3.5 Coastal Transport And Deposition Flashcards
What is longshore drift?
Most waves approach at an angle- generally from the same direction as prevailing winds. As waves advance, material is carried up the beach at an angle. The backwash then pulls material down the beach at right angles to the shore. The net effect of the movement is long shore drift
What is an example of a coastline being eroded due to long shore drift? Why does this happen?
Where the removal of sediment is greater than the supply of new sediment, the beach is eroded.
Strong prevailing south-westerly winds and large waves create sustained long shore drift in West Africa. Cotonou’s coastline in Benin has retreated 400 metres in just 40 years
How are spits formed?
- Longshore drift carries material along a beach
- A sudden change in coastal shape, means sediment will begin to deposit
- The presence of a river or extension to water currents will prevent the spit extending
- Recurved or double spits can be formed
How does a recurved spit form?
The end of the spit will begin to curve, as wave refraction carries material round into the more sheltered water behind the spit
What is an example of a spit?
Poole Harbour has a double spit
Holderness coastline at Spurn Head
How is a drift aligned beach formed?
- Sediment is transferred along the coast by Longshore drift
- These beaches tend to be longer and have graded material
What is an example of a drift aligned beach?
Mappleton, Yorkshire
What is a swash-aligned beach?
- Sediment moves up and down the beach with little lateral transfer
- These occur in tight bays and so are not affected by the effects of LSD
What is an example of a swash aligned beach?
Barafundle Bay, Pembroke
How are offshore bars formed?
- Destructive waves erode sand
- The stronger backwash pulls it offshore and then deposits it as a bar
- These cause waves to break further out to sea
What is an example of an offshore bar?
Scroby Sands, Norfolk
What is a cuspate foreland?
A triangular shaped headland that extends out from the main coastline
How does a cuspate foreland form?
- Longshore drift occurs in opposite directions
- Sediment is deposited where they meet
- Vegetation then grows and stabilises these landforms
What is an example of a cuspate foreland?
Dungeness in Kent
What is a Tombolo and how is it formed?
A beach that has formed between a small island and the mainland. Deposition occurs when waves lose energy and the tombolo begins to build up.
They may be covered at high tide.
What is an example of a tombolo?
St Ninian’s in the Shetland Islands
What is a barrier beach?
What is a barrier island?
Where a beach or spit extends across a bay to join two headlands
A barrier island occurs when the barrier beach and the mainland become separated
How does a barrier beach form?
- Longshore drift extends a spit between two headlands
- These barrier beaches trap water behind as a lagoon
What’s an example of a barrier beach?
Slapton Sands, Devon
What is traction?
Sediment rolls along, pushed by waves and currents
What is saltation?
Sediment bounces along, either due to the force of water or the wind
What is suspension?
Sediment is carried in the water column
What is solution?
Dissolved material is carried in the water as a solution
What is gravity settling?
Occurs when the energy of transporting water becomes too low to move sediment; large sediment will be deposited first, followed by smaller sediment
What is flocculation?
A depositional process, which is important for very small particles. Small particles clump together due to electrical or chemical attraction and become large enough to sink
What is plant succession and how does this begin?
This is when bare ground is gradually colonised by plants. The first colonising plants are called pioneer species. These species begin the process of plant succession, during which other species invade and take over until a balance is reached
Why is plant succession important?
It influences the shape of the coastline and can cause coastal accretion, protecting coasts from coastal erosion and coastal flooding
How does vegetation stabilise sediment?
- the roots of plants bind sediment particles together making them harder to erode
-when submerged, plants growing in sediment provide a protective layer so the surface of the sediment is not directly exposed to moving water and therefore erosion
-plants protect sediment from wind erosion by reducing wind speed at the surface due to friction with the vegetation
What are the different stages of plant succession called and what is the end of plant succession called? What is this?
Each step is called a seral stage
The end of plant succession is called a climatic climax community. This is when the final community is adjusted to the climatic conditions and become stable
What are the two different types of plants that often grow in coastal environments?
Halophytes
Xerophytes
What are halophytes?
They can tolerate salt water, either around their roots, being submerged in salt water or salt spray from the sea
What are xerophytes?
They can tolerate very dry conditions, such as those found on coastal sand dunes where the Sandy soil retains very little water due to drainage
What is a sand dune ecosystem called?
