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What Is a Flood and Why Is the Surge of Water So Dangerous?

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Water is one of the most useful things on Earth. We drink it, clean with it and use it to cook food. Most of the time, it is entirely benign. But in large enough quantities, the same stuff we use to rinse a toothbrush can overturn cars, demolish houses and even kill.

Flooding has claimed millions of lives in the last hundred years alone, more than any other weather phenomenon. As evidenced by disasters from Hurricane Katrina in New Orleans to the 2022 monsoons in Pakistan, flooding can cause widespread devastation. But what is a flood, exactly?

In this article, we’ll discover what makes water change character so rapidly and see what happens when it does.

Water, Water Everywhere

To understand how floods work, you have to know something about how water behaves on our planet. The total amount of water on Earth has remained fairly constant for millions of years (though its distribution has varied considerably in that time). Every day, a very small amount of water is lost high in the atmosphere, where intense ultraviolet rays can break a water molecule apart, but new water is also emitted from the inner part of the Earth, by volcanic activity. The amount of water that is created and the amount that is lost are pretty much equal.

At any one time, this volume of water exists in many different forms. It can be liquid, as in oceans, rivers and rain; solid, as in the glaciers of the North and South Poles; or gaseous, as in the invisible water vapor in the air. Water changes state as wind currents move it around the planet.

The heating activity of the sun generates wind currents. The sun shines more on the area around Earth’s equator than it does on areas farther north and south, causing a heat discrepancy over the globe’s surface.

In warmer regions, hot air rises into the atmosphere, pulling cooler air down into the vacated space. In cooler regions, the cold air sinks, pulling warmer air down into the vacated area. The rotation of the Earth breaks this cycle up, so there are several smaller air-current cycles all along the globe.

The Water Cycle

Driven by these air-current cycles, Earth’s water supply moves in its own cycle. When the sun heats the oceans, liquid water from the ocean’s surface evaporates into water vapor in the air. The sun heats this air (water vapor and all), which rises through the atmosphere, carried along by wind currents.

As this water vapor rises, it cools down again, condensing into droplets of liquid water (or crystals of solid ice). Collections of these droplets are called clouds. More water may condense onto these droplets if a cloud moves into a colder environment.

If enough water accumulates in this way, the droplets become heavy enough to fall through the air as precipitation (rain, snow, sleet or hail). Some of this water collects in large, underground reservoirs, but most form rivers and streams that flow into the oceans, bringing the water back to its starting point.

Overall, wind currents in the atmosphere are relatively consistent. Consequently, specific locations generally experience the same weather conditions year to year. However, climate change has led to more extreme — and less predictable — weather.

Since waterways are formed slowly over time, their size is proportionate to the amount of water that normally accumulates in that area. When there is suddenly a much greater volume of water, the normal waterways overflow, and the water spreads out over the surrounding land.

At its most basic level, a flood is an anomalous accumulation of water in an area of land.

A series of storms bringing massive amounts of rain is the most common cause of flooding events, but there are others.

Under the Weather

The sort of flooding that most people are familiar with occurs when an unusually large number of rainstorms hit an area in a relatively short period. In this case, the rivers and streams diverting water to the ocean are simply overwhelmed.

Seasonal Flooding

The varying temperatures of different seasons lead to different weather patterns. In the winter, for example, the air over the ocean might be warmer than the air over the land, causing the wind flow to move from the land out to the sea.

But in the summer, the air over the land heats up, becoming warmer than the air over the ocean. This causes the wind current to reverse, picking up more water from the ocean island carrying it over land. This monsoon wind system can cause intense rain that is entirely out of step with the climate for the rest of the year. In some areas, excess water from melting snow exacerbates monsoon flooding.

The best-known example of seasonal flooding is the annual expansion of the Nile River in Egypt. In ancient Egypt, monsoon rains at the river’s source caused the waterway to extend significantly during the summer. In this case, the predictable flooding was not a disaster but a godsend.

The expanding waters would l­eave fertile silt all along the banks of the river, making the area ideal farming land once the river had subsided again. Regular flooding is one of the main factors that allowed civilization to thrive in the Egyptian desert.

A dam blocks off the river these days, collecting the summer rain and doling it out throughout the year. The dam has extended the planting season so Egyptian farms can grow crops year-round.

