Inter-lake Connection

Monterey County Water Resources Agency is continuing to develop the project that will connect Lake Nacimiento with Lake San Antonio via a 11,000 foot ‘tunnel’ dedicated to transfering water between the reservoirs; the development schedule of the Inter-Lake Tunnel Project has been beset with schedule challenges due to:

Delays in the U.S. Geological Survey’s development of an operational baseline model from Monterey County’s recently completed and calibrated integrated groundwater-surface water model. This model is critical in evaluating the hydrologic and hydrogeologic benefits of the project and is necessary for the completion of the draft environmental impact report.
Creating a Memorandum of Understanding with California Department of Fish & Wildlife on the white bass issue (non-transference from Nacimiento to San Antonio).
Landowner resistance to granting access to geotechnical borehole sites.

Monterey County Water Resources Agency now anticipates the Proposition 218 funding election to take place sometime in 2024.

Biological and cultural field surveys for sites impacted by the project have been completed and the draft Administrative Environmental Impact Report (EIR) was circualted for public comment in early 2023. Certification of the draft EIR is dependent upon finalizing the Project Description which includes required hydrologic and hydrogeologic modeling and analysis of design and operational alternatives. The draft EIR is estimated to be available for public review and comment in late 2021.

Project History

On December 9, 2014, the Monterey County Board of Supervisors and Monterey County Water Resources Agency Board of Directors, in a joint meeting, authorized the Monterey County Water Resources Agency to proceed with the Interlake Tunnel Project, including spillway modifications to the dam at Lake San Antonio. They directed the Monterey County Water Resources Agency to use AB 155 (Alejo), meaning this will be a design-build project as approved through this legislation and utilizing a Project Labor Agreement (PLA).

The Board of Supervisors authorized proceeding with negotiation of a funding agreement between Monterey County and the Monterey County Water Resources Agency for an amount not to exceed $2.5 million to perform geotechnical and final design engineering; permitting and environmental approval; financing plan preparation and implementation; and, program management services all for the Inter-lake Tunnel project.

The amount of funds to be financed by rate payers (pending a Prop. 218 vote) will depend on the outcome of the design work (the current budget is nearly $200 million, including the spillway modifications) and if other funding sources can be identified from the state through the Water Bond (Prop. 1, passed in November 2014) or from federal grants.

Funding agreements were again approved at the Board of Supervisors meeting on December 8, 2015; an additional amount of $971,000 will be directed to Phase I of the environmental studies required for the environmental impact report. This brings total funding of this project by the County to $2.071 million. At one time, consideration of a Public Private Partnership process (P3) was brought to the Board of Supervisors that could provide an alternative method of devloping and constructing the project. As there was no definitive timeframe for the project EIR draft, as well as the engineering report necessary to establish beneift zones, there was no decision on P3.

In early 2017, MCWRA was approved for a DWR grant of $10 million for sutides associated with this project. The grant funding was made possible through a CA Assembly Bill sponsored by (then) Assemblyman Luis Alejo. Grant funds will be used to prepare engineering and environmental reports, as well as project management during this phase of the project.

During the 2019 state budget process, funding was secured from state general funds, that if the project is approved for construction, the costs of fish screens required by CA Fish & Wildlife (to prevent the transfer of white bass from Nacimiento to San Antonio) would be paid exclusive of any local funding methods for the project. The estimate for these fish screens is approximately $17 million. Because of subsequent delays in the project development, the state curtailed the use of these general fund grants under the budget emergency genearted by the COVID-19 pandemic.

Engineering and enviromental studies continue, with public review now scheduled for late 2021.

At a special Board meeting held at the Ag Center Conference Room on November 19th, 2014, MCWRA presented preliminary findings and study results on the proposed inter-lake tunnel project. Presentation was made by consultants to the agency: EPC Consultants Inc., ECORP Consulting Inc., and Hollembeck Consulting.

Benefits of this project were listed as:

Significant increase in flood control storage, thus a reduction in flood damage downstream
Additional surface water available to serve current and future suite of infrastructure projects
Provides a supply of surface water to help sustain groundwater supply by offsetting pumping
Provides environmental benefits through increased flows in the Salinas River

The project was outlined as a 12,000 foot tunnel between Lake Nacimiento and Lake San Antonio, at a point where the downhill gradient is approximately 20′ between the two reservoirs. A 10′ diameter concrete-lined pipe will be installed as part of the coring process for the tunnel. The water transfer will be entirely gravity flow.

