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79 <br />ORANGE COUNTY PLANNING S INSPECTIONS DEPARTAAENT <br />POST OFFICE BOX 8181 3p6-F RE1/ERE ROAD <br />HILLSBOROUGH, NORTH CAROLINA 2727$ <br />e~wlt a/ e <br />~ 'i <br />,~ s+ <br />b ~ <br />17 3 <br />~~~~ e~~~~~~ <br />EROSION CONTROL DIVISION <br />Carrboro & Chapel Hill 968-4501 Durham 688-7331 Hillsborough 732-8181 Mebane 227-2031 <br />Telephone Extension 2588 Fax (919) 644-3002 <br />Dired Line 245-2588 <br />Jan. 11, 01 <br />Mr. AI Dodson <br />7214 Sunset Drive <br />Chapel Hill, NC <br />Re: Trapezoidal Channel Design <br />Dear AI: - <br />I calculated that your 30" pipe of corrugated metal would have a MaxFlow of <br />about 30cfs (cubic feet/second) and a flow velocity of about Six feet per second. <br />Several. stable channels at 1 % slope are possible based on side slopes of 2:1 <br />• Bottom Width Oft Eft 8ft 10ft 12ft <br />Depth 1.5 1.3 1.1 1.0 1.0 <br />Velocity(FPS) 3.0 2:8 2.6 2.4 2.2 <br />Almost all grassed swales at 1 % slope are stable at up to 5 fps so any of the <br />above designs should be fine. <br />Sincerely, <br />lrau <br />Reynolds J. (Ren) Ivins <br />Erosion Control Officer II <br />Acting Supervisor <br />G: uvi ns\corrospondence <br />