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Physical Geography > 8.1 Coastal processes > Flashcards

8.1 Coastal processes Flashcards

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1
Q

Coast definition

A

The part of the land most affected by its proximity to the sea and that part of the ocean most affected by its proximity to the land

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2
Q

Zones of coastal zone (4)

A

Onshore (foreshore + backshore), inshore, offshore

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3
Q

Onshore classification

A

May extend to up to 60km inland
Broken into: - foreshore (area periodically exposed by tides)
- backshore/upper beach (backed by cliffs or sand dunes)

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4
Q

Offshore classification

A

Covered by water, generally up to the limit of 200 miles off shore (Economic exclusion zone (EEZ))

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5
Q

Coastal system brief synopsis

A

The coast is a very dynamic system made up of morphological components which are the stores (landforms e.g. cliffs, depositional features like beaches +spits) and the flows of energy and material (sediment) between them

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6
Q

Types of change coasts +egs

A

Rapid + short term (wave type changing from constructive to destructive due to a storm)
Cyclical (seasonal changes in dominant wave types, winter vs summer)

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7
Q

Dynamic equilibrium coasts summary

A

A change in an input, transfer or output of sediment, water or energy can cause feedback to other components in terms of change to the morphology

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8
Q

Coastal processes

A

Tides, currents, waves, wind
Weathering processes

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9
Q

Sediment deposition swash aligned vs drift aligned

A

Swash aligned - offshore
Drift aligned - Along the shore (LSD)

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10
Q

Sediment budget

A

Inputs + outputs of sediment into the coastal system

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11
Q

Positive sediment balance + effect

A

When more inputs of sediment than outputs
Leads to deposition
- spits, bars, saltmarshes, dunes, etc

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12
Q

Reason for more sediment outputs than inputs

A

High energy storm waves with strong backwashes

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13
Q

Negative sediment balance effect

A

Material will be eroded and moved offshore and the coast may retreat

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14
Q

Relaxation time

A

The time taken for the coastline morphology to change

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15
Q

Sediment cell definition

A

A well-defined length of coastline & immediate offshore area in which sediment movement is relatively self-contained

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16
Q

Coastal population stats (3)

A

44% of people live within 150km of the coastline and at less than 100m above sea level
Population density in coastal areas in 3x larger than average
Projected population growth in the coastal zone is the highest in the world

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17
Q

Physical factors which affect coastline (8)

A
  • Lithology (rock type)
  • Rock structure
  • Processes
  • Constructive vs destructive waves
  • Shape of coastline
  • Biodiversity
  • Sub aerial processes
  • Sediment supply
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18
Q

Lithology effect on coastline

A
  • More resistant rocks (granite + basalt) will cause rugged landscapes (Giant’s Causeway)
  • Softer rocks (sands/gravels) will lead to low flat landscapes (Nile delta)
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19
Q

Rock structure

A

Concordant (parallel to coastline) - relatively smooth coastline
Discordant (at angle to coastline) - headlands + bays

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20
Q

Place where deposition dominates + egs of result

A

Netherlands, sand dunes + mudflats

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21
Q

Place where erosion dominates + egs of result

A

East coast of England, rapidly retreating cliffs

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22
Q

Importance of waves in shaping the coastline

A

Waves provide about half the energy to do work at the coast – they are the principal agents shaping the coastline through erosion and transportation of sediment which may get deposited elsewhere

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23
Q

How are waves formed?

A

Waves are produced by wind blowing over the surface of the water- which creates frictional drag.
The friction of wind on the surface produces orbital movements of water molecules that get smaller with depth. This produces troughs and crests of waves.
Wave base = no further movement relating to wind energy at this depth.
This is the water depth at which the wave motion is no longer felt and is generally 10–20m below surface.
Beneath this, the sediment on the seabed remains undisturbed
As waves approach the shore and the water gets shallower, the wave base is affected which means that friction starts to slow down the wave until it breaks.
When the water depth is too shallow to support the whole oscillation/orbit, the orbits become more elliptical because the top of the orbit is travelling faster than the base
Wave form breaks and collapses when the wave crest advances over the top of the wave base as it approaches the shore and potential wave energy is translated into kinetic energy of water

