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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8. Proposed Near-Term Improvement and Operation of Pioneer Ditch, Sterling, Colorado, "Summary
Report" by Charles F. Leaf and Resource Consultants, Inc., dated July 1988.

9. Flood Insurance Study, City of Sterling, Colorado, Logan County by Federal Emergency
Management Agency, dated September 29, 1989.

to. Flood Plain Management Study, Pawnee Creek in Logan County, Colorado by U.S. Department of
Agriculture, Soil Conservation Service, dated April 1992.

11. Section 22 Planning Assistance, Major Drainageway Plan Pioneer Ditch at Sterling, Colorado by
U.S. Army Corps of Engineers, dated December 1995.
\...,

Description of Flooding Problems

Major flood events have occurred on Pawnee Creek in May 1935, June 1965, and July 1997. Minor
flooding occurred in 1967, 1990 and 1992. Flows in 1935 damaged the railroad between the City of
Sterling and Town of Atwood. Several hundred yards of HWY 14 west of Sterling were destroyed,
and approaches to the Sterling Bridge north of Atwood, HWY 6, and the bridge and tracks of railroads
at the Pawnee Bridge were also damaged. Discharge estimates from the 1935 event are not available,
but the Pawnee Overflow did operate sending flows into Sterling.

Flooding occurred on June 14 and 15 of 1965 as a result of heavy rainfall on the Pawnee Creek
watershed. Considerable damage occurred throughout the lower reaches of the watershed. South of
Sterling, near Town of Atwood, floodwater overtopped HWY 6 and the railroad tracks. Pawnee Creek
floodwaters also flowed in a northeasterly direction along HWY 6 and inundated portions of the
western section of Sterling. A peak flow estimate of 35,200 cfs was indirectly measured at the
Colorado Highway 14 Bridge approximately 10 miles upstream from Town of Atwood. The estimated
Pawnee Overflow was 5,000 cfs.

During the week of July 28, 1997, heavy rains caused significant flooding in several areas including
the communities of Fort Collins, Weldona, Town of Atwood, and Sterling (See Figures 4 and 5 for
Photos). Rainfall over the Pawnee Creek watershed on the evening of July 29th saturated
agricultural/range lands and caused major storm runoff in the tributaries and main channel floodwaters
in Pawnee Creek crested in Town of Atwood at approximately 3 PM on July 30. Floodwaters from the
Pawnee overflow first reached the south side of Sterling at about 4 PM, and then crested in the
downtown area around midnight on July 30th. Point rainfall amounts of up to 15 inches in a 4 to 8
hour time period were reported in the upper watershed areas of Pawnee Creek. Estimated rainfall
depths over the drainage basin are shown on Figure 6. These depths were based upon a rainfall bucket
survey conducted by the CWCB, Corps of Engineers, Omaha District, and Logan County staff. The
peak flow estimate on the mainstem above Colorado HWY 14 is significantly higher than the 1965
peak flow estimate. Based upon field-surveyed high water marks, flow estimates were made using.indirect flow measurement techniques and are presented on the following table.

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