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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 />General descriptions of flooding conditions along South Boulder Creek are <br />provided in the following paragraphs. <br /> <br />shallow flooding. <br /> <br />Colorado and Southern Railroad to Baseline Road <br /> <br />Baseline Road to State Highway 93 (South Broadway) <br /> <br />This 1.4 mile study reach is characterized by moderate development in and <br />adjacent to the floodplain. Sheets 1 through 6 show limits of the 100-year <br />floodplain. Water surface profiles are indicated on Sheets 22 through 23 and <br />29A. <br /> <br />Between Baseline Road and State Highway 93 (South Broadway), a distance of about <br />3.4 miles, South Boulder Creek flows in a northeasterly direction through a <br />broad, relatively flat floodplain. With the exception of a few scattered dwel- <br />lings, existing land is used for pasture and open space. Sheets 7 through l4 <br />present the lOO-year floodplain limits. Water surface profiles are shown on <br />Sheets 23 through 26. <br /> <br />Vegetation along the creek channel consists of moderately heavy brush and trees, <br />while channel overbank areas are fairly flat and open. Crossings of South <br />Boulder Creek in this study reach include Arapahoe Road, McSorley Lane, and <br />Baseline Road. Diversion headworks for Valmont Reservoir are also contained in <br />this reach as is the juncture of the Wellman Canal Ditch with South Boulder <br />Creek. The average 100-year floodplain widths vary from 1800 to 3300 feet. <br /> <br />Crossings and other structural features in this reach include bridges at: South <br />Boulder Road, U.S. Highway 36 (Denver-Boulder Turnpike), State Highway 170 <br />(Marshall Road), and State Highway 93, as well as irrigation diversion structures <br />for McGinn Ditch, South Boulder Canyon Ditch, Schearer Ditch, and Goodhue Ditch. <br /> <br />Between the downstream study 1 imit (C&S Ra il road) and Arapahoe Road a spec i a 1 <br />split flow analysis was performed to determine the divided floodway and distri- <br />bution of flood flows between South Boulder Creek and the adjacent Dry Creek <br />No.2 Ditch drainageway. North of Arapahoe Road, commercial-industrial <br />structures are situated in the floodplain. Between the C&S Railroad and Arapahoe <br />Road, the lOO-year discharge tributary to the South Boulder Creek channel was <br />estimated at 2600 cfs while the remaining 4030 cfs of the total 6630 cfs <br />(IOO-year) is conveyed by the Dry Creek No.2 Ditch channel. These results are <br />consistent with the previous analysis completed by Simons, Li and Associates, <br />Inc. (Reference 6). <br /> <br />Average 100-year flOOdplain widths vary from 600 to 2400 feet. Improvements are <br />planned for the South Boulder Road crossing which consist of a new bridge and two <br />additional lanes for eastbound traffic. These improvements were incorporated in <br />the hydraulic mOdeling. It should be noted that flow depthS indicate that during <br />a laO-year event, some flow may overtop North Cherryvale Road to the east just <br />downstream of South Boulder Road. This flow would leave the South Boulder Creek <br />basin and pass into the Dry Creek basin, flowing towards Baseline Reservoir. <br />However, hydraulic modeling for this study was completed assuming that all flow <br />remained in the South Boulder Creek basin. <br /> <br />Upstream of Arapahoe Road, residential dwellings located adjacent to South <br />BDulder Creek along Old Tale Road to the west and Gaptor Road to the east, are <br />affected by flooding. The dividing line between the Dry Creek No.2 Ditch and <br />the South Boulder Creek drainageways generally parallels Old Tale Road. During a <br />100-year flood, flow in South Boulder Creek overtops this divide and spillS into <br />the Dry Creek No.2 Ditch drainageway. The cumulative overflow to the Dry Creek <br />No.2 Ditch, which occurs all along Old Tale Road, equals 2300 cfs. Much of the <br />municipal golf course situated west of the creek channel will be inundated by <br /> <br />At U.S. Highway 36 approximately 1660 cfs of the total 6160 cfs (100-year) will <br />overtop the highway near its intersection with State Highway 157. The remaining <br />4500 cfs passes through the bridge opening. A split flow analysis was used to <br />evaluate this distribution of flow. The split flow returns just downstream of <br />U.S. Highway 36 near South Boulder Road. <br /> <br />Gravel mining activity presently occurs upstream of U.S. Highway 36 on the west <br />side of the drai nageway. AreaS landward of the embankment were assumed <br />ineffective for flow conveyance. <br /> <br />10 <br />