Flash Flood Safety: How Deep Is Too Deep and What to Do When Water Rises Fast

Key points

• Flash flood safety starts with three numbers: 6 inches of moving water can knock an adult down, 12 inches floats most cars, and 2 feet carries away SUVs and trucks.

• Over half of flood drownings happen in vehicles. Turn Around, Don't Drown isn't a slogan, it's the single decision that matters most.

• Watch means be ready. Warning means move now. Flash flood emergency is the rare top tier and means a severe threat to life is already happening.

• Go to the highest floor, never into a closed attic without a way out, and never onto the roof unless you have to.

• Check your flood zone at the FEMA Flood Map Service Center today. NFIP flood insurance takes 30 days to take effect, so buying it as a storm approaches won't cover that storm.

Flash flood safety comes down to a decision most people make in about two seconds, usually behind a steering wheel, usually at night, usually on a road they've driven a thousand times. The National Weather Service puts the numbers plainly: a mere 6 inches of fast-moving flood water can knock over an adult, 12 inches carries away most cars, and 2 feet of rushing water can carry away SUVs and trucks.

Those numbers are lower than almost anyone guesses. That gap between what people assume and what water actually does is where the deaths happen. Here's the whole picture, calmly, with the parts that matter in the next 10 minutes first.

How deep is too deep in a flash flood?

Shallower than you think. The verified National Weather Service figures:

• 6 inches of fast-moving water can knock an adult off their feet. That is ankle deep. Once you are down in moving water, you are not standing back up.

• 12 inches of rushing water carries away most cars. NWS also puts it as 12 inches of water can float a car or small SUV. Ready.gov says the same: one foot of moving water can sweep your vehicle away.

• 18 inches is where NWS says water can carry away large vehicles.

• 2 feet of rushing water carries away SUVs and trucks. Not "might." Carries away.

The physics isn't complicated. Water weighs about 62 pounds per cubic foot, and a moving vehicle is a big buoyant box with a lot of surface area. Once enough water gets under it, the tires stop gripping the road and the car becomes a boat with no steering. The heavier and taller the vehicle, the more surface the current has to push against. A truck doesn't save you. It gives the water more to grab.

CDC adds the part people forget about standing water: "Swiftly moving shallow water can be deadly, and even shallow standing water can be dangerous for small children."

Why do most flood deaths happen in vehicles?

Because a car makes people feel protected, and because the decision to drive through takes less effort than the decision to turn around. The National Weather Service cites CDC data showing over half of all flood-related drownings occur when a vehicle is driven into hazardous flood water.

Flooding is also a bigger share of the disaster picture than most people realize. NWS notes that roughly 75 percent of all Presidential disaster declarations are associated with flooding, and that since the 1970s, inland flooding has caused more than half of the deaths associated with tropical cyclones in the United States. During hurricane season, the water inland kills more people than the wind at the coast.

Speed is the other factor. NWS points out that flash floods generally develop within 6 hours of the immediate cause, and gives one example worth memorizing: a mountain creek that is normally only 6 inches deep can swell to a 10-foot depth in less than one hour. You are not watching a slow river rise. You are watching a drainage fill.

What does Turn Around, Don't Drown actually mean, and why do drivers misjudge?

It means you never drive or walk into floodwater, at any depth, on any road, in any vehicle. The National Weather Service's own summary is "It is NEVER safe to drive or walk into flood waters." Ready.gov repeats it as a flat instruction: do not walk, swim or drive through flood waters.

Drivers misjudge for four specific reasons, and knowing them is half the defense.

1. You cannot see depth in moving muddy water. NWS states it directly: "Water may be deeper than it appears and can hide hazards such as sharp objects, washed out road surfaces, electrical wires, chemicals, etc." Brown water at night with rain on the windshield gives you no depth cue at all.

2. The road may not be under there anymore. This is the one most people never consider. Moving water scours the roadbed out from underneath the asphalt. What looks like 8 inches of water over pavement can be 8 inches of water over a 6-foot hole where the culvert used to be. That is why NWS lists "washed out road surfaces" among the hidden hazards.

3. Familiarity reads as safety. A road you drive daily feels known. The water does not care that you know the road, and the low spot you have crossed a thousand times is exactly the low spot that floods.

4. The car in front made it. Water depth and current change minute to minute during a flash flood, and a vehicle that crossed 90 seconds ago crossed a different road than the one in front of you now.

Barricades exist because somebody already measured the risk. Driving around one isn't a judgment call, it's a decision to trust a guess over a measurement.

What is the difference between a flood watch, a flash flood warning, and a flash flood emergency?

They are three escalating levels, and each one tells you something different about the next 10 minutes.

• Flood Watch, or Flash Flood Watch. NWS issues a watch when conditions are favorable for a specific hazardous weather event to occur. The action word is "Be Prepared." A watch usually covers several counties and hours of lead time. This is when you charge devices, move the car uphill, and decide where you would go.

• Flash Flood Warning. A warning is issued when a flash flood is imminent or occurring. The action word is "Take Action." A warning covers a smaller, more specific area. If you are in a flood-prone area, you move to high ground now, not after you finish what you are doing.

