Flood damage - Suigai

Japanese: 水害 - すいがい

Disasters caused by heavy rainfall due to heavy rain, typhoons, thunderstorms, etc. Included are floods, inundation, submersion, mudslides, landslides, cliff collapses, and other damage. Floods, inundation, and submersion are common in river basins in plains, causing enormous material damage. Inundation and submersion can also be caused by high tides and tsunamis, which also cause great damage.

[Takao Ando]

Mudflows and landslides

Cliff collapses, debris flows, and landslides are common in mountain valleys, and many people lose their lives due to the collapse of houses. In urban areas, cliff collapses begin to occur when the rainfall reaches about 20 millimeters per hour. Debris flows occur when a large amount of precipitation saturates the mountain soil, causing a rapid flow of mud and rocks down the slope; they are also known as flash floods. Landslides are even larger in scale than debris flows, causing the mountain slope to collapse. Depending on the type of soil, the lower layers of the earth may become saturated with water, causing a landslide, in which the upper layer of the mountain moves with it.

[Takao Ando and Yoh Nomura]

Trends in flood damage due to development

In recent years, new housing sites have been developed on slopes due to the expansion of urban areas. New types of disasters associated with development are also on the rise, such as cliff collapses in newly developed residential areas on slopes. In addition, the construction of dams with flood control functions has progressed on major rivers, so floods in large rivers have become relatively rare, but on the other hand, floods in small and medium-sized rivers have become more frequent. As urbanization progresses, land that previously served as rainwater drainage outlets and reservoirs, such as small and medium-sized rivers and rice fields, has been reclaimed and turned into residential areas, and the flow of water has changed. Particularly in urban areas, the rate at which rainwater is absorbed into the ground has decreased significantly due to the complete paving of roads and the development of sewer systems, so even short periods of heavy rain can cause sudden flooding in small and medium-sized rivers, and homes along the coast are becoming more frequently flooded. Floods are also occurring in places that were previously thought to be unlikely to occur because they are on plateaus.

[Takao Ando and Yoh Nomura]

[Reference items] | Cliff collapse | Mudslide | Flood and wind damage | Landslide

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

Japanese:

豪雨、台風、雷雨などによる多量の降雨が原因となって生ずる災害。洪水、浸水、冠水、土石流、山崩れ、崖(がけ)崩れなどの被害がこれに含まれる。洪水、浸水、冠水などは平野部の河川流域に多く、莫大(ばくだい)な物質的な損害を与える。浸水、冠水には高潮や津波による場合も含まれ、これらの被害も大きい。

［安藤隆夫］

土石流と山崩れ

崖崩れ、土石流、山崩れなどは山岳部の谷間に多く、家屋の倒壊などによる人命の損失が多くなる。都会地では、1時間雨量が20ミリメートルくらいから崖崩れの発生が始まる。土石流は、多量の降水で山の土が飽和し、岩石ともども泥流が急激に斜面を流下してくる現象で、鉄砲水などともいわれる。山崩れは土石流よりさらに規模が大きく、山の斜面が崩落する。土質によっては、下層の地層が水分の飽和によって、上層の山をのせたまま移動する地すべり現象をおこすこともある。

［安藤隆夫・饒村　曜］

開発に伴う水害の傾向

近年、都会地の膨張によって傾斜面に新たに宅地を造成することが多くなった。こうした傾斜地の新興住宅地に崖崩れがみられるなど、開発に伴う新しい災害形態も増大している。また、主要河川には洪水調節の機能が備わったダムの建設が進んだことなどから、大河川の洪水は比較的少なくなったが、その反面、中小河川の洪水が多くなった。都市化が進み、中小河川や水田など以前は降雨の排水口や湛水池(たんすいち)の役割を果たしていた土地が、埋め立てられて住宅地に変わると、水の流れも変化した。とくに都会地では、路面の舗装の完備と下水道の整備とにより、雨水が地面へ吸収される割合が著しく少なくなったため、短時間の強い雨でも中小河川への急激な出水をみるようになり、沿岸部の住宅浸水が多くなっている。また、台地上にあって水害は起こりにくいと思われた場所でも水害が発生するようになっている。

［安藤隆夫・饒村　曜］

[参照項目] | 崖崩れ | 土石流 | 風水害 | 山崩れ

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