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<br />002HO <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 500,000 acre- feet <br />of surcharge storage space above the normal maximum water surface of 3 700 feet. While this is a <br />large amount of reservoir space, it is small in comparison to either the 7.8 MAF April - July inflow <br />or the 2.1 MAF forecast error term for June I (5 percent exceedence level). A buffer of this size <br />would affect primarily moderately high years in which bypasses were on the range of several hundred <br />thousand acre-feet. Such bypasses could be reduced or eliminated entirely by storing the excess <br />inflow behind the gate extensions until it 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 I <br />in 100 years. These are the types of years which would produce releases in excess of 45,000 cfs, <br />perhaps for an extended period of time as occurred in 1983. The volumes of bypasses in these types <br />of years are very large, 3.4 MAF in 1983 and 1.0 MAF in 1984. The greatest determining factor in <br />the amount of bypass is the forecast error associated with high inflow years. <br /> <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 could <br />be released through the outlet tubes in 3 to 25 days, thus limiting total releases to 45,000 cfs or less. <br />During these types of years, it would be very unlikely that use of the spillways would be required. <br />It appears from this discussion, that only inflow years' with a return period of about I in 100 years <br />would force the use of the spillways and release more than 45,000 cfs. Reclamation believes that <br />current operating practices would initiate high powetplant releases and bypasses early enough as <br />required to safely operate the dam, thus meeting the intent of the GCDEIS provision without <br />requiring either the additional storage buffer or the spillway gate extensions. <br /> <br />The positive value of the spillway gate extensions <br /> <br />Although the extensions are not required to limit spillway use to the I in 100 year return period cited <br />in the GCDEIS, some limited value can be gained from their installation during years in which peak <br />releases would be less than 45,000 cfs. In these cases, if the total bypass volume was expected to be <br />750,000 acre-feet or less, then the entire expected bypasSes could be stored behind the extensions and <br />. , <br />released later in the summer. This might produce s<?me enviromnental benefits by not releasing <br />greater than 30,000 cfs if such releases would cause ecological harm. <br /> <br />However, it would also carry the dam safety risks associated with purposefully storing more water <br />in the reservoir than was assumed during the design of the spillways. If an extremely rare high inflow <br />event occurred, it could conceivably overtop the dam, even with full use of the spillways. <br /> <br />Proposed Recommendation for the Spillway Gate Extensions <br /> <br />As a result of the limited value of the extensions in controlling extremely high runoff years, the ability <br />to control more common inflow events without the use of the spillways, and the risk associated with <br />