2013-05-28_REVISION - M1977311 (3)

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to arrive at a stable riprap size. Chart 2 in Appendix C provides a solution to Equation 8 and Equation 9 using correction factor C. The stability factor, SF, used in Equation 6 and Equation 9 requires additional explanation. The stability factor is defined as the ratio of the riprap material's critical shear stress and the average tractive force exerted by the flow field. As long as the stability factor is greater than 1, the critical shear stress of the material is greater than the flow induced tractive stress, the riprap is considered to be stable. As mentioned above, a stability factor of 1.2 was used in the development of Equation 6. The stability factor is used to reflect the level of uncertainty in the hydraulic conditions at a particular site. Equation 6 is based on the assumption of uniform or gradually varying flow. In many instances, this assumption is violated or other uncertainties come to bear. For example, debris and /or ice impacts, or the cumulative effect of high shear stresses and forces from wind and /or boat generated waves. The stability factor is used to increase the design rock size when these conditions must be considered. Table 1 presents guidelines for the selection of an appropriate value for the stability factor. Table 1. Guidelines for the Selection of Stability Factors ,Condition Uniform flow; Straight or mildly curving reach (curve radius /channel width > 30); Impact 1.0 -1.2 from wave action and floating debris is minimal; Little or no uncertainty in design parameters. Gradually varying flow; Moderate bend curvature (30 > curve radius /channel width > 10); Impact from waves or floating debris moderate. 1.3 -1.6 Approaching rapidly vary ing flow; Sharp bend curvature(10 > curve radius /channel width); 1.6 -2.0 Significant impact potential from floating debris and /or ice; Significant wind and /or boat generated waves (.30 -.61 m)); High flow turbulence; Turbulently mixing flow at bridge abutments; Significant uncertainty in design parameters. 4.1.1.2 Application Application of the relationship in Equation .6 is limited to uniform or gradually varying flow conditions. That is in straight or mildly curving channel reaches of relatively uniform cross section. However, design needs dictate that the relationship also be applicable in nonuniform, rapidly varying flow conditions often exhibited in natural channels with sharp bends and steep slopes, and in the vicinity of bridge piers and abutments.