8.2 Characteristics and formation of coastal landforms Flashcards
formation of wave-cut platform
- hydraulic action/abrasion erode the base of the cliff, creating a wave-cut notch
- as the wave-cut notch grows deeper, it undercuts the cliff, leaving the rock above unsupported
- eventually, the overhanging section of the cliff becomes unstable and collapses under gravity
- the eroded material is removed by wave action, leaving a flat, gently sloping wave-cut platform
Headland and bay formation
- on discordant coastline, e.g swanage bay
- different bands of different resistant rock, less resistant rock quicker toe rode, whereas more resistant rock slower to erode
- results in headland
- wave refraction around headland, means erosion heading in all directions around that headland
- likely to get arches, stacks and stumps as a result of wave refraction
Succession of a headland
- wave refraction concentrates the erosion around the headland
- large crack, opened up by hydraulic action
- crack grows into a cave by hydraulic action/abrasion
- cave become larger and breaks through to form an arch
- arch eroded/collapses leaving stack and then finally form a stump
- only on sedimentary rock (rock type important feature (sedimentary))
blowhole
- sedimentary rock with large joints
- sea exploits cracks
- sea fills the hole in rock, exerting huge amounts of air pressure
- causing bits of the rock to fall away, results in a channel up to the top of the rock
- during a very high tide, water may come out of the channel
different kinds of beaches
swash-aligned:
- wave crests parallel to coast, so limited longshore drift
- sediment doesn’t travel far along beach, wave refraction may reduce speed of high energy waves, leading to formation of shingle beach with larger sediment
- drift-aligned
- waves approach at a significant angle, so longshore drift causes the sediment to travel far along the beach, which may lead to the formation of a spit at the end of a beach
- generally larger sediment is found at start of beach and weathered sediment moves further down the beach through longshore drift, becoming smaller as it does, so the end of beach is likely to contain smaller sediment
role of sediment in formation of the beach
Sandy beach:
- small sediment size
- lower infiltration rates = greater backwash
- material removed, so flatter gradient
- waves dissipate over a wider area - less focused erosion
Pebble beach:
- large sediment size
- greater infiltration rates = less backwash
- material built up, so steeper gradient
what are neap and spring tides
- neap to spring - flat beach (tide coming in)
- spring to neap - berms (tide going out)
different zones of the beach
back shore zone
foreshore
nearshore zone
offshore zone
whats backshore zone?
- consists of storm beach made up of fairly large sediment
- breaking wave has enough energy to throw pebbles up the beach but backwash too weak to pull back down the beach
- berms formed fortnightly by spring tide and two/three slightly lower high tides that follow
describe foreshore
- intertidal/surf zone
- often made of sand
- low ridges running parallel to coastline formed by constructive waves at different stages of the tide and ridges are separated by runnels
- flatter areas of beach small-scale sand ripples can form as backwash runs back into sea
describe nearshore zone
(breaker zone)
- coastal area that extends from the** low tide mark to where waves begin to break**
- on beaches with low tidal range, often form at point that waves break and known as breakpoint bars
describe offshore zone
- always underwater
- longshore bars also found in this area, produced when the circular movement of water in the waves start to touch the sea bed
- friction slows the wave, changes in motion of water within the wave can sweep up sediment to form these bars
- in places with very gently sloping beaches and low tidal range, bars can become substantial features and contribute to formation of barrier beaches/islands
- coastal area between the mean low tide mark and the point where waves start to interact with the seabed and break.
