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I I I I . I I I I I I I I I I I I I I would inundate a portion of Grange Hall Creek and the northeast tributary above the reservoir, it was necessary to determine if Carpenter Dam was or was not a viable flood control facility and complete the Phase B study on that basis. A special study on the flood control feasibility of Carpenter Dam and Reservoir was made at the request of Thornton and U.D. and F.C.D. An economic evaluation of Carpenter Dam and Reservoir as a flood control facility and as a raw water storage facility was made and the results presented to U.D. and F.C.D. and the City of Thornton in a letter report dated July 13, 1977. Table IV-I presents a summary of the costs associated with three alternatives: (I) the City of Thornton buys into Blunn Reservoir at a one time cost of $1,600 per acre-foot; (2) a total of 2, I 00 acre-feet of raw water is bought or rented on a yearly basis at $100 per acre-foot; and (3) the costs of water supply and flood related costs with Carpenter Dam constructed. The results of the study indicate that if Carpenter Dam was constructed the flood control benefit as a percent of the total cost would be about 14.9%, assuming a turnover of 2, I 00 acre-feet of raw water a year in the foci Iity. The City of Thornton and U.D. and F.C.D. concluded, after reviewing the results of the separate study on Carpenter Dam, that no further investigations into the feasibility of Carpenter Dam were necessary. In addition, the improvements in the areas of the Grange Hall Creek drainage basin, potentially affected by Carpenter Dam if constructed, should be designed and incorporated into the Phase B study assuming Carpenter Dam is not constructed. Due to the direction given in relation to Carpenter Dam the improvements have been designed assuming Carpenter Dam wi II not be constructed and no further consideration of the dam and reservoir is made in this report. F. Improvement Costs For the Phase A portion of the Major Drainageway Planning Study a cost estimate of the improvements for each alternative was prepared. These net values were used in a benefit/cost analysis to determine the economically best alternative plan. The selected alternatives for the Phase B study have been indicated for each reach in the Synopsis of this report and d,~tcJiled descriptions of the i'nprovements have been made in this chapter. A detailed quantity take-off was prepared from the master plan drawings presented in Volume II and previously discussed in this chapter, ~ection D. A set of unit construction cost values has been i'lcluded as T ohle IV-2 for the Denver Metro area. The data reflects the best estimate of the cost for the summer of 1977. The cost estimate for each improvement reach was divided into the following categories: channel improvements, street crossing improvements, right-of-way costs, utility relocation costs, contingency costs, and operation and maintenance costs. A fifteen percent contingency was applied to the su,n of all the costs identified except the opel .jfi )ft rJ:ld ..jujn1ennn'"'I~ ('os1s. The su,nrlluf,' of all the costs for each reach is presented in Table IV-3. A'l ilemized cost estimJle for each reach is 011 file with the U.D. and F.C.D. as an appendix to this report. The cost tigures presented ill this report are applicable for the summer of 1977. If the projects described herein are to be scheduled for future constrUction the appropriate inflationary factor should be applied. For the purpose of future plunning the evaluation of the present worth of the drainage improvements is shown in the following table based on an 8% inflation rate. A 6 3/8% interest rate and a 50 year project life was used to determine the present worth of improvement costs. 22