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<br />, : <br />002436 <br /> <br />Glen Canyon Dam Spillway Gate Extensions <br />An Analysis of Their Need and Their Impact on Spill Frequency and Duration <br /> <br />Introduction <br /> <br />The two spillways at Glen Canyon Dam are large tunnels bored through the abutment sandstone <br />on either side of the dam and are controlled by a total off our 40- by 52.5-foot radial gates, two <br />on each side of the dam. The gates are raised and lowered through a powered cable system <br />located on the top of the dam. Figures I and 2 are plan and cross section views of the gates. In <br />contrast to the approximate 45,000 cfs combined capacity of the powerplant and outlet tubes, the <br />spillways can discharge about 210,000 cfs when fully opened with the reservoir at elevation 3700 <br />feet. <br /> <br />This large release capacity was designed as a critical part of the dam's capability to accommodate <br />large inflows. Because of the very large surface area of Lake Powell and the long duration of the <br />spring runoff from snowmelt, most extreme inflow events can be handled through anticipatory <br />p-owerplant releases in the months prior to the peak inflow. It was primarily the large scale <br />general storm events that require the use of the spillways. These and other large hydrologic <br />events produce inflows that challenge even the large capacity of Lake Powell. Inflows of the <br />magnitude of the Probable Maximum Flood (pMF) would produce peak inflows of about 700,000 <br />cfs, resulting in discharges exceeding 200,000 cfs for a 9-day period of time and in a maximum <br />reservoir elevation of about 3710 feet. Fortunately, the statistical frequency of such events is <br />extremely rare, perhaps only on the order of once in 10,000 years or longer. <br /> <br />The spillways were also designed to playa role in normal operations during high inflow years <br />when there was also a large error in the forecasted runoff, such as 1983. Unanticipated extreme <br />inflows could not be accommodated with the anticipatory powerplant releases cited above. There <br />simply isn't enough time in those situations to evacuate sufficient reservoir storage space. Use of <br />the spillways enables the safe passage of the peak inflows without overtopping the dam. <br /> <br />Since large dam releases have significant impacts on downstream resources, previous documents <br />such as the GCDEIS contained recommendations on restricting the frequency oflarge releases <br />above powerplant capacity, citing two options for controlling such releases. The ROD for the <br />GCDEIS selected the option of installing spillway gate extensions rather than the option of <br />providing a greater vacant storage space buffer to reduce the frequency of powerplant bypasses. <br />The Grand Canyon Protection Act (GCP A) also addressed powerplant bypasses and their impact <br />on the Grand Canyon. <br />The installation and use of these spillway gate extensions are the focus of this report. As in the <br />recent Beach Habitat Building Flow (BHBF) triggering criteria report, the report will include <br />discussions on the GCDEIS conclusions regarding spills, the recent change in thinking about <br />spills, and the agreement reached between the Secretary of the Interior and the Basin States <br />