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00255Q Repon of the Spike Flow Subgroup Glen Canyon Dam Technical Work Group October 27, 1997 Introduction A provision for a Beach/Habitat Building Flow (BHBF) was included in the preferred alternative of the Glen Canyon Dam Final EIS, As discussed in the GCDEIS on page 40, the BHBF would involve releases in excess ofpowerplant capacity and would be considered in low reservoir storage years to rejuvenate beaches and backwater areas, However, in the Record of Decision on the EIS, the Secretary of the Interior determined that the objectives of the BHBF were to be accomplished in high reservoir storage years using releases in excess of powerplant capacity required for darn safety purposes. Whereas the ROD established the framework for implementing BHBF's, it left unresolved the technical criteria for determining whether sufficient risk of releases above powerplant capacity existed to schedule a BHBF. It did not specify what risk of a spring spill needed to exist in order to implement a BHBF in the Marchi April timeframe currently favored by river resource managers, The purpose of this subgroup exercise is to evaluate alternative spill avoidance operations and risk thresholds and recommend specific criteria for determining when a BHBF can be prescribed as a part of spill avoidance operations. In this report we describe (I) the evolution of thinking regarding the role of spills in downstream resource management, (2) historic characteristics of powerpIant bypasses, (3) how spill risks from Glen Canyon Darn are modeled. (4) alternative BHBF decision criteria. (5) a recommendation for BHBF "triggering.' criteria. and (6) a recommendation for additional studies. In the following repon several terms are used to describe powerplant bypasses from Glen .Canyon Dam. Unavoidable bypasses are described as "spills" or '.flood flows" and usually occur in the May through July time period as the reservoir storage nears the full level. Intentional bypasses in anticipation of a high risk of unavoidable bypasses have been described in the past as a .'spike flow" but are herein labeled as a BHBF. A test of such a BHBF was conducted in Marchi April 1996. Following page 9 of this report are the two graphs and the table referred to in the text. An appendix containing the three sets of computer runs described in the alternative BHBF decision criteria section [ollows these graphs and table.