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24
Q

Wave height

A

Distance between crest and trough

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25
Q

Wave length

A

Length between two crests

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26
Q

Wave velocity

A

Distance travelled per unit time by the wave

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27
Q

Wave steepness + breaking stat

A

Wave height : wavelength ratio
- will break when ratio is 1:7

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28
Q

Wave period

A

Time taken for wave to travel through one wavelength

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29
Q

Wave frequency

A

Number of waves per minute - measured in hertz

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30
Q

Swash

A

Movement of water up the beach once wave breaks

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31
Q

Backwash

A

Water return down the beach after wave has broken, due to gravity

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32
Q

Plunge line

A

Point at which the wave ‘breaks’ (1:7)

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33
Q

Importance of onshore topography in breaking of wave

A

If the sea bed is steep then the wave velocity will reduce quickly as there is more friction – this will lead to more plunging/surging destructive waves . Important as determines way in which a wave will break and how much energy is translated onto the coast

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34
Q

Changes in wave profile as they move towards the shore (7)

A
  • Wave height/amplitude increases
  • Wavelength decreases
  • Wave steepness increases
  • Orbit of molecules advances faster in its upper part
  • Orbits become less circular
  • Wave crest advances over wave base nearer the shore/wave breaks nearer the shore
  • Wave interval gets shorter
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35
Q

Wave approaching shore process

A

As waves move towards the shore they enter shallower water. This subjects the orbiting molecules at the wave base to friction and their speed of movement is slowed. This causes waves to bunch up and reduces their wavelength. The faster moving water near the sea surface piles up, increasing wave height. Eventually, the crest of the advancing wave starts to spill over the lower part and eventually the wave breaks. The key, therefore, is the effect of friction on the molecules at the base

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36
Q

What determines wave height? (3)

A
  • Strength of wind
  • Duration for which the wind has blown in that direction
  • Length of fetch
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37
Q

Fetch definition

A

The distance over which the wind has passed over open water, blowing uninterrupted in that same direction

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38
Q
A
  • Positive relationship
  • Some variations
  • Non-linear
  • Some anomalies
  • Use of data for both anomalies + relationship
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39
Q

Where are biggest waves found?

A

40-60*N/S
The strong westerly winds here produce the world’s biggest waves as they have uninterrupted fetches
Waves average 5m in height - occasionally 10m in Southern Ocean
Seasonally, wave height may also be affected by Monsoon winds and also tropical cyclones or low pressure systems which bring storm or tidal surges

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40
Q

Where are smallest waves found?

A

Around parts of the Equator eg Indonesia where fetch is limited due to landmasses
Enclosed seas like Mediterranean and Caribbean due to small fetch

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41
Q

Swell waves generation + profile

A

Everyday waves generated in open sea by prevailing wind
They have travelled a long distance from where they were generated
Wind that has created these waves may have died down over the ocean but wave have continued to move in the same direction until they reach the coastline
Lower height. longer wavelength so less steep + lower frequency
These tend to build up the coastline
More important in summer

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42
Q

Storm waves

A

Produce a much rougher sea
Formed where strong winds blow directly on the ocean locally
Short wavelength, greater height, therefore steeper, high frequency
More destructive
More seasonal (winter)

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43
Q
A
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44
Q

What happens when a wave breaks

A

Their potential energy is converted into kinetic energy and released on shore
Depending on the way a wave breaks, energy will be transferred up the beach or down the beach and so will affect the shape of the beach

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45
Q

Littoral drift definition + size

A

The amount of sediment transported by longshore currents
Generally around 10,000-100,000m^3/year

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46
Q

Three main breaker types of wave

A

Spilling breakers - associated with wide, less steep beaches where energy is gradually dispersed - lead to constructive waves
Plunging and surging breakers - often break dramatically on steep beaches as they slow down quickly due to a rapid increase in friction as they enter shallow waters and significant amounts of energy may be reflected back to sea - lead to destructive waves

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47
Q

Types of waves of translation

A

Constructive + destructive

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48
Q

Constructive wave characteristics (5)

A

Strong swash
Weak backwash
Deposition occurs
Low wave frequency
Low wave height

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49
Q

Destructive wave characteristics (5)

