How Floods Happen, and How to Be Safe

Floods are capable of striking almost any river, creek, lake or coast. AC Rider/Shutterstock

People have been settling near water for millennia, relying on it for transportation, energy, food and, most importantly, water. But despite its many benefits, this prime real estate is also haunted by a hidden danger: floods. And thanks to growing human populations and wilder weather from global warming, the threat of flooding is forecast to rise in many parts of the world for decades to come.

Floods aren't a new problem, of course. Even the first human fishing camps likely suffered when rivers flooded. But the stakes grew higher as populations swelled, and when early farmers found rich soil along river banks — leading to permanent settlements in floodplains — the stage was set for future disasters. Man-made dams have since reduced death tolls from floods, but economic losses continue to surge as cities expand near water.

Global warming is expected to worsen this trend, since warmer temperatures make more water evaporate, pumping more moisture into the atmosphere. Still, individual floods can't be directly tied to climate change, points out Mike Halpert of the U.S. Climate Prediction Center.

Too many factors are involved, he says, including local geography, ground cover, long-term precipitation and climate trends like El Niño, La Niña or the Arctic Oscillation. The annual floods of the Red River in North Dakota and Minnesota, for example, are spurred by spring rains as well as months of winter snow. "When you put 4 feet of snow on the ground and then you get warm temperatures, or you get a nice warm rain on top of it, that's a prime setup for flooding," Halpert says. Elsewhere, record-breaking Mississippi River floods have been triggered by unusually snowy winters and intense spring storms, while many coastal states have been inundated as slow-moving tropical storms dump record amounts of rain.

Yet with U.S. weather forecasts expected to become more extreme and erratic — and U.S. watersheds growing more crowded and paved — Americans will likely face events like these long into the future. And while floods are famously difficult to forecast, people's precautions and reactions can still mean the difference between life and death.

There will be flood

Coast Guard rescues people in floods
U.S. Coast Guard members rescue people from flood water in Baton Rouge, Louisiana, in August, 2016. U.S. Department of Defense/Flickr

Floods are the most common natural disaster in the U.S., capable of striking almost any river, creek, lake or coast nationwide. They kill about 140 Americans each year and are often more destructive than the storms that caused them — water flowing at 10 mph exerts the same pressure on a structure as 270 mph wind gusts, according to the Federal Emergency Management Agency. Across the country, floods destroy some $6 billion worth of property every year.

Overflowing rivers are behind most U.S. floods, but anything from tsunamis and hurricanes to broken dams and urban runoff can cause one. A single flood may fit multiple categories, but floods are generally classified as one of the following:

Flash floods

Most deaths and damage from floods are due to flash flooding — "a rapid and extreme flow of high water into a normally dry area, or a rapid rise in a stream or creek above a predetermined flood level," according to the National Weather Service. Flash floods develop suddenly, often in just a few minutes, and while they occur in all 50 states, they're most common in hilly areas with steep valleys, or along small waterways in urban environments. Their speed, depth and element of surprise make flash floods highly dangerous, causing major damage while allowing little time to prepare or evacuate. Heavy rains are the top cause of flash floods, but urban runoff, "ice jams," dam failures and other factors may also be involved.

Slow river floods

flood on President's Island near Memphis
An old tractor sits in floodwaters from the Mississippi River outside a barn on President's Island near Memphis, Tennessee. U.S. Department of Agriculture/Flickr

Rising waters may spur flash floods in steep, narrow river basins, but in flatter, wider ones, flooding tends to be slow, shallow and long-lasting. Flat floodplains can remain inundated for days or even weeks, but these floods are at least usually easier to predict than flash floods. Spring snowmelt regularly swells northern rivers, and when big blocks of unmelted ice are floating downstream, they can become lodged under bridges or in narrow passages, creating an "ice jam" that sets off a flash flood on top of the slower, pre-existing flood.

Coastal floods

storm surge
Hurricanes push huge walls of sea water ashore, creating storm surges. .Martin./Flickr

Storms and earthquakes are the two leading causes of ocean floods. Hurricanes push walls of sea water ashore when they hit land, creating a saline flash flood known as a "storm surge." Storm surges are often responsible for the majority of deaths from tropical cyclones, as was the case in New Orleans after Hurricane Katrina in 2005. Despite hurricanes' strength, though, deep-sea earthquakes are capable of displacing even larger amounts of water, forming long-range waves called "tsunamis." Tsunamis can push floodwaters many miles inland, as seen after the 2004 Sumatran quake and the magnitude-9.0 temblor that hit Japan in March 2011.

Ground failures

This California home was lost to a mudslide.
This California home was lost to ground failure. Hector Sanchez/Flickr

Some floods attack from below, as the water table rises to the surface and washes away chunks of topsoil. This can cause a variety of ground failures, including "subsidence," or sinking soil, and "liquefaction," a process in which water-soaked sediment loses strength and acts like a liquid. Scientists also differentiate between "mudfloods" — a liquid flood that carries up to 50 percent solid sediment loads — and "mudflows" — solid landslides where the downward flow is viscous enough to support large boulders within a wave of smaller particles. Mudfloods and mudflows are most common in California and other Western states, since they tend to occur on hillsides burned bare by wildfire.

