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<br />"Selection of a base condition also should reflect our traditional <br /> <br />concern for economy. Modification costs in the vicinity of the scale <br /> <br />of improvement identified as the base condition should be examined for <br /> <br />sudden increases in the cost/scale of improvement relationship. This <br /> <br />type change could occur, for instance, when a costly highway reloca. <br /> <br />tion is encountered near the scale of improvement identified as the <br /> <br />base condition. An adjustment in the base condition may be warranted <br /> <br />in some cases. On the other hand. the large increase in costs may be <br /> <br />justified if a significant reduction in the hazard with versus without <br /> <br /> <br />dam failure is achieved." <br /> <br />Using this guidance, it is possible that a design goal less than that called <br /> <br />for by the two preceding criteria could be selected, with some acceptance of <br /> <br />risk, if there is a rapid increase in costs without a significant reduction of <br /> <br />risk. On figure 2, the dam raise alternatives of 9 and l5 feet avoid downstream <br /> <br />Cherry Creek damages up to 82 and 94 percent, respectively, of the PMF while a <br /> <br /> <br />dam crest raise of 19 feet will completely eliminate downstream damages. <br /> <br />PERFORMANCE RELIABILITY <br /> <br /> <br />For spillways located in the dam embankment, there remains a question of the <br /> <br />integrity of the embankment that does not exist for spillways sited more <br /> <br /> <br />remotely. Sound dam safety practice would avoid this option if there are <br /> <br />economical and practicable alternatives. On the basis of this performance <br /> <br /> <br />factor, a chute spillway in the embankment and hardening the dam face are not as <br /> <br /> <br />acceptable as the other alternatives that involve more remote spillway siting. <br /> <br />ENVIRONMENTAL ACCEPTANCE <br /> <br /> <br />The environmental impacts of each class of alternatives is discussed in the <br /> <br />preceding section. For Castlewood Dam, there are serious negative impacts <br /> <br /> <br />associated with animal and plant species in the pool area. Widening the present <br /> <br />spillway would impact negatively on the existing wildlife habitat within the <br /> <br /> <br />spillway channel and on the adjacent spoil banks. No significant negative <br /> <br />impacts are identified with the other alternatives. <br /> <br />66 <br />