Flash flooding is weather’s No. 2 killer, claiming more lives than anything but heat, so it’s important to understand what causes it and how to stay safe.
A flash flood can happen anywhere intense rain falls faster than the soil can absorb. And as the name says, it happens in a flash!
Geography and topography play important roles in how soil handles extreme rainfall. Water rises very quickly – with wide variation possible even over small regions – so pinpointing exactly where flash flooding will occur is a major challenge.
Flash floods often happen when storms have been “training,” or rolling over the same areas for hours. This can overwhelm creeks, rivers and even storm drains and send water rushing into streets and neighborhoods.
Excessive rainfall outlooks are issued by the National Oceanic and Atmospheric Administration’s Weather Prediction Center to give an idea of where flash flooding could happen on a given day based on rain forecasts. The agency’s National Weather Service issues more localized flash flood watches and warnings where the biggest flooding threat is expected.
What to do if you’re caught in a flash flood
If you find yourself in a flash flood, move to higher ground immediately. Get out of floodwaters as quickly as you can, as they could be contaminated or electrically charged – or harboring fire ants or alligators.
Never drive on flooded roads. It only takes 2 feet of rushing water to carry away most vehicles, including pickups and SUVs, the weather service says. Floodwaters also can hide a variety of dangers, including whether the road has collapsed or washed away.
If a flood strands you in your car, “abandon it immediately and seek higher ground,” NOAA suggests.
Flash flooding can be worse in mountains
Mountainous areas are extremely susceptible to flash flooding.
There’s not as much surface area for rain to soak into soil before steep inclines send it rushing down, where it can overwhelm streams and rivers and funnel into valleys and rise quickly with little warning.
Landscape scarred by wildfire, especially around mountains, can result in a greater risk of flash flooding in heavy rain.
“In some areas, where the fire burned hot enough or long enough, soils develop a layer that actually repels water, like rain on pavement,” NOAA says. “Rainfall that would normally be absorbed by the forest canopy and loose tree litter and duff on the ground will instead quickly run off.”
Often, charred soils from burned areas can quickly come loose and get carried downhill by rainwater, causing debris flows that race into valleys.
“Debris flows are fast-moving, deadly landslides,” NOAA says. “They are powerful mixtures of mud, rocks, boulders, entire trees – and sometimes, homes or vehicles.”
A dam or levee breach can unleash deadly flooding
Catastrophic flash flooding can also happen when water overtops a levee or dam, or a levee or dam gets damaged or compromised.
When a levee breaches or a dam fails, enormous amounts of water can spill out with very little warning. This kind of sudden release of water can drown entire towns.
A prime example of this was Hurricane Katrina in 2005, when numerous levee breaches across New Orleans unleashed historic flooding, sending people to their rooftops for safety and killing hundreds.