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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flood event would produce discharges less than the excess ditch
capacities. (See Table 1.)
TYIO calibration runs were assembled. The first run included
the drainage area above the Eaton Ditch and below the Larimer County
Canal. At the Eaton Ditch, the loss rates were to be adjusted so
that peak discharges for the 5-year event would not exceed 400 cfs,
the combined excess capacity of the Eaton Ditch and the Terry Inlet.
No adjustment of infiltration rates was necessary. Depression storage
was dropped from 0.3 to 0.2. The resulting 5-year discharge was 280
cfs.

The second calibration run included all areas above the Larimer
County Canal with peak discharges not to exceed 125 cfs, the excess
capacity of this ditch. No modifications to the preliminary loss
values were necessary. The resulting 5-year discharge was 60 cfs.
Above Douglas Reservoir, little information was available for
calibration. Known information was that the reservoir has not been
overtopped in the 70-year period of its existence. From field obser-
vation, erosion on the spillway indicates that the spillway has oper-
ated with a substantial flow, however, not large enough that erosion
is excessive.

In the basin above Douglas Reservoir, irrigation canals again
have the capacity to intercept significant drainage flows. The Poudre
Valley Canal, often referred to as Flood Ditch, is in use for only
short periods of time, and therefore would likely intercept drainage
flows up to its full capacity of 350 cfs. The North poudre Ditch
would intercept further excess flows at the confluence of Park Creek
and Spring Gulch. This information is of little use, however, since
we have no conclusive observations related to flOOding in this part
of the basin.
Therefore, only qualitative observations could be used to adjust
flows above Douglas Reservoir. The initial results at Douglas Reser-
voir for a 100-year event showed a peak outflow of 400 cfs, ignoring
any interception at the irrigation canals. This flow magnitude is
consistent with what would be expected from simple observation at
the spillway. However, this peak outflow was substantially different
from the 100-year peak outflow of 1,700 cfs reported in the Flood
Insurance Study Hydrology Report. Because the difference was so large
and because permeability rates seemed unusually high to be used for

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