A Primer on Flood Risk

This primer is excerpted from Mid-Atlantic Regional Climate Impacts Summary and Outlook: Summer 2025.

Introduction

Flooding is a significant and recurring hazard in the Mid-Atlantic region, affecting communities, infrastructure, and public safety. While flooding can occur at any time of year, this primer focuses on summer flooding, which is often driven by heavy rainfall, thunderstorms, and tropical systems. Understanding the causes, risks, and warning systems associated with flooding is essential for public agencies responsible for stormwater management, emergency management, public works, and infrastructure resilience.

What is Flooding?

Flooding occurs when water overflows onto land that is typically dry. There are several types of flooding:¹

• River Flooding or Fluvial Flooding: When rivers or creeks rise above their banks due to excessive rainfall or upstream runoff.²
• Urban/Street Flooding: When stormwater overwhelms drainage systems, causing water to flow over streets and into properties.
• Coastal Flooding: When ocean water inundates coastal areas, often due to storm surge or high tides.³
• Flash Flooding: A rapid and extreme flow of high water into a normally dry area, or a sudden rise in a stream or creek, typically within six hours of a heavy rain event. Flash floods are especially dangerous due to their speed and unpredictability.⁴

Floods can cause loss of life, damage or destroy homes, businesses, and infrastructure, and disrupt essential services. They can devastate local economies and pose public health risks by contaminating water supplies, spreading disease, and increasing mold and vector-borne illnesses.⁵ Flooding is also one of the deadliest weather-related hazards in the U.S.⁶ While we mention coastal flooding as a form of flooding above, it is not the focus of our discussion below.

What Causes Flooding?

Flooding is influenced by a combination of natural and human factors:

• Heavy Rainfall: Intense or prolonged precipitation can overwhelm natural and engineered drainage systems. The frequency of extreme precipitation events has increased in the Mid-Atlantic, raising flood risk.
• Antecedent Conditions: The state of the environment before a rain event matters. Saturated soils or already high river levels mean less capacity to absorb new rainfall, increasing flood potential.
• Topography: Areas surrounded by hills or mountains receive runoff from higher elevations, making them more prone to flooding. Downstream locations or river confluences are also at higher risk.
• Human Engineering: Urbanization increases impervious surfaces (roads, parking lots), leading to more runoff. Dams, levees, and managed lakes can mitigate or exacerbate flooding, depending on their condition and management. Failures of these structures can cause catastrophic floods. Coastal barriers may reduce, but not eliminate, coastal flood risk.

Conceptualizing Flood Risk

Flood risk is typically understood as a function of three factors:⁷

1. Hazard: The likelihood and intensity of a flood event (e.g., how much rain falls, how quickly).
2. Exposure: The presence of people, property, and infrastructure in flood-prone areas.
3. Vulnerability: The degree to which a system, community or asset is likely to experience negative consequences (e.g., damage, injury, etc.) due to exposure to a hazard.

The concept of flood risk allows us to understand both the likelihood and consequences of a flood for a particular location, community or infrastructure system. It is also helpful for prioritizing and selecting risk reduction measures aimed at lowering exposure (e.g., zoning, buyouts), reducing vulnerability (e.g., floodproofing, emergency planning), or reducing the hazard (e.g., stormwater management, upstream retention).^8,9

How is Flood Risk Changing?

Academic research has established a clear link between climate change and changes in extreme precipitation patterns, which in turn affect flood risk in the Mid-Atlantic and across the United States.^10,11 Warmer atmospheric temperatures increase the capacity of air to hold moisture, leading to more intense and frequent heavy rainfall events.¹² Studies show that the Northeast, including the Mid-Atlantic, has experienced a significant increase in the frequency and intensity of extreme precipitation over recent decades.¹³

Changes in extreme precipitation can heighten flood risk by increasing the likelihood and severity of flood events in areas exposed to flood hazards. However, flood risk is influenced not only by climatic factors but also by land surface dynamics—such as soil saturation, land use, topography, and the extent of impervious surfaces. Consequently, flood risk reflects both changing climate and the physiographic characteristics of the landscape, highlighting the need for integrated flood risk management approaches—including forecasting and warning systems—that account for both meteorological and land surface factors.

Flood Forecasting and Warning Systems

The National Weather Service (NWS) and the Middle Atlantic River Forecast Center (MARFC) monitor conditions and issue alerts to help agencies and the public prepare for and respond to flooding.¹⁴ Understanding these alerts is critical for timely action. The following definitions are from the NWS:

• Flood Watch: Be Prepared. Issued when conditions are favorable for flooding. Flooding is possible, but not certain or imminent.
  • Flash Flood Watch: Issued when conditions are favorable for flash flooding due to non-convective causes (e.g., dam or levee failure, ice jam), and/or flash flooding and debris flows caused by excessive rainfall on burn scars or in debris flow- and landslide-prone areas.¹⁵
• Flood Advisory: Be Aware. Issued when flooding is expected to be less severe but may still cause inconvenience or minor impacts. Caution is advised.
• Flood Warning: Take Action! Issued when flooding is imminent or already occurring. Indicates a serious threat to life or property, often with river stage forecasts.
• Flash Flood Warning: Take Action! Issued when a flash flood is imminent or occurring. Immediate action is necessary to protect life and property.

For more details, see: NWS Flood Safety

SOURCE: National Weather Service, https://www.weather.gov/mob/severe_flood

• Flood Watch
• A Flood Watch is issued when flooding is possible. Stay tuned to trusted news sources and be ready to seek higher ground.
• Be Prepared

• Flood Warning
• A Flood Warning is issued when flooding is happening or about to happen. Move to higher ground immediately! Never drive or walk through floodwaters.
• Take Action!

In 2019, the NWS added "threat tags" to their flash flood warnings. Flash flood warnings can have the following "threat tags":

• "Base – used most of the time, when flash flood impact is possible
• Considerable – Used rarely, when indications of flash flooding capable of unusual severity or impact are imminent or ongoing and urgent action is needed to protect lives and property.
• Catastrophic – Used exceedingly rarely, when a flash flood threat to life and catastrophic damage is occurring or is imminent, and floodwaters have risen or will rise to levels rarely if ever seen."¹⁶

Flash flood warnings must have either a "considerable" or "catastrophic" tag in order to trigger wireless emergency alerts. These tags are an effort to create more "impact-based warnings" and reduce the number of overall warnings that people receive on their phones.¹⁷ Flash Flood Emergency alerts are triggered with a "catastrophic" tag and Flash Flood Warning alerts are issued when they have a "considerable" tag.¹⁸

SOURCE: National Weather Service, https://www.weather.gov/mob/severe_flood

Flash Flood WARNING: This alert is issued when dangerous flash flooding is happening or will happen soon. Some roads will be flooded, and homes and businesses may flood. The guidance is to move to higher ground and never drive through flooded roads.

Flash Flood EMERGENCY: This alert is issued for exceedingly rare situations when there is a severe threat to human life and catastrophic damage from a flash flood is happening or will happen soon. This is described as a life-threatening situation. Individuals are strongly advised not to attempt travel unless fleeing an area subject to flooding or under an evacuation order.

Resources for Preparedness

• Middle Atlantic River Forecast Center (MARFC)
• NWS Flood Safety
• NWS Flood Infographics
• NRCC's Resources for Hazard Mitigation Planning
• Climate and Hazard Mitigation Planning (CHaMP) Tool