Lake floods

flooded lake
Lakes flood differently than the way rivers or streams do. Marc Berry Reid/Flickr

Most lakes experience fluctuating water levels, but they usually don't "flood" the way rivers do because lakes typically have outlet streams or rivers to help them drain. But not all lakes have such outlets, and these "closed-basin lakes" are prone to potentially catastrophic floods if their water level rises too high. Glacial lakes — which were carved and filled by glaciers, and make up most lakes in North America — are also at risk of drainage problems, and can undergo dramatic, long-term fluctuations in depth.

Opening the floodgates

While rain and snow cause most floods, they're also pawns of broader climatic trends that shape daily weather. Linking specific weather events to these trends is never easy, but climatologists can at least trace the origins of some recent U.S. flood problems to unusually heavy precipitation during preceding months.

The winter of 2010-'11 saw record snowfall in much of the U.S., for example, which was largely blamed on the Arctic Oscillation pushing Canadian weather south. That helped feed another big Red River flood the following spring, and when a low-slung jet stream later began producing frequent thunderstorms across the Midwest, it contributed to historic flooding on the Mississippi and other major rivers. A similar phenomenon also occurred between 2009 and 2010, when El Niño-driven rains were initially welcomed, helping end long droughts from California to South Carolina.

2009 flood in Atlanta
Flooding covered much of metro Atlanta in 2009. WhosThisValGirl/Flickr

Within a few months, however, much of metro Atlanta was underwater (above). That region's drought had faded, and by September '09 El Niño was already drenching the U.S., sending storm after storm east across the country. October '09 was the wettest month nationwide in 115 years of record keeping, and when winter arrived, all that rain began falling as snow instead. El Niño reached its peak in December, helping fuel one of the snowiest winters in recorded history — 63 percent of the U.S. had a white Christmas in 2009, and by February, the entire Northern Hemisphere's snow cover was more than 1 million square miles above average.

The effects were so extreme, in fact, that climatologists soon realized El Niño wasn't working alone. Its accomplice turned out to be the strongest negative phase of the Arctic Oscillation ever recorded, a phenomenon that essentially shoves cold Arctic air south into mid-latitude regions, while pulling their warmer air up north. That helped create unseasonably cold temperatures across the U.S. and Europe, which in turn transformed El Niño's barrage of rainstorms into blizzards. When that snow later melted, it released a huge amount of moisture all at once — and many soils were still too waterlogged in 2010 to handle it.

"Snow always melts; it just depends how much is on the ground," Halpert says. "[2010] was unusual in that both December and February had all-time record low values for Arctic Oscillation, and the seasonal value was also a record." While that was odd, though, nearly the same thing happened in 2011: A negative Arctic Oscillation fueled heavy winter snow, followed by frequent spring storms, which proved too much for many rivers to contain. In March, U.S. officials warned that half the country faced spring-flooding risks, including the Red and Mississippi river basins.

U.S. Capitol in the snow
Recent studies have linked intensifying winters in the U.S. to climate change. Katja Schulz/Flickr

Recent research has since linked intensifying U.S. winters to climate change, while two others even specifically related rising temperatures to rising floodwaters. Scientists typically discourage such specific attributions, but climate change could potentially affect flooding in several ways, depending on the region and type of flood in question. Most experts agree melting glaciers and warm, swollen seawater are raising sea levels, which may eventually flood low-lying coastal cities. NASA and other science agencies also predict weather patterns will swing more violently, possibly causing drought/flood cycles even more extreme than the ones seen recently in the U.S. And if climate change does create stronger storms as expected, many U.S. rivers could be in for unprecedented floods — suggesting that even if record flooding in 2010 and 2011 wasn't a reflection of global warming, it may still be a preview.

In fact, in 2015, according to NOAA, 176 lives were lost in the U.S. due to flooding, which was the most since 1995. By mid-October 2016, NOAA already reported 140 flood-related deaths.

Flood safety

Potomac River stream flood
The small stream is just part of the overflow from the main Potomac River at Great Falls during a flood. NOAA Photo Library/flickr

The main rule for staying safe during a flood is to never willingly go near the water, whether on foot or in a car. Just six inches of moving water can knock people off their feet, so FEMA warns against walking through flowing floodwaters, and points out that since even apparently dry land could be subject to ground failure in a flood, it's not a bad idea to use a pole or stick to test the soil before stepping on it. The best place to be is high ground, but if you're in a building when floods arrive, go to the roof or the highest floor, but be careful not to get trapped in an attic or other confined space by rising water.

More than half of all deaths in floods happen when vehicles are swept away, usually in flash floods. Many of the drivers are overtaken before they can react, but people also frequently overestimate their ability to drive through flowing water, often with tragic results. NOAA's "Turn Around, Don't Drown" campaign is aimed at reducing these preventable deaths by raising awareness of how dangerous road flooding can be.

Electrocution is another dangerous side effect of flooding, one more reason to stay away from the water. Avoid and report any downed power lines and electrical wires, and consider turning off your home's electricity and checking around for gas leaks.

Infection and disease can be major problems during a flood as well as long afterward. While water levels are still high, an array of contaminants can be mixed in with the flood, ranging from untreated sewage to toxic chemicals. But even after the threat of water-borne bacteria and synthetic pollutants wanes, respiratory risks arise from black mold and other fungi that flourish in water-damaged wood and fabrics. Anything that got wet from flooding should either be thoroughly cleaned or thrown away.

For more flood advice, see FEMA's FloodSmart page, which has safety tips as well as information on the National Flood Insurance Program.

Watch the video below to see one example of how quickly a flash flood can escalate out of control.