• Flash Flood Emergency. The rarest tier. NWS describes it as issued "for the EXCEEDINGLY RARE situations when extremely heavy rain is leading to a severe threat to human life and CATASTROPHIC DAMAGE from a flash flood is happening or will happen soon." A forecaster chose those words deliberately. If you get one, the correct response is immediate movement to the highest ground available, not a look out the window.

The practical translation, in one line each. Watch: get ready. Warning: go. Emergency: go now, higher.

Wireless Emergency Alerts push flash flood warnings to your phone automatically, but cell service is one of the first things to fail in a bad storm. A battery or hand-crank NOAA weather radio is the backup that keeps working, and it is worth understanding which radio actually does what during an outage before you need one.

What should you do in the first 10 minutes of a flash flood warning?

Move up, not out. If you are already home and water is rising around you, driving into it is the worst available option.

1. Get to the highest level of the building. Ready.gov's instruction is to get to the highest level if trapped in a building.

2. Do not climb into a closed attic. Ready.gov is explicit: "Do not climb into a closed attic to avoid getting trapped by rising floodwater." An attic with no window, no vent you can open and no way onto the roof is a sealed box. If the attic is your only option, take an axe or a pry bar with you.

3. Only go onto the roof if you have to. Ready.gov says to get on the roof only if necessary, and once there, signal for help.

4. Take three things up with you. A charged phone, a flashlight, and shoes. People end up barefoot in debris more often than you would guess.

5. Stay off bridges over fast-moving water. That is a specific Ready.gov instruction, and it is about the bridge failing, not the water rising.

6. Do not go back for anything. Not the car, not the documents, not the pet food. Bring pets up with you when you first move.

If you are away from home when the warning arrives, the answer is the same shape: go up or go away from the drainage, not through it. This is where a plan you already made does the work, because deciding under stress is the part people get wrong. A family evacuation plan with routes chosen before the water rises is what keeps you from picking the low road by default.

What do you do if your car is caught in rising water?

The order matters, and it changes depending on whether the water is moving.

If the water is still shallow and you have not stalled: stop, do not try to power through, and back out the way you came if that path is still clear. Reversing on a road you just drove is safer than continuing onto a road you cannot see.

If the vehicle is stalled in shallow, non-moving water: get out and get to higher ground on foot, moving away from the flow. Abandon the car. It is a car.

If the vehicle is caught in rapidly moving water: Ready.gov's guidance is to "Stay inside your car if it is trapped in rapidly moving water. Get on the roof if water is rising inside the car." Stepping out into a current that is already moving a 4,000-pound vehicle will take you off your feet immediately. The vehicle is the better raft until it is not, and the trigger for moving to the roof is water coming in.

Two things to do before any of that ever happens. Know how your windows work when the power is out, and keep a seatbelt cutter and window breaker in the door pocket, not in the trunk. Side windows break. Windshields do not, because they are laminated. And call 911 as soon as you are stuck, before the water gets high enough to make talking hard.

How do you find out if you are in a flood zone?

Look it up on the FEMA Flood Map Service Center. This is the single most useful thing in this whole article, and it takes about two minutes.

The FEMA Flood Map Service Center is the official public source for flood hazard information produced for the National Flood Insurance Program. You type in your street address and it returns the official flood map for that location. That map tells you what flood zone you are in and whether your property sits in a Special Flood Hazard Area.

Three things to understand about the answer you get:

• "Not in a high-risk zone" does not mean "will not flood." FEMA's own program reports that 29 percent of NFIP claims come from outside high-risk flood areas. Flood maps describe historical and modeled risk, not a guarantee.

• Maps change. FEMA notes that flood maps are continually updated, so a map you looked at years ago may not be current.

• Your local risk is smaller than the map's resolution. The storm drain at the end of your street, the creek behind the neighbor's fence, and the fact that your driveway is the low point on the block are all real and none of them show up on a federal map. Walk your own property during a hard rain and watch where the water goes. That is data no map will give you.

Why does flood insurance bought as a storm approaches not cover that storm?

Because National Flood Insurance Program coverage takes 30 days to take effect. The program's own wording is "Your flood insurance coverage will go into effect 30 days after your date of purchase." A policy bought when a hurricane enters the forecast does nothing for that hurricane.

The exceptions are narrow and none of them help you during an approaching storm:

• No wait if you buy flood insurance while making, increasing, extending or renewing a mortgage.

• No wait if you change your coverage while renewing an existing policy.

• A one-day wait if your property is in a newly designated high-risk flood zone and you buy within 12 months of the update.

• A one-day wait if the flood is caused or worsened by a wildfire on federal land and you buy within 60 days of the containment date.

The other half of this, which surprises people every single season: standard homeowners insurance does not cover flood. The NFIP states it plainly: "Most homeowners insurance does not cover flood damage. Only flood insurance covers the cost of rebuilding after a flood." Wind-driven rain through a broken window is usually a homeowners claim. Water that rose from the ground is not.

