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How are coastal landforms developed? - Coggle Diagram
How are coastal landforms developed?
Depositional features
Solution
- mineral dissolved in water
Suspension
- sand & fibre minerals are kept moving by
Saltation
- skipping movement of sand grains along sea bed, follow an arc trajectory. when grains land they move others
Traction
- large particles roll & hop on the sea floor
Sorting
is the selective grading of materials into different shapes and sizes, occurs during transport.
May result from
wave energy, higher wave energy can transport larger load further, lower energy selectively removed finer materials leaving coarse materials behind (called winnowing)
progressive reworking by waves, increases sorting
attrition, during sediment transport reduced pebble size, changes overall shape & increases roundness so they are nearly all the same size, shape & roundness - well sorted
at peak energy material is used for abrasion
Wind energy
wind energy is similar to water but has an upper energy limit, it an only transport from clay to medium sand size
Seasonal beach profiles
summer beach
weak backwash, swash>backwash (adding material on)
low energy
constructive
dune crest
berm
steeper beach profile
winter beach
backwash>swash
high energy
destructive
some dune erosion
berm erosion
bar formation
less steep beach profile
Low wave energy
in low energy conditions (fair weather) profiles are steep (c.11 degrees) & reflect wave energy back to sea
waves travel up the beach as swash & return as backwash BUT if there is a long term between waves the backwash will return before the next swash & does not interfere with the next wave. thus transport sediment up the beach: constructive wave action
therefore, summer beach profiles are steep, with a ride as the back called a berm
High wave energy
profiles are gentler (c. 5 degrees)
incline dissipates wave energy (feedback)
waves are closer together & the swash is interrupted by return of the backwash so cannot transport sediment up the beach
sediment is removed from the beach by destructive wave action
storm or winter profile
shallow beaches have shore-parallel ridges called runnels or gullies, cut by small rip currents
Negative feedback on beaches
A beach is in a state of dynamic equilibrium, when a storm removes material from a beach negative feedback returns the beach back to its former state
1) a beach in dynamic equilibrium
2) sediment is eroded from the beach during a storm
3) steep coastal profile is generated by high energy destructive waves
4) sediment is deposited offshore forming a bar
5) waves forced to break on the bar before reaching the beach dissipating their energy erosion when they reach the beach
6) when the storm calms normal wave conditions rework sediment from offshore bar back to the beach
7) lower energy constructive waves push sediment up the beach building a shallower beach profile
8) back to number 1
Lower beach = tidal imposed features, shallower gradients (1-2 degrees), finer materials of sand & mud.
Upper beach = wave imposed features, steeper angles (10-20 degrees), coarser materials
Importance of rivers & tides
1) fluvial systems input clay as sediment soruce
2) marine salt water provides salt particles
3) in the mixing zone, flocculation occurs causing deposition
4) as tidal range increases greater energy leads to increased creek bank erosion & reworking of sediment, deposited as tides reverse & energy drops
5) vegetation reduces tidal energy and vegetation traps sediment leading to accumulation and salt marsh growth
Depositional landforms
Factors
shape of coastline
beach aspect
offshore obstacles
beach gradient
Major landforms on central North Landscape
beaches (sandy & shingle)
sand dunes
shingle ridges
spits & barrier islands
estuaries & mudflats
salt marshes
Beaches
Factors that make beaches different
sediment type (geology)
wave energy
wave orientation (swash or drift aligned)
tidal energy
sediment supply
time (short, medium, long term
Comprised of
rivers
wind blown materials
longshore drift
onshore deposition
humans
cliff weathering & mass movements
Beaches are:
1) lithogenic (lumps of rocks)
2) biogenic (biologically made e.g. shells, corals)
3) mineralogenic (particular mineral)
Different beach profiles
sediment size
wave energy
tidal energy
time (seasonal changes)
Low energy -> high energy
sandy, shingle, storm
Down the beach
grain size decreases
roundness increases
sorting increases
Gradient of natural beaches rely on:
wave energy, constructive & destructive
particle size
Offshore -> nearshore -> foreshore -> backshore
destructive waves carry material down beach
constructive waves carry material up beach
material carried upwards on shingle beaches
material carried downwards on sandy beaches
Shingle beaches
larger gradient
smaller shingle near waves, largest further (different berms, height of different seasonal tides)
Sand beaches
gentle gradient
small particle size, compact when wet, restricted rate of percolation (most swash returns as backwash due to storage of water in pore spaces)
little energy lost by friction
longshore bar at low tide mark (waves break further from shore, wider beach)
Extreme narrowing of estuaries can concentrate the tidal rise so rapidly that an advancing wall of water, or tidal bore, may travel upriver
Beach formation
most common landform of deposition
accumulation of material deposited between tides & highest storm waves
beach material comes from cliff erosion (5%), offshore (5%), rivers (up to 90%)
shingle or sand
Beach landforms
Berms
smaller ridges that develop at position of mean high tide, deposition at top of swash (formed by wave action). determines beach width, high energy waves & high tidal ranges are wide. Location dependant on seasonality & tidal cycles.
