How Floods Work

People walking flooded New Orleans streets after the levees broke from the storm surges caused by Hurricane Katrina. See more storm pictures.
Michael Lewis/National Geographic/Getty Images

Water is one of the most useful things on Earth. We drink it, bathe in it, clean with it and use it to cook food. Most of the time, it is completely benign. But in large enough quantities, the very 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. Hurricane Katrina in New Orleans and the 2008 cyclone that struck Myanmar are recent examples of the widespread devastation that flooding can incur.

In this article, we'll find out what makes water change character so rapidly and see what happens when it does. We'll explore the negative impact of floods as well as some of the benefits. We'll also examine how human construction can contain flooding or, in some cases, cause it.

Water, Water Everywhere

Flooding of farmland in Missouri. Heavy rain in the spring and summer of 1993 flooded areas throughout the midwestern United States, leading the federal government to declare 500 counties in nine states as major disaster areas.
Flooding of farmland in Missouri. Heavy rain in the spring and summer of 1993 flooded areas throughout the midwestern United States, leading the federal government to declare 500 counties in nine states as major disaster areas.

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 ultraviolent 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 is 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 from state to state as it is moved around the planet by wind currents. Wind currents are generated by the heating activity of the sun. 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 surface of the globe. In warmer regions, hot air rises up into the atmosphere, pulling cooler air into the vacated space. In cooler regions, cold air sinks, pulling warmer air into the vacated space. The rotation of the Earth breaks this cycle up, so there are several, smaller air-current cycles all along the globe.

Driven by these air-current cycles, Earth's water supply moves in a cycle of its own. 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) so that it rises through the atmosphere and is 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. If a cloud moves into a cooler environment, more water may condense onto these droplets. If enough water accumulates in this way, the droplets become heavy enough that they fall through the air as precipitation (rain, snow, sleet or hail). Some of this water collects in large, underground reservoirs, but most of it forms rivers and streams that flow into the oceans, bringing the water back to its starting point.

Overall, wind currents in the atmosphere are fairly consistent. At any particular time of year, currents tend to move in a certain way across the globe. Consequently, specific locations generally experience the same sort of weather conditions year to year. But on a day-to-day basis, the weather is not so predictable. Wind currents and precipitation are affected by many factors, chiefly geography and neighboring weather conditions. A huge number of factors combine in an infinite variety of ways, producing all sorts of weather. Occasionally, these factors interact in such a way that an atypical volume of liquid water collects in one area. For example, conditions occasionally cause the formation of a hurricane, which dumps a large quantity of rain wherever it goes. If a hurricane lingers over a region, or multiple hurricanes happen to move through the area, the land receives much more precipitation than normal.

In 1927, the Mississippi River overflowed, flooding many cities along its coast.
Photo courtesy NOAA

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, this is what 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, but there are others. In the next section, we'll look at some of the ways floods start, as well as some of the factors that determine their magnitude.

Under the Weather

Heavy rains in the spring of 2001 flooded Davenport, Iowa. Until the waters subsided, locals had to get around town by rowboat.
Heavy rains in the spring of 2001 flooded Davenport, Iowa. Until the waters subsided, locals had to get around town by rowboat.

In the last section, we saw that floods occur when an atypical volume of water collects in an area. There are a number of ways this might happen, and there are a wide range of events that occur when it does.

The sort of flooding that most people are familiar with occurs when an unusually large number of rainstorms hit an area in a fairly short period of time. In this case, the rivers and streams that divert the water to the ocean are simply overwhelmed. The varying temperatures of different seasons leads 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 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, so that more water from the ocean is picked up and carried over land. This monsoon wind system can cause a period of intense rain that is completely out of step with the climate the rest of the year. In some areas, this flooding may be exacerbated by excess water from melting snow.

During the flooding of 1993, volunteers in St. Genevieve, Missouri, filled sand bags to build makeshift flood levees.

Perhaps the best known example of seasonal flooding is the annual expansion of the Nile River in Egypt. In Ancient Egypt, monsoon rains at the source of the river would cause the waterway to extend out a good distance during the summer. In this case, the 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. This is one of the main factors that allowed civilization to thrive in the Egyptian desert. These days, the river is blocked off by a dam upstream, which collects the summer rain and doles it out throughout the year. This has extended the planting season so that Egyptian farms can grow crops year-round.

