State money secured for Interlake tunnel project

The tunnel is proposed as a flood control mechanism diverting water from Nacimiento Reservoir to San Antonio Reservoir that would have otherwise been spilled at Nacimiento Dam. The Nacimiento River basin produces nearly three times the average annual flow of the San Antonio River basin, according to the Monterey County Water Resources Agency (MCWRA). Capturing high Nacimiento River flows and diverting those flows to San Antonio Reservoir increases the overall storage capacity of the system.

The MCWRA anticipates awarding a design-build contract for the tunnel project located in the County of San Luis Obispo. The agency is proposing an approximately 12,000ft-long (3.7km) tunnel, citing either TBM or roadheader excavation. Conceptual design envisions a reinforced concrete lined tunnel with an inside finished diameter of 10ft (3m) and a slope from Nacimiento to San Antonio of -0.4 percent. The tunnel will be designed to accommodate internal pressures and seismic activity in the region.

In addition to the conveyance tunnel between the lakes and intake and outlet facilities, the project concerns spillway modifications at Lake San Antonio to increase storage by approximately 60,000 acre-feet. Total project cost is estimated at USD 48M, including USD 3.4M in contingency. Depending upon the degree of environmental documentation required the Interlake Tunnel could be completed in two to three years. MCWRA’s schedule anticipates the environmental and permitting process will wrap up in 2017, and calls for an operational tunnel by 2020. Epc consultants signed a program manager and construction manager contract with the agency in July 2014.