C150163 Feasibility Study

• Contract/PO #: C150163
• Contractor Name: New Cache La Poudre Irrigating Company
• Contract Type: Loan
• Water District: 3
• County: Weld
• Bill Number: 04-1221
• Loan Projects - Doc Type: Feasibility Study

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1 OO-yr storm which would reduce the required spillway length substantially. If the
Drury Reservoir spillway were designed for the 100-yr storm, and the dam failed,
the Company would be liable for the incremental damage, but the project would
be more economically feasible. This feasibility study assumes that an
incremental damage analysis will be conducted and accepted by the State
Engineer's Office for Drury Reservoir. Therefore, the preliminary design and cost
estimates contained in this study are based on a 100-yr storm for Drury
Reservoir. The required spillway length for the 1 OO-yr design storm is 430 feet.

The spillway used for the cost estimate consists of a concrete cutoff wall with
rip rap extending approximately 10 feet beyond the toe of the dam. The spillway
cutoff wall has a trapezoidal cross-section with a 430-foot base width and
6(horizontal):1 (vertical) side slopes. Results of the preliminary hydrologic
modeling for Drury Reservoir are summarized in Table 5.

T bl 5 D

R

"Hd I 'Mdl' S

a e - rury eservolr IYI ro oglc o emg umman
Kn = 0.06
Dam Classification -+ Class II, Small Class II, Small
Design Storm 50% PMP 100-yr storm 1
Peak Inflow 47,890 cfs 11,512 cfs
Total Inflow 24,335 ac-ft 4,586 ac-ft
Peak Outflow 47,890 cfs 8,968 cfs
Peak StaQe 4,718'" 4,718'"
Required Spillway LenQth 2,250' 430'

1. Assumes Incremental Damage Analysis is accepted by SEO.
2. Peak stage elevation is one foot below the top of dam elevation in accordance with SEO requirements.

Cornish Plains Reservoir

The Cornish Plains Reservoir Dam can conservatively be classified as a Class II,
Intermediate dam. The required design storm for a Class II, Intermediate dam is
50% of the PMP. Because the Cornish Reservoir basin is small, the reservoir
would only require a 15-foot wide spillway to pass the 50% PMP storm. The
spillway used for the cost estimate consists of a concrete cutoff wall with riprap
extending approximately 10 feet beyond the toe of the dam. The spillway cutoff
wall has a trapezoidal cross-section with a 15-foot base width and
6(horizontal):1 (vertical) side slopes. Results of the preliminary hydrologic
modeling for Cornish Plains Reservoir are summarized in Table 6.

Table 6 - Cornish Plains Reservoir H drolo ic Modelin Summary
Dam Classification -+ Class II, Intermediate
Desi n Storm 50% PMP
Peak Inflow 8409 cfs
Total Inflow 1545 ac-ft
Peak Outflow 494 cfs
Peak Sta e 4702'
Re uired S illwa Len th 15'
1. Peak stage elevation is one foot below the top of dam elevation in accordance with SEO requirements.

New Cache Feasibility Study

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