FLOOD07346

• County: Logan
• Community: Sterling
• Stream Name: South Platte River
• Basin: South Platte
• Title: Sterling CWCB Construction Loan Application Form
• Date: 11/5/1998
• Prepared For: CWCB
• Prepared By: Sterling
• Floodplain - Doc Type: Miscellaneous

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Hydraulics

Hydraulic Analyses were completed for both the eXlstmg channel conditions and for proposed
improvements. The most recent version (4.6.2: May ]991) of the U.S. Army Corps of Engineers'
HEC-2 computer program was used to determine water surface profiles for the 10-year. 50-year, and
1 DO-year flood events. The model used for this study was the same model developed by the SCS as
presented in their April 1992 Report. For existing channel. conditions, L:uS model was duplicated and
the resulting water surface profiles matches those presented by the SCS. For proposed improvements,
some cross-sections and split flow areas were modified in order to accurately depict proposed bridge,
channel. and levee improvements. A more detailed discussion of these modifications is included in the
Project Formulation Section of this report.

Comparisons of the 100-year water surface elevations were made for existing and proposed conditions
in order to evaluate potential impacts of the proposed improvements. Table 8 presents a summary of
the 10-year. 50-year, and 1 DO-year flood events for proposed and existing conditions. As shown in the
table, flood elevations for the lOO-year event west of CR. 35 to just downstream of CR. 3 1 increase for
the proposed improvements. This can be expected since the proposed leve,es along the north overbank
of Pa"'l1ee Creek confine the flows to the main channel and increase the discharges through this reach.
Map Sheet 1 presents the existing floodplain along Pawnee Creek as presented in the SCS April 1992
Report. Also shown are the proposed improvements and resulting residual 1 DO-year floodplain. Flood
profiles depicting the existing and proposed 1 DO-year water surface elevations are presented on Figures
8 and 9. A detailed description of the proposed improvements is included in the following section of
this report.

Proiect Formulation

Based on an evaluation of existing studies and input from the project sponsors, it was determined that a
system of levees and channel improvements in combination with widening the existing bridges at
HWY 6 and the UPRR is the most economical approach to solving the Pawnee Creek Overflow
problems. Development of the Pawnee Creek improvements began with modifications to the existing
HEC-2 model to analyze the impacts of proposed levees, bridge, and channel improvements. The
following describes the modifications made to the existing HEC-2 model:

1. No changes were made to the existing model from the study limits upstream of CR. 3 I (cross-
section 3839) to just downstream of CR. 3 1 (cross-section 30.1) since no improvements are
proposed in this reach.

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The model was revised to eliminate the existing flow splits in the north overbank. downstream of
CR. 33 at cross-section 2728. This was accomplished by adding the flows which split out of
Pawnee Creek towards the City of Sterling back into the model starting at cross-section 26 and
continuing to cross-section 21. The discharges were also increased upstream of HWY 6 to account
for the flow splits draining from the Town of Atwood and to eliminate the existing flows which
currently drain past the bridges at HWY 6 and on towards the City of Sterling. The amount of
storrn water draining from the Town of Atwood to Pawnee Creek on the northwest side of HWY 6
was determined by the Colorado Water Conservation Board based on field surveys and utilizing a
weir flow equation.

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