Psammosere
What is a salt marsh ecosystem called?
Halosere
How do embryo dune pioneer species help the environment?
-Stabilise the mobile sand with their root systems
-reduce wind speeds at the sand surface, allowing more sand to be deposited
-add dead organic matter to the sand, beginning the process of soil formation
What is one of the key plants in the process of plant succession and how is it adapted?
Marram grass:
Long- roots of up to 3m so can get water from well below the surface
It has flexible, waxy leaves that can cope with being sand - blasted in gale force winds
They are designed to limit water loss via transpiration
Can withstand heat of up to 60 degrees and grow up to 1 m per year to keep up with the deposition of wind-blown sand
What do creeping plants and invaders do to modify the environment?
They help the sand or mud in dunes and salt marshes to retain moisture. These changes allow other species to colonise.
The invaders modify the environment by providing shade as well as improving the soil
What happens once an embryo dune has been formed?
Embryo dunes alter the environment conditions from harsh, salty, mobile sand to an environment that other plants can tolerate.
New plant species therefore colonise the embryo dunes creating a fore dune. Further environmental modification continues as a soil develops, nutrients and water become more available and plants that need better conditions can colonise the area.
What did grey dune areas used to be? Why has this changed?
Embryo dunes
Due to plant colonisation and succession, the dunes have grown upwards and out of the sea- causing seaward and vertical coastal accretion
What makes sand dunes very dynamic environments?
-periods of wind erosion can create low areas within the dune system called dune slacks; erosion stops when damp sand at the water table is exposed as this cannot be easily eroded by wind
-embryo and fore dunes are prone to wind and wave erosion, especially during major storms, but as long as the supply of sediment to the coast resumes, new embryo dunes will form and the dune front will stabilise
What are the different types of dunes in plant succession?
Mobile dunes:
-Embryo dunes
-Fore dunes
-Yellow dunes
Fixed dune:
-grey dune
-heath and woodland
What are the conditions needed for a sand dune formation?
There must be a prevailing onshore wind, a plentiful supply of sand and a shallow offshore area with low gradients, where lots of sand can dry out at low tide. There should also be an extensive back shore area where sand can accumulate
Where are the youngest dunes and where are the oldest dunes?
The youngest are those closest to the shore and they the oldest dunes are furthest back
What are dune slacks like?
Here, a higher diversity of species are able to survive due to shelter and smaller distance to the freshwater table. This leads to an increase in competition and weaker competitors such as the marram grass are outcompeted
How do sand dunes form?
Deposition is blown by the wind and is trapped in the sand dune. This is then stabilised by plant succession
What are grey dunes like?
Here, there is a higher diversity of late successional species. Vegetation gains height due to shelter so species become shrubby and eventually will reach the climax community of a woodland
What factors increase along the psammosere succession?
- vegetation cover and species diversity
- soil depth and humus content
-soil acidity
What are salt marshes?
Areas of flat, silty sediments that accumulate around estuaries or lagoons
Where do salt marshes develop?
-in sheltered areas where deposition occurs
-where salt and fresh water meet
-where there are no strong tides or currents to prevent sediment deposition or accumulation
What is an example of a salt marsh?
Eden Estuary of Fife, Scotland
What is the process of salt marsh formation? How does plant succession work within this?
As mud flats develop, salt tolerant plants (such as eelgrass) begin to colonise and stabilise them
Halophytes (salt tolerant species), help to slow down tidal flow and trap more mud and silt
As sediment accumulates, the surface becomes drier and different plants begin to colonise
Creeks (created by water flowing across the estuary at low tide) divide up the salt marsh
How do sediment cells act?
As a system, with sources, transfers and sinks. Larger sediment is not transferred between cells, but finer sediment in suspension out at sea can be transferred.
What is the amount of sediment available within a sediment cell called?
Sediment budget
How does a sediment cell’s equilibrium system work?
Depositional features build up in equilibrium with the amount of sediment available.
If the sediment budget falls, waves continue to transport sediment (and erosion may therefore increase in some areas, because the sea has surplus energy). One change has led to another, known as positive feedback
However, if the sediment budget increases, more deposition is likely. This is known as negative feedback (the sea returns to a situation where it can handle the sediment supply.
This system is known as a dynamic equilibrium
