Levees

Question 1

What are levees and how are they formed

Answer 1

Levees are raised embankments formed by deposition of larger alluvial particles during floods but can be artificially raised to prevent floods

Question 2

Where are levees found

Answer 2

Found on floodplains on the lower course of the river

Question 3

5 headers for how levees are formed

Answer 3

Increased river discharge
Bank full capacity exceeded
Deposition of sediment
Vegetation and stabilisation
Cyclic process

Question 4

1 Increased river discharge

Answer 4

In times of high rainfall or snowmelt, rivers sometimes exceed their bankfull capacity forcing water out onto floodplains

Question 5

2 bankfull capacity exceeded

Answer 5

Water is forced out onto the floodplain. As the water slows down due to increased friction by vegetation, the rivers capacity decreases and the rivers capacity to transport sediment is reduced so sediment is deposited

Question 6

3 deposition of sediment (graph)

Answer 6

The hjulstrom curve tells us that as energy decreases, the largest particles of sand and gravel that are carried by the river are dropped first, followed by the smaller silt and fine clays furthest away from the channel

Question 7

4 vegetation and stabilisation (structure and what grows and where is there further deposition

Answer 7

As a result of deposition a natural bank of the largest coarse deposits are found alongside the channel. These raised banks are called levees and overtime vegetation grows on levees, which further stabilises them. After repeated flood events the levees increase and there is further deposition in the bed and the river flows at a higher level than the floodplain

Question 8

5 cyclic process (repeated, difficulty for tributary’s)

Answer 8

Each flood event can deposit additional layers of sediment, gradually building up the levee banks. Natural levees can cause difficulty for tributary streams trying to break through to join the main channel as they may flow alongside the channel for miles before finding a break in the levee (deferred tributary called a yazoo)

Question 9

Example of levees

Answer 9

Mississippi River levees artificially increased to increase channel capacity and reduce flood risk

Question 10

What is a Yazoo

Answer 10

A deferred tributary runs alongside a river downstream until they find a break in the levee