Flood Safety Information

What is Flooding?

Prince William County is affected by five types of flooding:

• River Flood: Occurs when water levels exceed a river’s banks due to excessive rain from tropical systems, persistent thunderstorms over the same area, combined rain and snowmelt, or ice jams.
• Inland Flood: Occurs when moderate precipitation accumulates over several days, intense precipitation falls over a short time, or a river overflow from an ice or debris dam or dam or levee failure.
• Flash Flood: Caused by heavy of excessive rainfall in a short period of time, generally less than 6 hours. Can also occur when no rain has fallen, such as a dam or levee failure.
• Coastal Flooding: Inundation of land areas along the coast caused by higher-than-average tide, worsened by heavy rainfall and onshore winds.
• Storm Surge: Abnormal rise in water level in coastal areas, over and above the regular astronomical tide, caused by a severe storm’s wind, waves, and low atmospheric pressure.

Although riverine and coastal flooding are two of the most common types of flooding within the County, floods may not happen near a body of water. Heavy rains, overloaded drainage systems, and nearby construction projects can affect flooding events far from actual lakes or rivers.

The county's primary waterway basin is the Tidal Potomac River, which forms the eastern boundary of the county. The primary inland basin for the western and central areas of the County is the Occoquan River with three tributaries: Cedar Run, Broad Run, and Bull Run.  The Farm Creek, Marumsco Creek, Neabsco Creek, Powells Creek, Quantico Creek, and Chopawamsic Creek basins on the eastern side of the county drain directly into the Potomac River.

The following advisories may be issued by the National Weather Service during a flooding event:

• Flood Watch/Flash Flood Watch: conditions are favorable for a specific area to flood (scale may determine warning level).
• Flood Warning: flooding is imminent or occurring.
• Flash Flood Warning: a sudden, violent flood is imminent or occurring.

To ensure that you receive flood watches and warnings, sign up for free PWC Alerts at pwcva.gov/alerts. Be sure to add your home and/or work addresses to your PWC Alerts profile, and then select "flood" under the "weather alerts" section.

Steps to take During Flood Conditions

During a flooding event, the county follows the steps outlined in the PWC Emergency Operations Plan.  As part of this plan, the county maintains pre-designated facilities that can serve as emergency shelters or reception centers based on the nature and magnitude of the emergency.  Residents, business owners, and visitors should:

• Pay attention to flood warnings and alerts from the county; monitor pwcva.gov/emergency for updates, including evacuation instructions and the location of emergency shelters or reception centers.
• Do not walk through flowing water; it can knock you off your feet.
• Do not drive through a flooded area; the road surface or bridge may be washed out.  Just 12 inches of water is enough to float a small car.  Remember: turn around; don't drown!
• Should your home or business begin to flood, turn off electricity, water, and gas supply if you can safely do so. Stay away from power lines and standing water; electrical current can travel through water and lead to electrocution.
• Keep children and pets away from floodwaters, as they may contain debris and other hazards.

Frequently Flooded Areas

The map below shows areas that experience frequent flooding during heavy or extended rainfall, along with reported flooding-related road closures and swiftwater rescues, between 2018-2020. Click here to open this map in a new window. 

Floodplains and Historical Flooding

A History of Floods in Prince William County

Prince William County has experienced major floods in 1937, 1942 and 1972. By far, Hurricane Agnes in 1972 was the largest flood and exceeded any predicted 100-year floods.

According to newspaper reports, the storm caused severe damage in the Manassas area. Major flood damage occurred to businesses, homes, boats and automobiles.

Mobile homes were literally washed away. Water washed over numerous bridges, including the one across Bull Run. Currents broke up the roads, and road damage was intensified by stopped up culverts and other drainage ways. As the water rose, 700 people were forced to abandon their homes.

In the Occoquan River watershed, the storm wreaked havoc and caused $8.7 million in damage. Most of the damage was to residential property ($2.2 million) and to transportation facilities ($3.5 million). Three deaths were attributed to the storm.

Flood hazards pose serious threats to life and property. Take the steps to safeguard yourself, your family, your home and your property. If you have questions about whether you live in a floodplain, or for more information, please call the Department of Public Works at 703-792-7070 (TTY: 711).

Prince William County has taken steps to help protect people and property against flood concerns. The first step was to create Resource Protection Areas (RPA) and Resource Management Areas (RMA).

RPAs are areas along streams where floodplains exist. The County has several square miles of RPAs. Vegetative buffers are required within RPAs to help:

• maximize infiltration, which reduces storm water runoff into streams and the potential for flash flooding

• protect streams from development impact, which improves water quality

RMAs include floodplains, highly eroded soils and other sensitive areas. Our entire County is considered an RMA. Developers and builders working in an RMA are required to use best management practices, which are steps to minimize erosion, control runoff and prevent pollution. Many Prince William developers build storm water management facilities. These facilities incorporate the best management practices and help provide flood control.

Is your property in a floodplain?

You can view the Interactive Floodplain Viewer to determine if your property is located within a floodplain.

Before buying a property, you can also take these steps to learn more about floodplain issues:

• Ask the real estate agent if the property is in a floodplain, if it has ever flooded and if it is subject to any other hazards, such as sewer backup or subsidence.

• Ask the seller and the neighbors if the property is in a floodplain, how long they have lived there, if the property has ever been flooded and if it subject to any other hazards.

• Call Watershed Management for assistance at 703-792-7070 (TTY: 711).

If you live in a floodplain, there are restrictions on building on your property. You can learn more about requirements and restrictions in the Environmental Systems Section 700 of the Design and Construction Standards Manual.

Learn more about Protecting your Family and Property Against Flooding. 

100 Year Floodplain Hazard

Prince William County has experienced 100 year floods. Today, more than ever before, there are more controls such as storm water management ponds and drainage systems in place to protect people and property. However, you should monitor weather reports and heed emergency warnings during major storms.