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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 />CENTRAL CITY MASTER DRAINAGE STUDY <br /> <br />10 <br /> <br />sites as evident in Eureka Gulch. <br />on mitigation measures available <br />gulch. <br /> <br />Section 5 provides discussion <br />to stabilize tailings in this <br /> <br />3.2 Reservoir Routing <br /> <br />As noted above, Eureka Gulch hydrology was broken into the area <br />tributary to the Dorothy Lee Placer Dam site (Figure 1.1.1) and <br />the area tributary to Eureka Gulch at the confluence of Nevada <br />Gulch below Dorothy Lee. Flows tributary to Dorothy Lee routed <br />through the Dorothy Lee Reservoir with attenuated peaks then <br />added to the lower basin hydrograph by lagging them 1/2 hour <br />(hydrographs are included in the Appendix B). The 1/2 hour lag <br />time was based on average velocities of the HEC-2 Hydraulic <br />Mode I . <br /> <br />Flows were routed through the Dorothy Lee Placer Detention Pond <br />using a standard reservoir routing technique. Mapping of <br />existing terrain was planimetered at the reservoir site to <br />determine a stage-capacity relationship. Two outlet <br />configurations were considered to determine which reduced peak <br />flows for the desired floods (since Opera House flume <br />overtopping occurs at high-frequency storms, the outlets were <br />designed to reduce peaks in this range). The two outlet <br />installations considered were a 24-inch pipe at Eureka Gulch flow <br />line with an overflow weir 25-feet wide, and a 36-inch outlet <br />pipe with a IS-foot weir. Hydrographs for 10, 50 and 100-year <br />events showing the hydrographs for 24-inch outlet, 36-inch outlet <br />and unrouted flows are shown on Figure 3.2.1. Table 3.2.1 <br />summarizes the results. <br /> <br />For the purposes of this study, the 24-inch outlet pipe with 25- <br />foot overflow spillway was selected due to its greater reduction <br />in peak flows for more frequent storms. This will be discussed <br />in more detail in Section 3.3. <br /> <br />, <br />Table 3.2.1. Flood Routing Summary, Eureka Gulch at Nevada Gulch <br /> Confluence. <br /> , <br /> Return Discharges (cfs) <br /> Period Unrouted Routed 21" Routed 312) <br /> (yrs) Peak Outlet 1) Outlet <br /> 2 104 54 <br /> io 314 189 205 <br /> 50 595 364 437 <br /> 100 704 589 534 <br /> ( 1) Overflow spillway width 25 feet. <br /> ( 2) Overflow spillway width 15 feet. <br /> <br />HydroDynamics Incorporated <br />