Coastal Landscapes and Change Flashcards
Define Littoral Zone
- dynamic area of change
- area of dynamic equilibrium
- different parts of the coast are undergoing constant short-term change
- coasts vary in rate of change
- variety creates different classifications of coastline
Parts of Littoral Zone
- coast : land effected by sea e.g settlements/farmland
- backshore : area above high tide level + only effected by extreme weather events e.g high tide
- foreshore : zone between high/low tide marks + where wave processes take place
- nearshore : just off coastline + beyond low tide + has shallow seawater
- offshore : where waves start to break + water is deeper
Long term changes creates coastlines that are …
- emerging/submerging depending on falling/rising sea-level
- rocky/estuarine depending on weathering/erosion resistance
- concordant/discordant depending on a geological strata/wave action relative position
Short term changes creates coastlines that are …
- classified by tidal range
- retreating / advancing depending on erosion/deposition
- high/low energy depending on balance of erosion/deposition
- land/marine based processes
Microtidal coast
<2m tide range
Mesotidal coast
2-4m tidal range
Macrotidal coast
> 4m tidal range
High energy coasts
- large/powerful destructive waves
- exposure to strong winds + long fetches + steeply shelving offshore zones
- erosion > deposition BUT coastline geology = more resistant to erosion
- rocky coasts + sandy coves + rocky landforms (cliffs/stacks/arches)
- clear distinction between land and sea
- cliffs w/ high erosion levels = steep + little vegetation
➕waves undercut cliff + wash away debris ∴ marine cliff profile - cliffs w/ high weathering = less steep as weathered material at base ∴ subaerial cliff profile
Low energy coasts
- small gentle constructive waves
- created by gentle winds + sheltered location + short fetches + gentle sloping offshore zones + may be protected by reef or offshore island
- deposition > erosion
- sandy beaches + salt marshes + estuaries + tidal mud flats + NO cliffs next to beach
- Land w/ gentle relief + low elevation ∴ harder to identify parts of littoral zone
- hit deposition levels due to stillness of water in estuaries + nearshore zone
- sediment comes from land + sea
- coastal accretion takes place (expansion or land into sea)
How waves are formed
1) wind blows over surface of water which creates waves
2) friction between wind + sea surface gives water circulation motion ad energy is transferred
3) ripples created which turn into larger waves over time
4) as waves approach shore , they break
5) friction w/ seabed slows bottom of waves , making motion fore elliptical
6) crest continues at same speed + rises up + collapses (wave breaking)
Conditions of waves
- fetch
- wind speed
- ?
Destructive waves
- high + steep
- circular cross profile
- short wave length ∴ high frequency = 10-14 per min
- strong backwash ∴ removes material from beach
Constructive Waves
- low frequency ∴ 6-8 per min
- low + long = ellliptical cross profile
- strong slash ∴ deposits material
Short term change to waves
- storm event = more destructive waves
- wind levels can change wave characteristics e.g high speeds = taller waves
Long term change to waves
- seasonal changes e.g more destructive waves in winter + waves create berm in summer
- climate change = more storm like conditions ∴ more destructive waves
Sediment cells
- divide coasts into separate sections which are lengths of coastline that are self contained units
- 11 cells in England+wales
- sediments don’t move between cells
Sources of Sediment Cells
- rivers carry eroded sediment from inland
- cliff face erosion + weathering
- accumulation of organism’s crushed shells
- movement from offshore deposits(sandbanks) by waves/tides/currents
- erosion of sand dunes (but only small)
Transfers of Sediment Cells
- LSD
- Offshore currents move sediment inland
- wind
- tidal currents move material in/out of coast daily
Sinks of Sediment Cells
- flocculations of sediment in salt marshes
- spit/bar formation
- sand dunes form by wind action
- offshore bars beneath waves
Define Sediment Budget
Amount of sediment entering + leaving system
Positive sediment budget
Sediment entering > sediment leaving = +ve sediment budget
∴growth
Negative sediment budget
Sediment leaving > sediment entering = -ve sediment budget
∴shrink
Dynamic equilibrium in sediment cells
