In San Francisco there’s a unique secondary “high-pressure water supply system” which is fully independent of the regular water mains of the city. The Twin Peaks Reservoir that was built to supply it has two separate basins so that either one of them can be off-line for maintenance when a major fire hits. I guess all of this has to do with the history of San Francisco burning down more than once.

https://sfpublicworks.org/project/emergency-firefighting-wat...

https://waterpowersewer.wordpress.com/2019/07/01/a-look-back...