Floodplain - Floodplain

A plain that is flooded when river water overflows its regular channel during flooding. It is a type of alluvial plain and is also called a flood plain. The lateral movement of the river as it meanders causes the valley walls to retreat, expanding the area of ​​inundation. The sediments that make up the floodplain are made up of unconsolidated material carried by the river, and lateral deposition is particularly promoted by the development of meandering rivers. The deposition of material carried from upstream during floods is less than the lateral deposition, but it plays an important role in the formation of natural levees and back marshes. Hydrologically, it refers to the area that is periodically inundated by floods, and the area inundated by floods with a return period of 10 years roughly corresponds to the area of ​​the topographical floodplain. The area inundated by the annual maximum flow is sometimes called an active floodplain.

[Shigemi Takayama]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

洪水時に河水が常水路からあふれて氾濫する範囲の平野。沖積平野の一種で洪涵平野(こうかんへいや)ともいう。川の蛇行に伴う側方移動によって谷壁が後退し、浸水の範囲を広げる。氾濫原をつくる堆積(たいせき)物は川の運んできた未固結の物質からなり、とくに蛇行州の発達によって側方への堆積が促進される。洪水時に上流側から運ばれてきた物質の堆積は側方への堆積に比べて少ないが、自然堤防や後背湿地などの形成には大きな役割を果たす。水文(すいもん)学的には周期的に洪水によって浸水する範囲をさし、再現期間10年の洪水によって浸水する範囲がほぼ地形的な氾濫原の範囲と一致する。年最大流量によって浸水する範囲を活氾濫原とよぶこともある。

［髙山茂美］

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