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1 I I I I I I' I I I I I I I I I I -18- preliminary and is not to be used for construction purposes. reinforced concrete box culvert or be replaced with a double 8' x 6' reinforced concrete box culvert, either of which will be capable of passing the la-year discharge of 990 cfs. The existing 25' x 6.5' concrete flume crossing at the Church Ditch should be replaced with a 16' x 18' concrete box culvert which will allow the separation of the lOa-year flood from Van Bibber Creek from normal Church Ditch irrigation flows. Upstream Limit of Study to South Boulder Canal: (Sheet 2 to 6) Included in this reach are three street crossings and one irrigation ditch crossing. The la-year design discharges through this reach range from 890 cfs at the upper end to 970 cfs at the lower end. The existing structure at Foothills Road is an 18' x 5.5' bridge with approximately 100 square feet of flow area. This bridge is capable of passing the la-year flood and thus is considered to be hydraulically adequate. The existing bridges at Dunraven and Crestone Streets were constructed during the development of the Apple Meadows subdivision and are adequate to pass the lOO-vear discharge. These structures should also remain in place. The fourth major crossing along this reach is the South Boulder Canal which presently crosses Van Bibber Creek in a concrete flume. To separate the drainage and irrigation waters, a triple 12' x 4' concrete box culvert should be constructed under or as part of the flume, to pass the lOa-year discharge of 1,720 cfs on the Creek. Diking upstream of the culvert structure will also be needed. It will be necessary to regrade the existing channel invert approximately 700 feet upstream and downstream to properly accommodate this structure. Although the main channel is quite narrow through this reach, there is a large area to the south of the main channel located both east and west of Easley Road, which is susceptible to shallow flooding. The shallow flooding would result from overflows from both the main channel and the South Tributary. As this channel is capable of passing the la-year discharge without overflow, no channel improvements are proposed at this time. Should develop- ment pressures be placed on the shallow flooding area, however, a dike could be constructed along the south side of the channel to contin the lOa-year discharge and eliminate the shallow flooding problem and flood plain between the two streams. South Boulder Canal to Confluence with South Tributary: (Sheets 6 to 8) There are three major facilities which cross Van Bibber Creek in this reach. The first crossing, West 60th Avenue, presently consists of three 36" corrugated metal pipes. These pipes should be replaced with a double 13' x 5' reinforced concrete box culvert, capable of passing the la-year discharge of approximately 990 cfs. The second crossing, Easley Road, also known as Ulysses Street, presently consists of a 10' bridge with a clearance of approximately 6 feet. The existing bridge could be supplemented with a single-barrel 6' x 6' Confluence with South Tributary to West 58th Avenue: (Sheets 8 to 16) The Van Bibber Creek flood plain through this reach is generally quite wide ranging in width from approximately 300 to 1,000 feet. The inadequate crossings at the Farmer's Highline Canal, McIntyre Road, the Croke Canal, Indiana Street and the Juchem Ditch generate backwater conditions which cause the wide flood plain. In order to pass the lOa-year discharge of 2,820 cfs over the Farmer's Highline Canal siphon, certain improvements are necessary. First, it would be necessary to replace the existing 60-inch CMP at McIntyre Street with a triple 14' x 6' concrete box culvert. This structure would be designed to pass the lOa-year flood and lower the tailwater caused by the McIntyre Street. By combining the construction of a dike upstream from the Farmer's Highline Canal with the proposed 1\ I