PROJ00279

• Contract/PO #: C153572
• Contractor Name: North Poudre Irrigation Company
• Contract Type: Loan
• Water District: 3
• County: Larimer
• Bill Number: HB 85-1042
• Loan Projects - Doc Type: Feasibility Study

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The second spillway location will not require the purchase of
Right-Of-Way to get the flows into the natural drainage chann~l.
However, the Windsor Ditch creates a dam across the natural
drainage which will backup the spillway flows all the way to the
dam toe. Thus, a berm will have to be built along the spillway
channel to keep the flows retained until the ditch bank is
breached. This type of embankment is often hard to hold during
high flows and is not structurally appealing.

For both spillways proposed the entrance channel will be
approximately 300 feet wide, the control section 250 feet wide and
the channel beyond the control section will be approximately 300
feet wide. The extra width at the inlet and outlet will help
reduce the velocities and minimize the amount of riprap protection
required. The control section will consist of a concrete wall
that is riprapped for protection. Since both spillways will be
cut through natural materials at the abutments of the dams,
erosion protection other than seeding will not be required except
at the control section and at any changes in the channel align-
ment.

The proposed spillway section of 250 feet will flow 4.5 feet
deep at the peak discharge for the 0.75 PMP flood. The spillway
control section will be at Gage Height 24 feet with the peak at
Gage Height 28.5 feet which is U.S.G.S.Elevation 5233.1 feet. A
typical spillway section is shown in Appendix H.

E. Drainaqe. Special drainage problems occur at the site in
conjunction with the two spillway locations proposed but are
discussed in Section D above. With regard to the general site
drainage from local runoff, no special problems are expected on
the site due to drainage except at the toe of the East Dam. The
ponding of water at the toe between the dam and the Windsor Ditch
will have to be eliminated by filling the depression or by
providing a swale to the west to provide drainage away from the
dam toe. Surface drainage must be provided for in all areas of
new construction to prevent future ponding. Underseepage from the
reservoir will occur but for the most part will be controlled by
the toe drains which will intercept and divert any underseepage
into the natural drainage ways below the dams.

F. Sedimentation. Estimates of sediment yield are complex and
involve numerous assumptions and laboratory determination of
site-specific parameters. (Linsley, Ray K., Jr. et al: Hvdroloqv
for Enqineers, McGraw-Hill, New York, 1982, pp. 311-338. Bureau
of Reclamation: Desian of Small Dams, U. S. Government Printing
Office, Washington, 1974, pp. 767-796). Due to the difficulty of
conducting this type of study it was determined a reasonable
approach would be to utilize previous studies conducted by the
Soil Conservation Service for Dams B-2 and B-3. Both of these
dams are within 3 to 4 miles of the Reservoir No. 2 site and have
soil characteristics similar to Reservoir No. 2 The watershed for
B-3 is very similar to the drainage basin for Reservoir No. 2 in
which the flows through the basin are intermittent and occur only

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