Depositional coastal landforms Flashcards
Why does deposition occur?
Waves no longer have sufficient energy to continue to transport materials
- resistance eg. by groynes
- dissipation through refraction
- friction
How does deposition occur?
- gravity settling - occurs when the energy of transporting water is too low to move sediment, large sediment is deposited first
- flocculation - small particles such as clay are suspended in the water
What are offshore bars and how do they form?
Offshore bars are ridges of sand or shingle running parallel to the coast in an offshore zone
- they form from sediment eroded by destructive waves and carried seawards by backwash
- the sediment is deposited at the boundary of the offshore and nearshore zone, where the orbit of water particles ceases to reach the seabed which halts the transport offshore
- also called breakpoint bars because the offshore boundary is where waves first begin to break
- sometimes exposed during neap tides
What is longshore (littoral) drift?
- waves hit the beach at an angle determined by the direction of prevailing wind
- the waves push the sediment in this direction and up the beach in the swash
- the wave then carries sediment back down the beach in the backwash at a 90 degree angle due to gravity
- this moves sediment along the beach in a zigzag motion
What are barrier bars/beaches and how do they form?
Linear ridges of sand extending across a bay, connecting the land on both sides. This traps a body of water behind it, forming a lagoon
- form on drift-aligned coastlines, when LSD extends a spit across the width of the bay
- can also form due to rising sea levels, when constructive waves drive a ridge of sediment onshore to coastlines with a gently sloping shallow sea bed
eg. 9km barrier beach across Start Bay in Devon, Slapton Ley lagoon lies behind it
What are tombolos and how do they form?
Linear ridges of sand and shingle connecting an offshore island to the coastline of a mainland
- form on drift-aligned coastlines, when LSD builds up a spit out from land until it contacts with the island
- can also form on swash aligned coasts, when there is wave refraction around both sides of the island. This causes a collision of wave fronts on the landward side which slows velocity, creating an area of deposition, so sediment builds up between the island and the coastline
eg. St Ninian’s tombolo on the Shetland Islands
What are spits and how do they form?
Linear ridges of sand stretching into the sea beyond a turn in the coastline, but connected to the land on one end
- at the turn, LSD continues in the original direction, but its energy is dispersed, lost as the wave refracts and the current spreads, leading to deposition on the sea bed
- over time, sufficient sediment is deposited to break the surface, extending the beach into the sea as a spit
- the process continues until equilibrium is reached at the seaward end of the spit, between deposition and erosion by waves
eg. Spurn Point, Holderness coastline
What are recurved spits and how do they form?
A spit whose end is curved landwards
- may form due to wave refraction which transports and deposits sediment for a short distance landwards
- alternatively, it can form due to a change in prevailing wind direction, which generates short periods of LSD in the landward direction
- strong incoming tidal currents can also form a recurved spit
eg. Minion-Matsubara sand spit in Japan, west coast of Suruga Bay
Why are the gradients of sandy and shingle beaches different?
- in shingle beaches, water rapidly percolates through the rocks, causing the wave to lose energy so the backwash is weaker than the swash that is moving material up the beach. This limits the backwashes ability to transport materials back down the beach and therefore the gradient steepens. Sand has less percolation, so backwash is able to move material back down the beach
- the uneven surface on a shingle beach means that more energy is lost through friction, and therefore there is less energy for waves to erode and transport material back down the beach
What are cusps and how do they form?
Crescent shaped indentations that form on beaches of mixed sand and shingle
- formed when there is a junction between sand and shingle. Once the curving shape is created, swash is concentrated in the small bay that forms in the centre of the cusp. This creates a stronger backwash that removes material down the beach
What are storm berms/ridges and how do they form?
Often formed by spring tides and consist of the largest material thrown up by the strong swash of the larger waves. Tides often cannot reach these features so they remain relatively untouched
- there are often a series of smaller ridges formed beneath the storm ridge known as berms. These mark the successive high tides that follow the spring tide through to the neap tides
What are ridges and runnels and how do they form?
A wide area of beach producing ridges and intervening depressions called runnels
- form where the waves energy spread across a wide beach. They are particularly common on shallow and sandy beaches and form as a simple drainage route for tides. Water flows in and out via the runnel
Why are depositional features unstable?
- consist of unconsolidated sediment which makes them vulnerable to change
- during major storms, large amounts of sediment can be eroded or transported elsewhere
- then rely on a continuous supply of sediment to balance erosion, meaning that they change as dynamic equilibrium shifts
What are the 2 types of beaches?
- Swash-aligned - waves approach perpendicular to the coastline so there is limited LSD and limited movement of sediment
- Drift-aligned - waves approach at an angle so sediment travels further along the beach, which may form a spit
How do salt marshes form?
- A salt marsh begins as a mudflat. To begin with, mud is deposited close to high-tide line, and sinks to the seabed due to flocculation, when particles of sediment stick together and sink due to their combined mass.
- Pioneer plants such as eelgrass begin to colonise which are salt-tolerant and help to trap further deposits of mud.
- Gradually, mud rises above high tide and lower salt marsh develops with a wider range of plants.
- Soil conditions improve and vegetation succession continues to form a meadow. Eventually shrubs and trees colonise the area and succession reaches its climax.
