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I I I I I I I I I I I I I I I I ,I I I 7.0 PROJECT EVALUATION 7.1 BENEFITS 7.1.1 OPTION 1 Downstream drainage construction Is designed to bring Hourglass Dam up to current safety standards and thereby prevent possible premeture deterioration of the structure from excessive seepage. The minimal spillway improvements and left groin protection will reduce potential damage from unusually severe storms and flood flows. These improvements are intended to permit the continued operation of the dam as it has been operated in the recent past. No additional oependablt! long term storage capacity is provided, and no reduction is expected In seepage. There is a potential benefit of a firm yield of 430 acre-feet of water per year. During the annual operation Of the rese!"\'oir, followinq completion of the Cetion 1 'NO"',, "'.]hr I'~veh of she' :"r." storage will be used to evaluate the need for continuing reservoir level restrictions. If the full storage short term capacity of the reservoir can be safely used, the full benefit will accrue to the Option 1 improvements. The resulting cost of firm yield would be $504,533 plus $40,000 (feasibUity study) divided by 430, or $1,266 per acre-feet. 7.1.2 OPTION 2 Partial depth slurry wall is presented as the minimum cost altemative that may be considered for construction to further improve dam safety and possibly to reduce seepage losses after completion of construction of the Option 1 improvements. A large seepage loss was found to occur at a relatively low reservoir level. Losses at higher reservoir levels will be higher. With partial depth cut off walls ranging upwards from one million dollars, additional seepage and other study will be required. The recently constructed measuring flume in the Hourglass Reservoir supply channel from Comanche Dam can be used during regular reservoir operations to estimate seepage losses from Hourglass Reservoir. 7-1