The hyrdologic model prepared to this date incorporates was supply requirements and water rights limitations and generally operates as follows:

Under baseline conditions, each reservoir is operated within its water rights
Water type years are dry, normal, and wet
Block flows are released when called for
Two consumptive demands that diver directly from Nacimiento (17,500 afy)
Minimum flow requirements are met from each reservoir
Salinas River Diversion Facility (SRDF) is operated on water year type based release patterns
Reservoir balancing to meet SRDF requirements are achieved through releases from Nacimiento up to the capacity of the hydroelectric plan (remaining releases to be made from San Antonio)
If Nacimiento is lowered to minimum pool, then San Antonio would meet SRDF release requirements

Proposed operating concepts of the tunnel:

Nacimiento intake elevation is at ‘sweet spot’ of 745′
Tunnel will operate on head relationship between inflow and outflow in a pressure flow mode
Water conveyance through the tunnel will occur when the Nacimiento surface water elevation is above 760′ (15′ above the intake)
No water conveyance through tunnel when San Antonio is full

Cost estimate for the project originally was $48 million plus another $15 million for improvement of San Antonio Dam (raising dam by 10′). Expected capture of water through the tunnel averages from 46,000 to 50,000 afy, depending on dam improvements. Increase volume over baseline of conservation releases made available by the tunnel is 16,327 afy; subtracting out SRDF releases of 5,390, this indicates remaining water available for suite of future projects of 10,937 afy.

The then-timeline for the project shows that if a funding mechanism for the project requires a Prop. 218 vote, date to be determined. In the subsequent years since this Board meeting, costs have been revised considerably upward, and the amount of water storage capacity and other benefits have be refined. There also now appears to be a reservoir permit hurdle that needs to be overcome with the State Water Resouces Control Board.

On June 3, 2014 the Monterey County Board of Supervisors approved a initial $500,000 funding mechanism for initial environmental and engineering studies for a new project that would link the Nacimiento and San Antonio reservoirs together through a pipeline (interlake tunnel project). The results of the preliminary studies on water rights, storage capacity, and construction cost estimates were presented to the Board of Supervisors on October 28th, 2014; review the presentation on this project here. Upon consideration, the Supervisors have delayed action on approving the project until early December 2014; more information is needed from the technical reports yet to be issued, as well as a better understanding of how the design/build process could benefit this project (AB 155, sponsored by Assemblyman Alejo).

The project concept is based on the fact that the Nacimiento watershed provides three times the water levels of the San Antonio watershed, thus filling up the Nacimiento reservoir faster in the rainy season. This pipeline would allow water to be transferred from Nacimiento to San Antonio using a downhill gradient. Additional water storage and flood control management could be accomplished by balancing the reservoir levels in this manner.

The project requires a full engineering and environmental analysis; an intake structure will be constructed at Nacimiento in an area generally closest in proximity to San Antonio, with a estimated 12,000 feet of 10-foot diameter concrete-lined pipeline connecting to an exit structure at San Antonio. Gravity flow will be used to transfer water from Nacimiento to San Antonio, with an estimated annual average of 50,000 acre feet of water to be conveyed. The project is expected to be completed in 2018.

Estimated cost of this project is currently $48 million, including $9.5 million in contingency. Proposed financing is to conduct a Prop. 218 vote (most likely in November 2016), a tax levy on beneficiaries of this project. Estimated tax assements for Zone 2C landowners are between $11.71 to $15.14 per acre per year if the initiative is approved by voters.

This project would improve the benefits provided by the reservoirs and other agency projects to the Salinas River Groundwater Basin and the Salinas Valley, mitigating the impacts of potential flooding, managing for drought periods, environemntal benefits, and improving the viability of the Salinas Valley and its agricultural production.

MCFB continues to monitor this project and attempts to maximize water storage and flood control management for the Salinas Valley.

This project is not undertaken to provide additional water supplies to the Monterey Peninsula. Water in the Salinas River Groundwater Basin and Zone 2C cannot be exported to other areas under the Agency Act.