What is a flash flood and what causes it?

Flash floods can happen anytime a large amount of rainfall occurs and the ground cannot absorb it. That can be because the earth is already too saturated, because it is covered in asphalt and concrete with poor drainage in an urban environment or because the soil is too hard after a drought or in a desert, among other reasons. When the water cannot dissipate through the ground, it accumulates above it, causing flooding. Greg Carbin, the National Weather Service’s chief of forecast operations, told Vermont Public the region’s floods this week were caused by several “ingredients” mixing together. The ground was already saturated from the state’s summer downpours when it was hit with a slow-moving storm, he said, which combined heavy moisture gathered in the tropics with winds from a low-pressure system that moved in from the Appalachian Mountains. New York Gov. Kathy Hochul also stressed the role of climate change in the weather event in a tweet on Monday. Warmer air holds more moisture, which is then dumped all at once. While some places may not see more rain in terms of overall totals, what does fall is more likely to come in short, intense bursts. “Make no mistake: This is our new normal,” Hochul said. “We are the first generation to feel the impacts of climate change and the last generation with a shot at doing anything about it.”

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

What distinguishes a flash flood is the incredibly short time it takes to materialize: generally less than six hours, according to the National Weather Service. Other floods can occur over days or weeks, as water builds up in the ground and catchments and spills over. Annie Lau, a geography lecturer at the University of Queensland, said flash floods can catch people off-guard, particularly in areas not normally prone to flooding, and give them less time to evacuate. “Flash flood locations are also not limited to flood plains, unlike river floods,” she said. “Some flash floods can travel at high speed, particularly common in valleys where rainwater rushes downhill, resulting in what people may describe as an ‘inland tsunami.’”

How do flash floods affect the environment?

Low-lying areas are at the greatest risk of flash flooding, but it can happen in many environments. In urban areas, the lowest points are often streets and roads, which can turn into rivers. Amy Peden, a drowning prevention researcher at the University of New South Wales, said floods will pick up and deposit debris, and in some cases can change the landscape of waterways. “You may think you know your local waterway, but if a flood has come through, it’s likely going to be very different,” she said. Lau said they can leave long-term impacts on some areas, through rapid soil erosion, washing away of vegetation and exposure of tree roots, as well as through infrastructure damage. “How urban areas and drainage systems are designed play huge roles in mitigating or worsening floods,” she added.

What is a flash flood warning?

The National Weather Service has tiers of notifications it issues to help people prepare. In increasing levels of seriousness, they are: flood watch, flood advisory, and flash flood or flood warning. A flood watch is issued when a flood developing is possible because the right conditions are present: this is when evacuation plans should be developed. A flood advisory is issued when relatively minor flooding is present in an area: caution should be exercised. A flash flood warning or flood warning is issued when a flash flood or flood is imminent or already occurring: this is when evacuation plans should be put into place to move to higher ground if it is safe to do so.

What should you do about a flash flood warning?

Peden said the key to navigating a flash flood successfully is careful planning when heavy rainfall begins: Keep up-to-date with emergency warnings, place valuables high up, pack a bag and plan a route to higher ground with backups in mind to avoid floodwater. Authorities say you should never attempt to walk or drive through floodwater, which can be deeper and faster moving than it appears. “Most flood-related deaths occur within vehicles,” the NWS warned on Monday. After a flood, Peden said it’s important to keep the risk of infection from floodwater front of mind. Wear protective gloves, clothing and goggles while cleaning up; make sure abrasions or cuts are covered up; use plenty of disinfectants; and keep children out of floodwaters, she said. Most of all, she said, be careful not to underestimate floods’ danger. “People that experienced floods regularly, I would also caution them not to be complacent,” she added. “Just because you’ve lived through one doesn’t mean the next one is going to be the same.”

Flash flooding: What causes it and how to respond in an emergency

By Frances Vinall

July 11, 2023 at 5:11 a.m. EDT

Storms leave damage along Main Street in Highland Falls, N.Y., on Monday. (Jeenah Moon for The Washington Post)

5 min

About two months’ worth of rain has fallen on Vermont and New York state’s Hudson Valley since Sunday, causing flash flooding and at least one death, of a 35-year-old woman in New York, and damaged infrastructure.

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The National Weather Service expects the “significant to catastrophic flooding” to continue until the storm system exits the region on Tuesday.

Here’s what to know about flash floods, and how to protect yourself.

Northeast storms dump over 2 months’ worth of rain on Vermont

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