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002553 Historic Characteristics of Powerpiant Bypasses As a result of the concern in the late-1980's over continued spills, Reclamation extensivelv analyzed the causes, frequency and magnitude of spills. The period of forecasts and operations from 1966 through 1989 was used to model these three characteristics. We found that several parameters of reservoir operation most significantly affect spills. in the following orde~ of significance: the aggressiveness with which forecast changes are incorporated into monthly release schedules. the target storage levels each July 31, and the initial reservoir storage conditions each January I, Reclamation developed a simple operation model which allowed these variables to be altered and predicted the results in terms of monthly release patterns and spills, We found that as of 1986 the long term frequency of spills using the pre-GCES operating practices was about I year in 4. Moving from a frequency of 1 in 4 to I in 20 required changes to historic practices. The largest change was in the manner in which forecast changes altered monthly release patterns during the winter and early spring. Two of the primary reasons that the dam spilled as much as it did during the 1980's was the purposeful scheduling of (1) releases close to powerplant capacity during June and July and (2) storage near to the capacity of the reservoir at the end of July. Tbis practice minimized the operational flexibility during the peak of the spring runoff to accommodate the forecast increases which occurred in 1983 - 1986. By/adopting a July 31 target storage buffer of about 0.5 MAF and by incorporating forecast increases as soon as possible into the current month's releases, the frequency of spills was reduced. The historic spills which occurred in 1985 and 1986 could thus be eliminated and the , . those of 1983 and 1984 could be reduced. Another important factor in this analysis was the uprating of the Glen Canyon Dam generators which occurred in the mid- 1980's. increasing the powerplant releases capacity to 33,200 cfs from about 27,000 cfs. Tbis increased capacity is not fully available due to effects of reservoir head and transformer and generator operating factors. A more practicable capacity may be closer to 31,000 cfs, Tbis additional release capability increases the ability to avoid spills. and thus reduces the frequency and magnitude of such spills, Modeling the Risk of Spills from Glen Canyon Dam To further understand how spills occurred, we once again used the computer model developed in 1987 that Reclamation had used in initially reducing spill frequency, We updated the storage capacity of Lake Powell and the target storage levels for January 1 and July 31 to account for the recent sediment survey, and extended the data set through 1997. Upstream reservoirs were modeled according to recent constraints placed on them by consultations.,urider the,Ehdangered Species Act. Tbis resulted in scheduled high spring flows at the Flaming Gorge, Aspinall. and Navajo Dams. Glen Canyon Dam was modeled with an initial release pattern that scheduled higher t10ws in January and February and lower flows in April and 4