Coasts Flashcards
Coastal system
- Coasts - where land meets the sea
- Coastal systems are in dynamic equilibrium - inputs and outputs are balanced
Positive feedback - when a change in the system causes other changes to have a similar effect
Examples:
- Beach starts to form —> slows down waves —> causes more sediment to be deposited —> size of the beach increases
Negative feedback - when a change in the system causes other changes to have the opposite effect
Examples:
- As a beach is eroded —> the cliffs behind it are exposed to wave attack —> sediment from cliffs is deposited on the beach, causing it to grow in size
Coasts are systems:
Inputs:
- Energy and sediment coming into coastal system e.g. energy inputs come from waves, wind, tides and currents
Stores (stores of sediment):
- Erosional landforms e.g. cliffs
- Depositional landforms e.g. spits, beaches, dunes
Outputs:
- Sediment can be washed out to sea or deposited further along the coast
Flows/transfers (sediment moves from one store to the next):
- Erosion, deposition, transportation, weathering, mass movement
Sources of energy in coastal systems
Wind:
- Winds are created by air moving from areas of high pressure to areas of low pressure. During events such as storms, the pressure gradient (the difference between high and low pressure) is high and winds can be very strong
- Strong winds can generate powerful waves. If wind consistently blows from the same direction (prevailing wind) — this causes higher-energy waves than winds that change direction frequently
Waves:
How are waves formed?
1. Waves are created by the wind blowing over the surface of the sea. The friction between the wind and the surface of the sea creates small waves which gives the water a circular motion
2. When waves approach the shore they break. Friction with the sea bed slows the bottom of the waves (makes their motion more elliptical - squashed and oval shaped). The crest of the wave (top of wave) carries on moving - rises up and collapses
What is wave energy affected by?
The taller a wave, the more energy it carries —> wave height is affected by wind strength, duration and fetch of the wave
- Wind strength (link to pressure gradient)
- Wind duration
- Fetch of the wave (the maximum distance of sea the wind has blown over)
- Sea bed —> gently sloping sea bed — waves experience friction with the sea bed and slow down —> waves lose energy —> less erosion// steeply sloping sea bed —> less friction with the sea bed —> waves won’t lose as much of its energy —> more erosion
2 types of waves and characteristics:
Constructive:
- Low frequency —> 6-8 per minute
- Low and long
- Strong swash, weak backwash —> carries material up the beach and deposits it
Destructive:
- High frequency —> 10-14 per minute
- High and steep
- Strong backwash, weak swash —> removes material from the beach
- Constructive waves tend to deposit material —> size of beach increases
- Destructive waves cause erosion —> size of beach decreases
Anatomy of a wave (search up diagram):
- Crest - highest point of wave
- Trough - lowest point of wave
- Wave height - different in height between crest and trough
- Amplitude - half the wave height
- Wave length - difference between 2 crests/2 troughs
- Frequency - number of waves per minute
Tides:
- Tides - the periodic rise and fall of the level of the sea, caused by the gravitational pull of the moon and the sun
- Spring tide - when the moon, earth and sun are in a line, the combined gravitational pull creates the highest high tides and the lowest low tides —> greatest tidal range
- Neap tide - when the sun and moon are perpendicular to each other, their gravitational pulls interfere with one another, giving the lowest high tides and the highest low tides —> smallest tidal range
- Tides affect erosion and lead to the formation of different landforms
Currents:
- Currents - general flow of water in one direction
3 types of currents:
Longshore currents (littoral drift)
- Flow of water parallel to the coastline —> move material along the coast
Rip currents
- Strong currents moving away from the beach
- Waves cause a build up of water at the top of the beach —> eventually the waves finds a route back out to sea —> creates a strong current
Upwelling
- Winds drive water across the ocean surface, allowing cold, nutrient-rich water from the deep ocean to rise to the surface
