Mud flow - Dosekiryu

A flow that occurs in steep mountain streams, where soil, gravel, and water mix together. Its characteristics are quite different from those that transport gravel by the action of running water. A large amount of soil and gravel flows down rapidly and has great destructive power, often resulting in the total destruction of houses and the loss of life. In Japan, they occur somewhere every year during heavy rains, and have attracted social attention. There are various types of debris flows, from those that contain a lot of boulders and gravel to those that are mainly mud flows, and each has different flow characteristics. Debris flows that contain a lot of boulders and gravel generally have boulders and gravel at the tip, forming a step shape, and high-concentration soil flows and mud flows follow as subsequent flows. The impact force of the boulders at the tip has great destructive power. Debris flows that are mainly mud flows do not necessarily have gravel at the tip, but they often form a step shape. The amount of water flowing before the debris flow arrives is usually very small, and since the debris flow surges like a tsunami, it is also called a mountain tsunami.

Debris flows occur when a large amount of water is supplied to the sediments on the riverbed with a gradient of about 15 to 30 degrees due to heavy rain, causing surface flow and making the sediments unstable, but collapsed sediments may also develop into debris flows. Heavy rainfall over a short period of time is related to the occurrence of debris flows. When the gradient is about 10 degrees or less, the debris flow slows down and enters the deposition process, and the gravel stops at a gradient of about 10 to 3 degrees, and this is the area where damage from debris flows is concentrated. Subsequent flows (debris flows) flow further downstream and accumulate, which also causes disasters. Mud flows containing a lot of fine sediment often occur in areas of Tertiary (old and new) layers and active volcanic areas, and are highly fluid and flow fast. Debris flow deposits are a mixture of large and small grains and do not show layers, whereas sediment transported by the action of flowing water shows layers, making them easy to distinguish. Actual debris flows are generally accompanied by high-concentration soil and sand, and the combination of these two causes disasters.

In addition to the traditional sabo dams, permeable dams with slits or lattices that allow sand and gravel from small floods during normal times to pass through and only stop the sediment from debris flows are becoming more common as a measure against debris flows due to their advantages of protecting the environment and maintaining the capacity to stop debris flows. Channel construction (revetments, bed hardening, etc.) and training works (river conservation works and training embankments that safely guide debris flows down to safe places such as the sea) are also effective against mud flows. There are many dangerous areas for debris flows, and these direct methods alone are not enough to deal with them, so evacuation forecasts and warnings based on predictions of dangerous areas and predictions of their occurrence are also important, and efforts in this area are being made mainly by the Ministry of Land, Infrastructure, Transport and Tourism and local governments.

[Kazuo Ashida and Takahisa Mizuyama]

Kazuo Ashida, "Landslides in Rivers and Countermeasures - Sediment Flow, Debris Flows, Dam Sedimentation, Riverbed Changes" (1983, Morikita Publishing) ▽ Fumiko Oura, "Debris Flows" (1994, Koyo Publishing) ▽ Hiroshi Ikeya, "Debris Flow Disasters" (Iwanami Shinsho)

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

急勾配(きゅうこうばい)の渓流に発生する土砂や石礫(せきれき)と水とが混合して一体となった流れ。流水の作用によって砂礫を輸送するものとは性質がかなり異なる。多量の土石が急激に流下し、強大な破壊力をもつため、家屋の全壊や人命の犠牲を伴うことが多い。日本では豪雨時に毎年どこかで発生しており、社会的にも注目されている。土石流には巨石や砂礫を多く含むものから、泥流を主体とするものまで各種の形態があり、それぞれ流れの特徴を異にする。巨石や砂礫を多く含む土石流は一般に先端部に巨礫や石礫が集まって段波状を呈し、高濃度の土砂流や泥流が後続流として続く。先端部の巨礫による衝撃力は大きな破壊力をもっている。泥流を主体とする土石流では先端部にはかならずしも石礫を有しないが、段波状を呈することが多い。土石流の到達前の流水量は普通非常に少なく、土石流は津波のように押し寄せるので山津波ともいわれる。

　土石流の発生は、勾配が15～30度くらいの渓床に存在する堆積(たいせき)物に、豪雨などにより多量の水が供給されて表面流が生じ堆積物が安定性を失うことによるが、崩壊した土砂礫が引き続いて土石流に発展する場合もある。土石流の発生には短時間の強雨が関係する。土石流は勾配が10度程度以下になると減速、堆積過程に入り、石礫は勾配が10～3度程度の所で停止するが、この付近が土石流による被害が集中する場所である。後続流（土砂流）はさらに流下して堆積し、これまた災害の原因となる。微細土砂を多く含む泥流は、第三紀（新・古）層の地域や活火山地帯に多く発生し、流動性が高く流下速度が速い。土石流の堆積物は大粒径から小粒径まで混合して層状を呈しないのに対して、流水の作用によって輸送される土砂の堆積物では層状を呈するため区別しやすい。実際の土石流では一般に高濃度の土砂流を伴い、両者相まって災害の原因となっている。

　土石流対策として、従来から用いられてきた砂防ダムのほかに、平時の小出水による砂礫は通下させ、土石流による土砂だけを止めるスリットや格子形式の透過型ダムが環境保全と土石流をくいとめる容量の維持という利点から増加している。泥流などに対しては流路工（護岸・床固めなどの工事）や導流工（土石流を安全に流下させて海など安全な場所まで導く渓流保全工事や導流堤など）も有効である。土石流の危険区域は数多く、このような直接的な方法のみでは対応できず、危険区域の予測、発生の予知などに基づく避難予警報も重要であって、国土交通省や地方自治体を中心としてそれらの努力が払われている。

［芦田和男・水山高久］

『芦田和男著『河川の土砂災害と対策――流砂・土石流・ダム堆砂・河床変動』（1983・森北出版）』▽『大浦ふみ子著『土石流』（1994・光陽出版社）』▽『池谷浩著『土石流災害』（岩波新書）』

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