Tidal Activity

Another common reason floods occur is unusual tidal activity extending the ocean’s reach farther inland than usual. Tidal flooding may result from particular wind patterns that push the ocean water in an unusual direction or by tsunamis, large waves in the ocean triggered by a shift in the Earth’s crust.

Dam Failure

Sometimes, flooding occurs when a dam breaks. We build dams to modify the flow of rivers. Basically, the dam collects the river water in a large reservoir so that we can decide when to increase or decrease the river’s flow, rather than letting nature decide.

Engineers build dams that will stand up to any amount of water likely to accumulate. Occasionally, more water accumulates than the engineers predicted, and the dam structure breaks under pressure. When this happens, a massive amount of water is released all at once, causing a violent “wall” of water to push across the land.

In 1889, such a flood occurred in Johnstown, Pennsylvania. Many townspeople dismissed the flood warning as unfounded panic, so when the rushing wall of water did hit, more than 2,000 people were killed in only a few minutes.

Absorbency and Flood Risk

The severity of a flood depends not only on the amount of water that accumulates in a period of time, but also on the land’s ability to deal with this water. One element of this is the size of rivers and streams in an area. But an equally important factor is the land’s absorbency.

When it rains, soil acts as a sponge. When the land is saturated — that is, it has soaked up all the water it can — any more water that accumulates must flow as runoff.

Some materials become saturated much more quickly than others. To see how this works, just take a bucket of water outside and try wetting various surfaces. Soil in the middle of the forest is an excellent sponge. You could dump several buckets of water on it, and it would soak the water right up.

Rock is not so absorbent — it doesn’t seem to soak up any water. Hard clay falls somewhere in between.

Generally, soil tilled for crops is less absorbent than uncultivated land, so farm areas may be more likely to experience flooding than natural areas. One of the least absorbent surfaces around is concrete.

Take Me to the River

According to the United Nations Population Fund, over half of the world’s population lives in urban areas. Urbanization alters the landscape in many ways, including covering the ground in asphalt and concrete.

These surfaces are not the best sponges around: Almost all accumulated rain becomes runoff. In an industrialized area without a good drainage system, it may not take much rain to cause significant flooding.

Flood-relief Channels

Some cities, such as Los Angeles, have constructed concrete flood-relief channels to prevent this problem. During times of heavy rainfall, the water flows into these channels, which meander out of the city towards areas with more absorption capabilities.

These sorts of systems may cause flooding farther down the line, however. When you cover an area in concrete and asphalt, you are essentially cutting off part of the Earth’s natural sponge, so the rest of the sponge has a lot more water to deal with.


A similar problem can arise with levees, large walls built along rivers to keep them from overflowing. These structures extend the river’s natural banks so that much more water can flow through it. But while they may be effective at keeping water out of one area, they usually make problems worse for a spot down the line where there are no levees.

That area gets all the flood waters that would have spread out farther upriver. Another danger of levees is that, like dams, they can break. When this happens, a large amount of water flows out onto the land in a short period of time. Broken levees can cause some of the most dangerous flood conditions.

Coastal Flooding

People have yet to have much success with controlling flooding along coastlines. Excessive water in these areas is particularly destructive to manufactured structures because of the erosion it causes.

One method of controlling this erosion is to build fences and walls where the water meets the land, keeping the power of the waves at bay so they don’t wear down the beach.

However, the structures also interfere with the process of beach formation. When you block the water from moving against the coast, the ocean can’t distribute sand — and you don’t get beautiful beaches.

Another problem with fences and walls is that there is only so much they can do. Fundamentally, beaches are changing environments molded by the overpowering force of the ocean.

Beaches are, by their very nature, supposed to be eroded and moved by the dynamic action of waves. Flooding is a regular part of this process and most likely will continue to be, no matter what we do.

River Flooding

While a river may appear to us to be a stable, unmovable feature of the landscape, it is really a vibrant, dynamic entity. This is particularly true of big rivers, such as the Mississippi in the United States and the Yangtze and Huang He in China. Over time, these waterways expand, shift their path dramatically and may even change the flow direction. For this reason, the land around the banks of a river is highly susceptible to flooding.

Unfortunately, rivers are also natural draws for civilization. Among other things, they provide a constant supply of water, rich soils and easy transportation. When the water level is low, people build all along its banks and enjoy all its benefits.

