FS0018X Feasibility study

• Contract/PO #: FS0018X
• Contractor Name: Sterling, City of
• Contract Type: Loan
• Water District: 0
• County: Logan
• Loan Projects - Doc Type: Feasibility Study

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Table 1. Peak Discharge Estimates - 1997 Flood
Pawnee Creek and Pawnee Overflow

'lfocation .
Total Pawnee Overflow into Sterling just
northeast of the Riverside Cemetery
Portion of Pawnee Overflow into Sterling 2,500
Between the Riverside Cemetery and the Railroad
Embankment
Pawnee Creek Mainstream u/s ofHWY 14 65,000
Pawnee Creek mainstem at CR. 25 50,000**
Pawnee Creek mainstem at CR. 29 45,000**
* Assumes that the Sterling No. I Ditch is full prior to Pawnee Overflow peak discharge.
** This value is less than the upstream value due to valley storage and flow attenuation.

Flows overtopped the BNRR at several locations. At the Town of Atwood, an estimated 20,500 cfs
overtopped the railroad and returned directly to the South Platte River. Approximately 5500 cfs
overtopped the railroad just to the south of the Riverside Cemetery. Additionally, existing
bridges/culverts allowed floodwaters to go under the railroad embankment. The Pawnee Creek main
channel bridge was estimated to have carried only 1800 cfs, and the bridge approximately 1500 feet to
the north had an estimated 500 cfs. Location of these overflows is shown on Figure 2.

Flow widths for the surveyed cross-sections across the Pawnee Creek floodplain ranged from about
1,600 feet at the upstream end, over one mile at the downstream end. The flow depth in the confined
channel at the upstream area was over 11 feet, while the flow depth closer to Town of Atwood was
much more wide and shallow, with depths on the order of 4 feet in the community of Town of Atwood
and adjacent agricultural lands.

The 100-year floodplain for the Pawnee Creek Overflow for the existing channel conditions was
presented in the report entitled "Flood Plain Information and Drainage Plan For Sand Creek and the
Pawnee Overflow At Sterling, Colorado" dated May, 1983 and prepared by Charles Leaf and Resource
Consultants. This base engineering data was used for the Community's Flood Insurance map, and also
the basis for the damage analysis. Approximate limits of the floodplain are shown on Figure 7. Flood
flows for the 100-year flood coming into Sterling from the Pawnee Creek overflow were estimated to
be 5000 cfs. This compares to the calculated Pawnee Creek overflow peak of approximately 3800 cfs
during the 1997 flood.

IV. HYDROLOGY

A detailed hydrologic analysis was not completed as a part of this report. The hydrology from
previous reports were reviewed and utilized in this study. This feasibility study was prepared using
hydrology information taken from SCS, April 1992 document titled "Flood Plain Management Study,
Pawnee Creek in Logan County, Colorado." The following table summarizes the flow rates generated
at various locations.

Revised hydrology has since been prepared and will be used for future design efforts. The new source
for the Pawnee Creek hydrology is the report titled "Hydrologic / Hydraulic Analysis of Flood
Mitigation Alternative #5, Pawnee Creek Overflow, Logan County, Colorado" which was prepared by

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