A

Weak swash
Strong backwash
Erosion occurs
High wave frequency
High wave height

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50
Q

Constructive wave formation

A

Wave frequency is low and they arrive over a gently shelving sea floor which increases friction gradually and gradually steepens the wave
A spilling breaker is therefore formed, and its powerful swash surges up the beach as it breaks
The weak backwash percolates through the sand with little transport of sediment back down the beach

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51
Q

Destructive wave formation

A

Result from locally generated winds which create waves with high frequency
If these approach up a steeply shelving coastline, they will face a rapid increase in friction
As a result, a steep, plunging breaker is formed
Wave energy is transmitted down the beach, accelerated by steep beach gradient and so the wave becomes destructive with little percolation down through the sands and eroded material carried offshore and deposited as longshore bars

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52
Q

Constructive vs destructive waves (9)

A
  • Low frequency (6-8 arriving onshore per minute) vs high frequency (12-14 per minute)
  • High wavelength relative to low height vs low wavelength relative to higher wave height
  • Gentle sloping sea floor (fine material e.g. sand) vs steeply sloping coastline (e.g. gravel, shingle)
  • Spilling breakers formed (low, flat) vs plunging breakers formed (curled, steep)
  • Strong swash, weak backwash vs weak swash, strong backwash
  • Swash transports material up the beach, causing deposition vs wave breaks down beach material (beach cliffs collapse progressively over time) and material carried offshore (erosion)
  • More elliptical orbits vs more circular orbits
  • Low energy vs high energy
  • Generated far offshore (swell waves) vs generated from locally generated winds (storm waves)
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53
Q

Relationship between constructive and destructive waves

A

Dynamic equilibrium
- constructive waves push material onshore to create steeper beach profile
- this will then lead to destructive waves
- this will then lead to gradient being removed and so a less steep gradient
- process continues

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54
Q

Why is the classification of waves at particuliar locations an over-simplification

A
  • Destructive waves are more frequent in winter
  • Constructive waves are more common in summer
  • Prevailing winds vary daily + seasonally
  • Type of waves can even vary within the same day
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55
Q

Low energy coasts classification

A

Waves are not powerful
The rate of deposition exceeds the rate of erosion
Characteristic landforms include mudflats, salt marshes, deltas which are dominated by tides not waves

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56
Q

High energy coasts classification

A

Waves are powerful
The rate of erosion exceeds the rate of deposition
Characteristic landforms include headlands, cliffs and wave-cut platforms, beaches, spits - wave dominated rather than tides

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57
Q

Wave refraction

A
  • ‘Bending’ or distortion of wave fronts due to varying water depths
  • Where water shallows, water slows down due to friction
  • Wave height and steepness increases
  • Wavelength and velocity decreases
  • Waves in deeper water move ahead
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58
Q

Wave refraction at coastline

A

Wave fronts approach straight shoreline at angle - the front of the wave slows down so refracts which shortens and heightens wave so it becomes steeper and breaks on headland
- due to refraction, waves converge around headland i.e. wave energy is concentrated upon a headland enhancing this erosion
- this process also sets up longshore currents that move sediments from headlands into bays (deposition)
If refraction isn’t complete, it leads to LSD of sediment

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59
Q
A
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60
Q

Additional factors which affect waves (4)

A
  • Wave refraction
  • Storm surges
  • Tides
  • Sea level change
61
Q

Storm surges definition + significance

A

Temporary large scale waves caused by intense low pressure systems and high wind speeds
Although ocean waves are the principal agents for shaping the coast and driving nearshore sediment transport, the contribution of storm surge is significant - raised water levels may last several days

62
Q

Storm surge formation

A

During severe storms, the water level near the shore can be significantly higher than tidal predictions, and the difference between the measured and the predicted water level is the storm surge
For every drop in air pressure of 10mb, sea water rises by 10cm - so in a tropical sea, levels may rise by 1m in a storm
If coastlines are funnelled, the rise is intensified and if they coincide with a tide and espicially a spring tide, the effect may be devestating
If they approach gently sloping coastlines, they will have more impact

