Flooding: Coastal and Riverine - America's Costliest Natural Disaster

Flooding is the most frequent and economically damaging natural hazard in the United States, affecting every state and claiming more lives each year than tornadoes or hurricanes. Understanding the difference between coastal and riverine flooding - and knowing your specific risk - is the first step toward resilience.

What Is Flooding?

Flooding occurs when water inundates land that is normally dry. While the concept is straightforward, the mechanisms that produce floods are varied and complex. Hartree tracks two primary flood categories - coastal flooding and riverine flooding - because they arise from different drivers, affect different geographies, and require distinct preparedness strategies.

Coastal Flooding: When the Ocean Advances

Coastal flooding happens when elevated sea levels, storm surges, or extreme wave action drive seawater onto land. It is most common along ocean coastlines, bays, and tidal estuaries. The primary drivers include:

• Storm surge from hurricanes and nor'easters: A hurricane's winds and low-pressure system can push walls of water 10 to 20 feet or more above normal tide levels, inundating coastal communities within minutes.
• Astronomical tides combined with storms: When a storm coincides with a high tide cycle, flooding potential intensifies dramatically - a phenomenon known as a compound flood event.
• Sea level rise: Global mean sea level has risen approximately 4 inches (100 mm) since 1993, according to NASA satellite measurements. This baseline rise means that storm surges that once only reached doorsteps now routinely enter homes.
• King tides: Periodic extreme high tides - now occurring with greater frequency due to sea level rise - can flood coastal roads and infrastructure even in the absence of storms.

Key Statistic: NOAA data shows that the frequency of high-tide flooding events along U.S. coasts has doubled since 2000 and is projected to increase by 5 to 10 times by 2050 in many locations.

Riverine Flooding: When Rivers Overflow Their Banks

Riverine flooding - also called fluvial flooding - occurs when rivers, streams, or creeks exceed their natural or engineered capacity and spill onto adjacent floodplains. This form of flooding can affect areas far from the coast and often develops over hours to days following heavy rainfall or snowmelt.

Key drivers of riverine flooding include:

• Prolonged or intense precipitation: Rainfall that exceeds the capacity of soil and drainage systems saturates the landscape and sends excess water rapidly into river channels.
• Rapid snowmelt: When warm temperatures arrive suddenly after a heavy snow season, vast quantities of meltwater can overwhelm river systems within days.
• Upstream land use changes: Deforestation, urban development, and agricultural practices that reduce natural water absorption accelerate runoff into waterways.
• Aging infrastructure: Levees, flood walls, and stormwater systems built decades ago are increasingly inadequate for today's more intense precipitation events.

Flash Floods: The Deadliest Flood Type

Flash floods are a subset of riverine flooding characterized by a rapid and extreme surge of water, typically occurring within six hours of heavy rainfall. They are responsible for more flood-related deaths in the U.S. than any other type of flooding, primarily because they leave little time for warning or evacuation. Mountainous terrain, narrow canyons, and urban environments with impervious surfaces are particularly susceptible.

Health and Safety Impacts

The consequences of flooding extend well beyond property damage. Floodwaters carry significant health risks:

• Drowning and physical trauma account for the majority of flood fatalities.
• Contaminated floodwater, which often contains sewage, industrial chemicals, and pathogens, poses serious infection risks.
• Mold growth in flood-damaged structures can trigger respiratory illness weeks after water recedes.
• Mental health impacts, including post-traumatic stress disorder and depression, are well-documented among flood survivors.

How Climate Change Is Altering Flood Risk

A warming atmosphere holds more water vapor - approximately 7% more moisture for every 1°C of warming, according to the Clausius-Clapeyron relationship. This means that when precipitation does occur, it tends to be more intense. Research published in peer-reviewed journals consistently shows that the heaviest rainfall events are becoming heavier, increasing both the frequency and magnitude of flood events across much of the United States.

At the same time, sea level rise is progressively raising the baseline from which storm surges build, giving coastal floods a greater reach with each passing decade.

Protecting Yourself and Your Property

• Know your flood zone: FEMA's National Flood Hazard Layer maps show whether your property lies within a Special Flood Hazard Area (SFHA). However, more than 40% of NFIP flood insurance claims come from outside mapped high-risk zones.
• Consider flood insurance: Standard homeowner's and renter's insurance policies do not cover flood damage. The National Flood Insurance Program (NFIP) and private flood insurers offer policies that can be critical financial lifelines.
• Prepare an emergency kit: Include at minimum 72 hours of water, food, medications, and important documents stored in waterproof containers.
• Follow evacuation orders immediately: Never attempt to walk or drive through floodwater. Just 6 inches of moving water can knock a person down; 12 inches can carry away a small vehicle.
• Elevate and protect: Install check valves in plumbing, consider elevating electrical systems, and apply waterproof sealants to basement walls if you are in a flood-prone area.