Meteorological

Weather at 9 a.m. June 26 at the time of heaviest rainfall in northern Kyushu. Low pressure in Tsushima. Rainy season front extending toward central China and extending to south of Fukushima

Early June the Meiyu rain front had briefly come to a standstill over north-central Kyushu, raining over the city of Fukuoka and Nagasaki at the start before latter bringing rains over Kumamoto.^[2] Later the front moved south over Amami Ōshima before fluctuating between Amami Ōshima and Yakushima. Meanwhile from the south, the Pacific High in the area around the Philippines began to grow in strength and push the rain front into the Tsushima Strait. At the same time a mobile anticyclone from China began pushing the rain front back towards Yakushima. Trapped between these two competing forces the rain front became stabilized over Mount Aso around June 23. Moist, warm air from the high pressure areas stimulated the rain front while low pressure waves that would have normally passed through were instead redirected through the Tsushima Strait. Working in tandem these meteorological conditions generated the cloudbursts and prolonged rain that led to the unprecedented amount of precipitation over northern Kyushu.^[3]

Geographical

In addition to the heavy precipitation, geographical factors contributed to and exacerbated the flood. Mount Aso, one of the largest active volcanoes in the world, has produced throughout the surrounding area a lava cap of andesite which is poorly permeable to water. Additionally, deforestation during and after the Second Would War had decreased the local water retention capacity. These factors combined to allow the precipitation to quickly flow unimpeded into nearby waterways which subsequently exceeded their capacities. Furthermore, just two months prior on April 27, Mt. Aso had erupted and deposited 5.16 metric tons of ash which combined with the rain water to produce a debris flow.

Topographically, the rivers in northern Kyushu tend to follow steep grades which cause them to flow rapidly downstream. Additionally the river systems in the area have larger drainage basins upstream than middle- and downstream with the Shirakawa River and Chikugo River having drainage basin ratios of 80%^[4] and 70%^[5] respectively between their upstream and downstream systems. These topographical features resulted in the upstream systems quickly accumulating and transporting water in amounts that the lower stream systems couldn’t handle.

Flood in Kumamoto, 6.26 Great Flood

• In Kumamoto Prefecture, the central river Shirakawa flooded most severely, and people remember it by the name of 6.26 Suigai (flood). The factors of the 6.26 flood were: heavy rain in the Aso district (in Kurokawa mura), the rainfall reached 888.4 mm in 5 days, another factor being the soil of the areas; it contained layers rich in yona (volcanic ash) over lava; which easily liquefies on heavy rain.
  • A similar event was seen in 1993 Kagoshima Heavy Rain.
• On April 27, Mount Aso erupted and a great amount of volcanic ash which fell was mixed with rain; producing peculiar debris flow in the Kurokawa river and Shirakawa river merging into Shirakawa into Kumamoto.
• The third factor was the Kumamoto city; the river Shirakawa was the so-called tenjo-kawa, or a ceiling river, namely, the river bed is higher than its surroundings such as Shimotori. These factors worsened the damage of the flood.
• 70% of the then Kumamoto city was flooded, except the Kyomachi Hills and Kengun area. The central areas of Kumamoto City were 2.5 meters to 3 meters deep in muddy water. 15 bridges out of 17 in the city were carried away except Taiko Bridge and Choroku Bridge. Kokaibashi was carried away with 40 inhabitants.
• The disposal of 60 billion tons of muddy soil presented big problems; 116 areas including a moat of the Kumamoto Castle were used. The Kumamoto city gave subsidies to clean up the streets; ¥2,000 for bicycle trailers, ¥4,500 for 3-wheel cars and ¥10,750 for a truck; producing many small enterprises with trucks.^[6]
• A house for the aged was destroyed and 52 people were killed.
• The damage amounted to ¥17.3 billion (¥121.93 billion now)
• Even in the southern part of Aso gun, the dead and missing amounted to 66.^[7]

Flood in Fukuoka Prefecture: Chikugogawa Area

The Chikugo River (筑後川, Chikugo-gawa) flows through Kumamoto, Oita, Fukuoka and Saga prefectures in Japan. With a total length of 143 kilometres (89 mi), it is the longest river on Kyushu. It flows from Mount Aso and empties into the Ariake Sea. The river is important to industry, with twenty electrical power plants located along its banks, as well as the major city of Kurume in Fukuoka Prefecture.

• 147 people were killed in the areas of Chikugo River; all areas of Tsukushi plain were flooded, and the Yoake dam which was under construction, the bank of Kasegawa, and the bank of Yabegawa, the area of Homan Gawa were also flooded. 80 percent of Kurume city was flooded; in the center of the city, water was 1 meter deep, 3 meters deep at the deepest. This flood is called one of the three worst floods, including that of 1890 and that of 1921.
• The total damage amounted to ¥79.3 billion, which was equivalent to twice the yearly budget of Fukuoka Prefecture.

Estimates published by the National Regional Police Fukuoka Center