63
Q

Storm surge example

A

Ganges delta - storm surges may exceed 4m and storm waves may add another 4m to wave height which is also exacerbated by funnelling caused by Bay of Bengal - low pressure from tropical storms due to warm air from Bay of Bengal meeting cold air from the Himalayas - unstable air that rises, convects and leaves low pressure at the surface
Higher waves may also be experienced seasonally due to the Monsoon Winds

64
Q

Tides

A

Tides are produced by the gravitational pull of the Moon and Sun acting on a rotating Earth. This pull produces a very slight bulge in the ocean, which is known as a tide. This creates a regular upward and downward movement of the ocean caused by gravitational pull of the moon and sun

65
Q

Tide intervals

A

Takes just over 6 hours to come in and the same to go out
High tide occurs eery 12 hours and 25 mins

66
Q

Tidal variations

A

4 times per months the tide may deviate significantly from the norm - 2 spring tides when tidal range is much greater than the norm and 2 Neap tides when tidal range is much smaller

67
Q

When might tides be especially threatening?

A

When spring tides combine with storm waves and the physical geography leads to funnelling too e.g. in bays or between landmasses light between England and France in the English Channel

68
Q

Importance of tides in shaping the coastline

A

The tides and the currents they generate are responsible for about 50% of the marine energy delivered to the coast. Along with waves, wind and tides are also significant contributors in shaping the coastline, and are indeed dominant in coastal dune and estuarine environments ie low energy environments
Tides are different from waves, but do influence where on the beach or cliff profiles a wave will break. Therefore the impact where sediment accumulates and where erosion occurs. They also have an importance effect via tidal currents and scouring as tides ebb and flow
The major impact of tides is to shift the shoreline between high and low tide and to generate tidal currents either parallel to the coast or at tidal inlets and estuaries, currents flowing into the inlets and perpendicular to the coast
Tidal range also affects the levels at which waves may operate - zone of attack on a cliff face therefore moves up and down so if the tidal range is slight, the zone of attack will be especially concentrated maximising the impact of wave attack in that area of the cliff face - responsible for causing formation of a wave cut notch

69
Q

Where are tides greatest?

A

In bays and along funnel shaped coastlines where there will be funnelling of water - as the tide approaches, it is funnelled into a small area so rises rapidly and may produce a tidal bore

70
Q

Affect of hemispheres on tides

A

In the northern hemisphere, water is deflected to the right of its path due to the Coriolis effect which will affect tidal ranges locally

71
Q

Tidal range

A

Height between high and low water mark - this can vary from day to day for any given place and may be exceeded in storm conditions
Affected by shape of the coastline and can range from almost nothing in enclosed seas like the Mediterranean to 15m in the Bay of Fundy in Canada

72
Q

Low tidal range vs high tidal range examples

A

Almost nothing in the Mediterranean (closed sea) to 15m in the Bay of Fundy in Canada

73
Q

What does tidal range affect?

A

Vertical range of erosion and deposition
Weathering and biological activity affected by time between tides - as at low tide the wavecut platform becomes exposed and is more subjected to weathering than it is to erosion as a result of tides
Velocity of tidal flow as the tide ebbs and floods is affected by tidal range and has important scouring effect

74
Q

Macro-tidal range

A

4m+

75
Q

Meso-tidal range

A

2-4m

76
Q

Micro-tidal range

A

Less than 2m

77
Q

Sea level change impact

A

Determines the position of the shoreline on a slower and much longer-term basis

78
Q

Ice age overview

A

During the last ice age 18,000 years ago, sea level was 120m below present, and the continental shelves were exposed - this lead to an ‘emergent’ coastline with exposed features like raised beaches
It then rose, reaching present sea level around 6,000 years ago, after which it was relatively stable
Now, with climate change, it is beginning to rise again, and may rise as much as 1m over the next 100 years, triggering shoreline retreat, inundation (leading to submerged coastlines and the formation of distinctive coastal features like rias and fjords) and the degree and extent of erosion

79
Q

Describe the characteristics of breaking waves and explain the processes by which waves transport sediment on beaches. (10)

A
80
Q
A
81
Q

Diagram shows the movement of water in sea waves. Explain how such movement occurs and describe the differences between constructive and destructive waves

A
82
Q

Role of waves in generating sediment + eval

A

Aid coastal erosion, create longshore drift currents through wave refraction to transport it along the coast as well as transporting material up and down the beach via the swash and backwash
Most sediment is not derived from wave erosion, but more from the input of rivers

83
Q

Swash zone

A

Area that is alternately covered and not covered by breaking waves

84
Q

Breaker zone

A

Area where waves start to break

85
Q

Surf zone

A

Area between the swash zone and the breaker zone

86
Q

Where are the biggest waves in the UK found and why?

