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e ALTERNATIVE 3 -COMBINATION OF FLOOD STORAGE AND DIVERSION Because diversion of the peak flow out of the watershed and complete storage of the 100-year flow were not feasible, a combination of storage and reconstruction of a new channel is required for this alternative to be successful. Development of as much storage as possible was considered as a first option to reduce the size of the channel that would be required through the Lower Basin to accommodate residual flows. This flood control option seemed to be the most realistic based on water rights, impacts to the Poudre River and construction cost. Conceptually, a 500-cfs channel through or around the Lower Basin seemed realistic to design and construct. Suitable storage locations in the Middle Basin provide storage to limit residual flow to 500-cfs at the L- W Canal. This alternative requires the use of a SCADA system for remote control of the hydraulic gate operations. The SCADA system would include sensors to determine flooding in Dry Creek and then ensure adequate capacity in the L-W Canal by opening gages into the East Vine Diversion and closing the gates at the L- W Canal's diversion at the Poudre River. It also requires construction of the East Vine Diversion. ALTERNATIVE 3 COMPONENTS & OPTIONS e A combination of flood storage and diversion of the residual flows was identified in the initial work as the only feasible alternative for Dry Creek flood control. In summary, the plan would consist of a number of detention storage sites in the middle basin, diversion to L-W Canal, and the outfall channel (East Vine Diversion) downstream of the L-W Canal. Additional studies identified a number of storage and detention options that achieve the objective of limiting flood flow into the L-W Canal. Each option was built around a number of common features, some existing, some new features, and a number of separate features specific to that option. Eventually, four distinct options were defined. They were designated as Option A, B, C, and D. The common design elements amongst these storage and diversion options are described below. In conjunction with the common elements each option uses combinations of increased capacity in existing facilities and new detention storage at different locations to reduce flood flow to 500-cfs at the L-W Canal. Following the common features are descriptions of each option's unique features. Costs for each alternative option discussed in this portion of the report, are presented in Appendix F. . The largest and the first detention storage site at the start of the middle basin, a key component for all options, is created in Douglas Reservoir with the use of a spillway enhancement concept. With this concept the hydrologic analysis showed that Douglas Reservoir could be modified to provide control for floodwater originating from the Upper Basin. However, even with installation of the recommended Spillway Enhancement Regulating Weir (fuse plug), the middle basin still produces a peak discharge of approximately 4,000-cfs. Further analysis determined that sufficient storage could be constructed to accommodate the remaining middle basin discharge volume if it was built at several locations. Existing and new structures were considered when evaluating potential locations for this additional storage. These potential detention locations are shown on Figure 6. 16