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Long Hoilow Reservoir <br />Feasibility Study <br />An open-channel spillway will convey flood flows around the left abutment of the dam. The <br />open-channel spillway has a 60-foot-wide low-flow channel at elevation 6,312 and a 180-foot- <br />wide channel with a control weir at elevation 6,319 feet. The operating flood pool of the <br />reservoir will be controlled by the low-flow channel, which can pass a 50-year storm. The PMP <br />event will result in approximately 10 feet of head at the control weir and a reservoir surface <br />elevation of 6,329, which is 1 foot below the crest of the embankment dam. Providing a flood <br />pool between elevation 6,312 and 6,329 and the low-flow channel resulted in a narrower spillway <br />than a single spillway crest at elevation 6,312. A narrow spillway is more economical than a <br />wider spillway, and the savings justify an increased flood pool by raising the embankment by <br />approximately 10 feet. <br />The spillway channel downstream of the control section will be excavated in the Lewis Shale <br />that, based on results of the geotechnical investigation, heavy construction equipment will be <br />able to rip. The shale should provide reasonable resistance to the erosional forces of flowing <br />water in the spillway, which will have a maximum velocity of approximately 17 to 21 feet per <br />second (fps). Concrete slope protection is provided along the right side of the spillway to prevent <br />the spillway flow from leaving the channel and prematurely entering the valley downstream of <br />the embankment. No slope protection or energy dissipation has been provided at the end of the <br />spillway, and as a result, significant erosion is anticipated during a major spill event. <br />The geotechnical investigation of the site indicates that soil cover in the reservoir and at the dam <br />site is relatively thin in most areas. The principal material available for constructing an <br />embankment is the Lewis Shale formation. It appears that Lewis Shale can be ripped and <br />degraded for compaction in an earth embankment. The amount of durable rock for riprap and <br />relatively clean granular soil for roller-compacted concrete is minimal. <br />The embankinent will be a zoned earth structure with a core of Lewis Shale. The exterior zones <br />on the embankment will be constructed with undifferentiated overburden soil and portions of the <br />Lewis Shale including interbedded sandstone, which are less easily degraded to a soil material. <br />991-077.120 Wright Water Engineers, Inc. Page 3 <br />April 2005 <br />