The Hetch Hetchy Regional Water System collects surface runoff from rivers and creeks in three major watersheds that is stored in reservoirs that are created by dams, both large and small, for municipal water supply and hydropower generation. These dams and reservoirs are essential elements of our water delivery system. A dam is a structure (usually an earthen embankment or concrete) designed to hold back water, together with appurtenant works (such as a spillway). The California Division of Safety of Dams (DSOD) regulates dams in California that are of a certain size (generally above 25 feet high with a storage capacity of greater than 50 acre feet). There are 1,249 DSOD-regulated dams in California.
Dam Safety in San Francisco
The SFPUC has a robust dam safety monitoring and maintenance program to ensure the integrity of our dams in San Francisco and to protect the public. We use precise on-line instrumentation and alarms monitoring, conduct regular field inspections of the facilities, and perform emergency response planning to monitor the integrity of these structures.
SFPUC staff regularly monitor water levels within the reservoirs, and how much water is flowing into and out of them to ensure the operations at the dam remain safe. We measure underdrain systems to monitor for any indication of crack development in the floor or walls of a reservoir. Intrusion alarms also notify staff quickly of a potential security breach. In addition to these measurements and routine visual observations by our workers, we perform a complete visual inspection of each of our dams and reservoirs on a regular basis and after any significant nearby earthquakes. In addition, DSOD inspects all DSOD-regulated dams annually.
We also monitor the other reservoirs and tanks in San Francisco that are not classified as dams on a monthly basis to ensure safety, as required by the State Water Resources Control Board Division of Drinking Water.
Dam Safety Throughout Our Service Area
We place instruments called piezometers, in dams and foundations to monitor water pressure and the presence of water seepage. We are also on the lookout for changes in the rate of flow and changes in the turbidity of the water in the seepage which may indicate a developing structural issue for the dam. We measure survey markers to check for any potential change in the shape or dimensions of the dam as result of potential weaknesses. Measurements of the instruments are compared to long term data and trends to evaluate the condition of the dams and appurtenances.
In addition to these measurements and routine visual observations by our workers, we perform a complete visual inspection of each of our dams and reservoirs on a regular basis and after any significant nearby earthquakes. In addition, DSOD inspects all DSOD regulated dams annually.
Using all of these tools, SFPUC crews can be forewarned of facility conditions that could warrant further investigation long before a dam’s integrity is compromised.
SFPUC Emergency Planning and Coordination
- SFPUC maintains Emergency Action Plans for each of our dams; these plans include notification procedures and contacts with the Office of Emergency Services in each respective county, and other first responders needed in the unlikely case of a dam emergency.
- SFPUC’s Emergency Planning and Security Division coordinates and conducts regular exercises of the Emergency Action Plans.
- SFPUC staff regularly train and practice using these plans.
Are you near one of our dams?
Did you know that our water system includes 18 dams that are regulated by the DSOD? In fact, 7 of those dams are within the City and County of San Francisco, and you may live near one of them. Emergency Action Plans, which include inundation maps, have been submitted to DSOD.
Inundation maps show the area that would be inundated by flooding and the degree of flooding from an uncontrolled breach of a dam and/or the failure of an appurtenant structure, such as a spillway. The flooding portrayed in the map simulates the effect of an immediate release of all of the water behind a dam caused by the extremely unlikely scenario of a complete failure of the dam. These maps are essential tools for emergency response planning because they help determine which neighborhoods downstream of a dam might be impacted from a complete dam failure so that first responders can better prepare. Inundation maps are available from the DSOD.
Be Prepared, whatever the emergency
- Sign up with your local emergency notification system to receive public service announcements, text messages and email alerts, and emergency preparedness tips.
- San Francisco: SF72.org
- Alameda County: acgov.org/ready/
- San Mateo County: smcready.org/
- Tuolumne County: https://www.tuolumnecounty.ca.gov/308/Office-of-Emergency-Services
- Store emergency supplies such as drinking water, nonperishable food, first aid kit, clothes and shoes, critical contact information, and medications in a “go-bag.”