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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Open Space

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U.S. Army Corps of Engineers in Omaha, Nebraska, based on their hydrologic

analysis experience along the front range. Depression and detention storage

was selected to be consistent with the Fossil Creek and Dry Creek studies.

Subcatchment roughness characteristics were based on field investigation and

selected to be similar to the previous studies. Channel unit lengths, slopes,

and geometries were defined from field surveys and from 1:1200 scale

topographic maps from Larimer County, Colorado, where available (reference 14)

or U.S. Geological Survey 1:24000 scale topographic maps (reference 13).

Manning's roughness coefficients were based on field investigation. Table 6

lists the parameters used and the range of values selected. Storage discharge

curves for ponding behind channel crossings were defined from topographic

mapping and weir flow equations. The ponded areas along Cooper Slough were

upstream of the crossings at State Highway 14, Colorado and Southern Railroad

and Vine Prive. The ponded areas along Boxelder Creek were upstream of 1-25

near Fort Collins and Wellington, State Highway 14, abandoned Colorado and

Southern Railroad, Vine Drive, Larimer-Weld Canal and Willox Lane. Stage-

discharge curves were presented in the "Technical Addendum" (reference 10).

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Table 5. Percent Impervious for Fully Developed Zoning Conditions.

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Land Use

Density

Percent Impervious

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Industrial
Commercial

80
90

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Residential
(inside Urban Growth Area)

4.0 dwelling units
per acre

40

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Residential/Agricultural
(outside Urban Growth Area)

0.43 dwelling units
per acre

6

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Results

Rainfall-runoff data were not available for the Cooper Slough or Boxe1der

Creek basins. For this reason, the results of the Fossil creek and Dry Creek

studies were used to evaluate and justify the results of this study. The peak

discharges per square mile from Dry Creek and Cooper Slough are very similar.

The Fossil Creek basin produces higher peak flows per square mile because of

steeper slopes and lower infiltration rates than the Boxelder Creek and Cooper

Slough basins. The Boxelder Creek drainage basin is longer and flatter than

Dry Creek and has lower unit area peak discharges. A comparison of the study

results with the previous studies was presented in the Technical Addendum.

A summary of the peak flows for existing and developed conditions in the

Cooper Slough basin is given in Table 7. A summary of peak flows for the com-

bined Boxelder Creek-cooper Slough basins is given in Table 8. The discharge

profiles which show the change in flow rate along the study reach for Boxe1der

Creek are given in Figure 5. Figures 6 and 7 present the discharge profiles

for Cooper Slough for existing and developed basin conditions, respectively.

The flood hydrographs at State Highway 14 are given in Figure 8, for Boxe1der

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