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EQ2: How do characteristic coastal landforms contribute to coastal…
EQ2: How do characteristic coastal landforms contribute to coastal landscapes?
What are the characteristics of waves?
Waves are generated by the friction created by wind blowing over sea which transfers energy from the wind into the water. Characteristics are determined by the strength of the waves, Its duration and Its fetch (distance a wave travels). The stronger the wind, the greater the friction on the surface of the sea and therefore the bigger the wave.
Wave size depends on
:
The strength of the wind
The duration the wind blows
Water depth
Wave fetch
Fetch
: the uninterrupted distance across water over which a wind blows, and therefore the distance waves have to grow in size.
Waves break as the water depth shallows towards a coastline:
At a water depth of approximately half the wavelength, the internal orbital motion water within the wave touches the sea bed
This creates friction between the wave and the sea bed, and this slows down the wave
As waves approach the shore, wavelength decreases and wave height increases, so waves ‘bunch’ together.
The wave crest begins to move forward much faster than the wave trough.
Eventually the wave crest outruns the trough and the wave topples forward (breaks)
Constructive waves
:
Have a low wave height (less than 1m) and long wavelength of up to 100m
Are flat wave with a strong swash but weak backwash
Have strong swash that pushes sediment up the beach, depositing it as a ridge of sediment (berm) at the top of the beach
Have a backwash that drains into the beach sediment
Destructive waves
:
Have a wave height of 1m and a wavelength of around 20m
Have strong backwash that erodes beach material and carries it offshore, creating an offshore ridge
Depending on conditions, beaches experience both constructive and destructive waves over a course of time and this can mean significant changes to the beach morphology (beach sediment profile) on different time scales:
Over a day as a storm passes and destructive waves change to constructive waves as the wind drops.
Between summer and winter
When there are changes to climate (e.g. if global warming resulted in the UK climate becoming on average stomier, then destructive waves and ‘winter’ beach profiles would become more common.
What are the erosion and weathering processes along a coastline?
Erosion
= the breakdwon of material along a coastline, caused by the sea
Transport
= the movement of this broken down material along a coastline
Weathering
= the breakdown of rocks by chemical, mechanical and biological agents.
Deposition
= the dropping of this eroded along a coastline.
Processes
:
Saltation
= Sediment bounces along, either because of the force of water or of wind, e.g. sand-sized particles.
Traction
= Sediment rolls along, pushed by waves and currents, e.g. pebbles, cobbles and boulders.
Solution
(erosion) = Carbonate rocks (limestone) are vulnerable to solution by rainwater, spray from the ea and seawater.
Abrasion
= Sediment picked up by breaking waves is thrown against the cliff face. The sediment acts on the cliff like a tool, chiselling away at the surface and gradually wearing it down.
Attrition
= As sediment is moved around by waves, the numerous collisions between particles slowly chip fragments off the sediment, making it smaller and more rounded over time.
Hydraulic action
(quarrying) = Air trapped in cracks and fissures is compressed by the force of waves crashing against the cliff face. Pressure forces cracks to open, meaning more air is trapped and greater force is experienced in the next cycle of compression. This process dislodges blocks of rock from the cliff face.
Biological weathering
= Plants and animals can also have an effect on rocks. Roots burrow down, weakening the structure of the rock until it breaks away.
Chemical weathering
= Rainwater and seawater can be a weak acid. If a coastline is made up of rocks such as limestone or chalk, over time they can become dissolved by the acid in the water.
Mechanical weathering
= The process of breaking big rocks into little ones. This process usually happens near the surface of the planet.
Suspension
= Sediment is carried in the water column, e.g. slit and clay particles.
Solution
= Dissolved material is carried in the water as a solution, e.g. chemical compounds in solution.
How are landforms created along a coastline?
Depositional Landforms
: Longshore drift is a key source of sediment for depositional landforms on coasts. Sediment transported down river systems to the coast or from offshore sources is also important. Sediment is deposited when the force transporting sediment drops. Deposition can occur in two main ways:
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 (pebbles → sand → silt)
Flocculation is a depositional process that is important for very small particles, such as clay, which are so small they will remain suspended in water. Clay particles clump together electrical or chemical attraction, and become large enough to sink.
