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

I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I

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 6IUPRR -- 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 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 fannlands, 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 lO-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
the bank and follow a direct alignment east to CR. 35. In the area of the oxbow, the
Pawnee Creek channel contains the flows until they reach CR. 35 extended and at this
point combines back with the main channel flows.

32