What Are Those Brick Circles in SF Streets?

I'm especially interested in this given my new place-of-work.  Although, I must say, SF is full of architectural curiosities.  Makes me want to visit there - or at the very least, re-read Laurie King's Locked Rooms.

What Are Those Brick Circles in SF Streets?

via CALIBER by Stuart on 3/8/11

There's actually an interesting story behind them…


I've seen these brick circles, or sometimes half circles, in the center of various intersections through out the city. I thought they might be historical markers or previous landmarks. In fact, each brick circle outlines an underground cistern full of pressurized water. The cisterns, ranging in size from 10,000 gallons to 250,000 gallons, are supplemental water supplies for the SF Fire Dept in addition to the high pressure and low pressure systems. They are accessible by special green hydrant valves and are tested every few years to ensure integrity.
There are 172 cisterns strategically spread throughout SF, totaling 11 million gallons of auxiliary water, enough to cover the city 1.25″ deep, and are part of the San Francisco Fire Department Auxiliary Water Supply System, a subset of The San Francisco High Pressure Water System.
Fifty four of the oldest were built as early as 1860. They exist independently of other water systems in the city and were put in place following the 1906 earthquake. As you may recall, it was the fires following the quake that destroyed most of the city and lack of water supply was a key reason the conflagration was able to do so much damage. In the event we were to experience another catastrophic earthquake, the high pressure supply system could be compromised and fire fighters would need immediate access to large quantities of water. These cisterns provide that additional flow, and as part of the high pressure supply, are unique to any city in the United States.
Over the next few weeks, I'll be investigating this High Pressure System further. It's unique to this city, quite complex and is made up of a lot of interesting landmarks that are fun to hunt down.


sources and more info:
http://www.sf-fire.org/index.aspx?page=1003
http://ronslog.typepad.com/ronslog/2010/05/emergency-preparedness-in-san-francisco.html
http://www.sfmuseum.org/hist1/hpfs.html
http://www.sfmuseum.org/quake/awss2.html
http://www.sfcityguides.org/public_guidelines.html?article=344&submitted=true&srch_text=&submitted2=&topic=neighborhoods