11.7 Levées and floodplains

Question 1

What is a floodplain

Answer 1

An area of land which is covered in water when a river bursts its banks

Question 2

How are floodplains made

Answer 2

• formed due to both erosion and deposition
• erosion removes any interlocking spurs creating a flat, wide area on either side of the river
• during a flood, material being carried by the river is deposited (as river loses energy)
• over time, the height of the floodplain increases as material is deposited on either side of the river
• to is a wide, flat area because of meanders shifting along the valley

Question 3

What are floodplains made up of

Answer 3

Agricultural land because the area is very fertile because it’s made up of alluvium (deposited silt from a river flood)

Question 4

How are levees formed

Answer 4

• Levees occur in the lower course of a river when there is an increase in the volume of water flowing downstream and flooding occurs.
• Sediment that has been eroded further upstream is transported downstream.
• When the river floods, the sediment spreads out across the floodplain.
• When a flood occurs, the river loses energy. The largest material is deposited first on the sides of the river banks and smaller material further away.
• After many floods, the sediment builds up to increase the height of the river banks, meaning that the channel can carry more water (a greater discharge) and flooding is less likely to occur in the future.

Question 5

What are the characteristics of levees

Answer 5

• raised river banks (2-8 metres high in the UK)
• composed of gravel, stones and alluvium
• grading of sediments with the coarsest closest to the river channel
• steep sided, but steeper on the channel side than on the land side
• fairly flat top, naturally covered in grass and is often a footpath eg Severn Way