The Velocity of Flood Flow of the Kuma River in Heavy Rain of July 2020 and Flood Damages

Abstract

Considering the contribution to disaster prevention of national land, I developed a unique method to estimate the surface velocity of flood and the flow direction of the Kuma River during the heavy rain of July 2020 using the oblique aerial photographs linked to the GSI maps. After confirming the motion vectors of the drifting objects using the stereo pair of aerial photographs, an ortho aerial photograph is created from them. The distance of movement of drifting objects and the flow direction are measured from the ortho photograph, and the surface velocity is calculated by the distance and the time interval of shooting. In the distribution map with the theme of surface velocity and flow direction, the spatial features of the flood flow can be estimated and the background of the flood damage can be discussed. The center line of stream with high surface velocity is approximately near the center of the river channel, and the velocity gradually slows down toward both shores. The flow directions coincide with the direction near the center line of stream and vary near the shore. In the velocity distribution in the longitudinal direction, areas with high surface velocity at the center line of the stream appear repeatedly at intervals of about 150 to 200 m. In the distribution map, the three features of the flood flow were understood. The first is that the location of the center line of stream and flow direction change due to river bend. The second is that the surface velocity and flow direction change due to river width. The third is that the surface velocities of the main stream and the tributary are very different. This method for flood flow seems to be practical for use because there are spatial correlations between the surface velocity and severity of the damage, and between the low surface velocity and the sediment accumulation. Therefore, the surface velocity and flow direction could be analyzed by the combination of SfM and GIS using oblique aerial photographs taken urgently in the event of the disaster. I believe that the method and the result will be an effective method for obtaining basic materials to reduce flood damage in applied geology.