Flash Flood Safety: Turn Around, Don’t Drown

Ask someone to name the most dangerous thing a thunderstorm can do and you will hear tornado, or lightning. Almost nobody says water on the road. That gap between what frightens us and what actually kills us is the whole problem with flooding.

In the United States, floods kill more people in an average year than tornadoes, hurricanes or lightning, and more deaths come from flooding than from any other thunderstorm-related hazard. It is the deadliest thing most storms do, and it does it quietly, a few people at a time, on ordinary roads.34

Here is the detail that should change how you drive. More than half of all flood-related drownings happen when a vehicle is driven into floodwater. The next largest share is people walking into or near it. These are not victims caught by a wall of water in their homes. They are people who made a judgment call about a puddle, in a car, usually within a few miles of where they live.4

That is grim, but it is also the good news buried in the numbers. Nearly all of it comes down to a single decision that you control completely: whether or not you enter the water.

The National Weather Service is unusually blunt here because the real numbers are so much smaller than instinct suggests. Six inches of fast-moving water can knock an adult off their feet. Twelve inches can float a car or a small SUV. Eighteen inches can carry away large vehicles, and two feet of rushing water will take away most SUVs and trucks.45

Twelve inches sounds impossible for something that weighs two tons, until you think about what a car actually is. It is a sealed steel box full of air, which is a decent description of a boat. Once water rises up the doors, it starts pushing up as much as it pushes sideways, and the car gets lighter against the pavement. Traction goes before flotation does. By the time you feel the vehicle move, the tires have already stopped being in charge.

The second hazard is the one you cannot see at all. Moving water scours out whatever is underneath it, so the road you are looking at may not be there anymore. Culverts wash out, shoulders collapse, and the surface of the water tells you nothing about whether there is a four-foot hole under it. Barricades are placed by people who have already been there and seen it.

A flash flood is not simply a big flood. It is what happens when rain falls faster than the ground can absorb it, and it can arrive within minutes of the rain that caused it. That is the distinction that catches people out: river flooding gives you a forecast and often days of notice, while a flash flood can be a dry ditch one moment and chest-deep the next.3

It also does not have to be raining where you are. The water arrives from wherever the storm sat, which may be miles upstream and entirely out of sight. A creek behind your house can come up under a clear sky because a thunderstorm parked over the hills an hour ago.3

Cities make this worse by design. Buildings, highways, driveways and parking lots are impervious surfaces: rain that would have soaked into soil instead runs straight off them, so the same storm produces far more runoff over a developed area than it would over a field. It is why urban flash flooding can be severe in places with no river anywhere near them, and why underpasses and low bridges fill first.3

Rain is not the only trigger, either. Rapid snowmelt and ice jams, where broken river ice piles up and dams the channel behind it, both produce flash floods. So does infrastructure giving way. In 1889 a dam failure above Johnstown, Pennsylvania sent a wall of water 30 to 40 feet high into the town and killed 2,200 people in minutes, and most of the devastation in New Orleans during Hurricane Katrina came from levee failures rather than from the storm surge alone.3

Flood alerts use a vocabulary that sounds interchangeable and is not. The difference between two of these words is the difference between finishing your errand and turning the car around.

A Flood Advisory means a flood event is expected that may become a nuisance: be aware. A Flood Watch means conditions are favorable for flooding: be prepared. A Flood Warning means the flooding is imminent or already happening: take action. A Flash Flood Warning means a flash flood is imminent or occurring, and the National Weather Service instruction is to move immediately to high ground if you are in a flood-prone area.6

Then there is a tier most people have never heard of. A Flash Flood Emergency is issued only for 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. If one is ever issued for where you are, treat it as a different category of message from a warning. It is the National Weather Service saying the situation is already beyond the ordinary, and the time for deciding has passed.7

If you are in a flood-prone area and a warning is out, get to higher ground immediately, and if officials tell you to evacuate, go at once rather than waiting to see how bad it gets. Do not walk through floodwater, because six inches of it moving will take your feet out from under you.5

Indoors, the danger changes character. Do not enter a basement or any room where water has reached the electrical outlets or where cords are submerged, because the water is energized and gives you no warning that it is.5

Night is the worst case for all of this. Headlights flatten the surface of standing water and hide how fast it is moving, most flooded roads have no lighting, and a driver who would never wade into a river on foot will happily drive into the same water at 40 miles an hour because it reads as pavement with a puddle on it. If you cannot see the painted lines, you do not know what you are driving into.

One of the most dangerous flood setups in the country has nothing to do with how much rain falls, and everything to do with what burned there last summer. Where a wildfire has burned hot enough or long enough, the soil develops a layer that actively repels water, like rain landing on pavement. Ground that used to drink a downpour now sheds nearly all of it.8

The consequence is that much less rainfall is needed to produce a flash flood on a burn scar than on unburned land, and the effect lasts. A recent fire creates a heightened risk of flooding for years afterward, long after the smoke is gone and the story has left the news. A storm that would have been unremarkable before the fire can now send water down a canyon in minutes.8

The worst version has its own name: a debris flow. These are fast-moving, deadly landslides, powerful mixtures of mud, rocks, boulders and entire trees, sometimes carrying away homes and vehicles with them. They move faster than a person can run and they are unpredictable about where they start and where they go. If you live below a burn scar, the rain that matters to you is not the rain in your forecast. It is the rain falling on the slope above you.8

The first is the phrase "100-year flood." It sounds like a once-a-century event, something you can reasonably expect to sit out. It is not a schedule, it is a probability: a 100-year flood is one with a one percent chance of happening in any given year. Every year starts fresh with the same one percent. Two of them can land in consecutive years, and as the USGS points out, the fact that it rained ten inches in a day last year does nothing to stop it raining ten inches in a day again this year. Hydrologists dislike the term precisely because of how badly it reads.9

The second is insurance, and it is the expensive one. Most homeowners policies do not cover flood damage. Flood is a separate policy, generally through the National Flood Insurance Program, and there is typically a 30-day waiting period between buying it and the coverage taking effect. That combination matters enormously: you cannot watch a forecast, decide the risk looks real, and buy protection on the way home. The decision has to be made a month before you need it, which is to say on a day when flooding is the last thing on your mind.10

Put those two together and the shape of the problem is clear. People read a low annual probability as "not my house," and then find out at the worst possible moment that the policy they assumed covered them never did.

The key to flood safety is time: knowing a route is dangerous before you reach it. ZoneWatch provides flood-risk forecasting and high-resolution radar for your exact locations, so you can see heavy rain building and reroute early.

Set it up for home and the places you commute to, and you’ll get a warning while you still have options.