coasts Flashcards
coastal systems
coast- a narrow zone where the land and sea overlap —> it is a dynamic environment (constantly changing)
changes on coast can happen over different time scales:
- hours - tides
- days - weather —> influence type of wave
- weeks/months - erosion/transport —> changes profile of beach
- years - development of landforms
- millennia (thousands of years) - sea level change
coastal system:
- open system
inputs: energy and sediment coming into coastal system
- energy inputs come from waves, wind, tides and currents
- sediment
stores:
- erosional landforms e.g. cliffs
- depositional landforms e.g. spits, beaches, dunes
outputs:
- dissipation of wave energy —> when a wave crashes onto a beach, the energy is transferred from the sea to the land
- accumulation of sediment above the tidal limit (sea can’t reach sediment anymore)
- sediment removed beyond local sediment cells
flows/transfers: move sediment from one store to the next
- erosion, deposition, transportation, weathering, mass movement
dynamic equilibrium and example of positive and negative feedback
- Coasts are open systems and are generally in dynamic equilibrium because the inputs and outputs are balanced
Example of negative feedback:
- A beach in dynamic equilibrium —> sediment is eroded from the beach during a storm by destructive waves —> sediment is deposited offshore (forming an offshore bar) —> waves hit offshore bar so waves lose energy —> this reduces erosion of beach —> constructive waves return and redistribute sediment from offshore bar back onto the beach —> dynamic equilibrium
Example of positive feedback:
- Vegetation on a sand dune is trampled on by tourists —> sand becomes exposed —> sand is blown away by the wind —> vegetation struggles to regrow and hold the dunes together —> sand becomes exposed —> sand is blown away by the wind
offshore, inshore, foreshore, backshore, nearshore
offshore:
- movement of water is not touching seabed
inshore:
- waves interact with seabed
- area between the point where waves meet seabed and the low water mark
foreshore:
- area between high water mark and low water mark —> bit between where water gets to at high tide and low tide
backshore:
- area above high water mark —> waves don’t normally reach backshore area but changes can still happen here e.g. wind can create sand dunes
nearshore
- area between the high water mark and where waves begin to break
(picture of this in notes)
sediment cells
sediment cell- a stretch of coastline within which the processes of erosion, transportation and deposition operate and the movement of sediment is largely self contained (sediment is moved around in the cell however sediment doesn’t really move around from one cell to the next) —> closed system
- sediment cells are often separated from each other by boundaries such as headlands and stretches of deep water
- larger cells are divided into smaller sub cells
sources of sediment:
- weathering and mass movement —> adds material into coastline
- river and estuaries —> rivers bring fine sediment into coastal zone
(in some sediment cells, as much as 90% of input of sediment into coastal zone is from rivers)
- offshore currents e.g. waves and tides etc —> bring material in with them
- marine organisms (corals and shells)
- cliff erosion
- longshore currents move material from one sub cell to the next
littoral currents also redistribute sediment within the cell/sub cell —> they will add or remove sediment from sinks
- sink —> anywhere where sediment is stored for a period of time e.g. beaches, spits, bars, tombolos etc
- sources (similar to input of sediment) —> sediment is gained from eroding sections of the coastline
(example in notes)
sediment budgets
sediment budgets
- balance between inputs and outputs
inputs > outputs —> there’s a positive sediment budget and a surplus of sediment —> coast begins to grow
outputs > inputs —> there’s a negative sediment budget and a deficit of sediment —> coast begins to recede
coastal processes
coastal processes:
- sub-aerial processes (operate on land)
- marine processes (operate in the sea)
- aeolian processes (driven by the wind)
marine processes (erosion)
Marine processes (operate in the sea) - erosion
- Hydraulic action - power of waves hits the cliff face and loosens the interior of joints and bedding planes
- Cavitation- As waves smashes into a crevice, it forces air and water into the crevice which compresses air bubbles —> cause air bubbles to fizz —> this builds up pressure —> as wave recedes the pressure is released which creates a mini explosion of air and water out of crack —> loosens rock
- Wave quarrying - energy of wave is enough to detach bits of rock
- 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
- 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 —> eventually turns rocks into sand grains
Weathering
- Sub-aerial processes (operate on land) - weathering
- Weathering- breakdown of rocks at the earths surface, in situ (without the rocks going anywhere) —> this is different to erosion as erosion removes the material as well as making the rock weaker
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
2. When the sun shines, the water evaporates —> salt crystals are left behind in the cracks
3. Salt crystals grow over time, exerting pressure on the rock —> causes fragments of rock to break off
