FLOOD06827

• County: Logan
• Community: Sterling
• Basin: South Platte
• Title: Pawnee Creek Overflow Interceptor Channel
• Date: 10/1/2001
• Prepared For: Sterling
• Prepared By: ICON Engineering, Inc.
• Floodplain - Doc Type: Floodplain Report/Masterplan

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Proposed Conditions Models

The Pawnee Creek Overflow Interceptor Channel discharges into the South Platte
River. Starting water surface conditions for the Interceptor Channel were based on
normal depth calculations since the drainage basin for Pawnee Creek is much smaller
than the South Platte River. Our calculations indicate that the ration of drainage
areas between Pawnee Creek and the South Platte River is 0,038,

The floodway boundary is based on equal conveyance (Method 4), but for the
interceptor channel, the floodway boundaries are consistent with the 100-year
floodplain boundary. The new Floodway Data Table provides a water surface
elevation comparison and corresponding surcharge for each cross section, The
floodway boundary duplicates the floodplain boundary, since a no-rise condition has
been applied. The County should not allow any encroachment into the interceptor
channel in order to maintain the flood control capacity, Floodplain tie-in
requirements to effective flood hazard information have not been considered since
the 100-year discharge will be transported to the South Platte in the Interceptor
Channel and the interceptor channel is located south of the upstream limits of the
effective model. Therefore tie-ins are not possible. Similarly, the floodway widths
do not tie into the effective models.

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HEC-RAS Model Input and Output Files

The HEC-RAS input and output files used in this CLOMR request are listed below,

Prop V chan,prj
Prop V chan,f02
PropVchan,f04
Prop V chan,gO I
Prop V chan,p02
Prop V chan,p03
Prop V chan,rep
Prop V chan2,rep

10/26/0 I
10/25/0 I
10/26/0 I
10/23/01
10/26/0 I
10/26/0 I
10/26/0 I
10/26/0 I

Project File
FEMA flows
100-yr Flow for Floodway
Geometry File
Plan File for 500,100,50,IOyr
Plan File for Floodway
Floodway Report
Multiple Profile Report

6,2 MODIFICATIONS TO THE FLOOD HAZARD INFORMATION

The proposed construction of the Pawnee Creek Overflow Interceptor Channel
would result in modifications to the flood hazard information, Floodplains from the
Pawnee Creek Overflow, Sand Creek, the Pioneer Drainage Ditch, and the South
Platte River all converge in Sterling, The 100-year floodplain associated with the
historic Pawnee Creek Overflow would be removed from the City of Sterling and
directed to the South Platte River along the new interceptor channel located adjacent
to County Road 22'l2, The previous overflow will be designated as Zone X shaded,
since flows exceeding the 100-year event will follow the historic drainage path, The
FPIDP report was used as a guideline to determine the extent of the 100-year

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