FLOOD05819

• County: Logan
• Community: Sterling
• Basin: South Platte
• Title: City of Sterling Drainage and Flood Control Implementation Plan
• Date: 6/1/1997
• Prepared For: Sterling
• Prepared By: ICON Engineering, Inc.
• Floodplain - Doc Type: Floodplain Report/Masterplan

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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 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.

CR 33 to CR. 31 -- As in the lower reaches, the channel has limited capacity and cannot handle flows
of even the 10-year magnitude. The 100-year floodplain is generally wide with shallow depths and has
flows splitting off from the main channel. In the upstream area adjacent to CR. 31, the floodplain is
very wide (approximately 4,600 feet). The existing channel and bridge crossing are undersized, and
the road acts as a weir spreading out the flood waters. Along the south overbank, flows split in a
southeasterly direction away from Pawnee Creek toward HWY 6 near the Town of Atwood. The
highway and railroad redirect the flows northeast towards the Pawnee Creek main channel. A berm
has been constructed approximately one mile south of the Pawnee Creek crossing. This berm blocks
some of the flows and forces a portion of the water over the highway and railroad embankment. The
remaining flows overtop the berm and continues north to the highway bridge where they combine with
the flows along the main stem of Pawnee Creek. In the lower portion of this reach at CR. 33, the
floodplain is confined by the channel banks south of the bridge crossing, as well as by a gravel
driveway to the north and CR. 33 to the east,

Upstream of CR. 31 -- No floodplain mapping is available upstream of CR. 31. However, hydraulic
analysis was completed by the SCS for a distance of about 1,000 feet, Based on discussions with the
City of Sterling and the Colorado Water Conservation Board, the floodplain is very wide in the
upstream reaches. There is the potential for flooding of some local residences, as well as damage to
agricultural farmland. No major flow splits have been determined in this area which would impact the
City of Sterling.

Hydrology

A detailed hydrologic analysis was not completed for this study. Detailed hydrology was completed by
the SCS and presented in their April 1992 Report. Flow data for various storm events generated for
that report were used as a basis for development of existing floodplain information, as well as for the
evaluation of proposed improvements. Final flows were reviewed and approved by the Colorado
Water Conservation Board and the City of Sterling for use in this study. A summary of the discharges
used for existing conditions is presented in Table 6,

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