Natural levees - Shizen teibou

A slight elevation formed naturally on both sides of the regular channel of a river flowing over a floodplain. When river water overflows from the regular channel during a flood and spreads onto the floodplain, the flow rate of the floodwaters slows due to the influence of vegetation and the sudden shallowing of the water depth, causing the sediment carried by the floodwaters to accumulate on both sides of the regular channel. This creates natural levees that are high along the river channel and have gentle slopes on both sides. When natural levees are formed, parts of the floodplain become relatively low, causing the formation of back marshes.

[Shigemi Takayama]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

氾濫(はんらん)原上を流れる川の常水路の両側に自然にできた微高地。洪水時に常水路からあふれた川の水が氾濫原上に広がると、植生の影響と、急に水深が浅くなるために、洪水流の流速が減少して、常水路の両側へ運搬土砂を堆積(たいせき)する。このために、流路沿いに高く、両側に向かって緩傾斜をもつ自然堤防がつくられる。自然堤防が形成されると、氾濫原の一部は相対的に低地となり、後背湿地を生ずる原因となる。

［髙山茂美］

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