FLOOD07346

• County: Logan
• Community: Sterling
• Stream Name: South Platte River
• Basin: South Platte
• Title: Sterling CWCB Construction Loan Application Form
• Date: 11/5/1998
• Prepared For: CWCB
• Prepared By: Sterling
• Floodplain - Doc Type: Miscellaneous

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Table 6. Peak Discharges Along Pawnee Creek Main Channel Under Existing Conditions

Location 10- Y ear 50- Y ear 100- Year
(cfs) (cfs) (cfs)
'Mouth (South Platte River 100- Year Floodplain) 1517 2026 2226
-
'Upstream HWY 6 1517 2026 2226
'Upstream CR. 33 2945 5434 6645
-
'Upstream CR. 31 3264 6729 8611

, Split flows depicted in Figure 8A cause reduced discharge in the main channel

There are several areas where the flood flows exceed the capacity of tile channel for the 10-year, 50-
year, and I DO-year events. Flows which split from the main chamlel are subtracted out and are
accounted for under existing conditions; therefore, the discharges vary significantly from downstream
to upstream. For Pawnee Creek Overflow in the City of Sterling, 100..year flows are estimated to be
5.000 cfs.

As a result of the improvements proposed in this report, flood flows are contained in the areas adjacent
to the channel in the north overbank. Therefore, these flows were considered a part of the main
channel and were included in the analysis. Improvements were not proposed in the upper reaches of
Pav,l1ee Creek along the south overbank since they do not directly contribute to the Pawnee Creek
Overflow in the City of Sterling. Subsequently, the areas where flows split out of the channel and
travel southwest towards the Town of Atwood were considered valid for proposed conditions and were
subtracted out of the main channel at these locations. A portion of these flows eventuall.y drain back to
the main channel at HWY 6 and the Pav,.l1ee Creek crossing.

A field survey was completed by the Colorado Water Conservation Board in the area along HWY 6
south of Pawl1ee Creek to determine the amount of flows which gets back to the main channel. The
flows in this area are controlled by an existing berm which blocks the flow path. Estimates were made
by the Colorado Water Conservation Board as to how much of the flow overtops the berm and how
much would be forced over HWY 6 and the UPRR embankment. Flows expected to overtop the berm
were added into the main channel at the HWY 6 bridge crossing of Pawnee Creek. A summary of the
discharges used for developing the proposed improvements are presented in Table 7.

Table 7. Peak Discharges Along Pawnee Creek Main Channel Under Improved Conditions

[ Location l0.::Yrn .10- Year 100- Y ear
(cfs) (cfs) (cfs)
Mouth (South Platte River lOO-Year Floodplain) 3800 6500 8200
Upstream HWY 6 I 3800 6500 8200
Upstream CR. 33 2945 5434 6645
Upstream CR. 31 3264 6729 8611

The discharges presented in Tables 6 and 7 were used in the hydraulic analysis to develop flood water
surface elevations for the different scenarios that were studied. Figure 8A presents a diagram of the
peak discharges and flow splits along Pawnee Creek

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