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<br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />Canyon Boulevard, thereby lessening the damage potential to the private <br />properties located north of Canyon Boulevard. A typical cross section (Sheet 16) <br />indicates the level of protection provided by the berm. <br /> <br />from the 100-year floodplain. Shallow flooding areas are designated where the <br />floodwaters diverge from the effective conveyance area of the overflow section <br />and seek the lowest levels where ponding occurs until the floodwaters recede. <br />Other areas of shallow flooding are noted where floodwater may inundate depressed <br />areas outside the designated floodplain. <br /> <br />At 6th Street, the flood waters become confined by the 6th Street bridge and <br />the Boulder County Justice Center located just upstream. This constriction <br />of flows will produce considerable backwater effects during the occurrence of <br />the 100-year flow. Plans are currently underway to provide flood protection <br />for the Justice Center. Upstream of the Justice Center, the channel grade <br />continues to steepen and the floodplain becomes confined between Canyon Boulevard <br />and Arapahoe Avenue, producing canyon-like flooding for the remaining study reach. <br />At the Arapahoe Avenue crossing, flooding waters overtop the road grade east of <br />the bridge, less~ning the backwater effects of the low-capacity bridge. <br /> <br />The typical cross section shown on Sheet 15 represents the confinement of flows <br />due to the Crossroads Mall floodwall. <br /> <br />Arapahoe Avenue Overflow <br /> <br />The Arapahoe Avenue Overflow Study Reach begins at its confluence with the main <br />stem at Cross Section No. 16.1, and ends just upstream of Folsom at Cross Section <br />No. 36. Floodplain limits for this study reach are shown on Sheets 5 to 9, and <br />the water surface profile is shown on Sheets 14 and 15. The Arapahoe Avenue <br />profile is shown as the reference line with stationing beginning at the inter- <br />section of Arapahoe Avenue and 47th Street (Foothills Parkway) and ending just <br />upstream of Folsom. Cross sections are also identified separately to simplify <br />user interpretation. <br /> <br />The flow splitting assumptions at the upstream limit of the overflow reach <br />results from the limited-capacity Folsom Bridge passing only 60 percent of the <br />total discharge, with the remaining flow overtopping the street grade. Nearly <br />5,000 cfs will overtop Folsom, assuming a 25-percent blockage condition. Of <br />this, an estimated 3,500 cfs will become hydraulically disconnected from the <br />main channel and contribute to the Arapahoe Avenue Overflow flooding. The <br />remaining 1,500 cfs will be returned to the main channel between buildings <br />located east of Folsom and north of the creek channel. At 28th Street, an <br />additional 700 cfs will be contributed from the main stem to the overflow area <br />from between the new Safeway Store and the stl'eet grade, making the total flow <br />on Arapahoe Avenue 4,200 cfs. At the lower portion of the overflow reach, <br />west of the Arapahoe Avenue bridge, the energy level exceeds the main stem <br />profile and flood waters will have a gradual tendency to return to the main <br />channel. Twenty-seven hundred cubic feet per second was assumed to be returned to <br />the main channel before reaching the Arapahoe Avenue bridge. The remaining <br />1,500 cfs will be returned to the channel through streets and floodplain areas <br />north of Arapahoe Avenue. <br /> <br />A typical cross section for this reach is shown on Sheet 17. <br /> <br />Flooding in this reach affects numerous commercial properties. Special considera- <br />tion was given to the flood protection works currently under construction for <br />the Crossroads Mall area, between 28th and 30th Streets, north of Arapahoe Avenue. <br />The flood profile through this reach reflects the floodwall in place, representing <br />the confinement of flows to Arapahoe Avenue and the floodplain area to the south. <br />The floodplain delineation also credits the floodwall by removing the parking lot <br /> <br />Floodplain and Floodway Data <br /> <br />The Flood Hazard Area Delineation Maps show the outline of the 100-year flood <br />event for present channel and floodplain conditions. The maps are supplemented <br />by Table II, "Floodplain and Floodway Reference Data". <br /> <br />13 <br />