depositional landforms Flashcards
what is a beach?
the area of deposited material between the low water line to the limit of storm waves.
how are sandy beaches formed?
- usually formed in a sheltered bay
- low wave energy (most energy focused on surrounding headlands), constructive waves transport material to the shore
- gentle profile (tend to be quite flat)
how are pebbly beaches formed?
- more exposed beaches tend to be steeper and more pebbly.
- higher energy waves remove the sand, leaving the larger shingle behind, due to destructive waves having a large backwash.
what increases the amount of deposition?
- if erosion on the nearby coast is high, then lots of rock and sand will fall into the ocean. this will increase deposition on nearby beaches.
- the more sediment in the ocean, the more deposition there’ll be.
what are sand dunes?
small ridges or hills of sand found at the top of a beach (away from the reach of the waves).
what conditions do sand dunes need to form?
- large supply of sand
- large, flat beach
- an onshore wind
- an obstacle for the dune to form against e.g. driftwood
how do sand dunes form?
- wind transports sand across a beach through saltation.
- deposition occurs around obstacles such as rocks or seaweed.
- over time, the sand builds up, creating small embryo dunes.
- these dunes are stabilised by vegetation such as marram grass. the long roots bind the sand together.
- the dunes develop even more, and over time, the rotting vegetation adds organic matter to the sand, and so more vegetation is able to grow.
what is a spit?
a long, narrow finger of sand or shingle jutting out into the sea from the land.
how do spits form?
- prevailing wind blows at an angle to the coastline.
- longshore drift occurs, due to the swash and backwash of the waves, and material is transported.
- when the coastline changes direction (e.g. turns a corner), deposition continues out into the sea, forming a spit.
- the spit continues to grow overtime, until a cross wind creates a recurved end.
- the area behind the spit is now sheltered, so deposition occurs, and a salt marsh may form.
- spits are often formed at the mouth of estuaries (when the river meets the ocean). the spit is not allowed to reach across to the other headland, as the river still has enough power to move some of the sediment.
why is the sediment at the end of the spit lighter and smaller than at the beginning of the spit?
- longshore drift can carry light material much further.
- the sediment has been transported further, so it may have been eroded more through attrition.
what are bars?
when a spit has grown across a bay, trapping a lagoon behind it.
how are bars formed?
- a spit grows across a bay
- it joins two headlands and becomes a bar
what prevents bars from forming?
if there’s a river with a strong current that travels through the bay and into the water behind the spit, it can prevent a bar from forming.
