Occoquan Dam Siren

A test of the siren was held on Tuesday, November 14, 2023, at 10:00am.

To further ensure the safety of the residents, visitors and businesses along the Occoquan River shoreline, Fairfax Water installed sirens to warn people in the highly unlikely event that there is ever a massive failure of the Occoquan River Dam.

Who's impacted?

The impact zone includes areas in Prince William County & Fairfax County located immediately adjacent to the Occoquan River from the Town of Occoquan to Belmont Bay. This includes the town, marinas, Occoquan Regional Park, and other residences and businesses adjacent to the river.

What does the siren sound like?

The siren is a loud whooping noise. You can hear what it sounds like by clicking here.

How do I know if I'm in the impact zone?

Just because you hear the siren does not mean you are in the impact zone. Find out whether or not you are in the impact zone or the noise zone by typing in your address in the search bar below. If you are located within the red boundary then you are within the impact zone.

What should I do?

During the siren test, there is no need to seek higher ground. Any other time, if you hear the siren and are in the impact zone, seek higher ground immediately! If you are not in the impact zone, no action is necessary.

Siren Information

Purpose

The Occoquan Dam Siren system exists to alert persons below the Occoquan Dam of a structural failure of the Occoquan dam (an extremely unlikely event). If the siren sounds, seek higher ground if you live/work/are visiting inside the Impact Zone. The Impact Zone is a small area adjacent to the banks of the Occoquan River between the Town of Occoquan and Belmont Bay that will flood if the Occoquan Dam experiences a structural failure.

Background

Fairfax Water owns and operates the Occoquan Dam which forms the Occoquan Reservoir as a drinking water source for Northern Virginia. The Occoquan Reservoir holds approximately 8.3 billion gallons of water. The Occoquan Dam was built in the early 1950s to create the Occoquan Reservoir. The dam is inspected and maintained on a routine basis. The structural integrity of the Occoquan Dam is very sound. In fact, through rigorous maintenance and improvements, it is even stronger today than when it was constructed. As a precaution in the extremely unlikely event of a structural failure, Fairfax Water has installed a siren warning system to alert persons downstream of the dam in the event of dam failure. The sirens will only sound for two reasons:

1) Testing of the system

2) The Occoquan Dam has failed in a non-weather related event.

Questions and Answers

Why is the siren necessary?

The sirens were installed several years ago for two reasons. First, the Virginia Department of Conservation & Recreation, the regulatory agency for the dam, permits the use of a sound-related system to inform people downstream of the dam in the extremely unlikely event of a dam structural failure. The second reason is that when we worked with the communities closest to the dam, namely the Town of Occoquan, it was the Town’s feeling that a siren system would be the most effective and desired form of communication to alert people in the extremely unlikely event of a dam failure.

How strong is the dam?

Very strong. The structural integrity of the Occoquan Dam is very sound. In fact, through rigorous maintenance and improvements, it is even stronger today than when it was constructed. As a precaution in the extremely unlikely event of a structural failure, Fairfax Water installed a siren warning system to alert persons downstream of the dam in the event of dam failure. The concrete spillway is approximately 72 feet tall, 525 feet wide, and 70 feet thick base. It is anchored with post-tensioned anchors into the bedrock. It contains more than 100,000 cubic yards (400,000,000 pounds) of mass concrete.

What if I have other questions that I can not find the answer to here?

You can direct any further questions you may have to Fairfax Water's Public Affairs Department by email.

Please click here to submit a question.

How can I stay informed on local emergency information?

One of the best ways to stay informed is to sign up for local alerts in the areas that you live and work.

To sign up for Fairfax Alerts, visit: www.fairfaxcounty.gov/alerts.

To sign up for Prince William County Alerts, visit: www.pwcva.gov/alerts.