Abstract
A seasonal rain front hovering over Northern Kyushu from July 6 to 8, 2020, and warm, moist air flowing from
the outskirts of the Pacific anticyclone caused record-breaking heavy rains in a wide area. From 0:00 on July 7 to
24:00 on July 8, the Automated Meteorological Data Acquisition System in Omuta recorded 388.5 mm of rain—the
most daily precipitation and 48 h of precipitation exceeding 600 mm in several areas, including Minamichikugo
District in Fukuoka Prefecture and Yamaga and Kikuchi Districts in Kumamoto Prefecture, that expanded linearly
east–west 40 km and south–north 20 km across southern Hita in Oita Prefecture. The heavy rain caused inland
waters in Omuta to flood, two people died, and the number of damaged houses totaled 2,054, with 11 completely
destroyed, 1,341 inundated above the ground floor, and 713 inundated under the ground floor. In particular, in the
Mikawa District on the left bank of the downstream Suwa River, torrential rain overflowed the Mikawa pump
station’s drainage capacity of 64.4 mm/h, and when the drainage function collapsed, the station flooded, as did 800
houses—up to nearly 2 m in the Mikawa District—many of which have yet to be restored. Severe flood damage
occurred in Shioyamachi, Higuchimachi, and Kamiyashikimachi 1 chome and 2 chome in the Mikawa District.
Those areas, once low flatlands of lotus root fields and rice paddies before World War II, experienced rapidly
progressing division maintenance in the 1960s following reclamation. Such development at a low altitude with a
high risk of flooding further expanded the damage.
