PROJECT PROFILE
Client:
International Boundary & Water Commission (IBWC) and the City of San Diego

Constructed Value:
$ 133 Million

Project Duration:
1986 - 2000

Parsons Services:
• Evaluation of tunnel alternatives versus conventional outfall construction

• Risk analysis for seismic event

• Final design of 19,000-foot tunnel, 160-190 feet under the sea floor

• Design for 75-year service life

• Hydraulic modeling, transient analysis

• Risk management studies tunneling

• Pipeline design

• Construction services
San Diego South Bay Ocean Outfall

The Pacific Ocean near San Diego has been contaminated by seven decades of raw sewage flowing over the border from Tijuana, Mexico. Parsons designed the South Bay Ocean Outfall to meet California state ocean plan discharge requirements to eliminate pollutants from the ocean water, beaches and local wildlife preserves. The outfall conveys treated effluent flows from the IBWC's international wastewater treatment plant to the South Bay Wastewater Treatment Plant and the City of San Diego's proposed Wastewater Reclamation Plant, then to the Pacific Ocean for the final disposal.

Parsons began the project in 1986 with extensive oceanographic field studies, including modeling to ensure compliance with California ocean disposal standards. This process led to two options to meet the standards: a conventional marine outfall or a tunnel outfall.

SELECTION OF THE OCEAN OUTFALL DESIGN
The following considerations led to the selection of the ocean outfall:

  • A tunnel eliminated the use of a construction trestle to reduce the impact of sand migration within the surf zone.

  • Tunneling minimized environmental impacts to a local salt marsh and barrier dune habitat, a major refuge for birds and wildlife.

  • The ocean outfall reduced the potential for liquefaction and seismic impacts.

  • The 3.5 mile long outfall—200 feet below sea level in soft, saturated ground lowered project costs by $43 million.

  • The ocean outfall reduced energy and chemical usage by elimination of chlorination


The earth pressure balance machine being lowered into the tunnel.
The project includes a 19,000-foot-long combined tunnel outfall connected to 4,400 feet of conventional sea floor pipe outfall. The 132-inch internal diameter outfall tunnel was constructed approximately 220 feet below sea level and ends approximately 13,600 feet offshore. A riser conveys effluent from the tunnel up to the sea bed where it connects to the 120-inch internal diameter sea floor pipe. The outfall ends at a diffuser-wye structure in the water between 95 and 97 feet below sea level. The outfall is designed for a service life of 75 years, and has the capacity to handle average dry weather flows of 174 million gallons per day (mgd) and peak flows of 333 mgd.

This project pioneered many design and tunneling innovations including:

  • A specially designed earth pressure balance machine tunneled longer and at a higher external pressure than on any other project in the world.

  • Parsons designed a 160-foot-deep prefabricated steel riser assembly which was constructed in 80 feet of water to connect the tunnel with the conventional marine pipeline.

  • Parsons also designed a single pass segmented liner to withstand 200 feet of external pressure and a differential internal operating head of 89 feet.

Today, the new South Bay Ocean Outfall has restored San Diego beaches, freed local wetlands from pollutants and eliminated a potentially hazardous situation.

  Recognition
The South Bay Ocean Outfall has earned the following awards: ASCE Outstanding Civil Engineering Achievement Award of Merit, the AAEE 2000 Grand Prize in Design and the American Public Works Association Year 2000 Environmental Project over $10 Million.

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