FS0018X Feasibility Study 11/01

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

Table 1. Peak Discharge Estimates - 1997 Flood
Pawnee Creek and Pawnee Overflow
Location Estimated Peak Flow (cfs)
Total Pawnee Overflow into Sterling just 3,800*
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 of HWY 14 65,000
Pawriee Creek mainstem at CR. 25 50,000**
Pawnee Creek mainstem at CR. 29 45,000**
* Assumes that the Sterling No. 1 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.
13