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I I I I I I I I I I I I, I I 1 I 1 II I -20- layer of vegetated topsoil. approximately 6.2 feet per second. The fixed capacities and structural aspects of the reservoir embankment give rise to the need to recognize the limitations associated with these facilities. That is, certain natural events may occur which would exceed the planned capacity of the reservoir. Detailed design and strict adherence to construction specifications must be made to minimize the possibility of experiencing the disastrous hazards of dam failure under probable flood conditions. Regular and routine inspection and maintenance of the embankments should be made to assure intended operation. Van Bl'bber Creek J'oins the main stream in The North Tributary to this reach. Hyatt Lake provides valuable detention storage to both this tributary and the main stream of Van Bibber Creek. A flood storage agreement should be obtained to perpetuate this storage. With the construction of the proposed reservoir and the assurance that flood storage in Hyatt Lake will be maintained, the 100-year flood plain widths will be reduced from maximums of 700 feet to 1,500 feet down to less than 100 feet. Because detention facilities, such as the one proposed as a part of this project, have inherent limitations, there is no assurance that by constructing this facility, areas downstream from the dam and presently in the flood plain, will not be subject to flooding. To assure no development in these areas, property presently located within the 100-year floodway should be acquired along all open areas between West 58th Avenue and the proposed reservoir. The total area of the property to be acquired would include approximately 90 acres. No conservation pool or permanent lake is proposed for this facility. Approximately 7 to 10 days would be required to drain the reservoir following a 100-year flood. Byoonstructing the proposed detention facility, the 100-year flood plain widths below the proposed facility are reduced from maximums of 300 feet to 500 feet to less than 100 feet. The existing 11' - 14' - 11' x 6' reinforced concrete box culvert under Ward Road will pass the 100-year discharge of 350 cfs. West 58th Avenue to Confluence with Ralston Creek: (Sheet 17 to 18) Beginning above Station 60+00, it would be necessary to regrade the existing channel so that a portion of the flood flows will be diverted into a proposed 66" reinforced concrete pipe along the south side of West 58th Avenue. The remaining flows will follow the existing Van Bibber Creek channel along the north side of l'lest 58th Avenue. The 66" RCP must include the construction of a slope tapered inlet and approximately 1400 feet of conduit. Riprap will be needed at the outlet for energy dissipation. The 66" RCP will have a hydraulic capacity of 300 cfs. There remains the potential for greater floods, up through the 100-year event of approximately 1000 cfs. The post improvement 100-year event will continue to flood the area north of West 58th Avenue, between Oak Street and the easterly crossing of 58th and the creek. The next major obstruction to flood flows is the Juchem Ditch crossing located at Station 107+00. There are presently no facilities at this crossing which will separate flood flows from irrigation water. The master plan drawings call for the construction of a 108-inch corrugated metal pipe under Juchem Ditch. The upstream channel above Juchem Ditch should be regraded for a distance of approximately 800' so as to match the pipe. In order to meet these grade requirements, one drop structure will be required in this reach. The channel cross-section through this reach should typically be approximately 20' wide at the bottom with 6:1 side slopes with a minimum dpeth of 3 feet deep. Velocities during the 100-year flood with this channel cross-section will be