Storm beach / ridge
storm waves hurl pebbles and cobbles to back of beach
Beach cusps
small, semi-circular depression
temporary features formed by a collection of waves reaching the same point and when swash & backwash having similar strength
sides of cusp channel incoming swash into the centre of the depression, producing a strong backwash that drags material down beach from centre of cusps enlarging the depression
Ripples
further down beach, due to orbital movement of water in waves
beaches are dynamic and their profiles change overtime as wind strength & hence wave energy changes (equilibrium profile between erosion & deposition)
Bars & Tombolos
Onshore bars
Accumulations of sediment parallel and joined to the mainland, a spit which joins 2 headlands, often exposed at low tide.
swash aligned beaches, sand bank develops offshore & is pushed on shore by constructuive wave action processes during shore normal currents and wves
drift aligned beaches, when a spit formed by longshore drift spans a bay and joins two headlands
a lagoon forms behind the bar, brackish water, often filled with sediment at a later date
E.g. Slipton Sands bar in Devon
Tombolos
an elongate wedge/ridge of sand joining an island to mainland, often exposed at low tides
often formed from spits that continued to grow seawards until they reach and join an island, caused by refraction and diffraction e.g. Mendiata in Peru (refraction, around island & diffraction, in cove)
a tombolo forms when a salient joins the obstacle
distance from shore is less than 1.5x width of island, waves are weak due to this
Other landforms
Sand dunes
formed by aeolian processes (that require wind energy)
Require
strong onshore & offshore winds, a large sediment supply, gentle sloping beach, long beach exposure times (tidal range), obstacles for dunes to form against, vegetation growth reduces wind velocity and leads to deposition & dune stabilisation e.g. marram grass
embryo, fore, yellow, grey dunes
Holkham beach & dunes, Norfolk
Spits
Wave crest approach the beach at an angle (causing longshore drift), beaches fed by sediment grow outwards from the coast. An abrupt change in coastline results in a detached beach, spits & tombolos. A recurved edge may result in wind refraction around the end or a change in wind directions. Spits occur in areas of low tidal range around the UK.
Deposition at Blakeney Point
LSD currents enter deeper water near the spit end energy is dissipated, rates of transport are reduced & deposition occurs. Wave refraction around the headland taking material inland. In low energy conditions behind the spit sediment builds up into a lateral ridge. Spits may have curves or noons, representing spit development overtime
Scott Head Island (west end)
Salt marshes
accumulation of silt & clay, rates of 10cm/yr. vegetation traps sediment, reduces energy. fall out of silt & clay occurs. gradually surface is raised. plant succession occurs and stabilises salt marshes. allows less tolerant species to develop as a succession.
Factors influencing salt marsh growth
low energy condition for development, sediment supply, periodic tidal flooding.
Salt loving plants are called Halophytes
e.g. eel grass, samphire
Bars
an elongated accumulation of sediment, parallel to the coast, often submerged at high tide which can be swash or drift aligned. Small scale 1-5m or large 11kms (often called barrier islands, not submerged at high tide). Mudflats & salt marshes on landward side. Tidal inlets & river estuaries break up bars.
Factors influencing their development
gently sloping offshore gradient, limited tidal range, high wave energy
Ridges & runnels
form parallel to the shoreline in foreshore zone
ridges are areas of foreshore that are raised above the adjacent shore which dips into a runnel, runnels are disrupted by channels that help to drain the water down the beach
wave energy moves up beach in high energy environments as the swash, the backwash is often impeded (deflects water parallel and helps form these features) (tidal variations & currents also help create these)
Tides & tidal energy
Spring tide
has the highest tides, with the combined gravitational pull of the sun & the moon
Neap tide
has the gravitational pull of the sun & the moon in different directions (at right angles to eachother), weakest tides
Microtidal - ranges <2m open ocean coasts, (wave dominated) Mesotidal - 2-4m (combination) Macrotidal - >4m (tide dominated), with wide shelves where most erosion, transport & deposition are driven
Britain is mainly macrotidal
Norfolk
beach profiles got more cliffs on right, increasing tidal range (funnelling effect on right)
Spring tidal ranges in the UK
width of continental shelf, causes friction & increases wave height, a wider shelf allows back of the wave to catch up increasing the height making it higher & narrower
coastal configuration, bays & estuaries compress waves increasing heights, most in funnel shaped entrants causing bores. open coast have lower heights, significant around uk.
