Laserfiche WebLink
<br />J <br /> <br />Long Hollow Reservoir <br />Feasibility Study <br /> <br />An open-channel spillway was selected to convey flood flows around the left abutment of the <br />dam, The open-channel spillway has a 60-foot-wide low-flow channel at elevation 6,312 and a <br />180-foot-wide channel with a control weir at elevation 6,319. The operating pool of the reservoir <br />will be controlled by the low-flow channel, which can pass a 50-year storm, The PMP event will <br />result in approximately 10 feet of head at the control weir and a reservoir surface elevation of <br />6,329, which is I foot below the crest of the embankment darn. Providing a flood pool between <br />elevation 6,312 and 6,329 and the low-flow channel resulted in a much narrower spillway than a <br />single spillway crest at elevation 6,312. A narrow spillway is more economical than a wider <br />spillway, and the savings justify raising the embankment by approximately 10 feet. <br /> <br />e <br /> <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 be moving at approximately 17 to 21 feet per second (fps). <br />Concrete slope protection is provided along the right side of the spillway to prevent the spillway <br />flow from leaving the channel and prematurely enteriDg the valley downstream of the <br />embankment. No slope protection or energy dissipaiion has been provided at the end of the <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 darn <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 /> <br />The embankment will be a zoned earth structure with a core of Lewis shale. The exterior zones <br />on the embankment will be constructed with portions of the Lewis shale including interbedded <br />sandstone, which are less easily degraded to a soil material. A blanket drain will be placed on the <br />downstream face of the central core and beneath the downstream shell (Exhibit 3.1-5). <br /> <br />e <br /> <br />991-077.120 <br />March 2005 <br /> <br />Wright Water Engineers, Inc. <br /> <br />Page 3 <br />