Orange County Branch Newsletter
South Coast Water District Sewer Tunnel Rehabilitation Project
For the March OC Branch lunch meeting we had three speakers, Eric Jessen, Chairman, South Laguna Water & Sewer Advisory Commission, David Jurich, PE, South Central West Unit Manager for Hatch Mott MacDonald, and Joseph N. McDivitt, Director of Operations, South Coast Water District, Laguna Beach, California. The speakers discussed the a two-mile sewer tunnel built in 1954 runs inside ocean-facing cliffs from Three Arch Bay to Aliso Beach in South Laguna.
The tunnel houses a sewer line that conveys one million gallons of wastewater a day from South Laguna and north Dana Point by gravity flow to the Coastal Treatment Plant in Aliso Canyon. the volume of wastewater carried by the line is approximately 25% of the District's total daily load. The tunnel is undersized, deteriorating and unsafe, posing risks of injury to workers and risks of damaging or breaking the sewer line itself and causing a sewage spill onto the beaches along the two-mile South Laguna coast..
On average, the tunnel is 6 feet wide and tall, but much smaller in many places. The support timbers are deteriorating and the earth and rock walls are crumbling. The pipeline has sustained damage and is at risk. With 10-inch beams protruding into the tunnel and at 2-foot diameter pipeline running through it, there is not much room for maintenance workers to maneuver safely and perform necessary repairs.
In 2007, the District enlarged and stabilized a 750-foot segment of the tunnel as part of an emergency repair project at Thousand Steps Beach, which took eight months and cost $5 million.
The District estimates that the remainder of the tunnel rehabilitation, about 10,000 feet, and the installation of a new pipeline will take six years and cost about $50 million.
Tunnel stabilization. To stabilize the interior of the tunnel, the District must enlarge it to reach solid rock, due to the soil geology of the area. The enlarged tunnel also provides a safe, improved work environment. The reconstructed tunnel will be about 7.5 feet in diameter, possibly larger in sections, structurally reinforced and lined with concrete.
Pipeline replacement. The 24-inch pipeline installed in 1974 will be completely encased by a concrete floor throughout the length of the tunnel and retained for possible emergency use. A new 24-inch pipeline will be installed on the concrete floor above the old line.
Reserves, operating revenues and proceeds from the recent sale of District bonds will pay for a portion of the estimated $50 million project. Additional bonds could be sold in the future. Bond financing is a long-term funding mechanism that spreads costs to future customers who will also benefit from the project.
The reconstructed tunnel is expected to last 100 years without major rehabilitation.
See the full presentation below and/or the SCWD Webpage http://www.scwd.org/projects/tunnel.asp for more details.