Coasts Flashcards
What are the inputs of a coastal system?
- River sediment
- Sediment from cliff erosion or mass movement
- Transported by waves from offshore
What is the definition of a Coast?
A narrow strip of land where the land meets the sea
What outputs are there in a coastal system?
Sediment washed out to sea or deposited further along the sea, usually by longshore drift
What are sediment/littoral cells
Lengths of coastline that are pretty much entirely self contained for the movement of sediment
How do sediment cells work?
Each one is its own coastal system. Processes going on in one cell don’t affect the movement of sediment in another
How do waves form?
When wind blows over the surface of the sea, friction between the wind and the surface gives the water a circular motion
What is the fetch?
The maximum distance of sea the wind has blown over when creating the waves. The longer the fetch the more erosion occurs
How are waves with high wave heights created?
When there is a high wind speed and long fetch
Why do waves break as they approach the shore?
Friction with the sea bed slows the bottom of the waves down. The motion of the water goes from circular to elliptical. The crest rises and then collapses
What are the two types of wave?
Constructive and destructive
What are the characteristics of a constructive wave?
- Low frequency (6-8 per minute)
- Produces a gentle beach gradient
- Powerful swash weak backwash
- High deposition
- Large wavelength
What are the characteristics of destructive waves?
High frequency (10-14 per minute) Strong backwash weak swash Steeper beach profile High erosion Short wavelength
What are tides?
The periodic rise and fall of the ocean surface
Why do tides change?
The gravitational pull of the moon and the sun cause the changes.
What are sub-aerial processes?
Coastal processes that are not linked to the action of the sea
What is freeze thaw weathering?
Water enters cracks in the rock. If temperatures fall below 0 degrees Celsius the water freezes, cracking it as it expands.
How does wetting and drying of certain rocks cause weathering?
Rocks like clay expand when wet, exerting pressure which can break up fragments of rock
What is thermal expansion?
When the repeated action of heating and cooling of rock causes them to fracture and shed layers
What is biological weathering?
When plant seeds get into cracks and begin to grow. This exerts pressure on the rock, causing them to fracture
What is chemical weathering?
- Hydrolysis is the splitting of materials due to their reactivity with water. •Oxidation is when rocks rust.
- Corrosion technically is a form of weathering
What is mass movement?
Large scale movement of material down slope in response to gravity
Where do rockfalls most commonly occur?
Cliffs where there are lots of joints. The joints make it easier for the rock to break up
What is the slowest form of mass movement?
Soil creep - when soil is dampened its mass slightly increases, causing it to move downhill
Why does slumping and landslides occur?
Water increases the mass of the soil and acts as a lubricant, so material slides down the cliff
What are the four main forms of erosion?
- Abrasion
- Hydraulic Action
- Corrosion
- Attrition
What is abrasion?
Waves carrying sand and shingle break on the cliff, the sediment acts like sandpaper, breaking bits off and smoothing surfaces
What is hydraulic action?
Waves force air into cracks under high pressure. The cracks grow as this is repeated and rock breaks off it
What is corrosion?
Soluble rocks get dissolved by the water
What is attrition?
Beach material is knocked together, reducing their size and making them round and smooth
How does the width of a beach affect the rate of erosion?
Beaches slow down waves due to the friction between the two, reducing their erosive power. A wide flat beach protects cliffs
What happens if a wave breaks at the base of a cliff?
Most of the energy of the waves is transferred to the cliffs, causing the most erosion. Waves that break further offshore erode much less
How is material moved along a Coast?
Longshore drift
How does longshore drift work?
Swash carries sediment up the beach in the direction of the prevailing wind. The backwash carries sediment back down the beach at right angles. When the prevailing wind is at an angle to the backwash, sediment is moved along the beach.
How do wave cut platforms form?
- Erosion at the high water mark forms notches in the cliff.
- These continue to be eroded, forming caves.
- The rock above becomes unstable and collapses
- This continues to happen and the cliff retreats, leaving a wave cut platform
What is the name of a coastline where the rock type is perpendicular to the coastline?
Discordant coastline
How do headlands and bays form?
When there are bands of soft and hard rock on a discordant coastline. The soft rock is easily eroded, forming bays. The more resistant rock takes longer to erode, forming headlands
What happens when faults are eroded on a headland?
- They increase in size, forming caves
- Caves on both sides of a headland may join up, forming an arch
- Arches can collapse forming stacks
- As the base of a stack is eroded it can collapse forming a stump
Why are shingle beaches steep and narrow?
Waves easily flow through the coarse shingle. This decreases the backwash erosion so sediment remains on the beach
Why are sand beaches wide and flat?
Small particles are evenly distributed, making it harder for water to percolate into the sand. This increases backwash erosion and the amount of sediment removed from the beach
What are berms and how do they form?
They are ridges of sand and pebbles. They’re formed by deposition of coarse material at the limit of the swash
What are runnels?
Grooves in the sand running parallel to the shore. They’re formed by backwash draining into the sea
What are cusps?
Crescent shaped depressions in the sand
How do spits form?
