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Fallura From Gaotachnlcal Cau..a . e help 10 prevent tension cracks from fonning. and is a key faclor in maintaining the stability of composite river banks. Destruction of vegetation can make the river bank more susceptible 10 mass failure. Banks may fail suddenly, particularly if there is active surface erosion or toe scour, or if sudden additional loading is applied 10 the bank. By far the greatest nwnber of river bank failures occur during heavy rains or high river stages and shortly afterwards. Figure 3-1 shows examples of the different failure types. For each type, the conditions under which they may occur are summarized in the figure. Standard textbooks on geotechnical engineering are helpful in understanding the basic mechanics of failure: however, not all the factors pertinent to river banks are likely 10 be covered. The types of failures are as follows: Shallow Failure (Figure 3-I-a) occurs where the bank angle is shallow. 11 is most often seen in largely non-cohesive materials. The failure surface is usually approximately para1lello the slope angle. Failure of river banks along a plane or slightly curved surface is nonnally associated with non-cohesive soils or materials in which relatively deep tension cracks have developed (Figure 3-I-c).ln the deep tension crack case, a slab-type failure will occur with a large block slipping and/or rotation forward. The volume of material involved in such failures can vary considerably. Surface erosion failure occurs from the entrainment of individual particles or aggregates which is discussed in the hydraulic section. Additionally, in steep cohesive banks during periods of prolonged dry weather, the surface layers can dry and crack inlo small prismatic blocks. Alternate freezing and thawing may also produce a similar effect. These small blocks remain weakly attached 10 the bank: before they eventually fall inlo the river. Any weakly attached blocks are generally removed when they become submerged. Individual failures of this type involve only very small amounts of material although the cwnulative long-term effect of such failures can be significant Continuous erosion of material in this manner can result in an over-steepening of the bank: face which could then bigger a more massive failure. Deep-seated Rotational Failure is observed where the banks are steep and modeI3lely high, and where the material is cohesive. In some circwnstances (particularly where the e Colorado Erosion Control Manual 19