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DocuSign Envelope ID: F52521 E7-10C1-43F9-B62F-C1 E599A5D572 <br /> Practice Standards and Specifications <br /> f the permi ssi bl a shear stress, T p, i s greater than the computed shear, T c, <br /> the lining is considered acceptable. Values for permissible shear stress, Tp, <br /> for I i ni ngs are based on research conducted at I aboratory f aci I i ti es and i n the <br /> field. Typical values are given in Table 6.17b. The permissible shear stress, <br /> 2 p, indicates the force per unit area resulting from flowing water required to <br /> create i nstabi I i ty of the I i ni ng materi a1 and/or ad aunt soi 1. <br /> Manning's Equation and Roughness Coefficient, n The condition of <br /> uniform, steady flow in a channel at a known discharge is computed using <br /> the anning s quation below. umerous computer programs are available <br /> to facilitate the use of this equation since a trial-and-error solution relating <br /> channel width, B,and depth, D, isrequired. <br /> Q = (1.49/n) (A) (R)2/3(S)1/2 <br /> anni ng s equati on for determi ni ng vel oci ty: <br /> V= (1.49/n) (R)2/3(S)1/2 <br /> where: <br /> Q =discharge(cfs) <br /> V=average vel oci ty in cross section(ft/s) <br /> n=Manning's roughness coefficient <br /> A=cross-sectional area(ft ) <br /> R= hydraul i c radius=A/P (ft) <br /> P=wetted perimeter(ft) <br /> S = energy gradient (commonly to en as equivalent to the channel bed <br /> slope,ft/ft) <br /> The appropri ate anni ng s "n" to use when designing with PS <br /> depends on whether one is designing for bare soil retention and vegetation <br /> establishment(short-term)or for fully grassed conditions(long term),or both. <br /> The°n"values for RECP's can vary significantly with material type and flow <br /> depth, but they typi cal I y range from . to . and are usual I y provided by <br /> the manufacturer. <br /> In lieu of product-specific information, the following values can be used as <br /> approximations. <br /> nunvegetated = 0.02 <br /> nvegetated = refer to Table 6.17c and Figure 6.17a <br /> nrned = refer to Table 8.05e <br /> Table 6.17c Grass Retardance Categories <br /> Average Grass Length Retardance <br /> >24 in. A <br /> 10 in. to 24 in. B <br /> 6 in. to 10 in. C <br /> 2 in. to 6 in. D <br /> Less than 2 in. E <br /> Rev.6/06 6.17. <br />