Image courtesy Jocelyn Augustino/FEMA

Water is one of the most useful things on Earth. We drink it, bathe in it, clean with it an dused 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.

The total amount of water on Earth has remained fairly constant for millions of years (though its distribution has varied considerably in that time). 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. 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. As a result, 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. 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 (like what happened during Hurricane Katrina), or multiple hurricanes happen to move through the area, the land receives much more precipitation than normal.

The Cost of Water The average annual U.S. flood losses in the past 10 years cost more than $2.4 billion.

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 unusual accumulation of water in an area of land.

While heavy rainstorms are the most common cause of flooding, there are other causes. Varying temperatures of different seasons lead can lead to seasonal flooding. Manmade dams sometimes break. 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.

Water World When you hear the words “flood” and “flooding,” you automatically think disaster. But in the case of the Okavango Delta, flooding can actually be a good thing. Every year, water from the highland areas of Angola finds its way to the Okavango River, more than doubling its size, so that it spills out over the surrounding land. The floodplain goes from parched land to a water-filled oasis. It lures hundreds of species of birds and a variety of animals – antelope, buffalo, elephants, hippos, hyenas, monkeys and warthogs – that all thrive during this slow, yearly flood.

Super Soaked
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. 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. 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. Cities have flooding concerns too, but for different reasons.

Image courtesy Jocelyn Augustino/FEMA New Orleans, LA, September 2, 2005 - Neighborhoods throughout the area remain flooded as a result of Hurricane Katrina

Much of the ground in cities is covered by asphalt and concrete. Obviously, these surfaces are not great sponges: 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. To prevent this problem, some cities, such as Los Angeles, have constructed concrete flood-relief channels. 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.

Image courtesy Michael Rieger/FEMA New Orleans, La., September 8, 2005 -- FEMA's US&R teams in route by helicopter to conduct a search in St. Bernard Parish, view the flooding in New Orleans

A similar problem can arise with levees, large walls built along rivers, and sometimes lakes, to keep them from overflowing. Along rivers, these structures extend the natural banks so that much more water can flow through it. But while they may be effective at the levee area, it usually make problems worse for an area down the line, where there are no levees. That area gets all the floodwaters 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. This is what happened during Hurricane Katrina. The levees surrounding Lake Pontchartrain, which had been added on to after previous breaches during other storms, broke apart under pressure from massive storm surges during the hurricane.

Water Power
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 of water can move with enough force to wash a car away, and 6 inches 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. So why do floodwaters 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.

Energency Kit Check List If you live in a flood-prone area, or anticipate the chance of severe flooding, it’s good to have an emergency preparedness kit on hand. Here are some of the items FEMA says your kit should include: A 3-day supply of non-perishable food A 3-day supply of water (one gallon per person, per day) A portable, battery-powered radio or TV and extra batteries A flashlight and extra batteries A first aid kit and manual Hygiene items such as moist towelettes and toilet paper Matches and waterproof container A whistle Extra clothes Photocopies of ID and credit cards Money (bills and coins) Prescription medicine *If there is an infant in the family, include formula, diapers, bottles, etc.

Go here http://www.floodsmart.gov/floodsmart/pages/riskassesment/findpropertyform.jsp to find out what your flood risk is. Go here http://www.floodsmart.gov/floodsmart/pages/statistics.jsp to find out how many people currently hold flood insurance. Go here http://www.fema.gov/fema/csb.shtm to find out if your community participates in the National Flood Insurance Program. Go here http://www.floodsmart.gov/floodsmart/static/riskscenario/riskscenario.jsp to see an animation of several flood scenarios. Discuss with your teacher and classmates which flood scenario is most likely to happen where you live.