Technical Memorandum To: Northern Colorado Water Conservancy District From: GEI Consultants, Inc. Date: May 10, 2006 Re: Technical Memorandum No.1: Northern Integrated Supply Project (NISP) Preliminary Assessment of Glade Dam and Reservoir and Associated Facilities INTRODUCTION The proposed Northern Integrated Supply Project (NISP) is a new water project that will develop water resources of the Cache la Poudre River and the South Platte River to meet water needs of 13 cities, towns, and water districts in Northern Colorado. NISP will include several reservoirs as well as water conveyance pipelines and canals. Potential reservoirs include Glade, Cactus Hill, and Galeton (which are water storage reservoirs) the Glade Forebay, and the Galeton Forebay, which are small regulating reservoirs from which water will be pumped to fill available capacity in the storage reservoirs. Cactus Hill Dam and Reservoir is an alternative to the Glade Dam and Reservoir. The objective of this Technical Memorandum (TM-1) is to provide preliminary feasibility-level information concerning the Glade Dam and Reservoir for use by the NISP EIS consultant in evaluating alternatives. The assessments presented herein are based on existing data. The level of investigation is preliminary in nature and does not include subsurface geotechnical investigations or development of detailed engineering concepts. It also does not include an assessment of how the Glade Dam and Reservoir would be operationally integrated into the other facilities of NISP. Other TMs address the other storage elements being considered for NISP. This TM includes descriptions of Glade Dam and Reservoir, the Glade Forebay, and modifications to existing facilities that will be required to to divert water from the Cache la Poudre River into the Forebay for pumping into Glade Reservoir and to bypass the Munroe Canal around Glade Reservoir. This bypass would be required because the Munroe Canal will be inundated by Glade Reservoir. The existing Poudre Valley Canal (PV Canal) will be used to deliver water to the Glade Forebay and to bypass the Munroe Canal around Glade Reservoir. The primary purpose of the Glade Reservoir is to store up to 170,000 acre-feet (af) of water that would be diverted from the Cache la Poudre River. The location of Glade Reservoir and its components in relation to the other proposed NISP facilities is shown on Figure 1.1. The general project features are shown on Figure 1.2. The Glade Forebay would be located immediately below Glade Dam, as shown on Figure 1.2. The Forebay would be created by excavation and by berm construction around the excavation to create a reservoir of approximately 2,000 af. A pumping station at the Forebay would lift water from the Forebay into storage in Glade Reservoir. Water would be supplied to the Forebay via the PV Canal, which would be upgraded to convey the requirements of both the PV Canal and Munroe Canal water users. The canal upgrading would include diversion dam modifications and improvements to the 1 canal cross-section to provide adequate flow capacity. As shown on Figure 1.2, the PV Canal would be inter-connected to the Munroe Canal by a pumping station and pipeline. The conveyance facilities are described in the Phase II Study by MWH and in reports prepared by Integra Engineering. DESCRIPTION OF GLADE DAM AND RESERVOIR The proposed reservoir will store approximately 170,000 af, including 3000 af of inactive storage for future sediment accumulation and active storage contents of 167,000 af. Refinement of these storage allocations and selection of the total volume of storage to be provided at the site will be undertaken during subsequent stages of project design. By Colorado State statute, the proposed Glade Dam will be a jurisdictional dam, subject to regulatory authority of the Colorado Office of the State Engineer (SEO), Division of Water Resources. The dam must conform to applicable state dam safety rules and regulations as contained in the Rules and Regulations for Dam Safety and Dam Construction (SEO, 1988), and referred to herein as SEO Rules. The dam is classified as “large” based on a height greater than 100 feet, and “high hazard” based on the potential for loss of life if the dam were to fail. For a new large-size, high-hazard dam, SEO Rules require use of the Probable Maximum Flood (PMF) for the Inflow Design Flood (IDF), comprehensive subsurface geotechnical investigations, stability analyses, and other evaluations and analyses consistent with SEO Rules. Under SEO Rules and NISP objectives, the dam will require an emergency spillway to pass rare floods up to and including the PMF, a service spillway to pass smaller, more frequent floods, and an outlet works with intake tower and one or more valve houses to accommodate NISP operations, stream releases, and emergency reservoir drawdown requirements. The Glade Reservoir basin is approximately 1.2 miles wide at its maximum and almost 4 miles long covering portions of an area called Hook and Moore Glade and an adjacent drainage. The reservoir would be “off-channel” and would not inundate the Cache la