The practical takeaway is a calendar item, not an emergency action. If you are going to carry flood insurance, buy it in a quiet month. There is no version of this you can do in the 72 hours before landfall, which is one more reason to work through the rest of your hurricane preparation well before a storm has a name.

What is safe to do after the water goes down?

Less than you would like. The hazards after a flood are quieter than the ones during it, and they injure more people.

Stay out of the water. CDC's summary is "The best way to protect yourself is to stay out of the water." Floodwater can contain downed power lines, human and livestock waste, household, medical and industrial hazardous waste, coal ash waste that can carry arsenic, chromium and mercury, germs, physical debris and displaced wildlife. Ready.gov puts it more simply: avoid wading in floodwater, which can be contaminated and contain dangerous debris.

Treat every wire and every puddle as live. Ready.gov warns that underground or downed power lines can electrically charge the water, and that you should not touch electrical equipment if it is wet or if you are standing in water. CDC is blunt about the wire itself: "NEVER touch a fallen power line. Call the power company to report fallen power lines." CDC also advises shutting off electrical power and natural gas or propane at your home to avoid fire, electrocution or explosions.

If you cannot avoid contact, dress for it. CDC recommends rubber boots, rubber gloves and goggles, and washing any exposed skin with soap and clean water as soon as possible.

Do not run a generator indoors to dry things out. Post-flood outages and portable generators are a bad combination, and carbon monoxide kills roughly 100 people a year in exactly this situation. The rules are the same as always, and they are worth knowing before the power goes out: 20 feet from the house, never a garage, alarms with battery backup.

When to call for help, and what this article cannot do

Call 911 if you are trapped by rising water, if your vehicle is caught, if someone is swept away, or if you find a downed power line. Call early. Cell networks degrade as an event unfolds, and a call that goes through at minute five may not go through at minute forty.

Call your utility, not 911, for a downed line that is not immediately threatening someone. Report it and keep everyone back.

Call Poison Control at 1-800-222-1222 if someone swallows floodwater and gets sick, or if there is a chemical exposure from contaminated water or cleanup products. It is free and staffed 24 hours a day.

Where this article stops: nothing here tells you whether a specific road is passable, whether your specific house will flood, or how deep the water in front of you is. Nobody can tell you that from a distance, which is the entire reason the rule is a flat one. If you can see water on the road, the answer is to turn around, every time, without weighing it.

None of this requires courage or equipment. It requires knowing three numbers, knowing what a warning means, and being willing to be the person who turns around while other cars keep going. That last one is harder than it sounds, and it's the whole thing.

Frequently asked questions

How many inches of water does it take to sweep a car away?

About 12 inches. The National Weather Service says 12 inches of rushing water carries away most cars, and that 12 inches can float a car or small SUV. At 18 inches, NWS says water can carry away large vehicles, and 2 feet of rushing water carries away SUVs and trucks. All of those are less than the height of a car door.

What is the difference between a flash flood watch and a flash flood warning?

A watch means conditions are favorable for flash flooding and covers a broad area with hours of lead time, so the action is to be prepared. A warning means a flash flood is imminent or already occurring in a smaller, specific area, so the action is to move to high ground immediately. A flash flood emergency is a third, exceedingly rare level meaning a severe threat to human life is happening now.

What should I do if my car stalls in floodwater?

If the water is shallow and not moving, get out and walk to higher ground away from the flow. If the water is moving rapidly, Ready.gov says to stay inside the car and get on the roof only if water starts rising inside it. Call 911 as soon as you are stuck. Stepping into a current strong enough to move a car will take you off your feet.

Does homeowners insurance cover flood damage?

No. The National Flood Insurance Program states that most homeowners insurance does not cover flood damage, and that only flood insurance covers the cost of rebuilding after a flood. Water that rises from the ground is a flood claim, not a homeowners claim, which is why a separate policy exists.

How long does it take for flood insurance to take effect?

30 days. NFIP coverage goes into effect 30 days after the date of purchase, so a policy bought as a storm approaches will not cover that storm. The narrow exceptions are buying alongside a mortgage transaction, changing coverage at renewal, a one-day wait in a newly designated high-risk zone, and a one-day wait for wildfire-related flooding on federal land.

Is it safe to walk through floodwater after the water starts going down?

No. CDC says the best way to protect yourself is to stay out of the water, because floodwater can contain downed power lines, sewage, hazardous chemicals, germs, debris and displaced wildlife. Ready.gov adds that underground or downed power lines can electrically charge the water. If contact is unavoidable, CDC recommends rubber boots, rubber gloves and goggles, then washing with soap and clean water.

You never know, but you can always be ready.

Sources: NOAA National Weather Service, Turn Around Don't Drown; NOAA National Weather Service, during a flood; NOAA National Weather Service, flood watch and warning definitions; NOAA National Weather Service, flood hazards; NOAA National Weather Service Mobile, flood safety and flash flood emergency; Ready.gov, floods; FEMA Flood Map Service Center; FEMA NFIP, buy a flood insurance policy; FEMA NFIP, FloodSmart; CDC, floodwater safety guidelines; CDC, floodwaters contain many things that may harm health.