Coastal Landscapes and change
EQ2- Coastal Erosion and Deposition
1 Shaping Coatlines
Waves
Construstive and destructive waves are the two main types of wave. The characteristics of these waces are described below.
Destructive waves
-Responsible for erosional processes
happen at high frew (10-15 waves per minute)
-steep and high, with a circular motion so waves break at a greater height
\This causes the eave to 'plunge' and travel a shorter distance along the beach
Destructive waves remove material from coasts because the swash is less powerful than the backwash
Constructive waves
-Happen at a low freq (6-9 waves per minute)
-long and low
-over time, constructive waves will form gently sloping beaches
-constructive waves deposit material on coats because the backwash is less powerful than the swash
they move in an elliptical motion, with waves breaking with little height
\This results in them 'spilling' and breaking out further along the beach
One of the largest recorded waves occured off the coast of Portugal at Nazare in 2011. This wave, which measured a whopping 78 feet height, was surfed
Processes of Erosion
Erosion is the wearing away of rock. Several erosion processes happen at coatlines, with their ability to alter the shape of coats influenced by the wave type, the coatline shape and its lithology
Corrosion
-happens when there is a chemical reaction between the seawater (which contains a weak acid) and suusceptible rocks like limestone
Hydraulic action
-HA causes the breakdown of cliffs due to the force of the water being compressed into the cracks of the rock
The repeated action of the water forced in and out of the cracks in the rock leads to the breakdown of the surrounding cliff
Attrition
-it is where smaller pieces of bedload (material carried in the water) are hit against one another. This causes them to break apart and become smaller and more rounded
Abrasion
-Where pieces of rock are picked up by waves and hit against the bed, the beach or cliffs. This wears them away over time
Weathering and erosion
Weathering= The process that takes place as rocks, and other parts of the geosphere, are brocken down into smaller pieces
-can be caused by water, air, chemicals, plants, and even animals including us
-2 types
-Mechanical weathering= the process of breaking big rocks into little ones
can be caused by frost, ise, plant roots, running water, sun heat
-Chemical weathering= involves changes that some substances can cause in the surface of the rock that make it change shape, or colour
can be caused by carbon dioxide, oxygen, acids
Erosion= the movement of weathered rocks by natural forces
Weathering helps make a mess, and erosion helps clean it up
2 Coastal formations
Wave-cit platforms and cliffs
over time, cliffs are eroded by destructive waves, resulting in the formation of wave-cut platforms.
1 formation of wave cut notch
-cliffs are created when erosion, weathering and mass movement processes cause the undercutting anf collapse of unsupported rock.
-Destructive waves are responsible for most of the erosion at the base of cliffs
-when destructive waves lead to abrasion undercutting the cliff face, a curved indent in the cliff forms and a wawave-cut notch is created
2 collaapse of wave cut notch
-continued erosion of the wave cut notch causes the rock above it to become less and less stable, until it eventually collapses
3 formation of new wave-cut notch
-waves wash away the collapsed material and begin eroding awar at the cliff again, causing a new wave cut notch to form
4 Formation of wave cut platform
-over time, multiple collapsing events lead to the cliff retreating. what is left behind is called a wave cut platform
Erosion of Headlands
resistant rocks with weaknesses called joints or cracks are what headlands are made of
1 Corrosion and hydraulic action
-destructive waves can erode weaknesses in headlands by the processes of corrosion and hydaulic action
2 abrasion
-when the weaknesses in the rock widen, abrasion becomes more important
3 formation of coastal landforms
-over time, these erosional processes lead to the formation of coastal landforms such as caves, arches, stacks and stumps
Coastal landforms caused by headlands erosion
The erosion of headlands can create:
caves
-erosion attacks lines of weaknesses in the headland
-when the crack widens into a small hollowed out area, a cave has formed
arches
-when caves are eroded right thr way through the headland, an arch is created
-arches have no bacl and you can walk or swim through them like a doorway eg Durdle Door on the Dorset Coast
stacks
-when the top of an arch collapses because of gravity, a column called a stack is left behind
stumps
-continued erosion and weathering cause the stack to breakdown, forming a stump
