A 100-Year-Old Emergency Water Supply Put Out A Fire In SF This Week

The five-alarm fire that destroyed a San Francisco apartment building this week put the city's municipal water supply to the test: When water pressure began to dwindle, firefighters tapped an emergency system that was built below the city—all the way back in 1913.

The six-story building under construction in the city's Mission Bay neighborhood caught fire Tuesday and raged into the night, creating a mushroom cloud that billowed above the city. Over 90 fire vehicles were called to the scene, which quickly overwhelmed the water system and diminished firefighters' capabilities to pump water out of the city's hydrants.

Firefighters ran hoses several blocks away to a different set of hydrants, which are connected to the city's backup water network. Created in response to the 1906 earthquake, where 80 percent of the city's buildings were lost to fire, the system is, essentially, emergency-proof: It's powered by gravity from a pair of water tanks located atop the city's biggest hills: Twin Peaks and Nob Hill (a third reservoir, also on Twin Peaks, is currently under construction). The Twin Peaks Reservoir holds over 10.5 million gallons alone, which provided enough water pressure to put out the fire.

A 100-Year-Old Emergency Water Supply Put Out A Fire In SF This Week

Twin Peaks Reservoir, where the water came from that was used to fight this week's fire

The visionary Auxiliary Water Supply System was first proposed in 1903 by the city's fire chief Dennis T. Sullivan (who actually died in the 1906 fires), but it was the earthquake and subsequent fires that spurred the city to build it: Over 300 water mains broke in the earthquake, obliterating any chance of putting out the flames that engulfed the city.

What they needed was a system with shock-resistant piping that could maintain pressure and didn't require the water to be carried long distances. The system was installed by 1913, designed by city engineer Michael O'Shaughnessy, the same person who created the Hetch Hetchy water system, which delivers San Francisco's drinking water from the Sierras.

A 100-Year-Old Emergency Water Supply Put Out A Fire In SF This Week

A 2009 map of the system, the special fire hydrants seen throughout the city, via SFFD

Auxiliary Water Supply System is not as comprehensive as the city water supply; you can't find the 1,500 hydrants—which have black, red or blue caps to denote which tank they come from—in every neighborhood. But the system is amplified by two generator-powered pumping stations, a fleet of fireboats that can help pump seawater, and other underground cisterns (marked by brick circles), which make it a fairly resilient system, even by today's standards.

The brilliance of the system is that it's completely site-specific and an almost "natural" solution which takes advantage of both the hilly terrain and the fact that the city is surrounded by water. Los Angeles, for example, could easily lose access to several of its aqueducts during a major earthquake, yet it has no similar emergency water supply in place.

Many San Francisco residents don't even know this system is hidde under their feet, but hopefully the fire has demonstrated its importance: In June, San Francisco will vote on a $400 million bond measure to expand the network. [SF Chronicle via Five Intriguing Things]

AP Photo/Jeff Chiu