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

Photo courtesy NOAA

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, 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.