FLOOD08945

• County: Larimer
• Community: Fort Collins
• Stream Name: Dry Creek
• Basin: South Platte
• Title: Hydrologic Analysis Major Drainageway Planning Dry Creek
• Date: 7/1/1979
• Prepared For: Larimer County
• Prepared By: Gingery Associates, Inc.
• Floodplain - Doc Type: Floodplain Report/Masterplan

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Reach III

All basin lands upstream of Reach II. Use
existing basin conditions--Agriculture
(Reference 1).

Land use for each of these reaches was determined from land use
maps for Larimer County. Impervious drainage area was then estimated
using aerial photography that showed similar, existing development,
and from typical values for the land use described. Examples of
values used in this study are shown in Table IV.

TABLE IV

Impervious Area

Land Use

% Impervious
90 - 95
75 - 80
60
20 - 30
10
1 - 5

Commercial
Industry
Multi-Dwelling Residential Units
Residential
Parks
Undeveloped

Reservoir information for routing of flows through Douglas
Reservoir and Park Creek Reservoir was also taken from the U.S.G.S.
quadrangles and aerial photography, respectively. This information
was supplemented with estimates made during field inspection of
Douglas Reservoir.

Model Calibration. No rainfall-runoff data was available from the
Dry Creek Basin that could be used to calibrate the SWMM model.
However, historic information and field observation was available
to allow reasonable adjustment of the model.
There has been no significant flooding in Fort Collins below
the Eaton Ditch since the area developed 25 to 30 years ago. In
addition, the Irrigation Ditch Companies have had no serious over-
topping of their ditches in this time period. Therefore, the assump-
tion was made that the excess capacity in the irrigation canals has
intercepted all flood flows over the past 25 years. In this 25-year
period, there is a 99 percent probability that a 5-year flood event
has occurred. Therefore, the model was calibrated so that a 5-year

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