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DRAINAGE CRITERIA MANUAL MAJOR DRAINAGE . be estimated for ordinary riprap using: _ 1/6 n - .0395 dSO in which d50 = the mean stone size in feet. This equation does not apply to grouted riprap (n = .023 to .030), or to very shallow flow (hydraulic radius is less than or equal to 2 times the maximum rock size) where the roughness coefficient will be greater than indicated by the formula. 5.4.2 Rock Size and Lining Dimensions Table 5-5 summarizes riprap requirements for a stable channel lining based on the following relationship which resulted from Smith and Murray's model studies (24): VSO.17 d 0.5 (S _1).66 50 s = 4.5 (Equation 5-4) . in which, V = mean channel velocity in feet per second S = longitudinal channel slope in feet per foot Ss = Specific gravity of rock (minimum Ss = 2.50) d50 = rock size in feet for which 50 percent of the riprap by weight is smaller. The rock sizing requirements in Table 5-5 are based on the rock having a specific gravity of 2.5 or more. Rock having specific gravity less than 2.5 is considered unacceptable. Table 5-5 represents a significant departure from past practices by recognizing that rock size does not need to be increased for steeper channel side slopes, provided the side slopes are no steeper than 2h:lv (24). Rock lined side slopes steeper than 2h:lv are considered unacceptable because of stability, safety, and maintenance considerations. Proper bedding is required both along the side slopes and the channel bottom for a stable lining. The riprap blanket thick- ness should be at least 1.75 times d50 (at least 2.0 times d50 in sandy soils) and should extend up the side slopes at least one foot above the design water surface. At the upstream and downstream termi- nation of a riprap lining, the thickness should be increased 50 percent for at least 3 feet to prevent under cutting. . 5-1-84