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DocuSign Envelope ID: E39A7F34-1956-4F98-91AB-121A6B293FOE <br /> Practice Standards and Specifications <br /> Sample Problem Determine if an RECP-lined drainage channel will be stable for a long- <br /> 6.17b term peak flow (10-year design storm) of 70 cfs down a 20:1 slope <br /> (S=0.05)with a 4 foot bottom width and 1:1 side slopes.The duration of <br /> flow is 50 hours for long-term and one hour for short-term design. The <br /> grass cover is expected to be in retardance group D.Short-term stability <br /> can be checked using the two-year design storm, which produces a <br /> short-term peak flow of 45 cfs. <br /> on -term esi n = vegetated channel stability <br /> • Use Qpeak = Q10-year= 70 cfs <br /> • From Figure 6.17c: Limiting shear= 6 Ib/ftz <br /> • Assume nvegetated = 0.05 <br /> Solve for the depth of flow using iterations of Manning's equation. An <br /> Excel spreadsheet located on the internet at http://www.dIr.enr.state. <br /> nc.us/pages/sedimenttecassist.html or commercially available channel <br /> software is recommended. <br /> For trapezoidal channels: <br /> (bd +zd2) 2/3 Qn <br /> (bd + zdz) _ <br /> b + 2d (z2 + 1)ti2 1.49S1/2 <br /> From trial-and-error, d = 1.7ft <br /> Determine area of flow A, from A= (bd +zdz) <br /> = 9.8ftz <br /> Since slope < 1:10, calculate VR using: <br /> estimate = 7.1 ft/s; <br /> VR= (7.1ft/s) (1.11) = 7.88ft/s <br /> From Figure 6.17a: Use n = 0.032. Recalculate d = 1.34ft <br /> A= 7.14ft2 <br /> Check shear stress L =YDS <br /> = (62.4) (1.34) (0.05) <br /> = 4.18 Ib/ftz <br /> 4.18 < 6 Ib/ftz, therefore acceptable <br /> Rev.6/06 6.17.7 <br />