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<br />Long Hollow Reservoir <br />Feasibility Study <br /> <br />event will result in approximately IO feet of head at the control weir and a reservoir surface <br /> <br />elevation of 6,329, which is I foot below the crest of the embankment dam. Providing a flood <br /> <br /> <br />pool between elevation 6,312 and 6,329 and the low-flow channel resulted in a narrower spillway <br /> <br />than a single spillway crest at elevation 6,312. A narrow spillway is more economical than a <br /> <br />wider spillway, and the savings justifY an increased flood pool by raising the embankment by <br /> <br />approximately IO feet. <br /> <br />The spillway channel downstream of the control section will be excavated in the Lewis Shale <br /> <br />that, based on results of the geotechnical investigation, heavy construction equipment will be <br /> <br />able to rip. The shale should provide reasonable resistance to the erosional forces of flowing <br /> <br />water in the spillway, which will have a maximum velocity of approximately 17 to 21 feet per <br /> <br />second (fps). Concrete slope protection is provided along the right side of the spillway to prevent <br /> <br />the spillway flow from leaving the channel and prematurely entering the valley downstream of <br /> <br />the embankment. No slope protection or energy dissipation has been provided at the end of the <br /> <br />spillway, and as a result, significant erosion is anticipated during a major spill event. <br /> <br />The geotechnical investigation of the site indicates that soil cover in the reservoir and at the dam <br /> <br />site is relatively thin in most areas. The principal material available for constructing an <br /> <br />embankment is the Lewis Shale formation. It appears that Lewis Shale can be ripped and <br /> <br /> <br />degraded for compaction in an earth embankment. The amount of durable rock for riprap and <br /> <br />relatively clean granular soil for roller-compacted concrete is minimal. <br /> <br />The embankment will be a zoned earth structure with a core of Lewis Shale. The exterior zones <br /> <br />on the embankment will be constructed with undifferentiated overburden soil and portions of the <br /> <br />Lewis Shale including interbedded sandstone, which are less easily degraded to a soil material. <br /> <br /> <br />A chimney drain will be placed on the downstream face of the central core and beneath the <br /> <br />downstream shell (Exhibit 3.1-5). <br /> <br />The crest elevation of the dam will be 6,330 feet, with a camber for settlement making the <br /> <br /> <br />maximum elevation at the center of the dam approximately 6,336 feet. The upstream slope of the <br /> <br />dam will be constructed on 3 horizontal to I vertical slopes and will be covered with sandstone <br /> <br />991-077.120 <br />March 2005 <br /> <br />Wright Water Engineers, Inc. <br /> <br />Page 3 <br />