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FLOOD04855
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Last modified
1/25/2010 6:47:30 PM
Creation date
10/5/2006 1:05:42 AM
Metadata
Fields
Template:
Floodplain Documents
County
Boulder
Community
Boulder
Stream Name
South Boulder Creek
Basin
South Platte
Title
Flood Hazard Area Delineation
Date
7/1/1986
Prepared For
Boulder County
Prepared By
UDFCD
Contract/PO #
&&
Floodplain - Doc Type
Floodplain Report/Masterplan
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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 />100-year peak discharges were developed using the SWMM. The 500-year event was <br />derived from an extrapolation of a frequency curve drawn through the plotted <br />100-, 50-, 25-, and 10-year discharges for the respective locations. <br /> <br />Fi gure 2 presents the di scha rge-probabi 1 i ty profi 1 es for the 10-, 50-, 100-, and <br />500-year flood events obtained from the COE's hydrologic analysis and used for <br />this study. <br /> <br />To evaluate the distribution of flow downstream of the Wellman Canal and the <br />potential for Dverflow to the west through the residential area along Old Tale <br />Road, the split flow option in the HEC-2 program was utilized. Results of this <br />analysis indicate that overflow will occur over the entire reach along Old Tale <br />Road between Arapahoe Road and the Wellman Canal. Cumulative overflow from South <br />Boulder Creek to the Dry Creek No.2 drainageway equals 2300 cfs just upstream of <br />Arapahoe Road. At Arapahoe Road, the backwater created by the road embankment in <br />combination with the configuration of the bridge crossing causes additional <br />transfer of flows between the two drainageways. <br /> <br />Hydraulic Analysis <br /> <br />The hydraulic analysis of the South Boulder Creek study reach was completed using <br />the U.S. Army Corps of Engineer's step-backwater computer model HEC-2. Manning's <br />roughness coefficients for the channel were evaluated using a recently published <br />paper by Jarrett (1984) entitled "Hydraulics of High Gradient Streams" as a <br />guide. Selected channel "n" values ranged from 0.045 at the lower end to 0.085 <br />near Eldorado Springs, where the channel slope is in excess of four percent. <br />Overbank n values ranged from 0.05 to 0.10. Starting water surface elevations <br />for hydraulic modeling were based on a recent study by Simons, Li and Associates, <br />Inc. (Reference 6) as well as bridge rating curves for the C&S railroad crossing <br />developed by Leonard Rice Consulting Water Engineers, Inc. (Reference 4). <br /> <br />The Arapahoe Road bridge over South Boulder Creek has the capacity to pass <br />approximately 2600 cfs during the 100-year flood. The remaining 4030 cfs <br />overtops Arapahoe Road as weir flow, with most of this flow passing directly into <br />the Dry Creek No.2 Ditch drainageway. Additionally, bank full capacity of the <br />South Boulder Creek channel between the C&S Railroad and Arapahoe Road is about <br />2600 cfs. Since the topography slopes down from the South Boulder Creek <br />drainageway to the Dry Creek No.2 Ditch, flows in excess of South Boulder Creek <br />bank full capacity will tend to be conveyed by the Dry Creek ND. 2 Ditch. <br /> <br />Dry Creek No.2 Ditch Split Flow <br /> <br />Finally, the C&S Railroad grade has a low point near the Dry Creek No.2 Ditch. <br />Thus, flows in excess of the bridge capacities at the C&S Railroad crossings of <br />Dry Creek No.2 Ditch and South Boulder Creek will overtop the railroad at Dry <br />Creek No.2 Ditch. A concrete-lined interceptor channel was constructed north of <br />the railroad to intercept the weir flow and convey it to the Dry Creek No.2 <br />Ditch drainageway. For the area downstream of Arapahoe Road, this distribution of <br />flow between South Bou 1 der Creek a nd the Dry Creek No. 2 Ditch of 2600 cfs and <br />4030 cfs, respectively, is consistent with previous hydraulic analyses <br />(references 4 and 6). <br /> <br />Special Floodplain Analysis <br /> <br />A split flow analysis was utilized to define the floodplain limits from the C&S <br />Railroad crossing (downstream study limit) upstream to the Wellman Canal. The <br />Wellman Canal runs perpendicular to South Boulder Creek and joins the west side <br />of the creek channel about 3,000 feet south of Arapahoe Road. Due to the <br />existence of low embankments along the canal, most of the IOO-year flood is <br />confined to the main channel and right (looking downstream) overbank areas. Two <br />low points on the ditch embankment will allow approximately 500 cfs to flow to <br />the north into the Dry Creek No.2 Ditch drainageway, however 6130 cfs of the <br />6630 cfs 100-year flood will remain in the main channel area at this point. <br /> <br />Separate water surface profiles for this split flow area are shown for South <br />Boulder Creek and Dry Creek No.2 Ditch on Sheets 22 to 23 and 29A, <br />respectively. The floodplain area between the Dry Creek No.2 Ditch drainageway <br />and South Boulder Creek consists of an area of shallow floodin9 and is shown on <br />Sheets 1 and 2. <br /> <br />8 <br />
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