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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people per household, the population of the Local Basin is 4,037. In addition the Local Basin
has 424 businesses.

IV. Preliminary Engineering Investigation

Preliminary engineering for the project was conducted by URS Greiner Woodward Clyde
Engineering Company (URS). The study was published in a report entitled "Dry Creek Flood
Control Plan", April 2000. Larimer County engaged URS Greiner Woodward Clyde (URS) to
conduct a preliminary engineering investigation of alternatives for flood control on Dry Creek
pursuant to the terms of the IGA. Funding for this work was provided by Larimer County, the
City of Fort Collins, and a grant from the Colorado Water Conservation Board. The URS Plan
represents a conceptual solution to both flood control and local basin stormwater needs in urban
areas of northeast Fort Collins. The solution was determined from three alternative approaches
that were examined. One such alternative was to divert the entire Dry Creek flood flow from the
Willox Lane Dry Creek intersection south and west into the Cache La Poudre River as suggested
in the 1980 GAl Study. Other alternatives examined were detention storage of the entire flood
flow and detention storage of the partial flood flow and diversion of the residual flow. The
objective of the preliminary engineering study was to define improvements which would convey
residual upper basin flood flows, reduce the burden on irrigation canals and reservoirs, and
provide for disposition of Local Basin stormwater flows. The criteria for that solution was the
least cost solution which achieves these objectives.

Existing Basin Hydrology

Three previous hydrologic evaluations have been performed for the Dry Creek watershed by
Gingery Associates, Inc., (1977 and 1980) referred to herein as the GAl study, Resource
Consultants, Inc., 1992 referred to as RCI, and Lidstone and Anderson Inc. (1997), referred to as
LAI. 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 State Highway 287
as 2,900 cfs, based on the GAl Study. A second analysis of the watershed hydrology was
conducted in October 1992 by RCI under contact to the Omaha District, US 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-Weld Canal). Larimer County
and the City of Fort Collins contracted an independent review of the RCI hydrology by LA! to
verify the information contained in the RCI Study. The results indicated similar peak 100 year
discharges (approximately 5,600 cfs at the Larimer-Weld Canal). This study therefore uses the
existing HEC-l model developed by RCI for the US Army Corps of Engineers to determine and
compare the effectiveness of alternative flood control measures. Flood control alternatives were
evaluated using a design storm of 3.46 inches from a balanced storm with a 6-hour duration.

Figure I shows the hydro graph of the 100 year runoff flow produced from this model at the
Larimer- Weld Canal. The 100 year runoff hydro graph produced from this model shows two
distinct peaks. The earlier peak of slightly in excess of 4,000 cfs is generated from watershed
areas downstream of the Douglas Reservoir. The later peak of slightly greater than 5,000 cfs.
is generated by flows originating upstream of Douglas Reservoir.

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