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<br />. <br /> <br />. <br /> <br />. <br /> <br />Executive Swnmary . BHBF Triggering Critena <br />TWG Subgroup <br />December 10. 1997 <br /> <br />,ApprQue/1 <br />~ <br /> <br />The TWG has agreed to forward the BHBF subgroup triggering criteria recommendations to the <br />AMWG. The recommendations are intended to be clear and straightforward, but with this <br />complex an issue, a short summary of the criteria and their implications will improve <br />understanding. <br /> <br />Criteria <br /> <br />The criteria consist of two triggers or decision mechanisms that would allow a BHBF to be <br />released if then deemed appropriate from an environmental perspective. Provided either of the <br />following release criteria is met, the decisiOll to release a BHBF would be based on the expected <br />benefits or impacts to downstream resources. These triggers are intended to be in agreement <br />with the Secretary of the Interior's decision contained in the 1996 AOP, that of complying with <br />both the 1968 and 1992 Acts. The two triggers are: <br />~ <br /> <br />I - If the January forecast for the Jan~ - July unregulated spring runoff into Lake Powell <br />exceeds 13 MAF (about 140 percentofnomlal), or <br />~ Zl.;:~+( ~ ,0" 1:,;) <br />2 - anytime a Lake Powell inflow forecast would require a powerplant monthly release greater <br />than1.5MAF. 'l..5,.'v <-I'> ~.~r'jl- .."".-I-hl,d flow. <br /> <br />tJec/;s><:'J~ '.v....""h..D ,:." ~~<:l J~ #>/ _____./;/ ,-,-c/ /:....cc-~,$.I a-.-Ir!J-.I <br />Either of these triggers implicitly recognizes that there is a significant risk of an uncontrolled <br />spill during the peak of the spring runoff. The triggers were detennined such that the magnitude <br />of this risk was agreeable to all parties on the BHBF subgroup. Regardless of the decision to <br />release a BHBF, releases from Glen Canyon Dam would be expected to exceed 25,000 cfs, <br />perhaps for an extended period of time. <br /> <br />Implications <br /> <br />1 - Such a BHBF would not materially reduce the risk of future spills later in the year since the <br />volumes ofBHBF's are relatively small (about 150,000 AF), but would accomplish the goal of <br />moving sediment from the main channel to tile side channels and eddies. <br /> <br />2 - The value of 140 percent was chosen as tile threshold value in an attempt to balance the <br />number of false alanns (BHBF's that after-the-fact were not actually required) with the number <br />of missed spills (actual spring spills that were unforeseen earlier in the runoff season). <br /> <br />3 - Using the above triggering criteria, BHBF's could occur any month in the January - July <br />period. Our analysis shows that when a purposeful BHBF occurs, about 30 percent occur in <br />Jani;ary, 30 percent in March, and 40 percent in June. January BHBF's occur as the result of <br />extreme snowpacks early in the winter such as in 1984. March BHBF's occur due to high runoff <br />years that accumulate snowpack in a normal manner such as in 1986. June BHBF's occur when <br />late wet and cool springs bring large forecast increases such as in 1983 but these large forecast <br />increases do not necessarily occur only in high runoff years. BHBF's occur in the month in <br />which tile triggers were met. <br />