What Are Flash Floods? 11 Things (2024) You Must Know

Flash floods are a type of rapid flooding that occurs in low-lying areas like washes, rivers, dry lakes, and depressions.

If you commonly hear about flash floods in your area, here’s everything you should know about their causes and risk factors.

1. What are flash floods?

Flash floods are those that begin within 6 hours (or sometimes even 3 hours) of heavy rainfall or another cause.

They’re often characterized by raging torrents following heavy rains.

These rip through riverbeds, urban streets, or mountain canyons and sweep up everything before them.

In some cases, flash flooding can occur within minutes of excessive rainfall or even occur without rainfall at all if a levee or dam has failed. 

Flash floods are distinguished from regular floods by a timescale of fewer than six hours between rainfall and the onset of flooding.

2. What are the five main types of floods?

There are five primary types of floods.

The first is flash floods, which we’ll talk more about in this article.

We’ll also explain the other four types: river, coastal, groundwater, and sewage floods.

In general, flash floods cause a greater loss of life.

On the other hand, river floods often cause a greater loss of property.

Flash floods occur when runoff from excessive rainfall (or some other event) causes a rapid rise in water height.

These floods are common in areas with a dry climate, rocky terrain, or lots of manmade land features that cause the land to lack soil or vegetation.

Typically, this soil or vegetation would absorb the water, but when they aren’t present, the water often flows over the land instead.

River flooding is more common in areas with large rivers and wetter climates.

Excessive runoff from longer-lasting rainstorms and melting snow can cause the water level to rise over a large area.

Coastal flooding is an accumulation of water on land caused by waves, tides, storm surges, or heavy rainfall.

These areas are often prone to flooding because of how close they are to the ocean.

Groundwater flooding occurs due to inundated drainage systems by significant rain that causes water to overflow into the streets and nearby properties.

Generally, you’ll see groundwater flooding occur gradually, and the water level will be shallow initially.

This gives homeowners time to remove their possessions.

Unfortunately, this type of flooding can persist for weeks until the soil can absorb the standing water present.

Sewage flooding is caused when unexpected heavy rainfall overwhelms sewage systems or treatment plants.

This causes them to break down and release untreated sewage into water bodies.

This type of flood is particularly hazardous because the water often contains harmful bacteria and chemicals that cause infections, diarrhea, fever, vomiting, and other diseases.

3. What are the causes of flash floods?

 Flash floods can be caused by the following:

bulletHeavy rain

bulletSevere thunderstorm


bulletTropical storm

bulletMeltwater from ice or snow that flows over ice sheets or snowfields

bulletThe collapse of a natural ice dam, debris dam, or manmade dam

bulletVolcanic eruptions

bulletHuman activities

Flash floods often occur in the highest mountain ranges in the U.S. as well as arid plains in the Southwestern part of the country.

4. What are the impacts of flash floods?

Flash floods both occur and dissipate quickly.

While an overwhelming amount of water is often an issue, it’s not the only problem associated with this extreme weather event.

Here are the common impacts of flash floods.

bulletDeath or serious injury

Flash floods are fast and unpredictable.

They occur without warning, and people are often killed or seriously injured by these natural disasters.

They’re made even riskier by the fact that they often occur at night when people are sleeping.

According to the National Weather Service, flash floods are the #1 weather-related killer in the U.S.

bulletImmediate property damage

Flash floods can carry large debris like boulders in their waters.

This often causes heavy structural damage to homes and vehicles.

bulletLong-term property damage

Unfortunately, rooms, vehicles, and all other property that encounters flood waters are at risk for permanent damage.

None of these may be salvageable because of the moisture that invaded them, and trying to save them will only result in harmful mold or mildew.

Vehicles have similar issues, and they’re often irreparable because of water damage to the engine.

bulletLoss of critical infrastructure

Large debris and floodwater can cause structural damage to bridges and roadways.

This will make travel impossible, which challenges evacuation or later rebuilding.

Flash floods often take out power, telephone, and cable lines.

They can also disrupt or contaminate groundwater, which means the community isn’t able to consume tap water.

bulletDeposited sediment and silt

When a flash flood occurs, the accumulated water picks up and carries sand, silt, and other debris as they travel.

As the velocity of the flash flood begins to slow, it’ll deposit the debris that it picked up.

This silt and debris can make travel difficult, and it can also be expensive to remove from where it ends up.

bulletUninsured or under-insured homeowners

Flash floods aren’t often considered when homeowners insure their homes.

It’s important to know that regular homeowners’ and renters’ insurance doesn’t cover flood damage.

You must purchase flood-specific insurance through the National Flood Insurance Program (NFIP).

bulletEconomic losses

Unfortunately, flash floods can cause a lot of damage.

This will affect the local economy and prevent local businesses from opening or keep customers from getting to those businesses.

5. Are flash floods serious?

When people hear about flash flooding, most don’t worry.

They associate it with severe rain and think they should just stay off the roads for a little while.

However, flash floods are a more significant hazard than most people believe.

They cause more fatalities in the U.S. in an average year than lightning, tornadoes, or hurricanes.

These are severe weather events that should draw your attention.

If local officials say to evacuate, you should heed their advice without hesitation.

Flash floods may also deposit large quantities of sediments on floodplains.

This can be destructive to vegetation that isn’t adapted to frequent flood conditions.

6. What are the different flood warnings that you may hear?

Understanding the difference between flood warnings can help you know how to stay safe.

bulletFlood Watch or Flash Flood Watch

A “watch” means that flooding or flash flooding in your area is possible.

Be on the lookout for changing weather conditions, including flooding.

You may need to move to higher ground that isn’t impacted by flooding.

bulletFlood Warning

A warning indicates that a flood is about to occur.

You should avoid all low-lying areas and evacuate if necessary.

bulletFlash Flood Warning

Flash flood warnings indicate that this type of flood is imminent.

You should seek higher ground immediately.

7. What areas are most at-risk of flash floods?

Flash floods are more likely in densely populated areas.

This is because of the amount of area that’s covered by buildings, highways, driveways, and parking lots.

These features increase the runoff and reduce the amount of rain that can be absorbed by the ground.

8. Which cities in the U.S. are most vulnerable to flooding?

Are you worried about this natural disaster creeping up on you?

Check to see if your city is among the top impacted cities.

bulletSaint Petersburg, Florida

bulletSanta Clara, California

bulletBrownsville, Texas

bulletSalinas, California

bulletModesto, California

bulletCape Coral, Florida

bulletMidland, Texas

bulletFairfield, California

bulletSan Jose, California

bulletClearwater, Florida

bulletCorpus Christi, Texas

bulletHayward, California

bulletConcord, California

bulletSacramento, California

bulletStockton, California

bulletAntioch, California

bulletOrlando, Florida

bulletRoseville, California

bulletSan Diego, California

bulletSurprise, Arizona

9. What are flash flood safety tips?

Depending on whether you’re in a flash flood watching or warning, you should take different steps.

Here’s what you should keep in mind.

For flash flood watches, consider the following:

bulletListening to a local news station for the latest information

bulletFollowing instructions given by public safety officials

bulletStaying alert to changing weather conditions

bulletBeing ready to move to higher ground when/if necessary

bulletPostponing outdoor activities