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<br />002434 <br /> <br />not be required for hydrologic reasons) to years of high reservoir conditions and high inflows. <br />Thus a BHBF would occur in years when there was an expectation of having a hydrologically <br />induced spill. This agreement was institutionalized in the 1996 Annual Operating Plan for the <br />Colorado River, signed by the Secretary of the Interior in December 1995. A subsequent BHBF <br />test was conducting in April 1996, confirming the hypothesis that high flows could rebuild <br />sandbar deposits. In December 1996, the GCDEIS Record of Decision was signed by the <br />Secretary of the Interior and included this modification of the preferred alternative. <br /> <br />Impacts of Using Spillway Gate Extensions <br /> <br />GCDEIS expectati?ns related to spillway gate extensions <br /> <br />The CRSS modeling which formed the hydrologic basis for many of the GCDEIS decisions <br />determined that bypasses were rare events, and if a small amount of buffer space were provided, <br />such releases greater than 45,000 cfs could be avoided. Since it uses a monthly time step, the <br />CRSS model could not really estimate the peak bypass release other than to average the release <br />over the month in which it occurred. Thus some judgment was used in estimating the frequency <br />of releases greater than 45,000 cfs. <br />t <br /> <br />.~ <br /> <br />The limited value of the spillway gate extensions. <br /> <br />The GCDEIS commitment to install the 4.5-foot extensions would produce about 750,000 acre- <br />feet of surcharge storage space above the normal maximum water surface of 3700 feet. While <br />this is a large amount of reservoir space, it is small in comparison to either average April - July <br />inflow which is about 7.8 MAF or the 2.1 MAF forecast error term for June I (5 percent <br />exceedence level). A buffer of this size would affect primarily moderately high years in which <br />bypasses were oil the range of several hundred thousand acre-feet. Such bypasses could be <br />reduced or eliminated entirely by storing the excess inflow behind the gate extensions until it <br />could be released through the powerplant. <br /> <br />Inflow volumes of extremely high inflow years such as 1983 or 1984 had return periods of about <br />I in 100 years. These are the types of years which would produce releases in excess of 45,000 <br />cfs, perhaps for an extended period of time as occurred in 1983. The volumes of bypasses in <br />these types of years are very large, 3.4 MAF in 1983 and 1.0 MAF in 1984. The greatest <br />determining factor in the amount of bypass is the forecast error associated with high inflow years. <br />In contrast, moderately high inflow years such as 1985, 1986, and 1995 would cause bypasses of <br />about 100,000 to 800,000 acre-feet using current operating practices. These bypass volumes <br />could be released through the outlet tubes in 3 to 25 days, thus limiting total releases to 45,000 <br />cfs or less. During these types of years, it would be very unlikely that use of the spillways would <br />be required. <br /> <br />The need to reduce the frequency ofpowerplant bypasses <br /> <br />Current thinking among sediment experts is that, given high flow conditions resulting from large <br />runoff years, releases above 25,000 cfs should be preceded by BHBF's. The BHBF should be <br />greater in magnitude than the highest expected future release. This not only moves sediment igher <br />