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<br />F.llur. From Gaotachnlcal Cau... . <br /> <br />e <br /> <br />This section dea1s only with erosion causes which are directly attributable to hydraulic <br />forces. Examples include soil particle removal from direct cwrent attack,llIinfall runoff, <br />and wave action. Sections 3.3 and 3.4 will discuss the inBuence of hydrau1ic forces on <br />other causes of bank failure. <br /> <br />3.2.1 Influencing Variables. <br /> <br />Three principal factors affect bank erosion from hydraulic causes. They include; (I) the <br />resistance of the soil material to movement; (2) the hydraulic tractive force; and (3) the <br />lift forces exerted by the Bowing water on the soil. Each of these factors is controlled by <br />a number of interdependent variables. <br /> <br />Simons and Richardson (1963) reported upon a comprehensive list of these variables <br />and others affecting hydrau1ic erosion including resistance to Bow, Bow characteristics, <br />and their dependance and independence. The reader may refer to that reference for more <br />infonnation. <br /> <br />3.2.2 Current Attack - <br /> <br />Erosion from cwrent attack results when the shear exerted by the Bowing water exceeds <br />the shear strength of the sediment particles on the bank. This happens from relatively <br />straight Bow lines parallel to the bank, but usually only during BoOO Bows or on fine <br />non-cohesive soils. More frequently, it is the result of secondary helical cwrents or of <br />direct cwrent attack resulting from deBection of the primary Bow by an object in or <br />along the channel. <br /> <br />3.2.3 Rainfall Runoff - <br /> <br />Overland Bow of rainfall runoff occasionally leads to high rates of bank erosion. Direct <br />soil particle removal from sheet Bow of runoff is usually minor, but may be notable on <br />some banks. This mechanism may be a problem. however, when gullying occurs on the <br />bank. Gullies remove vegetation and create a condition of higher turbulence along a <br />bank during high Bow. Whenever evidence of gullying is found, it should be considered <br />a contributing cause of bank recession. <br /> <br />3.2.4 Wave Action. <br /> <br />Wave action can be a major contributor to bank erosion. However, it is usually a notable <br />factor only on navigable waterways. As such, it is not considered a principal factor on <br />any Colorado Waterways. <br /> <br />3.3 Failure From Geotechnical Causes. <br /> <br />e <br /> <br />Bank failure can involve geotechnical as well as hydraulic factors. Geotechnical factors <br />are important in determining locations and rates of erosion and therefore in selecting the <br />most appropriate type of bank protection. H failure is due mainly to geotechnical factors <br /> <br />COlorado Erosion COntrol Manual <br /> <br />17 <br />