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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Rainfall Data

Rainfall depths for the l-hour, 2-, 10-, 25-, 50-, and 100-year recur-

rence interval storms were obtained from the "Larimer County Storm-Water

Management Manu~l" (reference 11) which is based on data and procedures

described in the Precipitation Frequency Atlas of the Western United States,

Atlas 2, Volume,III, Colorado (reference 12). The 500-year rainfall depth was

estimated by extrapolation of the l-hour depths of the other recurrence inter-

vals on Extreme Value paper. The distribution of the point rainfall depth was

based on the distribution used in the previously mentioned studies in the area

and is shown in Table 1. The point rainfall depths used for each storm event

are given in Table 2. An areal reduction factor of 95 percent was used for

the storm over the Cooper Slough basin and a factor of 76 percent was used for

ths combined Boxelder Creek-Cooper Slough basin. Tables 3 and 4 contain the

rainfall hyetographs used for Boxelder Creek and Cooper Slough basins,

respectively.

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Table 1. Percent of One-Hour Rainfall Depth for
Five-Minute Intervals.

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End of Period Percent of One-Hour*
(minutes) Rainfall Depth
5 4
10 4
15 5
20 6
25 9
30 26
35 15
40 11
45 6
50 5
55 5
60 4

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Drainage Basin Parameters

The parameters required to determine runoff peaks and volumes from SWMM

are subcatchment area, width, percent impervious, infiltration rate,

depression and detention storage, overland roughness coefficient, channel

geometry, channel slope, channel roughness coefficient, and storage discharge

curves for ponded areas. The percentage of impervious area for developed con-

ditions was based on land use as defined by the Zoning Map for the Urban

Growth Area (Figure 4). The percent of impervious area by land use is given

in Table 5. The impervious area for existing basin conditions was based on

U.S. Geological Survey topographic maps and Larimer County topographic maps

(ref~rences 13 ,and 14). The hydrologic analysis of Boxelder Creek assumed

developed basin conditions within the Urban Growth Area and existing con-

ditions outside the Urban Growth boundary. Cooper Slough was analyzed for

existing and fully developed basin conditions.

Infiltration rates for pervious areas in the Cooper Slough basin were

based on soil types obtained from the "Larimer County Soil Survey" (reference

20) and selected to be consistent with the Dry Creek report. Infiltration

rates for Boxelder Creek were based on soil types obtained from the "Larimer

County Soil Survey" with a maximum rate of one inch per hour. The maximum

rate was recommended by the Chief Hydrologist of the Omaha District of the

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*Fossil Creek Hydrology Study
(reference 9)

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14

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15

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