FS0018X Feasibility Study 11/01

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

immediately south of the channel and east of the highway to provide access to homes
currently being served by Co Rd 39 that will be shortened and closed, riprap the
confluence with the South Platte, and modify irrigation channels and diversion structures.
Existing Flooding Conditions
Historically, Pawnee Creek has proven to be the most significant flooding source for the
City of Sterling and for Logan County. Flooding in the City of Sterling from Pawnee
Creek has occurred mainly as a result of insufficient channel capacity, roadway
structures, and the bridges at HWY 6 and the UPRR. The Soil Conservation Service
(SCS) completed a floodplain analysis of the Pawnee Creek overflow area in
unincorporated Logan County in April of 1992 that determined the 100-year floodplain
for existing basin and channel conditions. This report supplemented the previous
Leaf/Resource Consultants report of 1983, which was completed for the portion of the
Pawnee overflow in the City of Sterling. A floodplain map was prepared which
delineated the 100-year floodplain for Pawnee Creek through this study reach. Based on
the SCS Report, a description of the existing flood potential is presented below:
South Platte River to HWY 6/UPRR -- Flooding in this reach is limited by the capacity
of the existing bridge structures. However, the flooding can be severe and inundate much
of the highway and railroad. Flows split downstream of the UPRR bridge causing a very
wide (approximately 4,500 feet), shallow floodplain to the confluence with the South
Platte River. Two residences are potentially flooded in this reach: One residence is
located in the 100-year floodplain and is on the north bank of Pawnee Creek adjacent to
the railroad; and the other residence is located further downstream on a higher plateau
which could be impacted by potential shallow flooding. Additional flooding occurs to
the Sterling No. 1 Ditch, and adjacent agricultural lands.
UPRR/HWY 6 to CR. 35 -- Flooding in this reach is basically caused by: (1)
Insufficient capacity of the bridges at HWY 6 and the UPRR; (2) insufficient channel
capacity on Pawnee Creek; and (3) floodwater draining into the Town of Atwood, which
is a result of flow splits from the upper reaches of the basin. The greatest impact of
flooding occurs along HWY 6 to the north. Flood flows exceeding the capacity of the
bridges travel along HWY 6 damaging farmlands, inundating the Riverside Cemetery,
and eventually spreading out to the northwest into the City of Sterling. This area is
referred to as the Pawnee Creek Overflow, and is the driving force for the Community to
make flood improvements for the Pawnee Creek flood events.
CR. 35 to CR. 33 -- The channel in this reach has limited capacity (less than the 10-year
event). Once the flows exceed the channel capacity, they follow along the relatively flat
topography in a wide, shallow floodplain. Along the north overbank in an area
approximately 1,200 feet downstream of CR. 33, the flows split away from the main
channel and are conveyed to the north towards the City of Sterling contributing to the
Pawnee Creek Overflow. These flows mainly inundate agricultural fields with some
potential for flooding individual structures. Along the south overbank, the flood flows
are wide until the creek forms a large oxbow. At this location, the flood flows overtop
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