Coasts: Why are they different and dynamic? Flashcards
What is a coast?
Area where land meets the sea, affected by the waves and tides
What are the different zones on a shore?
- Offshore
- Nearshore
- Foreshore
- Backshore
- Beach face
- Berm
- Dunes
What are the factors leading to the differences and dynamism of coastal environments?
- Waves
- Currents
- Tides
- Geology
- Human activity
- Types of ecosystems
What are the parts of a wave?
- Crest
- Trough
What is wave length?
Distance from crest to crest or trough to trough
What is wave height?
Vertical distance between crest and trough
What is wave frequency?
The number of crests or troughs that pass a fixed point
What are the factors affecting waves?
- Wind velocity
- Wing duration
- Fetch- wind distance
What determines wave energy?
- Wave steepness
- Wave period (time taken to travel through one wave length)
Higher energy= steep+short
What shape do waves have in shallow water?
Elliptical shape
What shape do waves have in deep water?
Circular shape
What is the wave length in shallow water?
Short
Wave is the wave height in shallow water?
High
What is the wave length in deep water?
Long
What is the wave height in deep water?
Low
What is swash?
When waves break on the beach, water rushes up, carrying sediments up the shore. Loses energy due to gravity and friction
What is backwash?
When waves return to the sea, due to the pull of gravity it carries sediments back towards the sea. Loses energy due to friction
What are the types of waves?
- Constructive waves
- Destructive waves
What are constructive wave characteristics?
- Calm weather
- Gentle slope, sheltered coast
- Long wave length and low wave height
- Low wave frequency (6-8)
- Swash>backwash
- Deposition is dominant process
- Low energy environment
- Spilling breakers
- Produce gentle slope with fine sediments
What are destructive wave characteristics?
- Windy, stormy weather
- Steep slope and exposed coast
- Short wave length and high wave height
- High wave frequency (12-14)
- Backwash>swash
- Erosion is dominant process
- High energy environment
- Plunging/surging breakers
- Produce steep slope with coarse sediments
What is wave refraction?
- Process in which waves change direction (bend) when they approach an uneven coast
- Due to friction with the seabed causing a change in wave speed
- Waves converge on headlands and diverge at bays
- At headlands, increased wave height and greater erosive energy
- At bays, decreased wave height and lower erosive energy
- Uneven impact on shoreline
What are the different forms of erosion?
HAAS
1. Hydraulic action
2. Abrasion (corrasion)
3. Attrition
4. Solution (corrosion)
What is hydraulic action?
- Waves strike against rock surface
- Trap air in joints
- Air in compressed, exerts pressure on the joints
- Rocks shatter
What is abrasion (corrasion)?
- Waves break and sediments carried are hurled against the rock surfafces and coast
- Knock and scrape against coast and rock surfaces
- Repeated hurling action and scraping weaken the surfaces
- Surface breaks down
- Cliff is undercut (mostly by abrasion)
What is attrition?
- Waves carry rock particles
- Hit against each other
- Repeated contact break down and smoothen the surfaces of these rock particles
What is solution (corrosion)?
- Seawater reacts chemically with water soluble minerals in the coast and rock surfaces
- Dissolve them
- Rock surfaces weaken and disintegrate
- Eg. Limestone dissolved by carbonic acid in seawater
What are longshore currents?
- Waves approach coastline at an oblique angle
- Due to direction of prevailing wind
- Create longshore currents which are currents that flow parallel to the coast
What is beach drift?
- As waves break, swash carries sediments up the beach at the same angle as the wave approached
- Then brought perpendicularly down the shore through backwash due to gravity
- Zig-zag movement of sediments known as beach drift
What is longshore drift?
- A result of both longshore currents and beach drift
- Transport sediments along a coast for some distance
Difference between a sheltered and exposed coast in terms of sediments
Sheltered coast: Constructive waves–> Strong swash carry more sediments up the beach–> finer materials further inland, coarser deposited nearer the sea
Exposed coast: Destructive waves–> Strong backwash carries more sediments down the beach–> finer materials nearer the sea, coarser further inland
How does sediment size affect deposition?
Coarser ones are deposited first as when wave energy decreases, waves are unable to carrt the load anymore
What are the 3 types of erosional landforms?
- Cliffs and wave-cut platforms
- Headlands and Bays
- Caves, arches, stacks and stumps
What are the 2 types of depositional landforms?
- Beaches
- Spits and Tombolos
Describe the formation of cliffs and wave-cut platforms
- Hydraulic action and abrasion erode cracks on the rock surfaces, gradually enlargening the cracks to form a notch
- The notch is further eroded and deepened to produce a cave
- As the process continues, the roof of the cave collapses due to a lack of support
- Over time, a overhanging cliff forms which will also collapse due to lack of support
- The materials deposited at the foot of the cliff may be carried by waves and thrown against the base of the cliff to cause more erosion
- The cliff retreats inland and a gently sloping wave-cut platform appears at the base where the cliff used to be and is submerged at high tide.
Describe the formation of headlands and bays
- Coastlines with alternate bands of more resistant and less resistant rock layers arranged at right angles to the coast have different rates of erosion
- Destructive waves erode the less resistant rock more quickly than the more resistant rocks to form bays (wide, indented coasts)
- While the harder rocks are more resistant and are left protruding into the sea forming headlands
Describe the formation of sea stumps (and thus caves, arches and stacks)
- Within headlands, some rocks may be less resistant to erosion and thus are eroded more quickly by hydraulic action and abrasion
- Waves enlarge lines of weakness at the base of the headland to form a notch
- The continous action of erosion at the base and undercutting forms a cave when the headland is hollowed
- Wave refraction causes two caves at the opposite sides of the headland to be aligned and forms an arch (a wave eroded passage)
- Over time, the roof of the arch becomes unstable due to further erosion, gravity and weathering, causing its collapse to form a stack (a pillar of rock in the sea left behind when an arch collapses)
- Continued wave erosion and weathering of the stack will break the rock down to form a stump- smaller rock
Define a beach
Zone of sediment deposition, formed by loose sand, gravel, broken shells and corals
Why do the composition and size of beach materials vary?
- Composition depends on the source of materials (eg. eroded cliffs, wave sediments)
- Size depends on wave energy and source of materials
Why does the shape of a beach vary?
Depends on coastal processes
1. Constructive waves
2. Destructive waves
Why does the slope of the beach vary?
Depends on grain size
1. Finer grains- gentle gradient
2. Coarse grains- steeper gradient
Why do beaches form in bays?
- Due to wave refraction, waves approach the shallow sea in front of the headland first before they reach the adjacent bays
- As wave energy tends to concentrate at the headland, erosion occurs there
- Along the bays, waves are diverged, thus energy is spread out and weakened
- Deposition occurs along the bays and over time, forming a sandy beach
Describe the formation of spits and tombolos
- Spits are long, narrow ridges of sediments with one end attached to the land
- Formed by longshore drift
- When there is an abrupt bend in the coastline, longshore drift continues to transport sediments in the same direction rather than following the coastline. This transports the sediments out to sea
- As the strength of the drift weakens away from the coast, sediments accumulate and rise above the water surface forming a spit
- When the spit connects an offshore island to the mainland, a tombolo is formed
