When a car needs a tune-up, a mechanic simply pops the hood to gain access to the engine. But what happens when a massive, early-20th-century dam needs repair and its parts are nearly 300 feet under water? In installing a new bulkhead system for the O’Shaughnessy Dam last year, the SFPUC’s project team found that maintaining the dam involved going deep – literally. The team deployed divers and cutting-edge solutions including an unmanned cleaning tool to ensure that the dam remains in tip-top shape and continues releasing water as needed.
Built in 1923, the O’Shaughnessy Dam impounds the Hetch Hetchy Reservoir. Within the dam and further downstream are special valves that can release water in controlled amounts to manage changing reservoir capacities and the water that flows out into our drinking water supply and into the Tuolumne River. Part of the valve system includes bulkheads, which are underwater slide gates installed over 100 years ago. These are essential to isolating the valves, so that engineers and technicians can safely access the outlet system as needed.
Previously, making repairs to the dam’s aging conduits would have required us to significantly empty the Hetch Hetchy Reservoir. Instead, the SFPUC embarked on a three-year, $6.63 million project to deliver a modern bulkhead system that would allow us access without requiring reservoir dewatering. The Infrastructure team worked with the US Corps of Army Engineers and contractor Alltech and used 3D modeling to analyze the hydrodynamics and stress test the bulkheads in advance. With the gates fabricated and tested, the new bulkheads were installed in 2024.
Bring in the Divers
Phase two of the project started last year. The SFPUC’s project team enlisted the help of J.F. Brennan, a specialty marine construction firm, to inspect the new gates and refurbish the other underwater components of the dam. Four commercially certified divers joined on-site on a cool November day, with their scuba gear in hand.
However, before any divers entered the water, J.F. Brennan launched an unmanned cleaning tool that used high-pressure water jets to remove rust build-up and corrosion from the underwater inlet surfaces. This innovative tool dramatically reduced diver time in the water, improved safety, and increased efficiency, particularly in deep locations ranging from 150 to nearly 300 feet below the surface. The tool also allowed the team to achieve a level of cleaning consistency that would have been impractical or unsafe using diver-only methods.
“The O’Shaughnessy Dam is a feat of engineering and keeping it state-of the art is our responsibility,” said Jimmy Leong, principal project manager in Infrastructure's Project Management Bureau. “This key piece of infrastructure endures because the SFPUC is vigilant about maintaining and future-proofing our system, so that we can all continue to enjoy drinking water that is among the purest in the world.”
With the new bulkhead system in place, operators can now safely rehabilitate the dam’s existing outlets. For their innovation and collaboration, the project team was recognized with a silver award from the San Francisco Collaborative Partnering Steering Committee and two awards from the International Partnering Institute.
Photos courtesy of the San Francisco Public Utilities Commission/Robin Scheswohl.
