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River bank failure can be caused when the gravitational forces acting on a bank exceed the forces which hold the sediment together. Failure depends on sediment type, layering, and moisture content.〔(【引用サイトリンク】title=Riverbank collapse in northwestern Minnesota: an overview of vulnerable earth materials )〕 All river banks experience erosion, but failure is dependent on the location and the rate at which erosion is occurring.〔(【引用サイトリンク】url=http://your.kingcounty.gov/dnrp/library/archive-documents/wlr/biostabl/PDF/9305BnkStbCh3.pdf )〕 River bank failure may be caused by house placement, water saturation, weight on the river bank, vegetation, and/or tectonic activity. When structures are built too close to the bank of the river, their weight may exceed the weight which the bank can hold and cause slumping, or accelerate slumping that may already be active.〔〔 Adding to these stresses can be increased saturation caused by irrigation and septics, which reduce the soil’s strength.〔(【引用サイトリンク】title=Riverbank Erosion Control Products )〕 While deep rooted vegetation can increase the strength of river banks, replacement with grass and shallower rooted vegetation can actually weaken the soil. Presence of lawns and concrete driveways concentrates runoff onto the riverbank, weakening it further. Foundations and structures further increase stress.〔 Although each mode of failure is clearly defined, investigation into soil types, bank composition, and environment must be clearly defined in order to establish the mode of failure, of which multiple types may be present on the same area at different times. Once failure has been classified, steps may be taken in order to prevent further erosion. If tectonic failure is at fault, research into its effects may aid in the understanding of alluvial systems and their responses to different stresses. ==Description== A river bank can be divided into three zones: Toe zone, bank zone, and overbank area. The toe zone is the area which is most susceptible to erosion.〔 Because it is located in between the ordinary water level and the low water level, it is strongly affected by currents and erosional events.〔 The bank zone is above the ordinary high water level, but can still be effected periodically by currents, and gets the most human and animal traffic. The overbank area is inland of both the toe and bank zones, and can be classified as either a floodplain or a bluff, depending on its slope.〔 A river bank will respond to erosional activity based on the characteristics of the bank material. The most common type of bank is a stratified or interstratified bank, which consists of cohesionless layers interbedded with cohesive layers.〔 If the cohesive soil is at the toe of the bank, it will control the retreat rate of the overlying layer. If the cohesionless soil is at the toe of the bank, these layers are not protected by the layers of cohesive soil. A Bedrock bank is usually very stable and will experience gradual erosion. A cohesive bank is highly susceptible to erosion in times of lowering water levels due to its low permeability.〔 Failures in cohesive soils will be in rotational or planar failure surfaces, while in non-cohesive soils failures will be in an avalanche fashion. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「River bank failure」の詳細全文を読む スポンサード リンク
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