A Detection Method for Vulnerable Points of Riverbank Erosion Against Large Flood

Heavy rainfall that occurred in July 2018 caused severe damage along rivers such as bank erosion, collapse of dikes, etc. Those broke the evacuation routes. Therefore, it is important to detect the vulnerable points of river bank erosion against large flood and reflect it on the hazard map.

The information of river bank erosion, such as location, radius of curvature R, river width B, erosion length L, and circular arc angle curve θ, obtained by the disaster photographs from the Geographical Survey Institute. RRI (Rainfall-runoff-inundation) model is used to estimate flow discharge hydrograph. The flow characteristics in the damaged rivers are analyzed by the BVC method, which is a depth integrated model with the ability of calculating three dimensional flow structures.

Many river bank erosions are found in a tributary or upstream reach in the main river. It is revealed that the river bank erosion mainly occurred on the outer bank of the curved section. As a result of the analysis using the BVC method, the bank erosions are considered to be induced by the high turbulence energy induced by local three dimensional flow structure, such as secondary flow. Erosion length tends to increase with increasing in the intensity of turbulence energy. Then simple evaluation method for vertical velocity distribution which induce turbulence energy based two dimensional model is proposed and validated to detect the vulnerability to the bank erosions. Even in a one-dimensional model expressing the strength of the secondary flow using the unit width flow discharge q and the R, it roughly predicts that the erosion occurs in a place where the intensity of the secondary flow is large. While in the one-dimensional model, it is not possible to accurately represent the erosion length.