Wave Types & Their Formation
Wave Types & Their Formation:
The coast is a narrow contact zone between the land and the sea
- Waves are usually created by the wind blowing over the sea, friction with the surface of the water causes ripples to form and these develop into waves.
- Fetch = "the distance of open water over which the wind can blow".
- The UK's maximum fetch past is the south west, getting winds off the Atlantic
- Larger waves are formed by: Strong Winds for a long time over a large fetch
Formation of Waves & Important Terms
Formation of Waves:
- In the open seas, with water particles, there is a circular orbit (little horizontal transfer of water).
- When it reaches the shore the speed/velocity of the base of the wave slows down due to the friction of the seabed.
- Circular orbit changes to elliptical.
- The top of the wave is unaffected by the friction and is still fast.
- This seperates them and the top topples over and breaks the shore.
- Swash = The Forward movement of a wave up a beach / The transfer of water, energy and materials up the beach by the wave
- Backwash = Transfer of water, energy and material back down the beach
Constructive Waves (Smaller in height)
Constructive Waves (Smaller in height):
- A powerful wave with a strong swash which surges up the beach.
- Waves that surge up the beach with a powerful swash.
- They carry large amounts of sediment and 'construct the beach' making it more extensive
- They are formed by distant storms which can be hundreds of km away!
- These waves are well spaced apart and are powerful when they reach the coast
Constructive Waves (Smaller in height): Diagram
Destructive Waves (Large in height)
Destructive Waves (Smaller in height):
- A wave with formed by a local storm that crashes down on the beach and has powerful backwash,
- Formed by local storms close to the coast.
- They are named this because they 'destroy the beach'.
- Closely spaced and often intefere with each other producing a chaotic swirling mass of water!
- They rear up to form towering waves before crashing down on the beach.
- There is little swash when this wave breaks but a powerful backwash which removes sedimet therefore destroying the beach.
Destructive Waves (Large in height): Diagram
How the Coast is Shaped (Weathering)
How the Coast is Shaped:
The coast is shaped by weathering, mass movement, erosion, transportation and desposition
Weathering (sub-aerial process = taking place underneath the air):
Weathering is the breaking down of rocks in the same place. There are three types of weathering:
- Water - eg from rainfall or melting snow and ice - becomes trapped in a crack or joint in the rock.
- If the air temperature drops below freezing, the water will freeze and expand by 9-10 per cent putting pressure on the rock.
- The ice will melt when the temperature rises above freezing.
- If this process happens repeatedly, the rock will weaken and eventually shatter into angular fragments.
- The fragments may then be deposited as scree at the foot of a slope.
Repeated cycles of expansion (heating) and contrastion (cooling) can lead to the outer skin peeling away from the rest of the rock. The presence of water is important for this process to take place as it weakens the rock, making it more vunerable to flaking.
Chemical & Biological Weathering
Solution / Corrosion :
Salt Crystallisation of cliff faces from sea water disintegrates weaker layers
- Attacks coasts made of calcium
- Calcium Carbonate - Limestone & Chalk
Rain water picks up Carbon Dioxide from the air and becomes weak carbonic acid, then reacts with calcium carbonate to form calcium bicarbonate (which dissolves)
Cliffs are broken down by plants / seeds / tree roots / animals .
Mass Movement & Types of Mass Movement
Mass Movement is the downhill movement of material under the infulence of gravity.
- Slip Lanes/Slide Lanes - A line of weakness that often becomes lubricated with water causing rocks and soil to move downhill
Types of Mass Movement:
- Slumping/Rotational slips - Here, water can build up in soils and add weight to it. The material moves down slope along a curved surface, leaving behind an exposed scarp face below the head of the slump
- Rock falls - Here, large and small fragments of rock are continually weathered and eroded until they separate and fall from the cliff as whole parts. In Britain this is often due to freeze thaw weathering.
- Land slides - Where rocks are laid down in beds that slope (dip) down towards the sea whole layers can slide down slope along a slide plane.
- Mudflow - This is where saturated soil and weak rock flows down a slope and produces a lobe
Flow, Slides & Slumps
Flow - The velocity/speed is greatest at the surface and decreases to almost nothing at the bottom e.g. saturated soil and weaker rock flows down a stump
Slide - There is equal velocity from top to bottom of the moving mass (ie: the mass moves as one / the whole collum of material moves in one go) which slides down an inclined sliplane/slide. It does that more or less intact until it reaches the bottom where the impact usually breaks up
Slump - Usually occurs in weaker rocks than slides and are distiunguished from them having a rotational movement along a curved slip lane. The curvature of a slope causes the rock to rotate backwards as it slips (ie: the material makes a curved movement).
