INFLUENCE OF THE FLOODPLAIN CONDITION CHANGE OF UPPER KUMA RIVER ON THE FLOOD RUNOFF IN 2020

Abstract

 Comparing the 2020 flood in the Kuma River to the 1965 flood, the inundation of the upstream floodplain has decreased, while the downstream urban areas have suffered from more significant flood damage. In this study, therefore, the decline in the flood retention function of the upstream basin was investigated using the geographical data and the aerial photographs. The result suggested that the intensive farmland improvement on the floodplain and the terrace behind as well as the levee construction along the river possibly caused the decline in the flood retention effects. Considering the change of land condition, numerical simulation of the flooding was carried out on the simplified topography of the basin, and in addition, the effect of an “open levee system” for runoff control was examined using the same topographic model. The calculation results showed that the flood peak discharge was increased with 800 m³/s by the land condition change, but it would be recovered by the open levee system.