FLOOD05064

• Designation Number: 205
• County: Larimer
• Community: Fort Collins
• Basin: South Platte
• Title: Master Drainageway Planning - Cooper Slough, Boxelder Creek, Fort Collins, Larimer County, Colorado
• Date: 8/1/1981
• Designation Date: 1/1/1983
• Floodplain - Doc Type: Floodplain Report/Masterplan

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29

Creek, Figure 9 for Cooper Slough with existing basin conditions, and Figure

10 for Cooper Slough with developed basin conditions.

A detailed description of the methodology used and parameter values is

given in the "Technical Addendum."

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Hydraulic Analysis

The analysis of the hydraulic characteristics of Boxelder Creek and

Cooper Slough were conducted to determine the water surface elevation for peak

flows of the 2-, 10-, 25-, 50-, 100-, and 500-year floods. The water surface

elevations were computed using the U.S. Army Corps of Engineers' HEC-2 water

surface profile computer program (reference 17). Cross sections of Boxelder

Creek and Cooper Slough were taken from topographic maps (1" = 1 00', 2-foot

contours) provided by Larimer County, except for a one-half mile reach above

Vine Drive on Cooper Slough where the cross sections were obtained from a

field survey. Additional cross sections from a field survey were used in con-

junction with topographic mapping for the area south of State Highway 14 on

Cooper Slough. Bridge and culvert geometries were field measured. Elevations

for top of the road crossings and downstream and upstream channel inverts were

surveyed. Roughness coefficients were selected after field investigation and

analysis of photographs taken during field reconnaissance. The roughness

coefficients used have been presented in a separate report entitled "Docu-

mentation for Selection of Manning's Roughness Coefficients for Boxelder Creek

and Cooper Slough" (reference 16). The range of roughness values is given in

Tahle 9.

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At several locations along Boxelder Creek and Cooper Slough, some flow

leaves the channel because of the limited capacity of bridges or culverts. A

separate analysis was conducted to determine the flow split at these loca-

tions. Major flow splits on Boxelder Creek occur at 1-25 and Prospect Street.

Table 10 lists the flow split data for Boxelder Creek. The flow was not

traced since the water does not,return to the Boxelder Creek floodplain.

Splits occur on Cooper Slough at the Colorado and Southern Railroad and at

State Highway 14. The flood water that overtops Highway 14 flows along three

separate channels. Separate hydraulic analyses were performed to determine

the water-surface elevations along each individual char-nel. Table 11 lists

the location and quantity of these flow splits for Cooper Slough. These flows

were traced because the water returns to the Cooper Slough floodplain.

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