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. . . I . I . . . . . ., . I . I . -19- The existing 66" CMP at Indiana Street should be replaced with a triple 14' x 5' reinforced concrete box culvert. Dikes are also required to protect residences north of the channel, west of Indiana and south of the channel east of Indiana. The proposed embankment cross-section, provided by R. W. Thompson, Inc., Geotechnical Consultants, is based on a preliminary feasi- bility investigation of the suitability of the site for dam construction. The preliminary recommendations call for an embankment with maximum of 4:1 slopes upstream, 3:1 slopes downstream, a 12 foot minimum top width and a minimum 10 foot deep cutoff trench with a 12 foot base width. A 24 foot top width is proposed for the dam which will facilitate easier maintenance access and permit the embankments to be landscaped and provide a more multi-purpose use. An embankment drain and toe drain would be incorportated into the embankment. The upstream face of the dam should be protected with a minimum of l2-inch dumped riprap approximately l8-inch to 24-inch thick and underlain by 12 inches of gravel filter material. To avoid unstable foundation conditions, it may be necessary to stabilize and compact the foundations prior to construction of the embankments. The report, "Preliminary Feasibility Investigation for Flood Control Embankments, Van Bibber Creek, Arvada, Colorado", should be consulted for further information on the soil conditions of the sites. A copy of this report is on file at the office of the Urban Drainage and Flood Control District (Ref. 4). McIntyre Road crossing improvements, the 100-year flood peak will pass over the existing Farmer's Highline Canal siphon. The existing 36" RCP under the Croke Canal should be replaced with diversion structure similar to the one proposed for the Church Ditch. This diversion structure should be designed to allow only the decreed water right to be conveyed by the ditch. Excess flows should be diverted back into the Van Bibber Creek channel. Although the Croke Canal has a decreed water right of 944 cfs with a conditional decree to approximately 2,000 cfs, the channel is presently capable of conveying only 500 cfs. The diversion should be designed based on a ditch flow of 500 cfs. Of major importance to the downstream reach of this prolect is the proposed reservoir facility, to be located approximately 3,2PO feet west of the Ward Road. This proposed structure will be located in a topographically constricted area along the lower portion of the Van Bibber Creek main channel. The proposed embankment structure would be approximately 30 feet high to elevation 5510, yielding a storage capacity of 870 acre- feet at the spillway elevation of 5507.5. The embankment will be approximately 1,700 feet long with a 36" RCP outlet pipe and approximately 2.5 feet of freeboard for the 100-year flood event. The 100-year outlet discharge would be approximately 185 cfs. An emergency spillway would be constructed around the north abutment of the dam. Approximately 120 acres of right-of-way would be required for the dam and reservoir at the 100-year flood pool elevation. A portion of this area has already been acquired with Jefferson County open space funds. No detailed calculations of the probable Maximum Precipitation (PMP) event have been completed for Van Bibber Creek. However, based on studies of other streams, the probable maximum flood discharge might range from 50,000 cfs to 75,000 cfs. Due to the physical restraints of the location, a means of providing a spillway for this event, as would be required by present State Engineer criteria, would be to armor plate the downstream face and top of the embankments to withstand overtopping during such a flood. For the purposes of this report, the preliminary design calls for the construction of the downstream face and the crest of the dam to include a l2-inch layer of granular filter material covered with l2-inch gabion slope mattress covered with an 18-inch 1\ .