Marine Processes Shaping the Coast - Erosion
"When a material is destroyed/moved by wind/water"
Types of Erosion:
- Hydraulic action - Air may become trapped in joints and cracks on a cliff face. When a wave breaks, the trapped air is compressed which weakens the cliff and causes erosion.
- Abrasion - Bits of rock and sand in waves grind down cliff surfaces like sandpaper.
- Attrition - Waves smash rocks and pebbles on the shore into each other, and they break and become smoother.
- Solution - Acids contained in sea water will dissolve some types of rock such as chalk or limestone.
Coastal Erosion Land Forms - Bays & Headlands
Wave refraction refers to what happens to waves when they approach an uneven coastline. As waves approach an uneven coastline they reach the headlands first, this focuses a lot of energy on those forelands (destructive waves) and bends the waves into the bays where the energy expended is less (constructive waves).
Bays & Headlands:
If there are alternating bands of harder and softer rock running at right angles to the sea, the sea will erode these bands at different rates (called differential erosion). Hydraulic action, abrasion and corrosion are more effective at eroding the softer rock, particularly during storms, and this will erode further inland than the harder rock. During calmer weather and no stormy periods, the hard rock will absorb a lot of the wave energy and refract or bend the waves into the area with the softer rock, allowing sediment to be deposited and accumulate as beaches. The net result of this over long periods of time is that the hard rock is left jutting out to sea as a headland, and the softer rock is eroded into curved sand filled bays.
Coastal Erosion Land Forms - Caves -> Stumps
Stacks, Stumps and Caves
- These features are formed on cliffs or headlands.
- Waves attack vertical lines of weakness in the rock known as Faults.
- Processes such as hydraulic action and abrasion widen these faults into cracks and eventually the waves will penetrate deeply enough to create caves.
- Over time, the cave will be eroded into an arch, accessible to the sea on both sides.
- Weathering will also play a role, with physical weathering processes such as freeze thaw and salt crystallisation weakening the rock surrounding the cave or arch making it more susceptible to mass movement and collapse.
- Finally, the erosion and weathering continues and the arch collapses leaving behind a stack (a vertical column of rock) . These stacks can be attacked further, and eventually the stack may collapse to leave a low lying stump.
Coastal Erosion Land Forms - Cliff Erosion
- Marine erosion processes (Hydraulic Action & Corraison) attack the foot of the cliff and cause the erosion at a wave cut notch.
- This undermines the whole structure of the cliff.
- These processes are variable and depend upon the fetch of the wave (the distance it travels over open water), wind speed and how many storms there a year, but they are more or less continuous over long periods of time.
- At the same time that the base of the cliff is being eroded, the cliff face and its structure are being weakened by sub aerial processes.
- Oxidation and carbonation are some of the chemical processes that can weaken the structure of the rock, and depending upon the climate physical processes such as freeze thaw and water layer weathering can take effect.
- Over time this weakens the structure of the cliff face, and coupled with the erosion of the wave cut notch at a critical point this cliff face will succumb to the influence of gravity and collapse in a process of mass movement.
- This material will then be carried away by the sea in the process of long shore drift by the transportation process of solution, suspension, saltation and traction.
Coastal Erosion Land Forms - Cliff Erosion
Waves can approach the coast at an angle because of the direction of the prevailing wind. The swash of the waves carries material up the beach at an angle. The backwash then flows back to the sea in a straight line at 90°. This movement of material is called transportation.
Continual swash and backwash transports material sideways along the coast. This movement of material is called longshore drift and occurs in a zigzag. However LSD only occurs if the wind is approaching the wind at the angle and it can be controlled by groynes.
- Solution - Minerals are dissolved in sea water and carried in solution. (Invisible) Load can come from cliffs made from chalk or limestone, and calcium carbonate is carried along in solution.
- Suspension - Small particles are carried in water, Currents pick up large amounts of sediment in suspension during a storm (strong winds generate high energy waves)
- Saltation - Load is bounced along the sea bed, eg small pieces of shingle or large sand grains. Currents cannot keep the larger and heavier sediment afloat for long periods.
- Traction - Pebbles and larger sediment are rolled along the sea bed.
Deposition & It's Landforms
- Desposition is the dropping of sediment and material
- The sea deposits materials in bays (which explains beaches) because the wave has less energy there. There are no beaches on headlands because the waves energy is strong there.
- Spits are created by the process of LSD.
- Wherever there is a break in the coastline (e.g. across a river or a change in coastline direction) then material is deposited closest to the shore. This is because there are often counter currents and a loss in velocity, so material is dropped or deposited.
- Eventually this material builds up out into sea to form a spit. The spit often curves inwards towards the land as a result of the prevailing wind directing the waves which push the sediment in towards the shore.
- Spits often have salt marshes build up behind them because the spit offers protection from the stronger waves and the wind, allowing salt tolerant plants to grow.
