Effects of Existing Historical Second Embankments in the Floodplain for River Flood Inundation Control

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 before) especially in Yoshimi and Kawajima Town where severe floods were frequently occurred (around once in ten years). Because of construction of high embankments, dams, etc., large flood by breaching the embankment of main rivers has not been occurred 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 still existed in the area.

Two-dimensional nonlinear long-wave equations which can simultaneously simulate river flood and inundation were applied in the Arakawa River, Japan. River network was modelled using one main river and 8 branches. In each upstream, boundary conditions were set using water depth hydrograph estimated by the flood discharge of 200-years recurrent period floods. Two type of different flood patterns (1947, 1999) was selected considering the discharge ratio of the main river and the branches.

The differences of inundation pattern were analyzed under the condition at which secondary embankments exists or not. Inundation starts from the upstream of the confluences with branches. 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 course was changed, sensible risk is located in the similar area. The historical second embankments, Aiage Bank, Yokote Bank, Daikumachi Bank and Nagaraku Bank, have the capability to store the inundation flow and delay the flooding in the downstream region around 1-8hours for the simulated flood conditions. Large differences in the flood damage mitigation effects is simulated with respect to the second embankment height and the location.