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DRAINAGE CRITERIA MANUAL MAJOR DRAINAGE • be estimated for ordinary riprap using: n = .0395 d50 176 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): VS0,17 = 4,5 (Equation 5-4) d50 'S (Ss-1)•66 • in which, V = mean channel velocity in feet per second ~ 5 = longitudinal channel slope in feet per foot Ss = Specific gravity of rock (minimum Ss = 2.50) d50 = rack size in feet for which 50 percent of the riprap by weight is smaller. The rock sizing requirements in Table 5-5 are based an the rock having a specific gravity of 2.5 or mare. Rack having specific gravity ]ess 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:ly (24). Rock lined side slopes steeper than 2h:ly are considered unacceptable because of stability, safety, and maintenance considerations. Proper bedding is required both along the side slopes and the channel bottom fora stable lining. The riprap bianket thick- ness should be at least 1.75 times d50 (at ]east 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.