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<br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />Plan B would require an excavation through the knob to provide a total spillway crest width <br /> <br /> <br />of230 feet (see Appendix D, Results). Because of the height of the knob and the resulting depth <br /> <br /> <br />of cut it would be most economical to divide the excavation into two channels, one on each side of <br /> <br /> <br />the knob, leaving the highest prominence of the knob intact and reducing the volume of excavation. <br /> <br /> <br />Figure IV-I shows the configuration of the two channels which combined provide a total width of <br /> <br /> <br />230 feet. The two channels would join at the east side of the knob and a concrete crest wall would <br /> <br /> <br />be constructed to define the spillway crest. Below the crest a chute would be excavated down the <br /> <br /> <br />east slope of the knob terminating at an excavated stilling basin. A channel would be excavated to <br /> <br /> <br />convey the flow from the basin northeast away from the residential subdivision and toward the <br /> <br /> <br />natural drainage of Dry Creek. The side slopes of the chute, the stilling basin and the outflow <br /> <br /> <br />channel would be protected from erosion by placement of riprap. The floor of the chute would be <br /> <br /> <br />excavated to bedrock which is anticipated to be near the ground surface and would not be <br /> <br /> <br />anticipated to require erosion protection. <br /> <br />C. Plan C - Constructing a New Spillway <br />The primary element of this plan is the construction of a new spillway sized to provide <br />enough additional capacity such that the total spillway capacity (the combined capacities of the <br />existing and new spillways) would be sufficient to safely pass the design flood. The spillway would <br />be constructed over the north dam of the reservoir to convey the additional discharge from the <br />reservoir to the natural drainage located directly below the dam. Since the spillway would be <br />constructed over the dam embankment it will have to be erosion resistant, which on the 2.25: I <br />embankment face slope will require concrete construction. The concrete can be a traditional formed <br />concrete chute and stilling basin or a roller compacted concrete chute and stilling basin. For reasons <br />explained in Chapter V, Section C.2 a formed spillway is not technically feasible for this site. <br /> <br />The flowline crest width of the new spillway depends on the elevation of the crest and the <br />head of the water stored in surcharge storage in the reservoir. A small increase in head can cause <br />a significant decrease in the spillway width required to pass the same flow rate. In order to increase <br />the head and decrease the spillway width the dams would have to be raised. This can be <br /> <br />15 <br />