FLOOD08913

• Designation Number: 203
• County: Larimer
• Community: Fort Collins
• Stream Name: Dry Creek
• Basin: South Platte
• Title: Major Drainageway Planning - Dry Creek, Fort Collins, Larimer County, Colorado
• Date: 4/1/1980
• Designation Date: 1/1/1983
• Floodplain - Doc Type: Floodplain Report/Masterplan

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Hydrologic Procedure

The Stor~ Water ~~nagernent Model (SW~~), was selected for use in
this study because of the nature of the project and because of the
complexity of the basin.

The Dry Creek drainage basin was delineated on U.S.G.S. 7~ minute
quadrangle maps at a scale of 1"=2000' (Figure 2, Reference 3). The :Park
Cre3k Rese~voir w~s added to the maps from aerial photograp~y (Reference
4). The basin was then divided into 190 subbasins draining into 139
channel sections. (Figure 2). Basins were selected so that hydro-
graphs would be available at critical design points, particularly at
irrigation canal crossings and at on-stream reservoirs.

Rainfall information for this study was taken from the NOAA Atlas,
Volume III, Colorado (Reference 5). The SOO-year rainfall,. not
available fro~ the Atlas, was extrapolated assuming a Gumbel probability
distribution. The total rainfall depths for each frequency for a
I-hour duration storm were adjusted by an areal reduction factor, and
these adjusted values are shown in Table t. ~he I-hour duration
cloudburst event was assumed to be the typical storm producing flood
peaks along the Front Range of Colorado.

TABLE I
Rainfall Depths
One-Hour Duration Storm

Der>th

2-Year
0.62"

5-Year
0.90"

lO-Year
1.10"

50-Year 100-Year
1.60" 1.89"

SOO-Year
2.72"

Rainfall loss rates of depression storage and infiltration were
taken from existing information. The Corps of Engineers, Omaha District,
provided a depression storage rate of 0.3 inches from their experience
in studios along the Front Range. This value was modified in some
oil'li!'... to 0.2 i"":11,,s during IT.odel calibr"tion. l:>filtration rates
were assu~cd to equal perneability values from the Soil Survey for
tile Liuim..r County Survey An"" 644, dated September 2, 1971 (Refer-
ence 6).

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