Effect of Small Rivers for the Inundations Due to Levee Failure at Kinu River in Japan

On September 9, 2015, typhoon number 18 brought unprecedented rainfall to northern Japan, particularly to Kanto and Tohoku regions. As a result, flood damage and sediment disasters occurred in various locations including an overflow in Kinu River at Wakamiyado and levee failure in Kinu River at Misaka-cho. During the disaster, upstream of Kinu River has received more than 600 mm precipitation in 48 hrs which is more than twice the amount which usually received throughout the entire month of September. This unprecedented rainfall, has caused huge disaster to the people in Ibaraki prefecture particularly Joso city where the entire city has been recoded an inundation of about 1 - 2 m depth and it has been over 3 m in some areas. A small river (Hachikenbori River) flowing through the center of the flood plain was assumed to be the cause for earlier inundations in the downstream. We developed a coupled flood inundation model for the area with 1D and 2D hydrodynamic models, to study the effect of the Hachikenbori River for the inundations. The model includes the overbank flow and the flow due to river bank failure in Kinu River and Hachikenbori River. The developed model was validated with the observed data of inundations and water levels at Hachikenbori River. The results from the analyses performed with and without Hachikenbori River indicate that Hachikenbori River caused the early inundations in some places. Numerical results suggest that the Hachikenbori River has reduced the inundation area by 1 km² by allowing the flood water to pass quickly. However, there is no difference in the inundation heights of the cases carried out with and without Hachikenbori River.