shapes & depths of ocean basins, also affects tidal ranges e.g. in the North Sea a wave travelling south enters an area of shallower depth and narrow width causing rapid accumulation of funnelling of water, giving a higher tide
Large tidal range means
large expanse of shore platform meaning high capacity for weathering to take place
potential high impact of scouting over the exposed shoreline
strong tidal ebb and flood tides erode & transport materials
Small tidal range means
wave energy is concentrated over a small area
focus on erosion causes wave cut notches & caves
sediment is moved a shorter distance
Impacts of tides on coast
a tidal range influences where wave action will take place on a coastline
in exclosed seas, tidal ranges are low thus restricting the area where wave action takes place
tidal processes can be responsible for creating & modifying landforms at the coast
tides involve the movement of energy & materials at the coast
Tidal currents
as water rises and falls it produces tidal currents, a rising tide is called a flood tide, falling is called an ebb tide. they transport (entrain) and erode sediment (in high flow velocities) and deposit in slack low current positions, mid-point of high & low tides.
associated with wave action
1) shore normal currents; swash aligned - rip currents 2) longshore (drift) currents (at an angle to the coast) ; drift aligned
Rip-currents
strong backwash into ocean
Wave energy
Wave energy equation
P = H^2T
Constructive wave
long wave length so low-frequency (8-10 waves per minute)
low wave height (under 1m)
wave front is gentle sloping
gains a little height and breaks on the beach
water spreads a long way up the gently sloping beach
in ocean waves have circular orbit, near shore the orbit becomes elliptical due to shore friction slowing base of the wave
smaller longshore/offshore bar
strong swash
weak backwash
berm
Destructive wave
short wave length so high frequency (10-14 waves per minute)
wave height over 1m
steep wave front
breaking wave gains much height
wave plunges onto steep beach, energy directed downwards so does not travel far up the beach
in ocean waves have circular orbit, near shore the orbit becomes elliptical due to shore friction allowing the base of the wave
larger offshore/longshore bar where sand is deposited
weak swash
very strong backwash, erodes
storm beach
North Norfolk wave energy
wave energy strongest on right side compared to the left, dominant wind direction, long fetch
wave energy decreases from top of cliff downwards, this is as the majority of wave energy is exerted here
wave energy higher if storm wave energy also higher
Wave refraction
1) as wave travels from deep to shallow water, the wavelength shortens, the wave speed slows down, and the wave will refract or bend, towards the shallow area in order to conserve its energy
2) in bays, waves diverge due to refraction, reducing the relative amount of energy compared to a straight coastline
3) on the other hand, waves approach a headland coverage and concentrate energy, due to refraction
divergence reduces energy = deposition
convergence increases energy = erosion
waves refracted towards headlands, promontory effect
Wave diffraction
1) occurs when waves pass through an opening or around a barrier and change direction
2) or waves encounter an obstruction in its path and will change direction, or wrap around it
divergence occurs reduces energy
tombolos are a combination of refraction & dffraction
Diffraction
coves
Refraction
geo, bay
Diffraction & refraction
tombolo, headland, bay
Factors effecting rate of erosion
1) breaking point of wave
2) wave steepness
3) depth of sea / gradient
4) fetch & swell window
5) supply of beach materials
6) width of beach
7) rock resistance and presence of structures
Factors influencing wave energy
1) wind direction
2) fetch (maximum length of swell window)
3) swell window (amount of open ocean a coastline facing)
4) form of the coastline
5) beach gradient
6).storm surges
7) wind speed
8) shape of the coastline
Wave formation
Waves are generated by the transfer of wind energy to sea surface. Frictional drag on the surface is transferred to form waves. The amount of energy depends on wave pressure, high pressure next to low pressure causes strong winds.
wind energy depends on
1) wind velocity, 2) duration, 3) fetch
Storm surges
- caused by high winds & low atmospheric pressure. Combined with high tides this leads to higher than normal levels, leading to flooding
Tsunamis
- seismic induced sea waves. Earth-quakes cause sea floor displacements and drive a mass of water on land. Reaching heights of 35m & speeds of 600km/h they transfer energy inland.
