WSP11724 - Laserfiche WebLink

Home Browse Search

Fish Creek Reseruoir Expansion EIS ,~m.;~;:"";>>r-:->>%'i:..wm~.=<<m<::~;s?~W?',*~;W;:';':;:>>"::"~W;.<<.::'%W':w";:'%~~0~7;:;:W..J?::;:{:,"0.~~~::;%;~~:it:m~S;.%.W4.~;:;-AA~-:x:;.,-::':;:;::">>m:;w~m:~: The following discussion presents the impacts resulting from the implementation of the Proposed Action or project alternatives. The evaluation concentrates on the main and saddle dams, reservoir, ouelet works and spillway. The discussion also includes potential impacts to the primary borrow sites designated to provide materials required for construction and dam service roads. The potential impacts of the power supply options are also discussed. These options (as described in Chapter 2) are relevant to the aam safety/flood hazard discussion. 4.2.2 Direct and Indirect Impacts 4.2.2.1 Alternetive A . No Action The direct and indirect impacts associated with dam safety and flood hazard under Alternative A would result in status quo conditions. In terms of dam safety and flood hazard conditions, under Alternative A, the current structure would remain in operation with no alteration to the main dam, saddle dam, or spillway. No early warning system would be installed, and thus, the City would not be alerted to potential dam problems. Annual dam inspections by the State would continue. More regular dam inspections would not be performed, since access is unavailable for up to eight months of the year (November through June) due to snowfall and weather conditions. Since no power would be installed, operation and control of the Fish Creek Reservoir outlet works would continue to be done on-site through manual adjustments to the ouelet valve. Flood hazard potential downstream of the existing Fish Creek Reservoir, due to catastrophic dam failure, is discussed in Section 3.2, Dam Safety/Flood Hazard, and also in the Dam Safety/Flood Hazard Baseline Technical Report for the Fish Creek Reseruoir Expansion EIS (ACZ, 1992a) and the Water Resources Baseline Technical Report for the Fish Creek Reseruoir Expansion EIS (ACZ, 1992b). This discussion is based on the dambreak analysis done by Woodward-Clyde Consultants (WCC, 1992c) which is found in Appendix F. Peak flow discharges immediately downstream of Fish Creek Reservoir would be 63,200 cls. The study indicates that peak depth on Fish Creek at the water filtration plant would be 17 feet at 1.1 hours after dam failure. A flood wave of 20 feet would occur on Fish Creek at Highway 40 at 1.3 hours after dam failure. The peak depth on the Yampa River at Riverside Drive would be 10 feet at 1.9 hours after dam failure. Table 4-1, Results of Dambreak Analysis for Fish Creek Reservoir, presents the peak flow, peak depth and elapsed time from dambreak for each of the three alternatives. 4.2.2.2 Alternetive B . Proposed Reservoir Expension Alternative B is the proposal to raise and improve the existing dams and expand the reservoir from its current storage capacity of 1,842 AF to 4,122 AF (an increase of 2,280 AF). This alternative would result in both shon and long-term, as well as direct and indirect impacts to dam safety and flood hazard. Under Alternative B, an upstream raise design has been proposed for the enlargement of the main dam. This construction technique would result in the reduction of the quantity of fill materials and a decrease in the amount of seepage through the existing embankment. Additionally, to minimize seepage through the main dam embankment foundation and the underlying bedrock, a positive cutoff trench and a grout cunain in bedrock has been incorporated into the final design (WCC, 1992b). These construction methods would result in a safer dam structure. ~~::::':~W;<<1:m.-f"@~W~:::'::::i;:;:': ::;~~;:;~l:i~$.::::~:%.."W:t;:::m~~~:t.~:;::::~~~ffi~~;:;:~ Pag. 4.2 s.p,,,,,ber /993