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III IIIIIIIIIIIII III <br />999 <br />To: <br />Cc: <br />Bcc: <br />From: <br />Subject: <br />Date: <br />Attach: <br />Certify: <br />Priority: <br />Defer until: <br />Expires: <br />Forwarded by: <br />MathewsD~COAL,RouttenL~COAL <br />CrosbyEpCOAL~DNRML <br />Roadside Ditch Design <br />Monday, January 12, 1998 10:31:48 MST <br />N <br />Normal <br />1 reviewed Larry Reschke's response for the redesign of diversion above <br />CRDA 2. The two questions I had had to do with the CN used in the model <br />and the limiting velocity of 4.0 fps vs. 5.5 fps. <br />Reschke used a CN of 83 to represent a D-soil group with a 20~ sage-grass <br />cover. TR-55 shows a range CN 70 (fair cover) to 65 (poor cover). The CN <br />of 83 appears to fall within this range. Reschke's response is OK, and the <br />model does not need to be changed. <br />Reschke states the ditch above the refuse has a high suspended solids load, <br />so the ditch was designed for sediment laden water. This response is <br />adequate, but we should keep an eye on the grades over 1.4$ to ensure that <br />they are not erosive. If scouring is noted, we may need them to redesign <br />the channel for either vegetative lining or rip rap. The model does not <br />need to be changed. <br />