At some point, it is time for the water to shift, and the people who have built along the flood plains quickly discover that they live on unsound ground. The flood damage can be devastating if there is extensive construction in these areas.

Come Hell or High Water

The worst damage from floods — the loss of life and homes — is caused primarily by the sheer force of flowing water. In a flood, 2 feet (61 cm) of water can move with enough force to wash a car away, and 6 inches (15 cm) of water can knock you off your feet.

It may seem surprising that water, even a lot of water, can pack such a wallop. After all, you can peacefully swim in the ocean without being knocked around, and that’s a massive amount of moving water. And in most cases, a flowing river isn’t strong enough to knock you over.

So why do flood waters behave differently?

Flood waters are more dangerous because they can apply much more pressure than an ordinary river or a calm sea. This is due to the massive differences in water volume that exist during many floods. In a flood, a lot of water may collect in an area while there is hardly any water in another area.

Water is fairly heavy, so it moves very quickly to “find its own level.” The bigger the difference between water volumes across an area, the greater the force of movement. But at a particular point, the water doesn’t look so deep, and so doesn’t seem particularly dangerous — until it’s too late.

Nearly half of all flood deaths result from people attempting to drive their cars through rushing water. There is much more water in the ocean than in a flood, but it doesn’t knock us over because it is fairly evenly distributed — water in a calm sea isn’t rushing to find its own level.

Flash Flooding

The most dangerous floods are flash floods, which are caused by a sudden, intense accumulation of water. Flash floods hit an area soon after water begins to accumulate (whether from excessive rain or another cause), so a lot of the time, people don’t see them coming. Since there is a great deal of water collected in one area, flash-flood waters tend to move with a great deal of force, knocking people, cars and even houses out of the way.

Flash floods can be particularly devastating when a heavy thunderstorm dumps a high volume of rain on a mountain. The water moves down the mountain at tremendous speed, plowing through anything in the valleys below.

One of the worst flash floods in U.S. history occurred in 1976, in Big Thompson Canyon, Colorado. In less than five hours, thunderstorms in nearby areas dumped more rain than the region ordinarily experiences in a year.

The Big Thompson River, normally a shallow, slow-moving waterway, abruptly transformed into an unstoppable torrent, dumping 233,000 gallons (882,000 liters) of water into the canyon every second.

Thousands of campers had gathered in the canyon to celebrate the centennial of the state of Colorado. The storm surge happened so quickly that there was no time to issue a flash flood warning. When it hit, hundreds of people were injured, and 139 were killed.

Property Damage

A less catastrophic sort of damage is simple dampness. Most buildings can keep out the rain, but they aren’t built to be water-tight. If the water level is high enough, loads of water seeps into houses, soaking everything. But in most cases, the major damaging element is not the water itself, but the mud it brings with it.

As water flows over the landscape, it picks up a lot of junk. When the flood is over, the water level drops and everything eventually dries out, but the mud and debris stick around.

In 1966, a major storm flooded the Arno, an Italian river that runs through the city of Florence. The small city, one of the art capitals of the world, was overrun with water, mud and general slime. In addition to the loss of life and the damage to buildings, there was a great deal of damage to the city’s art collection.

Mud and slime covered almost everything stored in the city’s basements and ground-level rooms. Through many years of work, scientists and art historians have been able to restore most of the damaged artifacts to good condition.

Spread of Disease

Another sort of flood damage is the spread of disease. As water flows over an area, it can pick up all sorts of chemicals and waste products, leading to extremely unsanitary conditions. Essentially, everything and everyone in a flood is floating along in one big soup.

While diseases usually aren’t created by these conditions, they are more easily transferred (most diseases spread through water more readily than they move through the air).

If you are in a flooded area, it is very important that you drink only bottled or boiled water and observe other sanitation guidelines. To learn more about what to do in flooded conditions, check out this guide put out by the Center for Disease Control.

We’ll never be able to stop flooding. It is an unavoidable element in the complex weather system of our atmosphere. We can, however, work to minimize the damage inflicted by flooding, by building sophisticated dams, levees and canal systems.

But the best way to avoid flood damage may be to back out of flood-prone areas altogether. As with many natural phenomena, the most sensible reaction to flooding may be to get out of the way.

Lots More Information

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More Great Links

National Oceanic and Atmospheric Administration (NOAA)

Federal Emergency Management Agency (FEMA)

NOVA Flood Special

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