PROJ02027

• Contract/PO #: FS0044FX
• Contractor Name: Larimer County, Board of County Commissioners
• Contract Type: Miscellaneous
• Water District: 0
• Loan Projects - Doc Type: Feasibility Study

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SECTII.TWO

Hydrologic Mo..1

Three previous hydrologic evaluations have been performed for the Dry Creek watershed by
Gingery Associates, Inc. (1977 and 1980)(GAI Report), Resource Consultants Inc. (1992)(RCI)
and Lidstone & Anderson Inc. (1997){L&A).

The hydrologic information published in the effective Flood Insurance Study (FIS) for the City
of Fort Collins (March 18, 1996) indicates a peak 100-year discharge at S.H. 287 as 2,900 cfs.
The hydrology was reanalyzed in October, 1992 by Resource Consultants and Engineers Inc.
(RCI) for the U.S. Army Corps of Engineers. The hydrologic information contained in that
report indicates much higher 100-year peak discharges (approximately 5,022 cfs upstream of the
Larimer and Weld Canal). Larimer County and the City of Fort Collins contracted an
independent review by Lidstone & Anderson Inc. to verify the information in the RCI report.
The results indicated similar peak 100-year discharges (approximately 5,600 cfs at the Larimer
and Weld Canal). Therefore, the existing HEC-1 model developed by RCI for the U. S. Army
Corps of Engineers was utilized to determine the effectiveness of conceptual flood control
alternatives. Flood control alternatives were evaluated using a design storm of3.46 inches from
a balanced storm with 6-hour duration.

The source of Dry Creek is in the foothills and flows onto the agricultural plains immediately
north of Fort Collins. The total drainage area of the basin is approximately 75 square miles, with
a contributing drainage area of approximately 62 square miles. The basin is long and narrow,
with a major axis length of approximately 23 miles and a maximum width of approximately 6
miles. Elevations range from 7,000 feet MSL to 4,900 feet MSL at the confluence with the
Cache la Poudre River.

Stream channels in the uppermost basin are steep, narrow and deeply incised. Downstream, the
channels become wider and shallower with well defined floodplains. In the lowermost portion of
the basin (downstream ofthe Larimer and Weld Canal), parts of the channel have been
obliterated by development. There are two major tributaries to the Dry Creek basin: 1) Spring
Gulch and 2) Park Creek. Spring Gulch drains the upper northwest portion of the basin and joins
Park Creek in the upper one-third of the basin. Park Creek joins Dry Creek near the center of the
basin.

Land use in the basin is primarily agriculture. The uppermost areas are range land, with a
majority of the rest being irrigated pastures and hay meadows. The lowest portion of the basin in
the vicinity of Fort Collins consists of commercial, industrial, and residential development.

A system of irrigation canals and reservoirs makes the Dry Creek basin hydrologically complex.
There are 14 major reservoirs in the basin, 9 of which are large enough to capture a majority of
the local runoff resulting in a reduced area contributing to Dry Creek. The remaining 5
reservoirs are located on the mainstem or one of the major tributaries and affect the downstream
flows. Seven major irrigation canals cross the Dry Creek basin, intercepting flood flows and
having a significant effect on the Dry Creek flood hydrology. Although there are some records
available for flows in the irrigation canals and reservoir levels, no stream flow records exist for
Dry Creek.

The runoff hydrograph produced at the Larimer and Weld Canal exhibits a distinct twin peak.
Figure 2-1 shows the twin peaked hydrograph produced at the Larimer and Weld Canal. The
first peak is generated from runoff originating downstream of Douglas Dam. The second peak is
generated by flows originating above Douglas Dam, which are routed through the existing
spillway structure.

T:\ProjectsI6824660Dry CrkDiversionLarimerCountylSub_00\12.0 Word ProcIFlood Control Plan.Rptdoc 04/24100(11 :14 AM) 2-1