A survey of areas hit by flooding after the Kuma River overflowed during recent torrential rain in the Kyushu region of southwestern Japan has found that one area was flooded to a depth of 4.3 meters, making the disaster one of the biggest local floods in recent years, surpassing major flooding recorded in July 1965.
The survey was carried out by Kumamoto University's Center for Water Cycle Marine Environment and Disaster Management. Masahide Matsumura, a specialist in bridge engineering who heads the center's Disaster Mitigation Laboratory, and other researchers began surveying areas damaged by flood on July 5, the day after the torrential rains hit, in the city of Hitoyoshi and the towns of Ashikita and Tsunagi in Kumamoto Prefecture. Using flood estimate maps from the Geospatial Information Authority of Japan, they examined an area spanning about 5 kilometers from east to west and 1 kilometer from north to south, focusing on residential areas on the right back of the Kuma River in the city of Hitoyoshi.
The Kuma River is nicknamed the "Abaregawa" (raging river), as it is known as one of the three fastest-flowing rivers in Japan along with the Fuji River that flows through Nagano, Yamanashi and Shizuoka prefectures in central Japan and the Mogami River in Yamanashi Prefecture. The Kuma River, designated by the government as a class A river, has flooded several times in the past. In 1965, flooding swept away or damaged 1,281 homes, while 12,825 dwellings were flooded either above or below floor level. According to the Land, Infrastructure, Transport and Tourism Ministry's Yatsushiro River and National Highway Office, between then and June 2011, the Kuma River flooded at least nine times.
Matsumura and the other researchers examined watermarks left on the buildings and other structures in the area and compared them with the heights of levees and past flooding. One site that was examined was Aoi Aso Shrine in the Kamiaoimachi district of Hitoyoshi, whose offering hall and oratory (two of five structures at the shrine designated as national treasures) were flooded above floor level. During the July 1965 flood, a utility pole next to the shrine was submerged to a depth of 2.3 meters, while in another flood in August 1971, a water depth of 1.1 meters was recorded. This time the height of the water reached 3 meters.
Several hundred meters downstream from the shrine was another utility pole where the water reached 1.1 meters in the 1971 flood and 2.1 meters in the 1965 flood. This time the water depth at this spot reached 4.3 meters, watermarks showed.
Buildings near the Kuma River and one of its branches, the Yamada River, were submerged to a depth about 2 meters higher than the top of the levee, the researchers found. The Hitoyoshi Bridge in the center of the city stands at a higher elevation than the Kuma River levee, but the water still rose higher than the bridge, and logs and other debris were caught in the railing. The depth of the water north of the bridge reached around 3.5 meters.
Hazard maps that were prepared for the city of Hitoyoshi envisaged rainfall on the level of the July 1965 floods. It has been predicted that rainfall near upstream areas in Hitoyoshi would reach 440 millimeters over two days -- a level seen around only once every 80 years. But in actual fact downpours continued in the town of Yunomae near the upper reaches of the Kuma River for over three days, and the town was lashed with over 470 millimeters of rain in the space of 18 hours. In some places in the city of Hitoyoshi near the middle section of the river, areas were submerged to double the predicted depth, the researchers found.
"There's no doubt the level topped past floods for which records remain, and that inundation was beyond the level people had expected," Matsumura said. The center announced its findings on its website and will keep them up to date.
(Japanese original by Shotaro Asano, Nagasaki Bureau)