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e COMMON DESIGN ELEMENTS Douglas Reservoir - Soillwav Enhancement Regulating Weir Douglas Reservoir is located on Dry Creek between the Upper and Middle Basin. As such, any floodwaters generated in the Upper Basin must pass through the reservoir. Douglas Reservoir was designed for storage of irrigation water and does not supply explicit storage for flood control. The existing structure attenuates flood flows to some degree; however, additional storage in the reservoir could further attenuate peak flood flows and decrease flood discharge downstream. Because of the location of Douglas Reservoir within the watershed, the large volume of water storage, and because it is an existing structure ranked it as a prirne candidate for flood storage in the Middle Basin. Because it is an existing structure, it requires rninimal additional construction to achieve additional storage. Thus, the estimated construction cost ( per unit of storage) is much lower than for a new facility and would result in rninirnal incremental environmental impacts with providing additional flood storage. A zoned earthfill Regulating Weir at the Douglas Reservoir spillway would allow for additional storage of floodwaters in the reservoir during extreme runoff events without compromising the safety of the dam. e An earthfill Regulating Weir structure is typically an earth or mechanical structure, which provides additional storage to the crest of an emergency spillway but is designed to allow overtopping flow in a controlled manner, in case of extreme flows. The current emergency spillway of Douglas Reservoir has been designed to pass 75 percent of the Probable Maximum Flood (PMF) or 39,750-cfs. As such, in order to ~epuate flows up to the 100- year frequency event, a Regulating Weir approximately 3-~igh would be required. This height is based on a hydrologic model incorporating the routing of the upstream hydrograph through reservoir and spillway. The preliminary conceptual design is shown on Figure 7. In general, the design consists of dividing the weir into three (3) sections by constructing two (2) cast-in-place concrete piers. These piers will be designed to be stable under the design flood and to minimize hydraulic impacts on the ultimate capacity of the spillway. The first section will remain at the existing grade of the spillway crest and will therefore not affect the normal operating water surface elevation within the reservoir. All flood flows occurring upstream of the reservoir up to and including the 100-year frequency inflow design flood will pass directly through this opening. The second and third sections will have earthen structures constructed to different heights above the existing spillway crest. The second section will be constructed to a maximum height of 3-1/2 feet above the existing spillway crest. An approximately lO-foot wide notch is provided in the weir section so that controlled erosion will begin for water depths greater than 3 feet. A similar Regulating Weir will be constructed in the third section to a height of 4 feet. This staggered effect will minimize the magnitude of the release rate of the reservoir should one of the earthen structures be overtopped. Access will be accommodated along the upstream slope of the dam structure. . The cost for the Regulating Weir was determined to be approximately $300,000. Initially the Larimer and Weld Irrigation Company has indicated that they would be willing to consider this structure if Dry Creek was upgraded to convey a minimum of 75-cfs. This 17