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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IIvdraulics

Hydraulic .A..nalyses were completed for both the eXlstmg channel conditiol15 and for proposed
improvements, The most recem version (4,6,2: May 1991) of the U.S. Army Corps of Engineers'
HEC,2 computer program was used to determine water surface profiles for the IO-year. 50, year, and
] OO-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. rbis model was duplicated and
the resulting water surface profiles matches those ptesented by the SCS, For proposed improvements.
some cross-sections and split flow areas were modified I.n 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 repon,

Comparisons of the lOa-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] O-year. 50-year. and ] OO-year flood events for proposed and existing conditiol15. As shown in the
wble, nood elevations for the 1 OO'year event west of CR. 35 to just dO'-'"!lstream of CR, 31 increase for
the proposed improvements. This can be expected since the proposed levees along the north overbank
of Pawnee Creek COnIine the flows to the main channel and increase the discharges through this reach.
Map Sheet ! presents the existing tloodplain along Pav.rnee Creek as presented in the SCS April 1992
Report. Also shown are the proposed improvements and resulting residllili 100,year floodplain, Flood
proriles depicting the existing and proposed] OO-year warer surface elevanons are presented on Figures
8 and 9, A detailed descriprion of the proposed improvements is included in the following section of
this report,

Pr<Jiect Formulation

Based on an evaluation of exisring studies and input from the project sponsors. it was detetmined that a
s~'ste:n of levees and channel improvements in combination with widening the existing bridges at
HWY 6 and the UPRR is the most economical approach 1:0 solving the Pawnee Creek Overflow
problems. Deveiopment of the Pawnee Creek improvements began with modiiica:ions to the existing
HEC,~ model to analyze the impacts of proposed levees, bEcge, and channel improvements, The
rollowing desc~ibes 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] (cross-
section 38391 to just downstream of CR. 31 (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 o'lerbank downstream of
CR, 33 at cross-section 2728, This was accompiished by adding the flows which spiit out of
Pa\VTIee '~eek towards the City of Sterling back into the mocel starting at cross,section 26 and
continuing to cross-section 21. The discharges were also increased upstream of HWY 6 to account
for the !low spins draining from the Town of Atwood and 1:0 eliminate tile existing !lows which
currently drain past the bridges at HWY 6 and on towards the City of Sterling, The amount of
storm water draining from the Town of Atwood to Pawnee Creek on the northwest side of HWY 6
was determined bv the Colorado Water Conservation Board based on field sur/evs and utilizing a
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weir t10w equation,

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