AT U R EÊSF U N R Y What Is a Flood What Is a Flood Aflood happens when water from a river or the sea covers dry land But a flood is far more powerful than this description makes it sound Heavy rains can turn gently flowing rivers into terrifying torrents that sweep across the landscape submerging it under feet of muddy water Powerful storms and giant waves can send seawater cascading over low-lying coasts drowning towns harbors and people Flooding in Meissen Germany in 2002 Heavy summer rains caused the river Elbe to overflow its banks and spread onto the surrounding land Killer floods Floods kill more people than earthquakes famines volcanic eruptions and other natural disasters Over the centuries tens of millions of people have died in floods A single huge flood can bring catastrophe killing millions of people Floods are more frequent than other natural disasters and they cause as much damage to property as all other natural disasters put together Fast flowing floodwater washes away buildings cars trees and roads Even stagnant floodwater damages buildings electricity cables and other infrastructure Floods also alter the landscape by eroding soil carrying it away and dumping it elsewhere Flood types There are three main types of flood river floods flash floods and coastal floods Most floods happen on rivers when excess water overflows from a river onto flat land called the floodplain The most devastating floods happen along the world’s great 4
AT U R EÊSF U N R Y rivers such as the Mississippi in the United States the Ganges in India and the Huang Ho in China Flash floods happen suddenly when intense rain falls over a small area Coastal floods happen when storms and tsunamis raise the sea level above the level of the coast Despite being so destructive floods can also bring benefits The soil on river floodplains is extremely fertile making them good places to grow crops This is why millions of people around the world choose to live on floodplains despite the risk from flooding Car roofs poke above floodwater in New Orleans The flooding was caused by Hurricane Katrina in 2005 CASE STUDY The biblical flood In the Bible God sends a great flood that covers the whole earth Only a man called Noah and his family survive There are many other similar legends of great floods They may be based on a real flood that happened about 5,000 years ago in modern-day Iraq where archaeologists found a thick layer of sediment deposited by massive flooding W h a t I s a l F o o d 5
AT U R EÊSF U N R Y Flowing Rivers Flowing Rivers Before looking at why floods happen on rivers it is useful to understand how rivers flow The water on earth is constantly circulating between the oceans the atmosphere and the land This circulation of water is called the water cycle Water evaporates into the atmosphere from the oceans and land in the form of water vapor When the vapor rises and cools it forms water droplets that fall as rain and other precipitation Some of the rain forms rivers and flows back across the land to the sea Soaking in and running off Rain either soaks into the ground or flows across the ground when it is called runoff The amount of water that soaks in or runs off depends on the type of ground how wet the ground is and how heavy the rain is Water soaks into some soils faster than others For example it soaks quickly into sandy soil but very slowly into clay If rain falls so heavily that not all of it can soak in the surplus rain sits on top of the soil or runs off it downhill When the ground becomes saturated soaked with water all the rain runs off Forming a floodplain Runoff finds the easiest way downhill It collects into streams and rivers On hills and mountains rivers erode V-shaped valleys into the landscape The eroded soil and rock is washed downstream When the The flow of water down a river center is part of the water cycle Most of this water starts out in the sea and then returns to the sea through the mouth of a river 6
AT U R EÊSF U N R Y river reaches lowland areas the river naturally meanders bends from side to side Over time the meandering river slowly cuts a wide valley During floods the whole valley fills with water Some of the soil and rock transported by the river is deposited It builds up layers to make the bottom of the valley flat The flat area on each side of the river is called the floodplain i l F o w n g R i v e r s GROUNDWATER Water that soaks into the ground is known as groundwater It seeps slowly down through the ground saturating the rock below The top of the saturated layer is known as the water table In places groundwater comes out of the ground and forms springs Many streams and rivers start at springs It is groundwater from springs that keeps rivers flowing even after long spells of dry weather Meanders on the river Bynoe in Australia The flat area around the river known as the floodplain is formed from soil and rock left after floods 7
