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is less steep and erosive. Channel B would not be effective as a means to <br /> prevent gullying. <br /> Comments re. Channel C: Channel C is apparently a reconstruction of the east <br /> collector ditch. <br /> This has been a problem ditch for some time, and has been reconstructed <br /> previously using different types and placement of riprap materials, including the <br /> use of roof bolts to try to hold the riprap in place. <br /> In response to an NoV (C-91-015) written in 1991 for the failure of this ditch, <br /> MCR wrote DMG: <br /> The area involved is the east runoff collector ditch at the Sutey refuse <br /> disposal pile, and the design is found on map 11-8 App. 5-4. The design <br /> plan states that "riprap size for all other ditches [including runoff collector <br /> ditches]shall range from 3 to 15 inches in diameter D5o= 9 inches. <br /> Last year the ditch was rip rapped with 6"+ river rock. However, the <br /> combination of very incompetent subsurface soil with the larger, rather <br /> smooth rock material was not successful, as we have seen by the failure <br /> of the ditch this year. <br /> Accordingly, we feel it advisable to modify our previous approach. Prior to <br /> placement of the larger size rip rap, we intend to line the ditch with a 1' <br /> thick layer of gravel to cobble size rock material. While this may be <br /> somewhat small than specified in the design, we believe it will make for a <br /> more competent and durable ditch.... <br /> Despite modifications to the riprap design, this ditch remains a high maintenance <br /> problem because flows of water tend to wash through the riprap material to the <br /> soil beneath, which washes out and collapses due to the steep gradient and <br /> erosive nature of the soil. <br /> Because of this past experience we have concluded that riprap is not effective <br /> for this area. The approved plan (TR 25) calls for use of fabric in a <br /> reconstructed channel and, as indicated, decreased volume of flows to this ditch. <br /> The DMG bid package plan does not re-direct flows, and does use riprap in the <br /> reconstructed channel. <br /> Greg Lewicki comments: <br /> The Channel C ditch with its extremely steep grade will simply not last with <br /> a riprap D-50 of 12 inches. Past experience, as well as hydrologic <br /> MCR Objections/DMG Bid <br /> Page 6 <br />