Balance is maintained in a system w/ changing processes
E.g as waves erode beach , so does cliffs ∴ as material is taken away , material from cliffs is deposited so beach grows + system in balance
Positive feedback in coastal systems
As beach forms , friction causes waves to slow down + deposit. As beach grows bigger , even larger waves slowed down + more deposition continues
Coastal Landscapes of Discordant Coastlines
HEADLANDS and BAYS
- bands of alternating hard/soft rock perpendicular to shoreline
- once headland formed , it erodes quicker due to more exposure + wave refraction
- higher waves at headland as waves slow down , water behind crest builds up ∴ greater erosion power ∴ steeper cliff faces at headlands + arches/caves
- shape of bay dissipated wave energy = less height = less erosion
- over time difference between headlands and bays are less distinguishable
Coastal Landscapes of Concordant Coastlines
COVES
- bands of rock parallel to shoreline
- waves break through hard rock then easily erodes the exposed soft rock fast till they reach hard rock again which is harder to erode
Dalmatian Coastline
- valley lies parallel to coast + increase in sea level can cause Dalmatian coastline
- valleys formed by tectonic folding
- folding creates anticlines (slope down) + synclines (slope up)
- valleys flood as sea levels rise
Haff Coastline
- deposits of sand run parallel to to coastline on top of offshore bars
- lagoons from between bars and shoreline
Examples of Discrodant Coastlines
SWANAGE BAY:
- Isle of Purbeck , East Dorset
- formed from erosion of less resistant Wealden Clays
- faces east + sheltered from prevailing south westerly wind + highest energy waves.
Examples of Concordant Coastlines
SOUTH DORSET COAST
Strata
Layers of rock
Deformation of Strata
- Strata folding (bend/crumple) + dip (angle towards/away from sea)
- as strata fold/dip , diff amounts of pressure experienced
Joints
split rock into diff verticle blocks
Bedding Planes
split rock into diff horizontal planes
What does the profile of a cliff depend on
- resistance of the rock
- relative energy of a coastline
- position of rock strata in relation to coastline
Horizontal strata dip
- waves erode softer rock creating notches on cliff face
- if wave-cut notches = too large then cannot support rocks above ∴ rockfall
Strata dipping away from sea
- landward
- no undercutting ∴ stable cliff
strata dipping towards sea
- seaward
- gentle dipping = rocks breaking away in a wedge
- steep dipping = rocks mass movement
Coastal recession depending on bedrock lithology
- igneous rock = highly resistant (1-5mm yearly) as hard crystalline structure = few joints
- metamorphic rock = some resistance (1-10mm yearly) as crystalline structure aligned in one direction = more prone to folding + faulting
- sedimentary rock = limited resistance (2-6cm yearly) as made of clastic rock which has many fractures + porous = permeable + vulnerable to erosion
Clastic Rock
rock made form fragments of other rock
Coastal recession depending on lithological structure
- ROCK TYPE ORDER IN STRATA : if less resistant rock at bottom then more undercuttung = more cliff collapse = accelerating coastal recession OR multiple folds = more cracks + fractures = easier to exploit
- ROCK PERMEABILITY : impermeable rocks = more surface runoff , permeable rock results in pore water pressure = weaker rock structure = enlargerning cracks/joints or permeable rocks results in more weight taken in by rainwater ∴ slumping / landslips / slip planes
- ROCK REACTIVITY : some rocks contain compound that chemically react w/ seawater ∴ more vulnerable to erosion
Mechanical Weathering
- breaking rock material without changing chemical composition
- freeze-thaw weathering
- saline water causes salt weathering as water enters cracks at high tide , crystals from as rocks dry when tide leaves , expanding crack
- clay rocks expand when wet
Chemical weathering
- breakdown of rock by changing its chemical composition
- carbonation is where CO2 dissolves into rainwater to produce weak carbonic acid which reacts w/ calcium carbonate to slowly dissolve rock
- Oxidation is when rocks containing iron react w/ oxygen in air and form iron oxide that makes rocks crumble easily
Biological Weathering
- weathering through actions of plants + animals
- organisms may use cracks in rocks for burrowing/habitats which break apart rock
