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41 <br />vi en ranee , ~t)nr" <br />number snd-sixa...ofsa~ra~ culverts can ba reduced from what-is-given• in <br />Table 10: The culverts designed are all corrugated metal pipe. we recommend <br />that all culverts be installed on a slope of one percent or greater to prevent <br />plugging from sedimentation. <br />6.2.3 Proposed Culvert Design <br />Just east of the Newlin Creek drainage, the haul road crosses a small trib- <br />e utary drainage to Magpie Creek. This drainage area is delineated as Sw-LO on <br />Hydrology Map 2. Presently, no culvert exists and the runoff is stored by the <br />C depression (internal drainage Area iii). Due to the present and past mining <br />activities, the contributing drainage is small, approximately 14 acres and thus, <br />the runoff volume to the depression is small. The east side of this subwater- <br />r shed is an area that has been reclaimed and revegetated by G.E.C. The fact that <br />the present rnndition provides storage of the runoff promotes growth on the re- <br />vegetated area. Providing a culvert fot drainage under the haul road for the <br />present condition would eliminate this benefit. Also, the runoff would pond <br />on the northwest side of the main access road downstream. However, for the <br />post or future condition, drainage under the haul road will be required assuming <br />that the present mining areas and those areas disturbed by past-mining activities <br />. will be reclaimed. when a culvert is placed on this drainage under the G.E.C. <br />haul road, it should be designed for the reclaimed condition. Once this drainage <br />is provided, the runoff is still blocked by the main access road downstream and <br />1 some type of drainage outlet is required so the runoff can enter the Magpie <br />Creek drainage. This can be provided by placing another culvert under the main <br />road of equal capacity to the culvert under the G.E.C. haul road. The responsi- <br />bility for this second culvert is not certain since the main road was existing <br />prior to the beginning of G.E.C. mining operations and the road is used by and <br />provides access for other mining companies in the area. <br />The contributing drainage area for design of these culverts is represented <br />by the post-mining condition (Hydrology Map 3, Apxndix A). The resulting 10- <br />year, 24-hour peak discharge is 50 cubic feet per second. assuming inlet control, <br />projecting type entrance, and headwater/diameter ratio of one, the required <br />Sul ~•ert is 3.5-foot diameter corrugated metal pipe. The length of the culvert <br />should be approximately 75 feet. The culvert should also be installed at a <br />minimum slope of 0.01 to prevent deposition of sediment. The existing depression <br />