As a river continues its journey towards the sea, the valley cross section continues to become wider and flatter with an extensive floodplain either side of the channel. The river erodes laterally and deposition also becomes important. By the time it reaches the lower course the river is wider and deeper and may contain a large amount of suspended sediment .
Description of a floodplain:
The floodplain is the wide, flat area of land either side of the river in its lower course.
Floodplains contain sediments, often accumulations of sand, gravel, loam, silt, and/or clay. Features of a floodplain include: meanders, ox-bow lakes & levees. They can periodically be completely covered with water.
How does a floodplain form?
The floodplain is formed by both erosion and deposition.
Lateral erosion is caused by meanders and the slow migration downstream to widen the floodplain.
The river carries large quantities of suspended load.
The deposition on the slip off slopes provides sediment to build up the valley floor.
As the water floods onto the floodplain it loses energy, there is greater friction, the water is shallow and the rivers velocity falls so deposition of its suspended load occurs. Regular flooding results in the building up of layers of nutrient rich alluvium which forms a flat and fertile floodplain
Over thousands of years the deposits build up to form great thicknesses of alluvium which is why floodplains have fertile soil for farming.
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 do levees form?
When the river water bursts its bank, the shallower depth of water flowing over the surface means there is more friction and so the velocity (speed) is reduced.
This results in the loss of energy and therefore deposition occurs.
The heaviest materials are deposited first as these require the most energy to be transported and therefore build up around the sides of the river forming raised banks known as Levées .
Finer material such as silt and fine clays continuing to flow further over the floodplain before they are deposited.
Formation of levees http://www.eduonlinelive.co.uk/rivermidlower.html
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.
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’.
Floodplain - the area of land around a river channel which is formed during times of flood when the amount of water in a river exceeds its channel capacity and deposition of silt occurs. Levées - a raised river bank (can be natural features formed by deposition or artificial structures built to increase channel capacity and reduce flood risk)