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CITY OF EVERETf STORMWATER MANAGEMENT MANUAL <br /> � where: <br /> T� = time of concentration(minutes)and <br /> m = number of Flow se�nents <br /> ;neer Ffow: Sheet flow is flow over plane surfaces. It usually occurs in thc headwater areu of streams. <br /> With sheet flow,the friction value n,is used. The n�values arc for very shallow flow depths of about 0.1 <br /> foot and are only used for travel lengths up to 300 feet. Table SJ gives Manning's nT values for sheet flow <br /> for various surface conditions. <br /> Manning's kinematic solution is used to directly compute T�. <br /> T� = 0.42(n,L)oe/��pZ)osn�go�oa� <br /> whcre: <br /> T� = travel timc(min), <br /> n, = shect ilow Manning's eHectivc roughness cocCficient <br /> L = tlow length(fl), <br /> Pz = 2-year, 24-hour rainfall(in), and <br /> Sa = slopc of hydraulic gradc line(laud slopc, Nft) <br /> Shallow Concentrared F(ow: pRer a maximum of 300 fcet,shect tlow is assumed to become shallow <br /> J conccntratcd flow. The average vclocity for[tvs flow can be calculated using the ks valucs Gom in ���hich <br /> average velocity is a function of watercourse slope and type of channel. After computing the averagc <br /> velocity asing the velocity equation below, the travcl time(Ti) for the shallow concentrated Flow segment <br /> can be computed using thc lravel time cquation desctibcd above. <br /> Velociry Equation: The Collowing equation is commonly used for computing average (low velocity,once it <br /> has mcasurable depth: <br /> V = k(so)"i <br /> whc�c: <br /> V = velocity(tUs) <br /> k = timc of conccntration velocity facror(Ns) <br /> so = slopc of flow path(R/R) <br /> "k"is computed for various land covers and channel characteristics wiih ass�imptions made for hydraulic <br /> radius using the following rearrangement of Manning's equation: <br /> k = (1.49(R)oev��M <br /> whcre: <br /> R = an assumed hydraulic radius <br /> ❑ = hlanning's roughncss cocfficicnt for open channcl Flow <br /> Volume 1—Minimum �echnical Requirements <br /> S8 <br />