characteristics of a constructive wave
- long wavelength so low frequency
- gently sloping wave front and low wave height
- water spreads a long way up the gently sloping beach
- strong swash and weak backwash
characteristics of a destructive wave
- short wave length so high frequency (10-14 waves per minute)
- steep wave front
- breaking wave gains much height and so wave plunges onto steep beach, energy directed downwards so does not travel far up the beach
- weak swash and very strong backwash
compare summer/winter beach
Summer beach: constructive
- constructive waves move material up the beach as have stronger swash than backwash
- helps to create berms
- increase gradient of the beach (through the addition of material)
winter beach: destructive
- destructive waves comb material down the beach, so reducing gradient of the beach
- remove material creates offshore bars
formation of a spit
- prevailing wind comes from the south west
- material is carried eastwards by longshore drift
- where the coastline changes direction, sand and shingle are deposited in the water sheltered by the headland
- material builds upwards and outwards, forming a spit
- occasionally, strong winds blow from a different direction
- waves approach the land from the south east, causing the end of the spit curve round
- when the wind returns to its usual direction, the spit will continue to grow eastwards, developing further hooked ends during times of changed wind direction
- the spit cannot grow across the estuary due to the speed of the river carrying material out to sea
- spits become permanent when the prevailing wind picks up sand from the beach and blows it inland to form dunes
- a salt marsh develops in the sheltered water behind the spit
difference between simple and compound spits
Simple Spit:
- straight or slightly curved accumulation of sand or shingle extending from the coastline into the sea.
- Lacks complex features.
Example: Spurn Head, England.
Compound Spit:
- spit with a series of ridges or recurved ends formed by changes in wind direction or wave energy over time.
- May show evidence of multiple phases of growth and reworking.
- Often larger and more complex than simple spits.
Example: Hurst Castle Spit, England.
Key Difference:
Compound spits are** more intricate, showing multiple growth stages,** while simple spits are basic, single structures.
Tombolos
- formed by longshore drift
- after LSD carries sediments across a gap between the mainland and an island, forming a narrow long ridge of sand and pebbles
- where two beaches extend to an island they create a cuspate tombolo - wave refraction causes LSD to operate in opposite directions, altering the pattern of deposition
- e.g chesil beach connects the isle of portland
Offshore bar formation
- long ridge of sand/pebbles found short distance out to sea
- forms in shallow water, where destructive waves break before reaching the beach
- waves scour the seabed and throw material forward into a heap, currents in sea will also supply sediments for this
- sometimes exposed at low tides but when submerged at high tide can be a hazard to ships
- these shallow areas used for wind farms, e.g Scroby sands, also sometimes used for beach nourishment or dredged for building material
Bar formation
- form when a spit joins together two headlands
- bar particularly obvious at low tide when exposed
- can happen where there is no powerful river current to stop the spit growing all the way across the inlet
- lagoon forms between bar and original coastline
Barrier beach formation
- long sandy beaches detached from but parallel to the coastline
- form where there is a plentiful supply of sand, shallow nearshore and offshore areas, waves and winds with enough energy to move sand and a rising sea level so that waves move sand towards the shore
- also where the tidal range is low and where the sea bed and the adjacent coastline are very gentle sloping with no cliffs
conditions for growth of sand dunes
- wide, flat, sandy beach (supply of dry sediment above the tide mark)
- strong onshore winds
- an obstacle - e.g a stone, shell
formation of sand dune
- sand bounces along, due to pressure differences through saltation and piles up in zone of no wind, as high levels of friction
- sand bounces over obstacle and builds up on sheltered side (in zone of no wind)
- when wind direction changes, builds up more on sheltered side
Different sand dunes along beach
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1. Embryo dune:
- most hostile envt
- pioneer species
- halophytic - salt tolerant
- mainly sand and some marram grass
- Fore dune:
- marram grass and sea rocket
- sand and plants - yellow dune:
- more humus
- 80% plant cover dominated by marram grass - Grey dune:
- more sheltered, less acidic dune heath, red fescue, gorse - Dune slack:
- water-loving plants; willow, moss, reeds - mature dune:
- trees: birch, aider, oak
saltmarsh succession
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1. Algal stage: gut weed and blue green algae establish as they can grow on bare mud, which their roots help to bind together
2. Pioneer Stage: cordgrass and glasswort grow, their roots begin to stabilise the mud allowing the estuarine to grow
3. Establishment stage - saltmarsh-grass and sea asters grow, creating a carpet of vegetation and so the height of the salt marsh increases
