Investigation of the mechanism of the 2015 failure of a dike on Kinu River

A dike on Kinu River failed on September 10, 2015 during a heavy rain, flooding the downstream river basin and causing very serious damage. The mechanism of the break was estimated based on detailed investigations of the soil condition of the dike and the ground surrounding it, the process of overflow and breakup, and the leakage damage near the break. The broken section of the dike is located on a natural levee slightly elevated above the surrounding ground. The soil investigation showed that the dike was constructed of clayey soils deposited on the loose alluvial sand layer of a natural levee. According to reports from public employees patrolling the area at the time of the break and from inhabitants of the area, and based on the recording of a nearby CCTV camera, the authors estimated that the dike failed about two hours after water began to flow over it due to the overflow and to the seepage of river water into the levee. The authors concluded that the following processes occurred between the start of overflow and the break: i) the overflowing water scored the inland slope of the dike, ii) the area of the slope being scored expanded, iii) the loose sand layer became unstable its sand collapsed in continual increments until iv) the dike broke. Based on their understanding of this mechanism, they proposed a basic plan to restore the dike.