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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capacity curves for a 36 inch
For the size of pipe considered
will be controlled by the inlet

diameter concrete outlet conduit.
and the expected slope, the flow
conditions.

D. Structure Plans

1. Embankment. The embankment sections considered are at
slopes of 3 to 1 and 4 to 1. These slopes are considered to be
the minimum face slopes feasible for an earth dam. Thicker riprap
sections, problems with the downstream slope erosion control, and
stability under drawdown conditions become problems on slopes
steeper than 3 to 1.

For the stability analyses conducted, only 3 to 1 slopes were
analyzed as the factors of safety were all greater than the
minimum required and would be even greater for the 4 to 1 slopes.
The stability analyses are presented in the Geotechnical Report in
Appendix G and the minimum factors of safety for the cases
analyzed are summarized in Table 3 of Appendix G.

The 4 to 1 upstream slopes were considered due to the
practical consideration for long term maintenance and longer
riprap life. Riprap will last longer and less displacement and
less long term erosion will take place on a 4 to 1 slope than on a
3 to 1 slope. This is due to the wave energy or force on the
riprap being less on the flatter slope, and due to less likely
hood of the riprap rolling off the flatter slope. Having a 4 to 1
slope on the upstream slope is not required from a structural
stand point but is an alterative that needs to be considered due
to the high cost of riprap and long term riprap replacement costs.

Appendix H contains typical drawings of the
sections analyzed and considered feasible in this
three alternatives presented are as follows:

various cross
study. The

1. Rehabilitate the existing dams to 3 to 1 upstream
and downstream slopes and maintain the maximum
decreed water elevation of Gage Height 24 feet.

2.

Rehabilitate the existing dams
and 3 to 1 downstream slopes
maximum decreed water elevation
feet.

to 4 to 1 upstream
and maintain the
of Gage Height 24

3. Raise the dam to a crest height of 34 feet, five
feet higher than alternatives 1 and 2, to increase
the storage capacity. The upstream slope was
considered to be 4 to 1 with 3 to 1 downstream
slopes.

For the studies conducted, a small clay filled core trench
approximately five foot deep along with an upstream clay blanket
covering the entire upstream dam face was considered to cut off
any possible seepage between the existing embankment fill and the

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