-at times of high tide, the stump can become submerged
-eventually, the stmp will erode and there will be nothing left of it
Need headland, cracks/ joins for cave,arch,stack, stump
cliff reatreat results in wave cut platform
3 sediment transportation
longshore drift
LSD transports material along coatlines when waves approach the beach at an angle
swash direction
-wind approaches the coast at an angle ebcause of a prevailing wind direction
-waves are controlled by the wind and so this angle will be the directio the swash moves up the beach
backwash direction
-gravity is the only force that acts on the backwash, so it falls back tothe sea at right angles to the coastline
overall effect
-because of the diffrence between the angle of the swash and the angle of the beackwash, sediment repeatedly moves in the shape of a right angled triangle
-over time, sediment is carried along a beach
Depositional landforms
coastal landforms caused byt deposition include beaches, spits, bars, tombolos and cuspate forelands
spits
-when the coast changes direction at an estuary where a river meets the sea), LSD continues to move sediment across the inlet
-thr river doesnt let the spit completly join to the coast on the other side as the river has the eneergy to move the sedinent
-spits are the long fingers of sand sticking out from one side in a coastline that have been curved by secondary winds. they often have salt marshed behind them
Bars
-a bar is fored when a spit grows across a bay
-lagoons aften form behind bars
Beaches
-made of constructuve waves moving and depositing sand or shingle inland
-genrally, a more gently sloping beach tends to be formed from sand, whereas a steeper sloping beach is formed from pebbles
Tombolos
-if a spit stretches across the mainland and joins an offshore island, this causes the formation of a tombolo
cuspate forlands
-cuspate forelands form when sediment is deposited across a bay caused by LSD transporting sedient in 2 direction,
-this leads to the formation of 2 spits which eventually meet and this results in the trapping of sediment until eventually new land is formed
The sediment cell concept
the sediment cell concept describes a closed system operated by sources, transfers and sinks driven by erosion, transportation and depositional processes
sources
-the sources are subaerial processes, erosioal processes (breaking down cliffs) and sediment braught to the coatline by rivers
transfers
-they are LSD, onshore and offshore winds and tides
sinks
-the sinks are depositional landforms (spits, bars, beaches and sand dunes)
The UK
-The UK has 11 key sediment cells and seub-cells, each one with its own Shoreline managment plan (SMP)
4 Subaerial processes
processes of weathering
weathering describes the breakdown of rocks where they are. Mechanical, chemical and biological weathering are the three main processes.
mechanical weathering
-is whe rocks are broken down withought their chemical composition being changed
-salt weathering is the main type of mechanical weathering affecting coats:
\seawater gets into cracks in the rocks
salt crystals form when the seawater evaporates. these crystals then expand and put pressue on the rock
overtime the repeated evaporation of saltwater widens the cracks in the rock so much that the rock breaks apart
Biological weathering
-biological weathering is when rocks are broken down by living things
\eg plant roots can weather rocks by getting into cracks and pushing them apart as they grow
eg algae can cause the breakdown of rocks from the release of chemicals
-the activity of animals near the coast is also an example of biological weathering
\eg some animals scrap and secrete acid on the rock, like the piddock shells
chemical weathering
-when rocks are broken down because of their chemical composition being changed
-carbonation weathering is a type of chemical weathering that takes place when it is warm and wet:
\carbon dioxide dissolves in rain, making it a weak carbonic acid
the acidic rainwater dissolves types of rock tat contain calcium carbonate
mass movement
describes when rocks and loose material shift down slopes. this happens when gravity overcomes the force supporting the material. mass movements can cause rapid coastal retreat and are common when the material is saturated
slumps
-when material moves down a slope with a rotation
-happen slower than rockfalls and often happe when unstabke permeable rock overlays more stable impermeable rock
-a combination of heavy rainfall and erosional processes cause the rotational movement of the cliff
Slides
-when material moves down a slope in a straight line
Rockfalls
-when material breaks apart because of erosion and weathering and then rolls down a slope
-the fallen fragments at the base of the cliff create talus scree slopes