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2 <br />approximately 30 acre-feet from the heel to the spillway crest and the height is about <br />20.5 feet. <br />The Company was kind enough to FAX the March 1, 1991, memorandum to their <br />Flagstaff, Arizona, technical support office for evaluation during the course of the <br />meeting. During a subsequent telephone conversation with Peabody Senior Engineer, <br />Ronald J. Gehrke, Mr. Gehrke, conceded that the original ground upstream toe <br />elevation could possibly be 7062 where the embankment crosses the stream channel. <br />Furthermore, the impounded volume probably exceeds 20 acre-feet. Under these <br />circumstances, the company has two options. They can either lower the open channel <br />crest elevation to approximately 7070 from 7072.5 or bring the facility into compliance <br />and provide the required documentation to permit the dam. This decision is pending by <br />management authority above the engineering department. <br />INVESTIGATION: <br />In addition to the meeting, the author briefly examined the dam and appurtenant <br />structures. The cursory examination of the facility found the following. <br />1. The 24-inch corrugated metal pipe (CMP) principal spillway outlet is simply a <br />culvert with an inlet elevation of 7065.2 and an outlet of 7061.6. The calculated <br />gradient is 2.4 percent which implies that the slope is steep and that inlet controls. As <br />such, pressure flow is considered unlikely. A suitably sized, cubically shaped grid type <br />trashrack was placed at the inlet. Mr. McCulloh was informed that trashracks typically <br />exhibit available area approximately five times that of the inlet barrel area and should <br />be represented on the submitted drawings. In addition, Company personnel were <br />informed that the outlet conduit appeared to be lock-seam corrugated pipe which is <br />prone to exfiltration under very low heads. <br />Normally, lock-seam CMP pipe is not suitable for dam construction where the outlet is <br />expected to operate under pressure. Welded seam corrugated metal pipe is <br />recommended for pressure applications where the designer prefers corrugated metal <br />pipe. Further, watertight bands with O-ring gaskets should be used to preclude <br />infiltration and exfiltration. <br />2. The open channel may be properly designed but it is not properly constructed. The <br />channel shape should be well defined; the riprap lining should be smooth and free of <br />voids with all interstices filled; and the material utilized should be well graded and <br />exhibit a good variety of rock sizes. The channel examined did not exhibit any of the <br />foregoing qualities. In addition, the stone should be strong, durable, blocky material <br />&ee of thin elongated pieces. <br />Should the Company desire to permit the facility under 77.216, engineering calculations <br />will be required to support the design selected. <br />