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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Hydraulics

The hydraulic analysis for this report was completed using the U.S. Anny Corps of Engineers HEC-2
computer program, Version 4.6.2, (Reference 9). Existing HEC-2 models from previous studies were
used as a basis for the hydraulic analysis and modified to study the proposed improvements in more
detail. The source of the base HEC-2 models for Pawnee Creek is the 1992 SCS Report. The base
models for Pawnee Creek Overflow, within the City of Sterling and Sand Creek, came from the 1983
Leaf/RCI Report. For the Pioneer Ditch, base HEC-2 models were used from both the 1983 LeafIRCI
Report and the COE 1995 Report.

Proposed Improvements

Based on a review of the existing reports and discussions with the project sponsors, the following
improvements were selected for further study.

Pawnee Creek: A system of flood control levees appears to be the most economical approach to
eliminating the Pawnee Creek overflows. In general, the levees would follow along the north side of
Pawnee Creek beginning upstream of County Road 29 and continuing east to HWY 6. Since the levees
would be designed for the 100-year storm, they would be required to have 3 feet of freeboard by
FEMA. The existing bridges at HWY 6 would be improved to convey the 100-year discharge under
the bridges. For the southbound lanes, the existing concrete bridge would be expanded by adding 4
cells of 20' wide X 8' tall RCBs to the south of the existing bridge, The existing timber bridge along
the northbound lanes would be replaced. The Colorado Department of Transportation (CDOT)
currently has plans to replace the timber bridge within the next 3 to 5 years. In the summer of 19%,
the Burlington Northern Railroad replaced their bridge located just downstream of Highway 6 for
structural reasons. Design of the new railroad bridge was coordinated with this study so that the new
bridge will ultimately convey the 100-year peak discharge. Since the proposed bridge improvements
would allow more flow downstream of HWY 6, a second system of levees and a floodwall are
proposed to confine stormwater downstream of the highway to the 100-year floodplain of the South
Platte River. The estimated project cost of the proposed improvements along Pawnee Creek is
$1,135,500. This cost does not include the new bridge at HWY 6 by CDOT or the new railroad
crossing by UPRR.

These proposed improvements would eliminate the Pawnee Creek overflows from draining into the
City. The overflows are estimated to be about 5,000 cfs during the 100-year storm. Eliminating these
overflows removes a substantial amount of land and many structures from the floodplain in both Logan
County and the City of Sterling. The 100-year residual floodplain along Pawnee Creek and in the City
of Sterling is presented on Figure 3. This floodplain was delineated assuming that the proposed
Pawnee Creek Improvements have been constructed.

Sand Creek: A single flood control dam located just west of Pioneer Park appears to be the most
viable option to control flooding caused by Sand Creek. The dam and reservoir can also be used for
recreation if a permanent pool is established. In addition, a ground water recharge area can be
incorporated into the project. An outfall channel would be required to convey stormwater from the
dam to the Pioneer Ditch. The estimated project cost for the Sand Creek improvements is $6,487,600.

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