Another common source of flooding is unusual tidal activity that extends the reach of the ocean farther inland than normal. This might be caused by particular wind patterns that push the ocean water in an unusual direction. It can also be caused by tsunamis, large waves in the ocean triggered by a shift in the Earth's crust.

Downtown Johnstown, Pennsylvania, after the disastrous flood of 1889. In addition to destroying Johnstown, the wall of water also flooded towns farther 'downstream,' including Washington, D.C.
Photo courtesy NOAA Photo courtesy NOAA
Downtown Johnstown, Pennsylvania, after the disastrous flood of 1889. In addition to destroying Johnstown, the wall of water also flooded towns farther 'downstream,' including Washington, D.C.
Photo courtesy NOAA Photo courtesy NOAA

Floods may also occur when a man-made dam breaks. We build dams to modify the flow of rivers to suit our own purposes. 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 that is likely to accumulate. Occasionally, however, 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. The townspeople were warned that the flood was coming, but many dismissed the alert as unfounded panic. When the rushing wall of water did hit, more than 2,000 people were killed in only a few minutes.

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. As we've seen, 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 sort of sponge. When the land is saturated -- that is, 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 at all. Hard clay falls somewhere in between. Generally, soil that has been 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. In the next section, we'll see how concrete, asphalt and other human construction can affect flooding.

Take Me to the River

Flooded streets in St. Genevieve, Missouri. The small town was one of the many midwestern cities devastated by flooding in the summer of 1993.
Flooded streets in St. Genevieve, Missouri. The small town was one of the many midwestern cities devastated by flooding in the summer of 1993.

In the last section, we saw that the degree of flooding is determined by the amount of water that accumulates in an area, as well as the nature of the land surface. As civilization has expanded, human beings have altered the landscape in a number of ways. In the Western World, one of the most significant changes has been covering the ground in asphalt and concrete. Obviously, these surfaces are not the best sponges around: Almost all rain that accumulates becomes runoff. In an industrialized area without a good drainage system, it may not take much rain to cause significant flooding.

Some cities, such as Los Angeles, have constructed concrete flood-relief channels to prevent this problem. When it rains a lot, the water flows into these channels, which meander out of the city where the water can be better absorbed. 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 natural banks of the river 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 an area down the line, where there are no levees. That area gets all the flood waters that would have spread out farther up river. 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. This can cause some of the most dangerous flood conditions.

Breakwater walls in Maryland, built to slow beach erosion
Photo courtesy NOAA

People haven't had much success with controlling flooding along coastlines. Excessive water in these areas is particularly destructive to man-made 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. This keeps the power of the waves at bay, so they don't wear down the beach. But 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. They 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.

A breeched levee along the Mississippi River, during the great flood of 1927
Photo courtesy NOAA

The same can be said for many inland areas. 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 direction of flow. 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 an easy means for transportation. When the water level is low, people build all along its banks and enjoy all its benefits. At some point, it comes time for the water to shift, and the people who have built along the flood plains quickly discover that they are living on unsound ground. If there is extensive construction in these areas, the flood damage can be devastating.

In the next section, we'll look at the different sorts of flood damage to see how ordinary water can be a devastating force.

Come Hell or High Water

Cars piled up by a 1972 flash flood in Rapid City, South Dakota.
Cars piled up by a 1972 flash flood in Rapid City, South Dakota.
Photo courtesy NOAA

The worst damage from floods, the loss of life and homes, is caused primarily by the sheer force of flowing water. In a flood, two 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?

A house that was carried downstream by 1997 flooding in Arboga, California
Photo courtesy NOAA

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.

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.

A trailer, car and telephone pole piled up by a 1977 flood in Georgia
Photo courtesy NOAA

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 L) 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 flood happened so quickly that there was no time to issue a warning. When it hit, hundreds of people were injured, and 139 were killed.

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.

Rescue workers fight upstream against rushing waters in a 1975 flash flood that hit Rockville, Maryland.
Photo courtesy NOAA

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.

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.

Related Articles

More Great Links