Everything you need to Everything you need to know about themknow about them

LeveesLevees

The wonderful world of…

What are they?

Often several metres higher Often several metres higher than the river’s floodplain, they than the river’s floodplain, they are high natural embankments are high natural embankments of silt situated close to the river of silt situated close to the river

channel.channel.

Where are they found?

They are most commonly found in the lower course of a river where there is a floodplain, along rivers that flood periodically, have a along rivers that flood periodically, have a slow flow and carry large amounts of load. slow flow and carry large amounts of load.

How are they formed?How are they formed? The river before the flood at regular height.

The river whilst flooding

There is an increase in friction between the water leaving the channel and the floodplain when a river overflows. The water is shallower on the river banks and valley floor, thus this is why deposition of load occurs. After repeated flooding causing the deposition of silt onto the banks, this eventually settles which over time builds up to form a levee. The coarser material is deposited first, and is positioned closer to the river.

After repeated flooding, the river bed and levees are raised due to the constant process of the deposition of silt. Over time, this eventually causes the river to rise above level of flood plain. This is called an ‘aggraded bed’.

What are the purposes of artificial levees?

The main purpose is to prevent flooding of the river into the adjoining land.

What are the characteristics of artificial levees?

•They are built on a cleared, level surface using piled earth.

•They are broad at the base, triangular shaped with a level top on which sandbags are placed.

• Their surface must be protected from erosion to ensure the levees are maintained, thus vegetation such as ‘Bermuda grass’ is planted in order to bind the piled earth together.

Artificial levees

Existing artificial leveesExisting artificial levees

Examples of existingExamples of existing

artificial levees includeartificial levees include

along the Mississippialong the Mississippi

river and Sacramentoriver and Sacramento

rivers in the USA, asrivers in the USA, as

well as the Danube andwell as the Danube and

Loire in Europe.Loire in Europe.

The artificial levee along the banks of the Danube, Germany.

Mississippi, USAMississippi, USA It is the one of the It is the one of the

largest levee systems largest levee systems in the world. in the world.

The levees are 3,500 The levees are 3,500 miles long, extending miles long, extending 1,000 miles along the 1,000 miles along the MississippiMississippi river. river.

They measure 24 feet They measure 24 feet in height with some as in height with some as much as 50 feet.much as 50 feet.

What are the causes?What are the causes?

The breaking of leveesThe breaking of levees

•High rainfall causing higher peak flows.

•High winds which cause the ocean or river to swell, causing

it to crash over the levee.

What are the characteristics of a broken levee?

When the water has ‘overtopped’, or risen above the crest of the levee.

A broken levee in New Orleans, USA

A broken artificial levee on the Mississippi river, USA

A broken natural levee in New Orleans, USA

Both natural and artificial levees can be broken by natural force.

Created by:

Mark Bailey

Pictures and illustrations:

Geography resources webpage, wikipedia

The wonderful world of levees

Information obtained from: Geography resources webpage and ‘AS Level Geography’ textbook.