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<br />002439 <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 greater <br />in magnitude than the highest expected future release. This not only moves sediment higher on <br />beaches away from future releases, but also coarsens the main channel bed which reduces future <br />sediment transport. <br /> <br />The occurrence of high release years is fundamentally tied to the statutory operation of Lakes Powell <br />and Mead. Reservoir equalization and Upper/Lower Basin consumptive uses all affect the cyclic <br />drawdown and refilling of Lake Powell. High runoff years when the reservoir is full produces high <br />powerplant releases and increased risk of powerplant bypasses. <br /> <br />Some sediment experts believe that there is sufficient regeneration of main. channel sediment supplies <br />to allow BHBF's in all years that such events would be allowed by the 1996 agreement, even every <br />year if possible. Longer duration spills may have different effects than the short duration BHBF's, <br />so additional sediment transport modeling would help clarifY the allowable frequency of such spills. <br /> <br />The desired magnitude ofBHBF's <br /> <br />At a BHBF symposium held in April 1997, many researchers expressed the opinion that, while the <br />45,000 BHBF of April 1996 achieved some of the intended results, periodic higher releases would <br />be helpful to scour non-native vegetation, rework backwater areas, and deposit sand high enough on <br />the beaches to be less vulnerable to succeeding flows. Estimates of 60,000 to 90,000 cfs were <br />discussed as appropriate flow levels for these purposes. <br /> <br />Additional modeling and analysis should be devoted to this question of the magnitude of high <br />releases. This should address both the short duration BHBF's and the potentially longer duration <br />uncontrolled powerplant bypasses. <br /> <br />GCDEIS expectations related to spillway gate extensions <br /> <br />One of the GCDEIS conclusions was to reduce the return period of bypasses above 45,000 cfs to a <br />long term average of not more than I in 100 years. Thus, releases below 45,000 cfs were allowed <br />as part of normal operations, but the extreme spillway releases such as occurred in 1983 were <br />essentially forbidden. This threshold level of 45,000 cfs seemed to indicate that flows below that level <br />were acceptable from an ecological perspective while higher flows were deemed too damaging. It <br />is interesting to note that the current opinion of at least the sediment researchers is just the opposite. <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, such <br />releases greater than 45,000 cfs could be avoided. Since it uses a monthly time step, the CRSS model <br />could not really estimate the peak bypass release other than to average the release over the month in <br />which it occurred. Thus some judgment was used in estimating the frequency of releases greater than <br />45,000 cfs. <br />