• April 22, 2024, 4:42 pm

What is a flash flood? SoftAIT

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Update : Monday, November 6, 2023

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The main thing about a flash flood is its speed. By definition, the rushing waters begin within six hours after an intense storm—and often within three hours, which is why it’s called a flash. Sometimes, the water can come within minutes. These sudden torrents bring devastation: After heat-related deaths, floods are the deadliest weather event in the US. 

Flash floods can follow record-breaking rainfall, such as what happened in Kentucky in July 2022, killing 37 people. Floods have repeatedly burst across the US in recent years: At the tail end of 2021, 17 inches of rain fell on Tennessee in one day; 22 people were killed in the deluge. At the start of summer 2022, flash flooding shut down Yellowstone National Park. When heavy rains washed across the Northeast in summer 2023, it brought devastation to Vermont and placed 13 million Americans under a flood watch or warning.

Where do flash floods happen?

Flash floods differ from river floods or coastal floods. River flooding occurs when excessive runoff from rain or melting snow causes water to spill over the edges of riverbanks and onto the surrounding floodplains, over the course of many hours or even days. Coastal floods happen when seawater comes up on land, frequently during storms or storm surges. Flash floods, however, aren’t bound to where water already exists. In fact, though storms often precede flash floods, you don’t always have to see rain before flooding—snowmelt or precipitation might occur at a higher elevation and flow downhill. Levees, dams, or ice jams can break and, likewise, send walls of water into typically dry areas. 

Steep, mountainous topographies are particularly prone to flash floods, which happen in all 50 states. Urban areas are especially vulnerable, too. Dense concrete and other impervious surfaces in cities prevent water from sinking into the ground—instead, there’s 2 to 6 times more runoff than what would occur over absorbent dirt or other natural terrain. 

[Related: One of the world’s driest places just saw record flooding]

Excessive runoff in cities, without anywhere to go, often has the heaviest impact on those who can least afford to deal with it. It doesn’t take much rain to damage deteriorating or inadequate infrastructure, especially in places where large volumes of water overwhelm local stormwater drainage. Similarly, low areas like underpasses, garages, and basements can quickly become death traps. 

These kinds of fast-moving floods become dangerous quickly. It’s bad news if a flash flood comes when people are caught unawares, stuck at home, or in traffic. More than half of the deaths occur when people are trapped in their cars. Flash floods often manifest as walls of water that can wash away most things in its path and carry debris; they can reach heights of 30 feet or more and trigger equally dangerous mudslides. But the water doesn’t need to be very high to have severe consequences: 6 inches of fast-moving water can knock people off their feet. Two feet of water can sweep cars away. A flood in Big Thompson Canyon, Colorado, in 1976—moving at a velocity of 30 feet per second—transported boulders that weighed as much as 250 tons.

Are flash floods becoming more severe?

Natural disasters are generally getting worse—and that’s true for floods, too. 

As climate change warms the atmosphere, extreme rainfall is rising, which increases the risk for flash floods. Heavy precipitation events are projected to increase by 2 to 3 times the historical average—and hurricanes and storm surges will increase other kinds of flooding, too. The Federal Emergency Management Agency expects the nation’s floodplains to expand by 45 percent by the century’s end, as the agency reported in a recent study. A study in early 2021 found that increased precipitation—resulting partially from climate change—costs the US an additional $2.5 billion each year in flood damage. And some studies show that flood frequency is increasing in the Mississippi River valley and across the Midwest in the last century, as well as in the Northeast over the past 50 years.

What’s the best way to prepare for a flood?

There are several steps you can take to prepare for flash floods—and stay safe should they happen in your neighborhood. First, know the level of risk by looking at flood maps. You may be federally required to have flood insurance if you live in a high-risk area.

The US Geological Survey has compiled a list of additional map resources, too, and the nonprofit First Street Foundation created a tool that provides additional context to assess your property risk from environmental threats. 

[Related: Rain, storms, and mudslides batter California]

Pay attention to flash flood watches, which the National Weather Service issues to indicate when conditions could result in flash flooding. If you’re in the affected area, be ready to take action—particularly if the NWS announces a flash flood warning. That’s issued when flash floods are imminent and, at that point, you and everyone else should immediately evacuate the area. 

But be particularly careful when traveling by car. Don’t drive through flooded streets—it’s difficult to gauge the water depth, and roadways hidden below the water can collapse from erosion. 

If flood waters cause your vehicle to stall out, abandon it and seek higher ground. Rapidly rising water can sweep the vehicle away. Know your surroundings and head to higher ground, and listen to NOAA radio updates when in a flood. 

It’s also important to develop an emergency preparedness plan with your family or those you live with to have a few days’ worth of resources. The US Department of Homeland Security has a handy guide that will help you create a plan

For more tips like these, check out FEMA’s flood information sheet

What is a 100-year flood?

To describe the likelihood of weather disasters, meteorologists might use terms such as a “100-year flood” or “100-year storm”—or reference an even longer timescale to indicate a greater rarity. April’s South Florida flooding, for instance, which swamped the region after it received 26 inches of rain in a day, was labeled a “1-in-1,000 year event.”

But those descriptors can be somewhat misleading, as the US Geological Survey explains. Instead, hydrologists prefer to think of these as statistical “recurrence intervals,” in which a “100-year flood” occurs with about 1 percent probability in any given year. (A 50-year storm has a probability of 2 percent, while a 500-year storm translates to odds of 0.2 percent that this much rain would fall.) 

To put it another way: A 100-year flood, while rare, allows for the possibility that multiple such floods could occur within the same rainy season. It doesn’t mean that, if such a flood passes through an area, that spot is safe from a deluge for another 99 years. 

This story has been updated. It was originally published on August 3, 2022.



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