02000000008 Feasibility Study

• Contract/PO #: 02000000008
• Contractor Name: Lake Canal Reservoir Company
• Contract Type: Grant
• Water District: 0
• County: Larimer
• Loan Projects - Doc Type: Feasibility Study

Photographs are included in Appendix H showing the conditions of the dam and its appurtenant
structures.
, ALTERNATIVES EVALUATED
' During our study of Gray Lake No. 3 we evaluated the following altematives: 1) No action
alternative which may require breaching the dam, 2) Complete rehabilitation of Gray Lake No.
' 3 Dam and its appurtenant structures, and 3) Abandoning Gray Lake No. 3 and raising the dam
on South Gray Lake to increase the storage by the 111 acre-feet that is decreed for Gray No. 3.
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Following is a summary of our assessment of each alternative.
Alternative Hl
If Gray No. 3 is to be taken out of service long tecm, the State Engineer may require the dam be breached
in accordance wi[h Rule 7. This ru(e 7.A.(2)(a) requires the breach be sized to pass a 100-year flood.
Work to be conducted
1. Prepare plans for breaching the dam including a 100-year flood determination.
2. Breach the existing dam.
3. Spoil the excavated material.
4. Seed work area and spoil area.
Alternative N2
' This alternative consists of a complete rehabilitation of Gray Lake No. 3 dam and its appurtenant
structures. This work includes repairing the dam embankment, replacing the existing outlet and installing
a spillway. The dam is constructed of low density sands and would be expected to leak without some
� measures to control the seepage. An ups[ream PVC liner on the main dam embankment was included in
this study with a toe drain placed at the downstream toe to collect any seepage. The liner will be covered
with soil ro protect it from puncrures. The upstream face of the main dam will be covered with riprap and
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bedding for erosion protection.
The south dike for the reservoir will need ro be raised approximately 2-feet to provide for proper freeboard
and will require riprap placed on it for erosion protection. The dike between Gray No. 3 and South Gray
will not have to be raised but will have to have riprap placed on it for erosion protection.
Work to be conducted
i. Breach main dam embankment at existing ouflet.
2. Replace outlet works with 18-inch concrete pipe with concrete bedding.
3. Construct a gate tower intake structure with a bridge to access and a headwall/energy dissipater at the
downstream end of the outlet pipe.
4. Construct a 50-foot wide, 5-foot deep concrete spillway wall with 12-ioches of riprap and 6-inches
of bedding on the upstream and downstream sides to control erosion.
5. Install a 50-mil PVC liner on the upstream face of the main dam; flatten the upstream face of the main
Gray Lake-Feasibility Study Page 5
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