Forbes Hill Reservoir

Cost: $3.8 million
Location: San Rafael, Calif.
Year: 2016-12-12
Size: 30,653 sq ft
Owner: Marin Municipal Water District
Designers: Beyaz & Patel Inc.
Contractor: Aztec Contractors

When one pictures “an­tiques,” collectibles gen­erally come to mind—pieces of furniture, old clocks, first-edition books. But water infrastructure can be antique, as well—and when aging infrastructure starts to deteriorate, problems arise. Such was the case in San Rafael, Calif., a city of approximately 50,000 people located 10 miles north of San Francisco. 

The city’s 120-year-old Forbes Hill water reservoir, which serves 187,500 customers in central and southern Marin County, was suffering from structural and maintenance issues. The reservoir’s uneven surface encouraged rainwater pooling, increasing the risk of non-potable water leaking into the clean water supply. Years of exposure to the California sunshine had caused its cover to crack, allowing dust and debris to gather on the surface. Remedying these issues required constant, costly maintenance. Eventually, the decision was made to transform the reservoir into a 4-million-gal tank with a maintenance-free roof structure.

There were formidable hurdles to the project’s completion. For one, the project had to be completed in only six months, during the reservoir’s winter shutdown period. In this short span, the project’s contractor had to demolish the reservoir’s existing roof and liner, prepare its floor for the installation of column footings, and install the new roof and liner. Another hurdle entailed making the new tank earthquake-proof. To do this, the tank’s roof had to be elevated to meet seismic standards.

The project began in October 2014 and was completed in April 2015. Special care was taken to place the tank’s roof high enough and at the proper slope to mitigate the threat of water coming in contact with the roof’s underside. Because of the reservoir’s oval shape, primary supports had to be installed on the outside. 

The roof for the structure was an OptiDome from CST Covers, a Div. of CST Industries. CST manufactured the dome’s components at its Conroe, Texas, facility, then shipped them to the site for assembly. The OptiDome’s seamless, flush batten design allows it to shed water and eliminate ponding on the cover’s surface. Enclosed gaskets are less susceptible to ultraviolet exposure and degradation. Made of aluminum, the dome can support work crews, snow loads and high winds, and can last 50 to 100 years.

Today, the reservoir is in service and is providing water to customers of Marin Municipal Water.

“The Forbes Reservoir provided a unique challenge because the shape of the reservoir was oval, and the primary supports were to be outside of the reservoir,” said Hector Moreno, regional sales manager – municipal sales for CST Covers. “Our engineering team, along with the consulting engineer, did a fantastic job in coming up with the most optimum design, which also turned out to be aesthetically pleasing.

“This project is important because it shows the collaboration between client, consulting engineer and supplier in the most productive and constructive way. It is a great example of the sum of the parts being greater than any individual component.”