Wetting/drying (physical/mechanical) - certain rocks absorb water e.g. rocks with high clay content —> absorption of water makes rocks expand and drying out of rocks makes them contract —> rocks become weaker
Mechanical:
Freeze thaw weathering:
1. Water enters cracks in rocks
2. When temperatures drop, the water freezes and expands causing 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 break down
Chemical weathering:
- Carbonation - carbon dioxide in the atmosphere dissolves in rainwater —> makes it a weak carbonic acid —> carbonic acid reacts with limestone —> rocks are gradually dissolved
mass movement
- Sub-aerial processes (operate on land) - mass movement
- Mass movement- movement of loose material down a slope due to gravity
- Mass movement is an important input of sediment into the coastal system
Type of mass movement depends on:
- Type of material (e.g. consolidated rock or loose soil)
- Angle of the slope (gentle or vertical)
- Level of saturation of rocks
(Consolidated rock)
Rock fall:
- Loose fragments of rock break off a cliff face and fall onto the beach below
- Creates large fragments at the base of the cliff
Landslide:
- Rocks break off and slide down the cliff face
- Occurs when the bedding planes dip towards the sea
(Unconsolidated rock)
Mudflow:
- Saturated soil and rock flows down a cliff face
- Happens after heavy rainfall on unconsolidated cliffs
Rotational slip (slump):
- Saturated soil and rocks slides down a cliff face with a rotational (curved) movement
- Creates a stepped profile in the cliff
aeolian processes (driven by the wind)
- surface creep —> larger particles of sand may be dragged along the beach (similar to traction)
- saltation —> smaller grains of sand may bounce along the beach
- suspension —> even smaller particles might be carried through the air
factors affecting erosion:
wave strength:
- controlled by fetch and wind strength e.g. long fetches and stronger winds create bigger and powerful waves —> more erosion
bathymetry:
- underwater topography of the seabed impacts the strength of waves
- gently sloping sea bed —> as waves enter inshore zone, it experiences friction with the sea bed and slows 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
beaches:
- beaches increase the distance a wave travels before it reaches the cliffs —> beaches act as shock absorber and absorbs some wave energy before it reaches cliffs —> waves energy is reduced —> less erosion
- headlands refract waves —> causes erosive power to be directed at the headland so waves in bay have less power —> less erosion in bays
weathering:
- weathering creates weaknesses in rocks which can be further exploited by the processes of erosion
- weathering rates are higher —> rates of erosion will be faster
human activity:
- dredging (removing material from sea bed) —> material dissipates wave energy —> if material is removed then more wave energy reaches the shore —> erosion increases
- coastal management can reduce rates of erosion in one location but increase them down coast
erosional landforms
coves:
- formed on concordant coastlines
- more resistant outer band rock is eventually breached (waves will find a way through resistant rock)
- erosion speeds up when waves reach the less resistant bands of rock —> it spreads out laterally
- once harder rock is reached again, erosion slows down
headlands and bays:
1. alternating bands of hard and soft rock at right angles to the coast (discordant coastline)
2. soft rock is eroded quickly, forming a bay
3. hard rock is eroded more slowly and forms a headland which sticks out
-
beach develops in a bay —> headlands are sheltering bay and offering protection from high energy waves (can be due to wave refraction) —> less erosion
- eroded soft rock from headlands and bays —> eroded rock forms sediment that becomes apart of the beach - wave refraction —> concentrates wave energy on the headland —> waves in the bay are less powerful —> constructive waves in bay —> encourages deposition in the bay, further developing the beach
wave cut platform:
- sea attacks base of cliff forming a wave cut notch
- sub-aerial processes e.g. freeze thaw weathering or carbonation will weaken rocks at the top of the cliff
- eventually wave cut notch will increase in size until the cliff can’t be supported —> it will collapse through mass movement
- material that has fallen off the cliff will be broken down by attrition and also smooth the wave cut platform through abrasion
- the process repeats and the cliff continues to retreat
(the larger the wave cut platform becomes, the slower the rate of erosion tends to be —> wave cut platform absorbs some wave energy before it hits the base of the cliff —> waves experience friction with wave cut platform —> waves lose energy)
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
(wave refraction causes wave energy to be concentrated on the side of the headland)
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
blowhole:
- waves approach the bottom of the headland where there’s a crack —> they compress water into cracks and force it upwards —> water spurts out the top, forming a blowhole
beaches
beaches:
- beaches are accumulations of sediment —> act as an important shock absorber in front of cliffs
- the shape and characteristics of a beach are influenced by the type of rocks and sediments in the area
