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77 <br />• <br /> the channels. Thus, riprap is required to provide the protection that the <br /> vegetation and boulders provided in the natural condition. <br /> The main reason for the large riprap and steep slope for major diversions <br /> PM-CH-15 and PM-(]i-17, is the present location and slope of the Dorchester Magpie <br /> diversion. The natural Magpie Creek channel had a slope of approximately 0.050, <br /> whereas the Dorchester Magpie diversion has an existing slope equal to or less <br /> than 0.010. With the lesser slope, the drop in elevation along the Dorchester <br /> Magpie diversion is much less than occurred naturally. Thus, to make up for <br /> the loss in elevation drop, the G.E.C. portion of the Magpie diversion (PM-Cli-17) <br /> had to be designed steeper than the natural slope and required significant rip- <br /> rap protection. Also, the location of the Dorchester Magpie diversion cut off <br /> C the natural channel (PM~H-15) which flowed approximately 850 feet before joining <br /> Magpie Creek. The cutoff of this drainage created an elevation difference of <br />r- 6.5 feet between the natural channel invert of PM-CH-15 and the Dorchester Magpie <br /> diversion invert. Again, PM-CH-15 had to be designed with a steep slope to meet <br /> the invert of the Dorchester Magpie diversion and also required a transition <br /> with significant riprap protection. <br /> The channels have been designed for the 10-year, 24-hour event with rip- <br /> rap protection extending onto the overbanks which are sloped at 10:1. Some <br /> damage to the channels is likely during a 25-year event, but it should be <br /> minimal. During an extremely rare event (one equal to or greater than the 100- <br /> year event), it is likely that the channels will be altered and degradation <br /> will occur. The natural condition of the area is degradation occurring gradually <br /> because of infrequent flows. The proposed channel designs are as practical as <br /> present technology allows and represent natural conditions as much as possible. <br /> ;. ::iajor concern for the stability of major diversions PM-C3-17 and PM-CH-15 <br /> is the stability of the Dorchester Magpie diversion. The slope of this diversion <br /> does nct pose any stability problems; however, the side slopes of the diversion <br /> are euite steep and unprotected. During a significant runoff event, bank in- <br /> stability can promote lateral movement of a channel and will affect both up- <br /> stream and downstream channel responses. <br /> A significant amount of riprap is required for restoration of drainage <br /> chanre!s. Care during mining operations should be taY.en to collect and keep <br /> separate armoring layers and boulders from the natural channels. These cobbles <br />. and borlders can be used along with angular riprap for reclamation of disturbed <br /> drainage channels. <br />