Twin Peaks Reservoir, sometimes referred to as “Summit Reservoir,” is a 10.5-million-gallon storage reservoir assembled from 6-inch-thick (150 mm) reinforced-concrete slabs. Fresh water is delivered from the City’s domestic water system by two 750-US-gallon-per-minute centrifugal pumps. For safety, the reservoir is broken up into two tanks, and each tank can be emptied separately so that in case of a pipe breakage only half of the reservoir is lost. The tank is set at 758 feet.
The Twin Peaks are two prominent hills with an elevation of about 925 feet located near the geographic center of San Francisco. Only 928-foot Mount Davidson is higher within the city. The North and South Twin Peaks, also known as “Eureka” and “Noe” respectively, are about 660 feet apart. The peaks form a divide for the summer coastal fog.
Twin Peaks Reservoir acts as the backbone of the Emergency Firefighting Water System, also referred as the Auxiliary Water Supply System (AWSS). Alternatively, the Emergency Firefighting Water System is often referenced on manhole covers and hydrants as High Pressure Fire System. It is a supply network built for the city of San Francisco in response to the failure of the pre-existing emergency water system during the 1906 earthquake.
The 1906 earthquake crippled the City’s water supplies, and the people of San Francisco blamed the destruction of the City on the failed water system. Insurance rates soared, and, in some areas, coverage was unobtainable.
The Emergency Firefighting Water System was originally proposed by San Francisco Fire Department chief engineer Dennis T. Sullivan in 1903, before the earthquake. However, construction did not begin until 1909 and was finished in 1913.
In 1908, Marsden Manson, the City engineer, conducted a study of 250 cities throughout the world to develop plans for a guaranteed water supply system for the sole purpose of fire protection. Exhaustive tests of pipe materials, valves and hydrants were conducted, and many eminent engineers of the period were consulted. All arguments were for a separate fire combat water system and against a private system outside the jurisdiction of the Fire Department.
The Emergency Firefighting Water System is made up of a collection of water reservoirs, pump stations, cisterns, suction connections and fireboats. While the system can use both fresh or salt water, it is preferential to not use salt water, as it commonly causes galvanic corrosion in fire equipment.
The large, white oversized hydrants that are supplied by the AWSS/HPFS are visible throughout the city. The hydrants have painted tops that are color-coded by zone:
- Black-topped hydrants west of Twin Peaks zone are fed by the Twin Peaks Reservoir.
- Red-topped hydrants are in the upper zone fed by the Ashbury Street tank.
- Blue-topped hydrants are in the lower zone fed by the Jones Street tank.
There are two emergency pumping stations present within the Emergency Firefighting Water System . To supplement any outright failure of the pumping stations or reservoirs, three fireboats can be utilized to deliver salt water into the system.
As a final measure to counter any outright failure of the Emergency Firefighting Water System piping, a network of 177 independent underground water cisterns is present. Sizes vary from 75,000 US gallons to over 200,000 US gallons depending on location with a total storage capacity of over 11 million gallons of water. These cisterns are easily spotted at street level by red brick circles or rectangles complete with manholes labeled “CISTERN S.F.F.D.”
The mains in the lower zone at the City’s base elevations are under a static pressure of 160 pounds per square inch (psi). If, during the process of a fire in the lower zone, the incident commander needs an increase in pressure above that normally supplied to the lower zone, a radioed command will put Ashbury Tank on the line; gate valves between the two zones will be opened by a tank attendant at Jones Street Tank, and the pressure in the mains at the city’s base will increase to 214 psi. Should it be necessary to increase the pressure further, another order will place the Twin Peaks Reservoir on the line and increase the static pressure to 328 psi. If for any reason the quantity of water delivered in either zone is not sufficient, the supply can be increased by placing either one or both of the saltwater pumping stations in service.