Survey of agricultural reservoirs damaged by the July 2017 torrential rains in northern Kyushu, Japan
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.
KeywordsDisaster mitigation of agricultural reservoirs Slope collapses Discharged soil Drift wood Spillway
This paper summarizes the results of the JSIDRE survey on the 2017 torrential rains in northern Kyushu, Japan. For the survey, we were fortunate to have the cooperation of the Disaster Prevention and Restoration Division, Rural Infrastructure Department, Rural Development Bureau of the Ministry of Agriculture, Forestry and Fisheries, Japan, and the Rural Environment Division, Rural Development Bureau, of the Kyushu Regional Agricultural Administration Office. Moreover, we obtained valuable information from the Institute for Rural Engineering, NARO. We would like to express our gratitude to these agencies.
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