Flows of materials & energy
A coastline is an
open system
with various inputs and outputs. A change in input leads to a change in outputs. Inputs -> Stores -> Outputs
Feedback cycles
Positive
- the effect is to increase or amplify the cause of the disturbance and further change occurs
Negative
- where changes within a system, show down / reduce causes of distruption leading to stability
Source
e.g. terrestrial, fluvial, offshore
Movement
e.g. longshore drift, aeolian processes
Deposition
e.g. loss of energy
Modification
e.g. change in wind direction
Wave energy
Dominant winds
- perpendicular to coasts, usually storm winds
Prevailing winds
- most common direction
Onshore
- winds blow from sea to land
Offshore
- winds blow from land to sea :
Flows of materials
Weathering
, weathering provides sediment, gravity moves sediment, marine & wind erode, transport & deposit, tide & currents move sediment
rock store -> scree store -> beach store -> offshore store
Flows of energy
solar energy -> photosynthesis biological weathering
-> mechanical & chemical weathering
-> wind energy -> aeolian processes & wave energy -> erosion processes
gravitational energy
Equilibrium
is achieved when the amount of energy entering a system is equal to energy dissipated without changing morphology
seasonal changes, equilibrium steady overall but fluctates
meta-stable equilibrium, caused by storm or other event, triggers on either side
dynamic equilibrium, longer time scale, gradual changes e.g. sea level rise
Negative feedback loop example
rapid erosion by wave action lead to cliff instability, rock fall occurs -> rock fall reduces wave energy, protects cliff & slows erosion -> rock fall slowly removed by wave action processed & exposes the cliff to rapid erosion ->
Cliffs & Mass movements
Mass movements classified by
1) speed , 2) water content, 3) internal structure
Rockfall, rockslide, creep, rotational slump, slump, solufuction, debris flow, density current
Cliffs
lithology, less or more resistant rocks
juxtaposition of rock types, differential erosion
structures, faults & joints, bedding planes
dip of bedding planes & presence of faults
wave activity, cliff foot processes
weathering, weakens rock & provides sediment
presence of water, permeability & porosity
Cliff form & movements
impermeable over permeable = stable, permeable over impermeable = unstable
joints opened by weathering and pressure release cause toppling
diagonal bedding planes cause rock slabs to slide down cliff, rock slide
uniform horizontal strata, or rocks dip inland, produce steep (more) stable cliffs
Hunstanton failure mechanism
1) weathering & wave erosion undercutting of less resistant layers
2) undercutting & failure of weak carstone, falls of red chalk
3) further undercutting leading to overhang of white chalk
4) undercutting leads to slumping / toppling of chalk as cliff is unstable
Geological controls at Hunstanton
chalk and carstone are both well jointed and this is a very important property which controls both the processes on the cliff face and the way it retreats
the chalk is especially well jointed close to the surface (caused by periglacial weathering and pressure release) and this leads to toppling of blocks of rocks forwards and down onto the beach. undercutting in the carstone aids rock falls
Landforms of erosion
Concordant
trends parallel to the shore, coves, islands & arches
Discordant
trends at a right angle to the shore, headlands & bays
Structure
is the way the rocks are disposed or geographically arranged
Lithology
is the 'make-up' of each individual rock type e.g. faults
hardness is important as harder = more resistant
permeability, water inside rocks increases resistance to subaerial processes
Differential erosion
is the erosion at different speeds, caused by different structure & lithology
Caves
- energy is concentrated at headlands and any points of weakness (e.g. faults) are exploited by erosional processes, causing small caves. wave attack is concentrated between high and low tides, it is here waves form.
Arches
- if a cave enlarges (erodes) until it reaches the other side of the headland then an arch is formed
Stacks
- erosion widens & weakens support aided by weathering the arch collapses causing a stack
Stumps
- further erosion at the base of the stack can cause collapse leaving only a stump
Blowholes
- sea caves grow towards the land & upwards creating a vertical shaft exposed at the surface
e.g. La Bufadora, Mexico
Geos
- narrow steep sided inlets, when waves smash against the roof of a narrow sea cave, the roof may collapse formed a geo OR weak points eroded more rapidly than the resistant rock around them
e.g. Calder's geo, Scotland