- seeds may fall into cracks + plants grow ∴
breaking rock open
Mass Movement on Coasts
- material moves downhill due to gravity
- can occur when wave action undercuts cliffs and unsupporting overhand collapses
- landslides + rockfall + rotational slumping
- more likely for unconsolidated rocks as little friction between particles
- heavy rain saturates rocks ∴ reducing friction
Landslides
- material moving in a straight line down a slope
- quick
- occur on steep slopes after heavy rainfall / storm conditions
- rainfall reduces friction ∴ creating slip planes
- landslide scar left behind (area of unvegetated rock)
Rotational Slumping
- materials sliding down slope at curved angles
- material retains shape whilst sliding
- takes long period of time
- slip plane created when permeable soft rock lies ontop of impermeable hard rock
Rockfall
- broken blocks moving down slope
- more common on steep cliffs w/ many joints + bedding planes
- rock topple results in regolith (material at base of cliff)
Weathering
- weakens cliff face ∴ more vulnerbale to wave action
- provides scree for waves to use as erosive tool (abrasion)
Mass Movement
- adds rock material to beaches
- strengthens beaches by increasing size ∴ waves have more friction = less energy = less erosion
Erosive Processes
- hydraulic action = waves compress air into cliff face cracks = more pressure = rocks break off
- abrasion = rock + sediment transported scrapes against cliffs = breaks sections of cliffs away + smoothens surface
- attrition = rocks smash against each other into smaller/rounder pieces
- corrosion = seawater dissolves soluble rocks (limestone/chalk)
Formation of a Wave Cut Platform
- wave erosion causes notch to form at high water mark
- further erosion causes notch to get bigger
- eventually forms cave
- rock above cave = unstable ∴ collapses
- new notch forms etc , leaving behind a wave cut platform
Wave refraction
as wave approaches headland ,it enters shallow water + slows down due to fricion. waves far away are faster due to deep water ∴ refract
Wave refraction landmarks
- caves on opposite sides meet to form arch
- sea spray erode top of arch + create blowhole
- further erosion of blowhole makes arch collapse = stack
- exposure of stack makes it smaller
- undercut stack = stump
Factors for Rate of Erosion
- WAVE TYPE = destructive waves = more energy = strong backwash = more erosion
- WAVE SIZE = larger waves = more energy = quicker erosion
- LITHOLOGY = soluble rocks = softer + less resistant AND defined strata = more weaknesses
Factors for Rate of Recession
- TIDES : high tide = more area to erode AND frequency of spring/neap tides by climate change
- WIND DIRECTION AND FETCH : long fetch = stronger waves AND winds from SW = stronger as fetch = 6500km long
- WEATHER SYSTEMS + SEASONS + STORMS : low pressure = strong winds + more common in winter AND High pressure more common in summer + weak winds AND depression = low pressure AND climate change = more storms
Human actions influencing coastal recession
- building a dam interferes w/ sediment movement + less sediment for LSD to move ∴ quicker erosion further along coastline
- coastal defences e.g groynes can retain sediment ∴ preventing LSD + more erosion further along
- sand/shingle dredged from offshore bars = more erosion as sand removed from cell permanently
Economic Loss of Coastal Recession
- low lying coastlines = densely populated
- coastal settlements have long term economic problems as lack of investment
- land has different values e.g agricultural land is £23,000 per hectre wheres residential land in Dorset is £3.8m
- heavy industry + essential services(power plants) not located near receding coasts as expensive to build
- small businesses on receding coastline are abandones = less jobs + services
- selling coastal property = hard + impossible to get insurance + property quickly loses value w/ a knock on effect of devaluing whole village
- infrastructure on receding coasts = hard to reroute ∴ high economic burden on local govt
Social loss of coastal Recession
- isolation or disconnectedness as people + services leaving
- amenity value lost as coasts recede + areas abandoned = visually unattractive
- spiral of decline as people leave = service close down = loss of jobs = more people leave etc
- rarely cliff collapse = fatalities
Factors influencing sediment transport
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