2021-07-22_REVISION - M2010071 (6)

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Design Report Goose Haven Reservoir Cell 2A May 14, 2021 Page 1 of 7 1 INTRODUCTION This report presents the design of the proposed Goose Haven Reservoir Cell 2A (Cell 2A). The project objective is to provide the owner, the City of Lafayette, with approximately 700 acre-feet of additional water storage capacity. Cell 2A is located off-stream and approximately 2 miles north of Lafayette city limits, west of Highway 287, south of Boulder Creek and the Sullivan Ditch, and north or Isabelle Road. Cell 2A will be owned and operated by the City of Lafayette. Brierley Associates (Brierley) was retained by the project contractor, Rock Products of Colorado (Rock Products), to design the zoned earthen embankment dam for the proposed Cell 2A and act as the prime designer for the reservoir. Merrick & Company (Merrick) was retained by the City of Lafayette and Rock Products to evaluate the project hydrologic characteristics, perform the dam hazard classification, and design the hydraulic elements of the project. Close Associates, LLC was retained by Merrick to design the hydraulic transfer box structures. Weiland, Inc. was retained by Rock Products for surveying and Civil 3D drafting services for the project. At the Goose Haven Site, four reservoirs, referred to as Cells 1 through 4, are either planned or already exist to the east and the northeast of the proposed Cell 2A reservoir, as shown in Figure 1. Cell 1 and Cell 3 have been completed and are currently holding water. Cell 2 and Cell 4 are currently under construction. Cell 2 is directly east of the proposed Cell 2A reservoir. The proposed Cell 2A will be constructed by excavating up to 29 feet below existing grade and constructing a zoned earth embankment back to existing grade and up to 17 feet above existing grade, depending on the existing topography. Cells 2 and 2A will be connected with a 24-inch diameter pipeline that runs through a transfer box that incorporates an overflow structure. Flood flows are designed to route through the overflow structure, and no additional spillway is planned for the reservoir. This report is organized in sections consistent with report requirements presented in the 2020 Rules and Regulations for Dam Safety and Dam Construction (Rules and Regulations). 2 PROJECT COMPONENTS The Goose Haven Cell 2A reservoir is classified as a Low Hazard dam and will be comprised of: • A zoned earthen embankment around its west, north, and east sides, • An intake pipe from other cells onsite, and • An outlet pipe connecting to adjacent Cell 2. General characteristics of Cell 2A are as follows (all elevations given are relative to the project vertical datum described in the Project Construction Drawings): • Embankment Crest Elevation = 5065.0 ft (5065.5 ft including minimum overbuild) • Maximum Design Water Elevation = 5062.0 ft • Reservoir Bottom Elevation = 5055 to 5022 ft • Invert Elevation of the Outlet Pipe at the Outlet Structure = 5024.0 ft • Volume of Storage = 702 acre-ft BRIERLEY ASSOCIATES Creating Space Underground