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(.I) ~ '- \ as for the Sand Creek reservoir. The 100-year, standard project, and probable maximum floods were selected for analysis and the storage requirements, benefits, and incremental costs were developed and compared. A Toll Gate Creek reservoir capable of storing the standard project flood volume was found to gener- ate the maximum net tangible benefits; however, because of the significant intangible benefits to be derived from greater amounts of storage in the project, Toll Gate Creek reservoir was sized to store the probable maximum flood below the spillway crest. The gross storage of the reservoir would be 34,300 acre- feet and the spillway crest would be perched 26 feet above the standard project flood pool. f. ~d Creek channel improvements. As in the Cherry Creek basin, the need for safeguarding the operational capa- bilities of the Sand Creek and Toll Gate Creek reservoirs was given primary consideration in formulating a scale of develop- ment for the Sand Creek basin channel improvements. The resi- dual flood hazards which would remain even with Sand Creek and Toll Gate Creek reservoirs operating were also considered. For the reaches downstream from the dams, three design discharge levels were analyzed as potential designs for the channel improvement works. These discharge levels ranged from a minimum of that needed for reservoir operation to a maximum which was equivalent to the standard project floodflow. A discharge level equivalent to the I-percent floodflow was also analyzed. The results of these studies indicated that with a design dis- charge approximately equal to the I-percent flood peak, the net tangible benefits attributable to the channel work would be maximized. When consideration was given to the significant amount of intangible benefits to the developed areas along Sand and Toll Gate Creeks, however, it became apparent that a channel improvement designed to pass the standard project flood would be desirable and warranted. 29 . SUMMARY The hydrologic interrelationship of the Sand Creek and Toll Gate Creek reservoir and the Sand Creek channel improvement dic- tates that they be considered an integral unit. The Sand Creek r~servoir was formulated without a permanent recreation pool and with a perched spillway to control the standard project flood volume plus 36 percent of the probable maximum flood volume. The Toll Gate Creek reservoir was also formulated without a permanent recreation pool and with a perched spillway. The channel improve- ment works on Sand and Toll Gate Creeks are provided essentially to assure operation of the reservoirs, but were enlarged where 54