRAIN_GIS) and discharge from the two dams in the basin are used. We conducted a 2D numerical analysis with the shallow water equation in the Nuta River to investigate the effect of riverbed sedimentation, bridge docks and tree growth in river channels.

The RRI model for the upper basin from the dams is checked to adjust the initial conditions and model coefficients. It is found that the RRI model succeeded in reproducing the inflow discharge hydrograph. The calculated the discharge by the RRI model are used for a 2D calculation and verified with the trace water levels measured along the outer and inner bank after the flood. It was confirmed that the decrease in the cross-sectional area due to sedimentation, the increase in flow resistance from bridge piers and tree growth attributed to increase the flood damage in the Funaki district.