Author(s): Norio Tanaka
Linked Author(s): Norio Tanaka
Keywords: Flood risk; Fluid force; Inundation depth; Flood mitigation;
Abstract: In the Arakawa River, Japan, many secondary embankments for controlling flood inundation in the hinterland of river embankments were constructed after Edo Era (around 420-150 years ago) especially in Yoshimi and Kawajima Town, where severe floods frequently occurred. Because of construction of high embankments, dams, etc., large flood by breaching the embankment of main rivers did not occur in the area around 100 years. However, from the river capacity, sensible flooding risk is still located there when design level flood occurs. Therefore, the objective of this study was to quantify the role of the second embankment that still exists in that area. Two-dimensional nonlinear long-wave equations were applied in the river system composed of one main river and 8 branches. In each upstream, boundary conditions were set at the flood discharge of 200-years recurrent period floods. Two type of different flood patterns (1947, 1999) was selected. The differences of inundation pattern were analyzed under the condition at which secondary embankments exists or not. In the main Arakawa River, flooding starts from the upstream of the narrow river-width location at Yoshimi Town. The flooding situation is similar to 1910 flood. Although the embankments height and river course were changed, sensible flooding risk is located in the similar area. The historical second embankments have the capability to store the inundation flow and delay the flooding in the downstream region around 1-8 hours for the simulated flood conditions. Large differences in the flood damage mitigation effects is simulated with respect to the second embankment height, the location and flood type.