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I I I I I I I I I I I I I I I I I I I Selection of Alternatives In Phase A of the Little Dry Creek drainage study five alternative plans of drainage improvement were developed and their costs and benefits were evaluated. Following completion of the Phase A study, the local govern- ment entities together with the Urban Drainage and Flood Control District selected a plan for development of preliminary designs and cost estimates under the subsequent Phase B study. The drainage plan selected was Plan E. It consists of: 1. Rehabilitating Englewood Dam to meet current flood detention dam standards. 2. Constructing Holly, Berry, Prentice, Blackmer, and Quincy flood detention dams. 3. Improving the present entrance reach of Cinderella Conduit by adding a l2-foot high by 4-foot wide conduit section to in- crease its capacity to 4,700 cubic feet per second. 4. Extending Cinderella Conduit by adding a l2-foot by l8-foot, 4,700 cubic foot per second capacity conduit section from the present conduit entrance some 1,900 feet upstream to Sherman Street. 5. Channelizing reaches of Little Dry Creek above and below Cin- derella Conduit, in Englewood. 6. Channelizing stretches of Greenwood Gulch and Little Dry Creek in Cherry Hills Village. All flood detention dams are designed to have spillway capacities large enough to pass the probable maximum precipitation, or PMP, flood without overtopping or otherwise endangering them. The flood plain limits and all major channel improvements and conduits are designed for the 100-year re- turn period flood. This is the flood flow that has a one percent change of occurring in anyone year. As previously described, the lOO-year flood flow was for the estimated future development of the Little Dry Creek water- shed. Two flood plains are shown, one with the proposed improvements (Plan E) completed and one under existing conditions with only Englewood Dam in operation and with unimproved channel conditions. -49-