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