Floods

A flood is a body of water that covers land which is normally dry. Floods are common natural disasters that can affectmillions of people around the world. They destroy houses and buildings, and carry soil away from valuable farming land.  Floods can also contaminate drinking water and lead todiseases. They are often caused by rivers, but overflowinglakes and seas can also cause flooding.

Flooding has always been a part human history. Many ancientcivilizations developed along waterways and rivers because people needed water for their fields.

Floods are not always destructive natural events. Before the Assuan High Dam was built yearly floods in Egypt brought along nutrients and made the land around the Nilevery fertile.  Every year floods during the monsoon season in Bangladesh deposit fertile soil but also kill thousands of people and leave millions homeless.

How do floods occur?

At least once a year the plains around large rivers are flooded. This is due to the amount of water that rivers bring with them, because of heavy rainfall or melting snow in the mountainous regions. Thunderstorms can cause flash floods, in which small rivers can swell quickly and carry up to ten times the normal amount of water.

Rivers that flow slowly carry water, sand and silt. They build up their own beds, making them higher than the land around them. The Huang He, or Yellow River,  in China and the Mississippi in North America are examples for such rivers. Flooding here builds up slowly but causes more damagebecause more land is affected.

Coastal regions can also be affected by flooding. Afterearthquakes on the ocean floor tsunamis can bring up to 15-metre high waves and flood the coast many miles inland.  In 2004, a devastating tsunami in the Indian Ocean killed over 250,000 people in Indonesia, Sri Lanka, Thailand and other countries.

Tropical storms, cyclones and hurricanes also lead to flooding. Hurricane Katrina caused a massive flooding of the whole Mississippi Delta in 2004. Most of New Orleans had to be evacuated because of widespread flooding.

Low-lying countries are in permanent danger of being flooded. A large section of The Netherlands, for example, lie below sea level. In the past, ocean water from the North Sea flooded much of the country. Today a series of dikes and dams protect the land behind the coast. 

Floods are also caused by humans. Trees and plants normally help absorb too much water. When forests are cut or burned down, water from rainfall flows down barren land and produces mudslides. Too much water pressure on dams can lead to cracks in the concrete or even cause a dam to break completely.

Flood protection

Today flood protection has a high priority in countries that are in danger.  Dams are built along rivers toregulate the flow of water. They are often connected with hydroelectric power plants. In some areas rivers are dredged and their beds are laid deeper. In alpine regions reservoirs are built to hold back water and control the flow of small rivers.

London is protected from flooding by the Thames Barrier, a construction that moves up and stops water from getting in to London when it reaches a certain height.

In many areas, authorities provide quick and unbureaucratic help for people who have suffered from flooding. Special boats pick up people who are trapped on roofs or on the upper floors of buildings. Sheltersare set up for people who are left homeless. Rebuilding after massive floods often takes months and sometimes even years.