Monterey County Water Resources Agency (MCWRA) is proposing building a tunnel to divert water from Lake Nacimiento into Lake San Antonio.
The Nacimiento Regional Water Management Advisory Committee (NRWMAC) is concerned how the tunnel might affect Lake Nacimiento lake levels, recreation activities, property values, and water availability.
This document briefly explains the tunnel project, potential benefits, and questions landowners and lake enthusiasts may have.
Here is what has been said by Monterey County Water Resource Agency
Interlake Tunnel Project
Technical Memorandum
The proposed Interlake Tunnel is a gravity flow water conveyance tunnel approximately 12,000 feet long connecting Nacimiento and San Antonio reservoirs in San Luis Obispo and Monterey Counties respectively. Conceptual design of the tunnel project envisions a reinforced concrete lined tunnel with an inside finished diameter of 10 feet and a slope from Nacimiento to San Antonio of 0.4%. The tunnel will have an invert elevation in Nacimiento Reservoir at approximately 745’ MSL* with an inlet structure equipped with debris racks and stop logs to facilitate tunnel maintenance. The outlet structures in San Antonio will include a valve facility housing and a spherical valve for operation of the tunnel. The outlet structure will terminate in San Antonio reservoir with an energy dissipation structure.
*MSL refers to Mean Sea Level
NRWMACs primary goals for Lake Nacimiento water management
Maximize water levels up to the ideal level 790 ft., which stores more water for recreation & agriculture.
Maintenance of constant lake levels from May 1 through September 30 to enhance the habitat for fish.
Maintain year-to-year elevations through prudent management of transfers and releases.
Utilize transferred water from Nacimiento, being stored in San Antonio, prior to releasing water from Nacimiento.
Recreation water elevation at Nacimiento
People around the lake feel that the minimum lake level for good useable recreation is 780 feet msl, below 760 ft-msl many ramps are not useable and areas of the lake become too narrow for boats to pass
NRWMACs Concerns for the Tunnel Project
Physical Design
What design requirements or thoughts drive the size, location, elevation, shape, of the tunnel, the inlet and outlet works?
What is the priority of these requirements?
What are the design drivers for?
- Location of the inverts (tunnel inlet and exit)?
- Elevation of the inverts?
- Size of the tunnel?
- Size and Shape of other features?
Operational Design
Is there a prescription or rule for the transfer of water from one lake to the other? Any information on timing, amount of transfer, flow rate, or other parameters? What logic was used to model the flows between the lakes?
A concern of NRWMAC is why would MCWRA want the tunnel so deep? If the objective is to capture the 27,000 to 35,000 acre-feet of potential annual spill, it can be accomplished with the tunnel at the higher elevation. There appears to be no reason for transferring water to San Antonio when Nacimiento is only at 35% capacity (elevation 745).
Operationally, an agreement needs to be in place that clearly outlines the rules of operation of the lakes after the tunnel is installed. NRWMAC must be certain that new operational rules protect Nacimiento from unnecessary transfers.
NRWMAC fully supports the Interlake Tunnel Project as long as the transfers and releases benefit recreation.
Nacimiento Regional Water Management Advisory Committee
Questions
- Why not consider the Jerrett Dam Project? It has the potential for 135,000 acre-feet of additional storage.
- Will this project require a vote by SLO County voters as well as Monterey County voters for approval and funding?
- Are there Tunnel alternatives? What are the costs?
- Will Lake Nacimiento be drained for the years of construction? What are the construction impacts to the water, Canyon road and lake water elevation?
- Is it possible to modify other existing dams in the region to increase capacity without the Interlake Tunnel?
- What are the impacts to the local economy, boating and camping?
- What are the current financing sources? Who is paying for this?
How You Can Help
- Send us your questions, concerns, and comments by filling in the contact form on this page.
- Share with friends, neighbors, and others through word of mouth or social media.
- Donate to NRWMAC so that we can continue our efforts to retain enough water for lake property owners and lake enthusiasts on Lake Nacimiento.