High and low energy coastlines
- High-energy coasts receive high inputs of energy in the form of large, powerful waves. These can be caused by strong winds and long fetches. High-energy coastlines tend to have rocky landforms e.g. cliffs, caves, stacks and arches. The rate of erosion is often higher than the rate of deposition
- Low-energy coasts receive low inputs of energy in the form of small, gentle waves. These can be caused by gentle winds and short fetches. Low-energy coastlines often have saltmarshes and tidal mudflats. The rate of deposition is often higher than the rate of erosion
Sediment
Sources of sediment in coastal systems:
- Rivers carry eroded sediment into the coastal system
- Sediment is eroded from cliffs by waves, weathering and landslides
- Sediment can be formed from the crushed shells of marine organisms
- Waves, tides and currents can transport sediment into the coastal zone from offshore deposits
- Sea level rise can flood river valleys, forming estuaries (river and sea meet). Sediment in the estuary becomes part of the coastal system
Sediment budget:
- Sediment budget - difference between the amount of sediment that enters the system and the amount that leaves
- If more sediment leaves than enters, there’s a negative sediment budget and overall the coastline retreats
- If more sediment enters than leaves, there’s a positive sediment budget and overall the coastline grows
Sediment cells:
- The coast is divided into sediment cells (also called littoral cells)
- These are lengths of coastline (often between two headlands) that are pretty much entirely self-contained for the movement of sediment (sediment doesn’t move between cells). This means that each cell is a closed coastal system
Wave refraction
- Occurs when waves approach a coastline that is not a regular shape (headland and bay)
- Wave energy becomes concentrated on the headland, causing greater erosion
- In a bay, the waves lose power, causing deposition
Coastal processes
- Sub-aerial processes (operate on land)
- Marine processes (operate in the sea)
- Aeolian processes (driven by the wind)
Erosional processes
Erosional processes:
- Abrasion/corrasion - bits of rock/sediment carried by the sea are picked up by strong waves and thrown against rocks and cliffs, breaking bits off and smoothing surfaces
- Wave quarrying - energy of wave is enough to detach bits of rock
- Solution (corrosion) - soluble rocks get dissolved by the seawater e.g. limestone
- Attrition - Bits of rock in the water smash against each other and break into smaller bits
- Hydraulic action - force of water crashing into rocks, compressing air in cracks, and breaking the rock apart
- Cavitation - occurs when fast-flowing water creates bubbles in cracks. These bubbles collapse with force and break rocks apart
Factors affecting erosion:
Wave strength (strong waves = more erosion)
- Controlled by fetch and wind strength/duration e.g. long fetches and stronger/longer winds create bigger and powerful waves —> more erosion
Bathymetry:
- Underwater topography of the seabed impacts the strength of waves
- Gently sloping sea bed —> waves experience friction with the sea bed —> waves lose energy —> less erosion
- Steeply sloping sea bed —> waves experience less friction with the sea bed —> waves won’t lose as much of its energy —> more erosion
Beaches:
- Beaches increase the distance a wave travels before it reaches the cliffs —> absorbs some wave energy before it reaches cliffs —> waves energy is reduced —> less erosion
Weathering:
- Weathering creates weaknesses in rocks which can be further exploited by the processes of erosion
- Weathering rates are high —> rates of erosion will be faster
Rock type:
- Sedimentary rocks e.g. limestone have lots of faults, making them weak and vulnerable to erosion whereas igneous and metamorphic rocks are made up of interlocking crystals, making them more resistant to erosion
Transportation
Transportation (eroded material being moved):
- Solution - substances that can dissolve are carried along in the water e.g. limestone is dissolved into water that’s slightly acidic
- Suspension - very fine material e.g. silt and clay particles are carried along in the water (most eroded material is transported this way)
- Saltation - larger particles e.g. pebbles or gravel are too heavy to be carried in suspension so particles bounce along the sea bed
