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DocuSign Envelope ID: F52521 E7-10C1-43F9-B62F-C1 E599A5D572 <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 /> Long-term design = vegetated channel stability <br /> • Use Qpeak = Q10-year= 70 cfs <br /> • From Figure 6.17c: Limiting shear= 6 Ib/ft2 <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.dlr.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)v2 1.49S112 <br /> From trial-and-error, d = 1.7ft <br /> Determine area of flow A, from A= (bd +zdZ) <br /> = 9.8ft2 <br /> Since slope < 1:10, calculate VR using: <br /> Ve'timate = 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'Lc=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 />