Orange County NC Website
Browse       Search
DocuSign Envelope ID: E39A7F34-1956-4F98-91AB-121A6B293FOE <br /> Practice Standards and Specifications <br /> f the permi ssi bl a shear stress, T , i s greater than the computed shear, T , <br /> the lining is considered acceptable. Values for permissible shear stress, T , <br /> for linings are based on research conducted at laboratory f aci I i ti es and in the <br /> field. Typical values are given in Table 6.17b. The permissible shear stress, <br /> T ,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 acent 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, isrequired. <br /> Q = (1.49/n) (A) (R)2/3 )1/2 <br /> anni ng s equati on for determi ni ng vel oci ty: <br /> V= (1.49/n) (R)2/3 )1/2 <br /> where: <br /> =discharge(cfs) <br /> =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 /> =wetted perimeter(ft) <br /> = 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 /> n1ined = refer to Table 8.05e <br /> Table 6.17c rass Retar ance Cate ories <br /> Average Grass Length Retardance <br /> >24 in. A <br /> 10 in. to 24 in. B <br /> 6 in. to 10 in. <br /> 2 in. to 6 in. <br /> Less than 2 in. <br /> Rev.6/06 6.17. <br />