FLOOD01452

• County: Larimer
• Community: Fort Collins
• Stream Name: Fossil Creek Drainageway Basin
• Title: Master Drainageway Planning Study
• Date: 8/1/1982
• Prepared For: Larimer County
• Prepared By: Simons Li & Associates Inc.
• Floodplain - Doc Type: Floodplain Report/Masterplan

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feet topographic maps (ref. 21). Manning's roughness coefficients were field

checked. Table 6 lists the parameters used for the hydrologic and the range

of values for SWMM.

Table 6. Drainage Basin Parameters for SWMM.

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Storage discharge curves, as determined by the COE for ponding behind

channel crossings, were checked from topographic maps, culvert capacity calcu-

lations and weir flow equations.

Parameter

Range

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3.4 Results

Subcatchment

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

Fossil Creek basin are shown in Tables 7 and 8. The discharge profiles which

show the change in flow rate along the study reach for Fossil Creek, stanton

Creek, Lang Gulch, Smith Creek and Burns Tributary for existing and developed

conditions are shown in Figures 5 through 14.

Flood hydrographs for Fossil Creek above Lemay Avenue (County Road 13)

and at Fossil Creek Reservoir for existing and developed conditions are shown

in Figures 15 through 18. The hydrograph for Fossil Creek at Fossil Creek

Reservoir (Figures 15 and 16) has attenuated compared to Fossil Creek above

Lemay Avenue (Figures 17 and 18) due to the ponding behind the Union Pacific

Railroad. Flood hydro graphs for the tributaries for existing and developed

conditions are plotted at the following locations: Stanton Creek at the

confluence with Fossil Creek, Lang Gulch at the confluence with Fossil Creek,

Smith Creek at the confluence with Fossil Creek, and Burns Tributary at the

confluence with Fossil Creek (Figures 19 through 26). Stanton Creek

hydro graphs (Figures 19 and 20) exhibit double peaks for all storms except

both 2-year and 500-year, existing conditions. For the 500-yp.ar storm,

existing conditions, the timing of the peaks are close enough to eliminate the

second peak. For the 2-year storms, the hydrographs are flat and two peaks

are not evident. For Smith Creek, the hydrographs are flat due to the atte-

nuation at Shields Street. The 500-year hydrographs (Figures 23 and 24) have

sharp peaks because the flow overtops Shields Street and the peak is not

attenuated.

Slope (foot/foot)

0.01 - 0.35

Width (feet)

400 - 17,640

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Area (acres)

6 - 713

Impervious Area
(percent)

10 - 90

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Roughness (n)
(impervious)

0.013

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Roughness (n)
(pervious)

0.120

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Depression and Detention
(impervious) (inches)

0.2

Depression and Detention
(pervious) (inches)

0.3

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Infiltration Rate
( inches/hour)

0.4 - 3.0

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Infiltration Decay Rate

0.001

Channel

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Width (feet)

2 - 8

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Slope (foot/foot)

0.0013 - 0.1504

Roughness (n)

0.035 - 0.045

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The time to peak of the hydrograph for Fossil Creek at Lemay Avenue is

about 1 hour and 30 minutes. Within 8 hours, the 100-year flood has receded so

that the discharge is about equivalent to the 2-year peak. Time to peaks for

the tributaries also are on the order of 1 to 2 hours.

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