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<br />3.3 C.hon",'.lmprov..m..nf <br /> <br />3.3.1 The following narrative summarizes the results of the channel improvement <br />analysis perfonned for Pioneer Ditch. The study was based on a future 10-year local drainage <br />flood event. It was assumed that the detention pond was in place and that the alignment of the <br />channel was modified as shown on Plate 10. <br /> <br />3.3.2 An HEC-2 backwater model of Pioneer Ditch, using files received from Re- <br />source Consultants & Engineers of Fort Collins, Colorado, was used to calculate the design water <br />surface profile. HEC-2 cross section locations correspond to those used in the Leaf studies <br />referenced in paragraphs 2.1 and 2.2. A copy of some Pioneer Ditch flood boundaries is included <br />in Appendix A (from the 1988 Leaf study) to reference the cross section locations along Pioneer <br />Ditch. The starting water surface elevation was 3914.8 feet msl, corresponding to the South Platte <br />River 10-year flood elevation at the Pioneer Ditch confluence. The starting discharge was 478 cfs <br />downstream from the railroad bridges, decreasing to'300 cfs at the detention pond weir and <br />increasing to 1040 cfs upstream from the weir. The discharges for the remaining channel were <br />based on those shown in Table 1 in section 3.2.3. The channel improvement option was used to <br />change the configuration and elevation of the channel where flows were out of bank. There are <br />two locations where berms are required on both sides of the channe1--the detention pond area and <br />the downstream end of the channel realignment reach. Existing bridges were left in place or minor <br />changes made where the flow was not significantly obstructed. Locations where bridges need to <br />be significantly upgraded in size are at State Highway 14, a bridge approximately 1000 feet <br />upstream from State Highway 14, and Phelps Street. Bridges that require minor channel <br />improvements under the existing deck are at Sidney Avenue and the bridge about 1000 feet <br />downstream from State Highway 14. More detailed infonnation on the bridge and channel <br />improvements may be found in Appendix A The channel left and right banks, old and new inverts, <br />and the future 1O-year water surface profile are shown on Plates 11 and 12. <br /> <br />3.3.3 Table 5 shows the water surface elevations for a 1O-year Sand Creek flood eve\lt <br />in the existing Pioneer Ditch. Table 5 also shows water surface elevations for the future 10-year <br />local drainage flood event with channel improvements and the detention pond in place along <br />Pioneer Ditch. These two events are not directly comparable, but, by the absence of large jumps <br />in the water surface elevations, they show how the realignment and the bridge improvements have <br />significantly decreased backwater effects. Table 6 shows discharges for the same events described <br />in Table 5. Again the two events are not directly comparable. The Sand Creek discharges in the <br />existing Pioneer Ditch show the irregularity of the channel, backwater problems, and resulting <br />overbank flooding by the inconsistent discharge values. In the improved Pioneer Ditch, the 10-year <br />.. future local drainage discharges increase consistently in the downstream direction, except where <br />flow is diverted into the detention pond. This is a sign of more adequate channel capacity for the <br />design event. <br /> <br />9 <br />