- Longshore drift continues to deposit material across the river mouth, leaving a bank of sand and shingle in the sea
- If the dominant wind changes the spit may begin to curve, several of these curves may develop over time
- The area behind is sheltered from waves, so mudflats and salt marshes develop
What are bars?
When a spit joins two headlands together, across a bay or river mouth
What is the area behind a bar in a bay called?
A lagoon
How are sand dunes formed?
When sand deposited by longshore drift is moved up the beach by wind
How does the sand remain stable in a sand dune?
They are colonised by plants and grasses like marram grass. It holds the sand together, allowing more sand to accumulate here
How do mudflats develop?
Silt and mud are deposited by the river or the tide
How do mudflats become salt marshes?
The mudflats are colonised by vegetation that can survive high salt levels and be submerged in water for long periods of time. Over time more mud and silt builds up creating marshes
By how many mm are sea levels rising?
2mm
What are the two types of sea level change?
Eustatic and Isostatic
What is Eustatic sea level change?
Sea level change when there is a change in volume of water in the sea or a change in shape of the sea bed. This usually happens on a global scale
How do tectonics affect Eustatic sea level change?
Tectonic movements alter the shape of the ocean basins, a decrease in volume would cause sea level rise
What happens to sea levels when the temperature of the climate increases?
Ice sheets melt, which increases sea level. Water expands when it’s heated which increases sea levels
What happens if the temperature of the climate decreases?
Increases the volume of water in glaciers, reducing volume of water in the sea, which decreases sea level
What is Isostatic sea level change?
Vertical movements of the land relative to the sea.
•Downward movement of land causes sea level rise
•Upward movement of land causes sea levels to fall
How does land compress?
- Accumulation of ice sheets causes the land beneath to fall, rising sea levels
- Sediment at a river mouth
How does land up lift?
When ice melts the pressure on the land is released, so land is allowed to rise
What are rias?
River valleys that are submerged by a rise in sea level. The lower course and its floodplain are drowned, leaving the higher ground above
What is a fjord?
They are submerged glaciated valleys with steep cliff-like valley sides.
How do fjords form?
Glaciers eroded below sea level, creating the valleys. The glaciers melted and the valleys flooded
How do Dalmatian coastlines form?
Valleys that run parallel to the coastline are submerged when sea level rises. This leaves islands that were previously areas of high land
What are landforms of emergence?
When sea levels fall, revealing features previously submerged by water
How do raised beaches form?
Former wave cut platforms and their beaches are revealed when the sea level falls
What are relict cliffs?
Cliffs that appear raised by sea level fall slowly get covered by vegetation. They often feature wave cut notches, caves, arches and stacks
What is the aim of coastal management?
To protect homes, business and the environment from erosion and flooding
What is the cost benefit analysis?
Choosing where to place defences by evaluating their benefit
What is holing the line?
Maintaining the existing coastal defences
What is advancing the line?
Building new defences further out to sea than the existing line of defence
What is doing nothing?
Build no coastal defences and allow nature to do what it wishes
What is retreating the line?
Build no defences but move people away from the coast
What are hard engineering methods?
Defences that involve the engineering and construction of structures
How do sea walls work?
The wall reflects waves back out to sea preventing erosion, it acts as a barrier to prevent flooding also
What are the disadvantages of sea walls?
They are expensive, creates strong backwash which erodes under the wall
How do revetments work?
Revetments are slanted structures built at the foot of cliffs. When waves break against them their energy is absorbed by the revetments rather than the cliff
How do Gabion boxes and riprap work?
They absorb energy at the foot of a cliff, preventing erosion
What are groynes and how do they work?
They are fences built perpendicular to the coastline. They trap material transported by longshore drift, creating wider beaches - which Absorb the energy of waves
What are offshore breakwaters?
Concrete blocks or boulders built out to sea which cause waves to break before they reach the coastline
What are tidal barriers?
They are built across river estuaries, they contain retractable floodgates that can be raised to prevent flooding from storm surges
How does coastal management affect areas further downdrift?
They protect the area where they are built but can make it worse further downdrift. Groynes trap sediment, reducing sediment in other areas, leaving them susceptible to erosion
What is a main problem with hard engineering options?
They are often ugly and spoil the landscape
What are soft engineering defences?
Working with natural process to protect the coast
What is beach stabilisation?
Reducing the slope of a beach’s and planting vegetation and old tree trunks to stabilise the sand and maintain wide beaches
What is dune regeneration?
When sand dunes are created or restored to maintain the barrier between land and sea
What is land use management?
Managing environments to protect certain parts, eg wooden walkways to protect sand dunes
How can the creation of marshland reduce erosion?
By planting appropriate vegetation a marsh can be created which reduces the speed of waves and absorbs the erosive energy
What is coastal realignment (managed realignment)?
Breaching a sea wall and allowing the sea to flood the land behind. Over time vegetation will colonise the land behind and become marshland
Why are soft engineering methods more sustainable?
They have a lower environmental impact and a lower cost
Why must management strategies think about the future?
- Rising sea levels make storms more frequent and severe
* Improve sustainability
What is the integrated coastal zone management?
The process of trying to come up with an integrated, sustainable management plan.
What is beach nourishment?
Where sand and shingle are added to beaches from elsewhere, creating wise beaches which reduce erosion
