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Mechanl..na of Slrumbank Fallura e 3.1.2 General Modes of Failure - Banks fai I and erode because they exist in a dynamic environment which is constantly subjectin!: them to various forces. River beds and banks fail in one of three ways: 1) hydraulic forces remove erodible bed or bank material, 2) geotechnical instabilities result in hank failures, or 3) a combination of hydraulic and geotechnical forces cause failure. E lCh of these modes of failure have distinct characteristics. An investigation must be conducted to detennine the mode of failure since it is indicative of the specific cause of t~e problem. When belld or bank erosion occurs because water 1I0wing in the l:hannel exerts a stress that exceuls the critical shear for soil erosion, the mode of failure is hydraulic (Simons, 1982). C.itical shear is dependent upon the type and size of the material. It can be exceeded by tangential shear stress caused by the drag of water or by direct impingerr,ent of water against a bank. Bed degradation is an example of the first, while local scour induced by debris is an example of the second. Hydraulic failure is usually characterized by a lack of bank vegetation, high boundary velocities. and no mass soil wasting at the toe of the bank. This is tbe least common single mode of failure. but hydraulic forces playa role in nearly all types of bank failure. Geotechnical failures that are unrelated to hydraulic erosion are nearly always a result of bank moisture problems. Moisture can affect both the stresses and the ability of the bank mat,:rial to withstand stresses. Failures are usually the result of the shear strength of the ballk material being exceeded. Mass wasting of soil at the toe of the bank is one indicatior that a geotechnical failure has occwred. But the appearance of the failed bank can vary somewhat with the material type and the precise cause of the failure. Geotechn leal failures occur frequently, but are not the most common mode of failure. Bank failllTCs that result from a combination of hydraulic and geotechnical forces are more corr,mon than failures from either force alone. These forces combined are the third mode of lailure. Numerous scenarios can be developed whereby a combination of these two forces result in bank failures. Bed degradation leading to an oversteepening of the banks and a subsequent geotechnical failure is an example. Another is when successive slip planl failures occur on a geotechnically unstable bank and hydraulic forces erode mass W31ted material at the toe which is resisting further slips. While failures occur through one of the three mechanisms identified, the actual causes of channel erosion are complex and varied. They involve streamflow characteristics. stream bank properties including groundwater conditions, and the effect of man's activities. Successfully controllir, g the erosion requires a determination of the specific cause of the failure. The followin~ sections discuss these causes. 3.2 Erosion From Hydraulic Causes - In one w lY or another, hydraulic forces play a part in virtually all bank erosion. Be it streamllow. rainfall runoff, piping, hydrostatic pressure, or freeze-thaw, it is difficult to avoid the inlluence of water in the riparian environment. e 16 Colorado Erosion Control Manual