The wonderful world of… Levees Everything you need to know about them
What are they? Often several metres higher than the river’s floodplain, they are high natural embankments of silt situated close to the river 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 slow flow and carry large amounts of load.
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’.
Artificial levees 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.
Existing artificial levees Examples of existing artificial levees include along the Mississippi river and Sacramento rivers in the USA, as well as the Danube and Loire in Europe. The artificial levee along the banks of the Danube, Germany.
Mississippi, USA • It is the one of the largest levee systems in the world. • The levees are 3,500 miles long, extending 1,000 miles along the Mississippi river. • They measure 24 feet in height with some as much as 50 feet.
The breaking of levees What are the characteristics of a broken levee? When the water has ‘overtopped’, or risen above the crest of the levee. What are the causes? • High rainfall causing higher peak flows. • High winds which cause the ocean or river to swell, causing it to crash over the levee. A broken levee in New Orleans, USA
A broken natural levee in New Orleans, USA A broken artificial levee on the Mississippi river, USA Both natural and artificial levees can be broken by natural force.
The wonderful world of levees Created by: Mark Bailey Information obtained from: Geography resources webpage and ‘AS Level Geography’ textbook. Pictures and illustrations: Geography resources webpage, wikipedia