- Traction - very large particles e.g. boulders are dragged along the sea bed
Longshore drift:
1. Swash carries sediment up the beach in the direction of prevailing wind
2. Backwash carries sediment back down the beach at right angles
3. Overtime, sediment is moved along the beach
Deposition
Deposition (process of dropping eroded material):
- Marine deposition - when sediment carried by seawater is deposited
- Aeolian deposition - when sediment carried by wind is deposited
- Both marine and aeolian deposition happen when the sediment load exceeds the ability of the water or wind to carry it. This can be because sediment load increases, or because wind or water flow slows down (so it has less energy)
Wind and water slow down for similar reasons:
- Friction increases — if waves enter shallow water or wind reaches land, friction increases, which slows down the water or wind
- Flow becomes turbulent — if water or wind encounters an obstacle, flow becomes rougher and overall speed decreases
Weathering
- Weathering is a sub-aerial process (operates on land)
- Weathering - weakens rocks and makes them more vulnerable to erosion
Different types of weathering:
- Physical/mechanical - when rocks break up with no chemical changes
- Chemical - rock breakdown due to a chemical reaction
- Biological - rock breakdown due to organic activity
Salt weathering (physical/mechanical):
1. Salt water enters cracks in rocks at high tide
2. As the tide goes out the rocks dry and the water evaporates —> salt crystals are left behind in the cracks
3. As the salt crystals form they expand, exerting pressure on the rock - this causes pieces to fall off
Wetting/drying (physical/mechanical):
1. Some rocks contain clay
2. When clay gets wet, it expands and the pressure caused by this breaks fragments off the rock
Freeze thaw weathering (physical/mechanical):
1. Water enters cracks in rocks
2. When temperatures drop below 0°C, the water freezes and expands which causes the crack to widen
3. The ice melts and more water fills into the cracks
4. The process repeats itself until the rock breaks
Biological weathering:
1. Plant roots growing in cracks of rock —> widens cracks —> can cause rocks to breakdown
Chemical weathering:
1. Carbon dioxide in the atmosphere dissolves in rainwater, forming a weak carbonic acid. This acid reacts with rock that contains calcium carbonate e.g. limestone and the rocks are gradually dissolved
Mass movement
- Mass movement is a sub-aerial process (operates on land)
- Mass movement - shifting of material downhill due to gravity
Type of mass movement depends on:
- Type of material (e.g. consolidated rock or loose soil)
- Angle of the slope (gentle or vertical)
- Saturation of rocks
Types of mass movement:
- Slides - material shifts in a straight line (consolidated rock)
- Rockfalls - material breaks up and falls (consolidated rock)
- Mudflows - material flows downslope (unconsolidated rock)
- Slumps - material shifts with a rotation (unconsolidated rock)
- Unconsolidated rocks e.g. clay are prone to collapse as there’s little friction between particles to hold them together
- Heavy rain can saturate unconsolidated rock, further reducing friction and making it more likely to collapse
Coastal landforms caused by erosion
Caves, arches, stacks and stumps:
1. Headlands have cracks —> abrasion and hydraulic action widen the cracks
2. Repeated erosion of cracks causes cave to form
3. Continued erosion deepens the cave until it breaks through the headland to form an arch
4. Arch is eroded until the roof collapses leaving a stack
5. A wave cut notch forms at the base of the stack, eventually causing it to topple over and collapse —> leaving behind a stump
Example: arch, caves and stacks on the shore of Loch Bracadale, Scotland
Headlands and bays:
1. Headlands and bays form where there are alternating bands of hard and soft rock at right angles to the coast (discordant coastline)
2. The soft rock is eroded quickly, forming a bay
3. The hard rock is eroded more slowly and forms a headland which sticks out
Example: headlands and bays on the Cape of Good Hope, South Africa
Wave cut platform:
1. Sea attacks base of cliff forming a wave cut notch
2. Repeated erosion causes rock above notch to become unstable and it eventually collapses
3. Collapsed material is washed away and a new wave cut notch starts to form