4. Stabilisation - sea thrift, scurvy grass and sea-lavender grow and so salt rarely ever gets submerged beneath the marsh
5. Climax vegetation - Rush, Sedge and red fescue grass grow since the salt marsh is only submerged once or twice a year
zones of a saltmarsh
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- influence of tide decreased further inland (period of inundation reduces)
-
mudflats:
- inter tidal - only a few hours a day exposed to air
- the only plants are algae growing on the mud
- vegetation is halophytic (salt tolerant) because it can tolerate being submerged by the sea -
Lower marsh:
- the marsh is covered at each high tide and channels known as creeks are cut as the water receded
- typically characterised by spartina, salicornia -
Middle Marsh:
- as more mud is deposited, the first plants appear which help trap more mud and silt, building up the marsh
- salt pans can be related by impeded drainage
- water evaporates, making them extra saline -
upper marsh:
- level of the marsh is higher so period of inundation is reduced
- these can be tolerated by a variety of plants, so diversity increases -
Terrestrial vegetation:
- eventually land builds up until it is always above the level of the tide, so shrubs and woodland can grow
- vegetation is no longer affected by the sea, so not halophytic
Mangroves
- salt-tolerant trees and shrubs that grow in coastal intertidal zones
- stilt roots that anchor the plant into the mud
- slow down wave movement, encouraging further deposition
- halophytic and well adapted to live in salt water
- **prop roots **for stability
- pneumatophores: suck air down into root system, due to low oxygen levels in the mud, grow from roots vertically
- similar to salt marshes but develop salt tolerant forests and shrubs and cover 25% of tropical coastlines
tidal sedimentation in estuaries
- in tidal estuaries the incoming freshwater flow of a river will be opposed by both the mass of the seawater and the tide
- short period of time between the rising and falling tides when the flow comes to rest and turbulence of the water in the estuary is at a minimum
- gives heavier sediment particles a chance to settle at the bottom
- as tide current increases again, the particles are lifted in into the water column again
- but unlike river water which continues to flow through the estuary in the upper layer, the sediment particles are now in the lower layer of the estuarine circulation where the mean water movement is from the ocean towards the head of the estuary
- means large part of the sediment will therefore never make it to sea but accumulate in estuary
- sediment may eventually leave the estuary during flood events
when do saltmarshes form?
- low-energy wave environments which encourages the deposition of fine silts and clays
- large tidal range that produces strong tidal currents which can transport large quantities of silt and clay around
- large, constant supply of silts and clays - can come from coastal erosion of cliffs
what causes a salt marsh to form?
- when fine-sized clay material mixes with salts in seawater, chemical reaction causes clay particles to come together (flocculation) to form larger, heavier particles which fall to the sea bed
- the accumulated mud is then colonised with pioneer halophytic plants which can tolerate being covered during tide cycles where there is lack of oxygen/high pH value
- these plants trap sediment and lead to inter-tidal mudflats developing further
role of sea level change in formation of coastal landforms (eustatic and isostatic changes)
- eustatic changes occur when there is a change in the volume of water in the oceans
- affect sea level across whole planet and may be due to thermal expansion/contraction or changes in glacial processes
- isostatic changes occur when there is a movement of the land relative to the sea
- localised change
- often result of isostatic subsidence (glaciers weigh down the land beneath and so the land subsides)
- melting of glaciers after last ice age has led to isostatic recovery causing coastline to rebound and rise again in areas that were covered by ice
emergent and submergent coastlines and how they are affected by sea levels
Emergent:
- form due to fall in sea level
- where land has been raised in relation to the coastline
- arches, stacks and stumps may be preserved
- wave cut notches
Submergent:
- occur when sea levels rise or coastline sinks in relation to the sea
Rias: formed when rising sea levels flood narrow winding inlets and river valley
- Fjords: formed when rising sea levels flood deep glacial valleys to create natural inlets and harbours
- Dalmatian Coasts:
- occurs when valleys running parallel to the coast become flooded due to sea level change
- leaves series of narrow, long and rugged islands
Case study - Jurassic Coast
- durdle door (Arch) - Wave action has eroded through the resistant Portland limestone
- Lulworth Cove (bay and Headland): sea broke through a narrow band of resistant limestone and eroded the softer clays and sands behind.
- Old harrys rocks (stacks and stumps)
- Chesil Beach (Tombolo): shingle beach connecting the Isle of Portland to the mainland.
Spit example
examples
o Examples - Spurn point - 46m across, found at the coast of East Riding in Yorkshire- out into the sea for 5.5km across the Humber Estuary - caused by LSD in the North Sea.