- waves, tides, and currents constantly shape the beach by eroding and depositing sand
- human activity like construction can also change the beach’s shape
storm beach- found at the back of the beach —> composed of the largest sediments thrown by waves above the usual high water mark (during storms, waves have high energy and can throw material right to the back of the beach)
- more prominent on shingle beaches
berms- a series of ridges marking the successively high tides as the cycle moves from springs to neaps (highest spring tide and highest neap tide)
- they’re built by constructive waves (strong swash so pushing material up the beach which builds berms)
- more prominent on shingle beaches
cusps- semi-circular
- waves approach beach —> as waves approach the horn of the cusp (picture headland and bay —> looks like bay), the wave energy is deflected in 2 directions —> swash of waves will meet —> as they return back to the sea, a powerful backwash is created —> removes material from embayment
ripples- very small ripples in the sand caused by the movement of waves and tides over the sand —> most commonly found on the foreshore between HWM and LWM
- very short lived —> when next tide comes in, it washes over that part of the beach and ripples may change shape or disappear
ridges and runnels- drainage routes for the tide
- as tide comes in, it floods onto the beach and makes little channels that washes water onto the beach
- as tide goes out, water flows back down the channels and flows back out to sea
- ridges are the raised section next to the runnels
depositional landforms
spits- a finger of beach material extending out to sea
compound spits- several hooks
simple spit- one curved end
- longshore drift moves material along the coastline
- when the coastline changes direction e.g. at a headland, longshore drift doesn’t change direction
- sediment builds out to sea —> this creates a spit
- a change in wind direction will cause the spit to curve at the end —> recurved end
- behind a spit, the 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
bars (barrier beaches):
1. a bar forms when a spit joins 2 headlands together
2. lagoon forms behind the bar
tombolos:
- if a spit joins up to an island, it creates a tombolo e.g. angel road of shodo island, japan
offshore bars:
- destructive waves carry sediment from the beach and deposit it offshore —> covered at high tide and exposed at low tide
barrier islands:
- similar to offshore bars expect they are permanently above the level of water
- more deposition on offshore bar —> waves will hit offshore bar which will slow waves down —> waves lose energy and deposit material —> eventually the bar will poke out of the water —> vegetation might colonise this area of sand —> traps even more sediment —> eventually barrier island is formed
sand dunes (depositional landforms)
vegetation succession: evolution of plant communities at a site over time- from pioneer species to climax community
psammosere: name for the succession of vegetation which takes place on a sand dune
pioneer species: first plants to colonise an area of bare ground e.g. marram grass
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
what’s needed for dune formation?
- large, flat beach
- large tidal range (large amount of sand exposed at low tide)
- plentiful supply of sand
- onshore wind (wind blowing from the sea onto the shore)
- an obstacle (to give sand something to build up around)
- vegetation
(large flat beach and large tidal range allows beach to dry out)
psamosere succession:
stage 1: strandline/obstacle
- there’s lots of obstacles along the strandline e.g. seaweed and driftwood act as an obstacle and sediment begins to build up
stage 2: embryo dunes
- overtime, an embryo dune develops which may become vegetated by pioneer species such as marram grass
- marram grass is adapted to the dry, salty, windy conditions
- vegetation stabilises the dunes in 2 ways —> roots bind the sand together// stems of marram grass help slow the speed of wind —> wind loses energy —> causes sediment to be deposited
stage 3: yellow dunes
- eventually several embryo dunes will coalesce (join together) to create foredunes and yellow dunes —> this is the tallest of the dune succession
stage 4: grey dunes/slacks
- grey dunes are fixed dunes which have been fully colonised by vegetation and may eventually develop into a climax community of heathland or woodland
- marram grass will die and decompose which adds organic matter back into the soil —> this increases amount of nutrients in the sand and also helps to retain moisture —> allows vegetation to change overtime
- depending on the height of the water table, areas between dunes (slacks) may be damp or even contain standing water
things that change as we go inland (dunes):
- dunes inland are older
- soil colour changes from yellow —> grey —> brown
- soil pH goes from alkaline (high percentage of calcium carbonate near the sea from shells etc) to acidic
- percentage of humus (organic matter) increases
- more vegetation cover as we go inland (becomes larger and more complex)
- slacks become more pronounced as we go inland —> ones towards back become filled with water
- mobile and fixed dunes —> ones further inland are less likely to be changing shape and more likely to be fixed