4. The process repeats and the cliff continues to retreat, leaving behind a wave cut platform
Example: wave cut platform near Lannacombe Bay in South Devon
Blowhole:
1. Waves approach the bottom of the headland where there’s a crack
2. Water is forced and compressed into the cracks
3. Eventually. water spurts out the top, forming a blowhole
Example: the world’s largest blowhole is found in Nakelele Point in Hawaii
Coves:
1. Formed on concordant coastlines
2. Resistant outer band rock is eventually breached
3. Erosion speeds up when waves reach the less resistant bands of rock —> erosion spreads out laterally
4. Once harder rock is reached again, erosion slows down
Example: Lulworth Cove in Dorset
Coastal landforms caused by deposition
Spits:
- Spit - a finger of beach material extending out to sea
1. Longshore drift moves material along the coastline
2. When the coastline changes direction e.g. at a headland, longshore drift doesn’t change direction
3. Sediment builds out to sea and this creates a spit
4. A change in wind direction will cause the spit to curve at the end. This is known as a recurved end
5. Over time, several recurved ends form as the waves return to their original direction and then change again. A spit that has multiple recurved ends is called a compound spit
6. The area behind the spit is sheltered from the waves and often develops into mudflats and saltmarshes (sheltered area allows for the deposition of river sediment —> over time, this sediment builds up to form a mud flat —> as vegetation begins to grow on the mud flat, it transitions into a salt marsh)
- Example: Spurn Head on the coast of East Yorkshire
Bars:
1. A bar forms when a spit joins 2 headlands together
2. A lagoon forms behind the bar
- Example: Slapton Sands in Devon
Tombolos:
- If a spit/bar joins up to an island, it creates a tombolo
- Example: Angel Road of Shodo Island in Japan
Offshore bars:
- Destructive waves remove sediment from the beach and form the offshore bar. They are covered at high tide and exposed at low tide
- Example: Scroby Sands, Norfolk, England
Barrier islands:
- Barrier islands are long, narrow islands of sand that run parallel to the shore and are detached from it
- They tend to form in areas where there’s a good supply of sediment, a gentle slope offshore (more friction with seabed —> waves loss energy —> increased deposition), fairly powerful waves and a small tidal range
- It’s not clear exactly how barrier islands form, but scientists think low sea level that they probably formed after the last ice age ended, when ice melt caused rapid sea level rise. The rising waters flooded the land and transported sand offshore, where it was deposited
- Example: Horn Island in Mississippi
Sand dunes (depositional landform)
- A vegetation succession is a plant community that changes over time - from pioneer species (first plants to colonise an area of bare ground e.g. marram grass) to climax community (a community of plants which have reached a steady state over time and vegetation has evolved to the end point of succession e.g. deciduous woodland)
Conditions needed for dune formation:
- Large tidal range (large amount of sand exposed at low tide)
- Large supply of sand
- Large tidal range and large supply of sand allows the sand to dry so that it is light enough to be picked up and carried by the wind to the back of the beach
- Wind
- Obstacle
- Vegetation
Sand dune formation:
1. Sand is deposited around an obstacle e.g. seaweed and driftwood
2. An embryo dune develops which may become vegetated by pioneer species such as marram grass
3. Roots bind the sand together// marram grass help slow the speed of wind —> wind loses energy —> sediment is deposited
4. Several embryo dunes will join together to create foredunes and yellow dunes. This is the tallest of the dune succession (yellow dunes)
5. Grey dunes form as the dunes become more mature. As plants die, nutrients are added to the sand dune, so more complex plants can grow. Eventually, the climax vegetation is reached e.g. heathland or woodland
6. Areas between dunes (slacks) may be damp or even contain water
Characterises of marram grass:
- It is tough and flexible, so can cope when being blasted with sand
- It has adapted to reduce water loss through transpiration
- It has mechanisms to tolerate high salinity
Example: Sand dunes at Cape Hatteras in North Carolina
