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<br />I <br />(J <br /> <br />, <br /> <br />.J <br /> <br />,J <br /> <br />-J <br /> <br />-J <br /> <br />,..1 <br /> <br />J <br /> <br />J <br /> <br />J <br /> <br />Table 6. Peak Discharges Along Pawnee Creek Main Channel Under Existing Conditions <br /> <br />Location i I 0- Year I 50, Y ear - I <br /> (cfs) (efs) <br />'Mouth (South Platte River 100- Y ear Floodplain) I 1517 I 2026 I <br />'Upsrream HWY 6 ! 1517 ! 2026 , <br />1 <br />'Upstream CR. 33 i 2945 I 543.:1 I <br />. L' psrream CR. 3 I ! 3264 I 6729 ! <br /> <br />100, Year <br />(cfs) <br />2226 <br />2:26 <br />6645 <br />8611 <br /> <br />, Split flows depicted in Figure 8A cause reduced discharge in th.: main~hanne' <br /> <br />There are several areas where the flood flows exceed the capacity of the channel for the 10-year, 50- <br />year. and 100,year events, Flows which split from tlle main chmmel are subtracted out and are <br />accounted for under existing conditions; therefore, the discharges vary significantly from do\\>nsrream <br />to upst~eam, For Pawnee Creek Overflow in the City of Sterling, 100,year flows are estimated to be <br />5,000 cis, <br /> <br />As a result of the improvements proposed in this report, f100d flows are comained in the areas adjacent <br />to the channel in the north overbank, Therefore. these flows were considered a part of the main <br />channel and were included in the analysis, Improvements were not proposed in the upper reaches of <br />Pa\\>nee Creek along the south overbank since they do not directly contribute to the Pawnee Creek <br />Overflow in the City of Sterling, Subsequently. the areas where flows spilt out of the channel md <br />travel southwest towards the Town of Atwood were considered valid for proposed conditions and were <br />subtracted out of the main channel at these locations A portion of these: flows eventually drain back to <br />the main channel at H\VY 6 and the Pawnee Creek crossing, <br /> <br />A field survey was completed by the Colorado Water Conservation Board in the are:, along HWY -5 <br />south of Pa\vnee Creek to determine the amount of flows which gets back to the main channel. The <br />flows in this area are conn-oIled by an existing berm which blocks the flow path, Estimates were made <br />bv the Colorado Water Conservation Board as to how much of the flow ove:1ops the berm and how <br />much would be forced over HWY 6 and the UPRR embankment. Flows expected to cve:-top the berm <br />were added imo the main channel at the HWY -5 bridge crossing of Pawnee Creek, A summary of the <br />discharges used for developing the proposed improvements are presented in Table 7, <br /> <br />Table 7. Peak Discharges Along Pawnee Creek Main Channel Under Improved Conditions <br /> <br />Location <br /> <br />10- Year <br />(cfs) <br />3800 <br />3800 <br />2945 <br />3264 <br /> <br />50- '\'"eJ[ <br />(efs) <br />6500 <br />6500 <br />5434 <br />6729 <br /> <br />.1 00- Ye3.[ <br />(cfs) <br /> <br />! Mouth (South Plane River] 00- Year Floodplain) <br />!l'pstrearn HWY 6 <br />I Lpstream CR, 33 <br />l'pstrearn CR, 3] <br /> <br />8200 <br />3200 <br />6645 <br />861 I <br /> <br />, <br />,-- <br /> <br />The discharges presemed in Tables 6 and 7 were used in the hydraulic analysis to develop flood water <br />surface elevations for the different scenarios that were studied. Figure 8A presents a diagram of the <br />peak discharges and flow splits along Pawnee Creek <br /> <br />15 <br />