- If a spit extends from headland to headland then a bar will be created.
Case Study - Rising Sea Levels - East Anglia
Case Study - Rising Sea Levels - East Anglia:
- Sea levels are rising because of global warming and thermal expansion of the sea. Also the UK is experiencing isostatic reajustment as a result of the last Ice Age (The SE going down by 1-2mm a year). East Anglias has flat land is effected by isostatic reajustment.
Impacts of Rising Sea Levels:
- The Ferns & The Norfulk Broads boating & other area will be lost -> Less jobs
- Risk of storm surge which killed 350 people in 1953
- Less income from farming & tourism
- Cost of costal protection
Salt Marches are under threat and we care about it's bio-diversity (natural flood defence).
Case Study - Cliff Collapse - Barton-On-Sea
Case Study - Cliff Collapse - Barton-On-Sea:
- Geology of arangement of rocks (sandstome at top and clay at bottom)
- Fetch - strong prevaling wind from the Gulf of Mexico slamming against weak cliffs
- Longshore Drifts
- Buildings ontop of the cliff
- Tourism -> Trampling
- Smaller Beaches - dredging (getting sand for industrial stuff), groynes = more erosion
Social & Economic Effects:
- Houses worth less
- Loss of tourism & farmland -> Loss of money and jobs
Case Study - Cliff Collapse - Barton-On-Sea
- Loss of cliff habitat for wildlife
- Loss of fossils
- Reveals new fossils
Coastal Management: Hard Engeneering
- People make demands of the coast with: tourism, industry, transport, farming and fishing
- Sustainable Management: good for the environment, people and the economy
- "Hold the line" = Taking Actions to keep the line of the coast as it is now
- Hard Engeneering: Artifical Structures such as:
- Sea Walls: Which protects the base of cliffs, land and buildings against erosion and van prevent coastal flooding in some areas. However is expensive to build. Curved sea walls reflect the energy of the waves back to the sea. This means that the waves remain powerful. Over time the wall may begin to erode. The cost of maintenance is high
- Groynes: Prevents the movement of beach material along the coast by longshore drift. Allows the build up of a beach. Beaches are a natural defence against erosion and an attraction for tourists. Can be seen as unattractive and costly to build and maintain.
- Rock Armour: Absorb the energy of waves and allows the build up of a beach. But can be expensive to obtain and transport the boulders.
Coastal Management: Soft Engeneering
- Soft Engeneering: Sustainable Approach without artificial structures:
- Beach Nourishment: Relativley cheap and easy to maintain and blends in with the existing beach and increases tourist potential. However its need constant maintainance
- Dune Regeneration: Relativley cheap and maintains a natural costal environment however it is time consuming to place the grass and fences & damaged by storms.
- Marsh Creation: Cheap option which creates habitats for wildlife however land is lost for it.
- Managed Retreat: Allowing controlled flooding of low lying costal areas or cliff collapse in areas where the value of land is low.
- Costal Benefit Analysis: Weigh up the cost of managed retreats and the advantage of marsh habitats and the value of the land and decide on if we should allow the land to flood.
Case Study - Coastal Management & Habitat - Abbots
Case Study - Coastal Management & Habitat - Abbots Hall Farm:
- Sea walls were build around the Essex coastline in the 1940-50's because the government changed this grazing march into arable land = risk of flooding agricultural land
- Intertidal Land is areas of bare/plant covered sediment between high-low water marks
- Managed Retreat is needed because sea levels are pressuring intertidal land
- In 2002, five beaches in the farm were created and allowed 80ha of low lying land behind a 3.5km sea wall to be flooded. They carried out an environmental assesment.
- Created a salt marsh which is an important intertidal habitat with high primary productivity.
- Acts as a floodwater storage therefore having smaller floods
- Increases in biodiversity and used to graze sheep on -> higher price for meat
- Soft Engerneering -> Sustainable
- Creates a Tourist Attraction -> More Money
Case Study - Coastal Management & Habitat - Abbots
- Loss of farmland/food produces and affects Oysters -> No jobs for Oyster Men/ maintenance
- Some people think it gives us less protecton against storm surges
- Keyhaven marshes has formed behind Hurst Castle Spit. This spit provided a sheltered place for sediment accumulation and for eel grass to accumulate away from the impact of strong winds and coastal erosion.
- The pioneer colonising plant, eel grass, helps to stabilise the area further by trapping more sediment.
- Gradually, halophytes (salt tolerant plants) such as glass wort and sea blite colonise the accumulating mudflats. These plants trap more sediment and contribute organic matter when they die. These processes help the salty marsh to grow.
- Eventually the salt marsh will grow further and an even more complex set of plants will colonise the area, until trees grow!
Vegetation Succession: Diagram