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<br />54 <br /> <br />Table 6,5. Multiplying Factors for depth of Scour S for Skewed Walls, <br />\ <br /> <br /> Length to Width Ratio of Pier <br /> in Flow <br />Horizontal Angle of <br />Attack e 4 8 12 16 <br />0 1.0 1.0 10. 1,0 <br />15 1.5 2.0 2.5 3.0 <br />30 2.0 2.5 3.5 4,5 <br />45 2,5 3,5 4,5 5.0 <br />60 2.5 3.5 4,5 6.0 <br /> <br />6.5.3.3 fill Embankments <br /> <br />Local scour at the toe of fill embankments is calculated in the same manner using <br /> <br />S = d [1.1 (~) 0.4 (V~d) 0.33] <br /> <br />where a is the distance in feet measured normal to the flow along the impinging edge of <br />the fill (see Figure 6,7). <br /> <br />The second major concern along fill embankmE;nts is erosion protection of the fill slope. Par- <br />ticularly errosive currents occur at bends, slope changes, or other transition areas of channel <br />banks, When available in sufficient size, rock riprap is usually the most economical material <br />for protection against velocities greater than 5 fps. Where the velocity is expected to be less <br />then 5 fps, vegetation using various types of grasses should be adequate. When riprap pro- <br />tection is used, an adequate design includes slope of riprap protection, adequate sizing, <br />thickness of riprap and underlying filters. For detailed design of bank protection, the reader is <br />referred to Urban Storm Drainage Manual, Volume 2 and Criteria for Channels and Hydraulic <br />Structures on Sandy Soils. (References 44 and 33, respectively) <br /> <br />6.6 Embankment Stability <br /> <br />Embankment or slope stability is a major conc"rn especially when the conditions result in <br />saturated soils. The following discussion on slope stability is taken from "Floodproofing Non- <br />Residential Structures, Preliminary Draft" (R.eference 8), <br /> <br />Slope stability of an earth fill or levee embankment may be defined as the resistance of a <br />given embankment to soil slippage or a tendency to move to a more stable (flatter) slope <br />angle. Slope stability analysis techniques may be used to establish adequate safety factors to <br />ensure that a given embankment will perform in a satisfactory manner. The "safety factor" is <br />generally defined as the ratio of all stabilizing (resisting) forces to the driving forces (the for- <br />ces tending to cause movement), The slope on the verge of failure is considered to have a <br />safety factor of 1.0, For normal loading cases, an acceptable safety factor would be between <br />1,3 and 1.5. For extreme loading cases, it may be as low as 1.1, The stability analysis should <br />be performed for the worst loading conditions that are expected to develop, <br /> <br />It is recommended that two modes of sheer failure be investigated, the rotational (Figure <br />6,10) approximated by circular arc and the translatory slide (Figure 6.11) that occurs along a <br />definite plane of weakness near the base of thE; embankment. <br />