Landforms
:
Spit
= Sand or shingle beach ridge extending beyond a turn in the coastline, usually greater than 30°. At the turn, the longshore drift current spreads out and loses energy, leading to deposition. The length of a spit is determined by the existence of secondary currents causing erosion, either the flow of a river or wave action which limits its length.
Bayhead Beach
= Waves break at 90° to the shoreline and move sediment into a bay, where a beach forms. Through wave refraction, erosion is concentrated at headlands and the bay is an area of deposition.
Tombolo
= A sand or shingle bar that lines the coastline to an offshore island. Tombolos form as a result of wave refraction around an offshore island which creates an area of calm water and deposition between the islands and the coast.
Barrier Beach/bar
= A sand or shingle beach connecting two areas of land, with a shallow-water lagoon behind. These features form when a spit grows so land that it extends across a bat closing it off.
Hooked/recurved spit
= A spit whose end is curved landward, into a bay or inlet. The seaward end of the spit naturally curves landward into shallower water and the ‘hook’ may be made more pronounced by waves from a secondary direction to the prevailing wind.
Cupate foreland
= Roughly triangular-shaped features extending out from a shoreline. There is some debate about formation, but one hypothesis suggests they result from the growth of two spits from opposing longshore drift directions.
Erosional Landforms
:
Erosion produces a suite of distinctive coastal landforms. The most well known is the cave-arch-stack-stump sequence. The most fundamental process of landforms formation on a coastline is the creation of wave-cut notch. This is eroded at the base of a cliff by hydraulic action and abrasion.
As the notch becomes deeper, the overhanging rock above becomes unstable and eventually collapses as a rock falls.
Repeated cycles of notch-cutting and collapse cause cliffs to recede inland.
The former cliff position is shown by a horizontal rock platform visible at low tide, called a wave-cut platform.
Some landforms are influenced by structural geology. The location of fault lines and major fissures influences the location of caves and therefore arches.
How does a sediment cell work?
Sediment cells have inputs, transfers and outputs of sediment. Under natural conditions, the systems operate in a state of dynamic equilibrium, with sediment inputs balancing outputs to sinks. For short periods of time (e.g. during a major storm that erodes a spit) the system's equilibrium might be disrupted but it will tend to return to balance over time. Negative feedback mechanisms help maintain the balance by pushing the system back towards balance:
During a major erosion event a large amount of cliff collapse may occur, but the rock debris at the base of the cliff will slow down erosion by protecting the cliff base from wave attack
Major erosion of sand dunes could lead to excessive deposition offshore, creating an offshore bar that reduces wave energy allowing the dunes time to recover.
Source
: places where sediment is generated, such as cliffs or eroding sand dunes. Some sources are offshore bars and river systems and these are an important source of sediment.
Transfer zones
: places where sediment is moving along shore through longshore drift and offshore currents. Beaches and parts of dunes and salt marshes perform this function.
Sinks
: Locations where the dominant process and depositional landforms are created including spits and offshore bars.
What are the main types of mass movement?
Mass movement
= the downslope movement of rock and soil. It covers specific movements including landslides, rockfall and rotational slide. This is the dominant of cliff collapse.
Fall
: Rockfalls are a rapid form of mass movement. On coasts, blocks of rocks can be dislodged by mechanical weathering, or by hydraulic action erosion. Undercutting of cliffs by the creation of wave-cut notches can lead to large falls and talus scree slopes at their base.
Topple
: Geological structure influences topples.
Where rock strata have a very steep seaward dip, undercutting by erosion will quickly lead to instability and blocks of material toppling seaward.
Rotational Slide/Slumping
: Mass movement can occur along a curved failure surface. In the case of a rotational slip, huge masses of material can slowly rotate downslope over periods lasting from days to years. Water plays an important role in rotational slips - create a back-scar and terraced cliff profile.
Flow
: Flows are common in weak rocks, e.g. clay or unconsolidated sands. These materials can become saturated, lose their cohesion and flow downslope. The heavy rainfall combined with high waves and tides can contribute to saturation.