Poudre River or other perennial streams. The reservoir rim is relatively steep, except at the upper end of the reservoir basin. While the reservoir rim slopes are expected to be stable during reservoir filling and draining, periodic drawdowns below normal pool level will leave relatively large portions of the reservoir rim and upper portions of the reservoir bottom exposed. Regional and Site Geology The proposed site for the Glade Dam and Reservoir is located in the Southern Rocky Mountains physiographic province near the border of the Great Plains physiographic province to the east. The Southern Rocky Mountains in Colorado consist predominantly of north-south trending mountain ranges once covered with sedimentary strata. The individual ranges have central cores of Precambrian rocks that were uplifted during mountain building episodes causing the Paleozoic and Mesozoic sedimentary rocks to dip away from the central cores. Uplift, faulting, folding, and erosion from Laramide time (tertiary) through late Pliocene time have modified the topography to its present configuration. The Front Range, the easternmost range of the Southern Rocky Mountain system, extends from the Colorado-Wyoming state line to the Arkansas River. The oldest rocks in the region are Precambrian metasedimentary, metavolcanic (metamorphic), and intrusive (granitic) rocks. In general, these rocks consist of schists, gneisses, and granites of the Idaho Springs Formation. Overlying the Precambrian rocks are mostly late Paleozoic to Mesozoic 2 sandstones, shales, and limestones of continental and marine origin. The dam and reservoir sites are located primarily within these folded and faulted Paleozoic sedimentary rocks. A deposit of Quaternary alluvium covers the floors of the valley formed in the Paleozoic sedimentary rocks. Numerous north-northwest to south-southeast-trending faults bisect the region. Three of the more prominent faults proximate to the project site include the Bellvue Fault, the Livermore Fault, and the North Fork Fault. All of the faults within the vicinity of the project are considered to be inactive. The Livermore Fault, which was once hypothesized to be potentially active, is now considered to be inactive (Unruh, 1996, and Kirkham and Rodgers, 1981). The bedrock units at the Glade site include the Precambrian metamorphic and granitic rocks occurring in the upper part of the right abutment, faulted against Lykins siltstone in the right portion of the valley section. The central valley section consists primarily of Sundance and Jelm sandstone, and the steep part of the left abutment contains the Morrison shales and sandstones. The upper left abutment extension consists of two units of the Dakota Group mapped as the Lytle Formation conglomeritic sandstone and the South Platte Formation shales and sandstones. The crystalline rocks and sandstone/conglomerate units are usually strong and resistant to weathering but they are jointed and fractured and therefore contain potential zones of high hydraulic conductivity. During construction of the dams for Horsetooth Reservoir, excavation into the Sundance sandstone revealed several “open and fractured joints” which subsequently were suspected of diverting drilling water during foundation grouting (USBR, 1950). These zones were grouted with curtain holes spaced on 5-foot centers. The siltstones and shales are generally weaker rocks forming some of the strike valleys. These materials have generally lower hydraulic conductivity due to the content of silt and clay and the plasticity of these constituents. Several zones of “soft materials” were encountered in the foundation cut-off wall excavations for Horsetooth Reservoir and were over-excavated and backfilled (USBR, 1950). The Lykins Formation is estimated to be approximately 800 to 1,000 feet thick in the Colorado- Wyoming area (Broin, 1957). The upper 500 to 700 feet of the formation is the Red Hill shale that contains predominantly red shales, siltstones and sandstones and only a few thin limestone layers. The Lykins Formation has been the subject of several geotechnical investigations at Horsetooth Reservoir involving reservoir seepage through one or more of its limestone, gypsum, or anhydrite sub-units contained in the lower 250 feet. The southern end of the North Fork fault is mapped as intersecting the dam axis in the right valley section (Braddock, 1988). The displacement of this fault may have caused enough offset of the previously described rock units to have removed the lower problematic units of the Lykins from the dam foundation. Shear zones associated with the fault may cause additional issues regarding the construction and performance of the proposed dam, such as poor rock quality and potential seepage paths through the foundation. Additional information concerning the foundation bedrock conditions will be needed to evaluate these issues and finalize designs of a grouting program and other foundation treatment precautions. Another fault mapped at the site involves the left abutment foundation bedrock and may be related to landslide deposits occurring in that area.