The torrential rains that hit northern Kyushu, Japan, on July 5–6, 2017, caused serious damage, especially in Asakura City, Fukuoka Prefecture, and in Hita City, Oita Prefecture. The primary meteorological mechanism that caused these torrential rains can be summarized as record-breaking heavy rains caused by linear rainbands in northern Kyushu. These heavy rains triggered hillside collapse, with the massive volume of driftwood and soil destroying downstream areas. Shortly after the immediate damage caused by the torrential rains was confirmed, the agricultural reservoirs were suspected to have exacerbated the flood damage in downstream areas. However, it is considered that the reservoirs helped mitigate flood impacts. To verify the disaster mitigation effect of the reservoirs on these torrential rains, the Japan Society of Irrigation, Drainage and Rural Engineering organized a survey team to investigate the damaged agricultural reservoirs in Asakura City. From the survey, four of 11 surveyed reservoirs were confirmed ravaged because of the massive driftwood and soil volume from the hillside area. However, the other reservoirs remained intact, and two distinctive characteristics were found to have helped prevent the flood from worsening. One was the reservoirs’ capacity to store soil; they had a large water capacity, and their strong walls kept the soil inside at their full water level. The other was spillway type and location. The reservoirs that had side-overflow spillways were able to discharge driftwood. Therefore, the water storage capacity of the reservoirs, dike strength, and spillway type and location are important factors that may help mitigate flooding.
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