Atmosphere And Weather

A Scientific Explanation of how Floods Occur



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Heavy rainfall is one of the most common causes of flooding.  There are, however, several other events that can cause floods.

When rain falls, it is either absorbed into the ground (infiltration) or flows along the surface of the ground (runoff).  If the soil is permeable and not waterlogged, then the rain will seep downwards until it reaches the underground water table.  The water table is the area of the soil that is saturated with water most of the time.  If there is too much rainfall, however, the water table can rise until it reaches the surface of the ground.  When this happens, any further precipitation will stay on the surface of the ground.  This can cause flooding if there is enough rain.  The excess water, for example, can accumulate in local waterways, causing them to overflow their banks.

Runoff occurs when rain does not seep into the soil.  As mentioned above, surface runoff can be the result of the ground already being saturated with water.  It can also happen when the ground is not permeable enough for water to get through.  This can happen when soil is dense or tightly packed.  It also happens in paved areas, such as roads and parking lots.  The surface runoff water flows until it reaches a waterway or area with permeable soil.

Floods most commonly occur when bodies of water overflow.  The areas near to rivers, lakes, creeks and other bodies of water are the regions most commonly affected by flooding.  Much of North America and other parts of the world are covered by watersheds.  A watershed is the area that drains into rivers, oceans and lakes.  Smaller streams and creeks flow into rivers.  Rivers sometimes flow into larger even rivers.  The waterways are also often fed by underground water.  Ultimately, most rivers flow into the ocean.  These drainage systems are called watersheds or river basins.  Generally flooding takes place in watersheds when they get more water than they can drain.  Then waterways flood surrounding areas.

Heavy rainfall can overwhelm watersheds and cause flooding.  There are different kinds of rain storms that commonly cause flooding.  One slow moving thunderstorm can dump a large amount of water over a relatively small area.  Another storm pattern that can cause flooding is known as ‘training’.  This is when a series of thunder storms come one after the other, like boxcars on a train.  Collectively, these storms can dump a large amount of water, causing flooding.  Another storm pattern that can cause flooding is back building.  This is when a storm’s back edge keeps developing while the front of the storm moves forward.  The result is a long period of non-stop rain.  In short, any storm pattern that brings a sufficient amount of precipitation over a short period of time can cause flooding.

Other storm systems that can cause flooding are tropical cyclones and hurricanes.  Hurricane winds can cause floods in coastal areas.  Hurricane winds can whip up storm surges.  This is when heavy winds blow large amounts of ocean water onshore.  Coastal flooding can also be caused by underground seismic activity.  Offshore earthquakes can cause huge waves, known as tsunamis, to hit the shore.

In arid desert regions, flooding can also be a problem.  Dry desert soil often becomes hard, which prevents water from being absorbed.  So when rain falls in arid regions, it often forms surface runoff.  This water forms temporary streams or rivers in channels known by various names around the world.  This surface runoff can cause flash floods.  Because desert soil often does not absorb water, very small amounts of rain can cause flash floods.  These kinds of floods are common in the arid regions of the Southwestern United States.  Flash floods are most deadly in canyons.  Rain water flows quickly over the hard canyon walls.  People who are down in the canyon can be trapped.

Floods can also happen when man-made dams break or fail.  These can result in the release of a large amount of water over a short period of time, flooding areas downstream.  One well-known dam failure occurred in the Italian alps in 1963.  A landslide pushed water over and around the Vaiont Dam.  Although the dam itself did not break, it failed to stop the flood.  About two thousand people died.

Other potential causes of flooding include ice jams and erosion.  In colder climates ice can accumulate in watersheds during the colder months.  This ice then thaws in the spring and summer.  Chunks of thawing ice are washed downstream.  These pieces of ice can become clogged in narrow parts of a river system, obstructing water flow and causing flooding.  Ice jams can also be caused by ice accumulating at man-made obstructions, such as bridges and dams.  Erosion can also cause flooding.  Soil can be washed into river systems.  This soil may then be deposited in slower moving sections of the river system.  As soil is deposited on the river bed,  the river becomes more shallow and therefore more prone to overflowing its banks.

Flooding is often seen in a negative light.  It is true that floods have killed millions of people and have destroyed a great deal of property and farmland.  Nevertheless, flooding is also helpful to people.  One of the reasons why floods are often so lethal and costly is that regions prone to flooding often support large populations.  The reason so many people live in areas prone to flooding is that flooding helps to enrich soil.  Flood waters often deposit rich soil in the regions surrounding large rivers such as the Nile, the Mississippi, the Tigris and Euphrates, the Indus, the Ganges, the Yangtze, the Mekong and other great rivers around the world.  This is why the flood plains of these rivers often support dense populations.

Flooding is generally the result of a watershed having to take in a larger amount of water than it can drain.  This excess water is usually the result of heavy rainfall somewhere in the watershed.  In coastal areas, flooding can be caused by tropical cyclones and hurricanes or by offshore underwater seismic activity.  Storm winds and underwater earthquakes can cause water from the ocean to flood coastal areas.


Source:

Jean Allen.  Floods.  Mankato, Minnesota: Capstone Press, 2002.

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