A

In the South-West, as they have travelled over 7,000km across the Atlantic Ocean

87
Q

Spilling breakers

A

Associated with gentle breach gradients and high-steepness waves (e.g. during storms)
A gradual peaking of the wave occurs until the crest becomes unstable, resulting in a gentle forward spilling of the crest and the production of bubbles and foam

88
Q

Plunging breakers

A

Occur on steeper beaches than spilling breakers, with waves of intermediate steepness
The shoreward face of the wave become vertical, curls over, and plunges forward and downward as an intact mass of water

89
Q

Surging breakers

A

Found on steep beaches with low-steepness wave (e.g. summer swell waves)
The front face and crest of surging breakers remain relatively smooth and the wave slides directly up the beach without breaking or producing foam

90
Q

Longshore currents

A

Shore-parallel flows within the surf zone
They are driven by waves entering the surf zone with their crests aligned at oblique angles to the shoreline
The larger the wave angle, the stronger the currents

91
Q

Wave energy stat

A

A wave with a height of 2m and a wavelength of 14m breaking along 2km of coastline has approximately 45kWh of energy

92
Q

Erosion

A

Mostly caused by the action of waves so a marine process - dependant on wave energy
Hydraulic action, cavitation, abrasion/corrasion and attrition are the main erosional processes but solution can be important in areas of limestone/chalk
Operate at the foot of the cliff
Can create a wave-cut notch and overhang which may lead to mass-movements being more likely
Depends on geology + management strategies
Operate in the inter-tidal zone between high water mark and low water mark - this will concentrate on the wave-cut platform which are exposed at low tide as waves and tidal currents erode material through attrition making pebbles smaller and solution works to create rock pools creating a jagged coastline

93
Q

Erosional processes

A
  • Hydraulic action
  • Cavitation
  • Corrasion/abrasion
  • Solution
  • Attrition
94
Q

Hydraulic action at coasts

A

Sheer force of the waves exerts a pressure which can be up to 30,000kg/m^2 in storms

95
Q

Cavitation at coasts

A

The compression of air in openings in the rocks at the coast as the wave hits - weakness the structure and increases surface area of cracks and joints for other forms of erosion to capitalise on
Large blocks can be ‘quarried’ and removed from the cliff face
Common in jointed, well-bedded rocks eg limestones and poorly consolidated clays and glacial deposits
11 tonnes/m^2 of cliff face is removed each year through cavitation

96
Q

Corrasion/abrasion at coasts

A

The load carried by the breaking waves acts as a tool like sandpaper, smoothing the rock
Plays an important role in landform formation too
- important in producing the notch at the cliff base and in shaping and smoothing wave-cut platforms and so contibrutes to more mass movement and weathering (sub-aerial)

97
Q

Solution at coasts

A

Active in calcareous rocks like chalk and limestone especially on wave-cut platforms in the inter-tidal zone where carbonation solution creates soluble material which is carried away by the waves
(acid from organisms like molluscs will do the same and will be important when tides are low and the wave-cut platform is left uncovered)

98
Q

Attrition at coasts

A

Leads to a reduction in the calibre of the load - shape, size smoothness
- material moved along the coast by LSD is especially susceptible
- more attrition further from the source

99
Q

Where and when do marine processes operate?

A
  • Dominate at the cliff face
  • Especially at the foot of the cliff - the erosional process create a wave-cut notch which then undermines the cliff and makes mass movement more likely causing the cliff to steepen and retreat
  • Generally a constant factor causing change but can cause cliff collapse
  • Impact will vary throughout the year - waves will erode more in summer but may deposit and build up the coastline through beaches during summer when constructive waves dominate
  • Operate in the inter-tidal zone - therefore tidal range is important - energy will be more focused on areas with a micro-tidal range (sub-aerial will dominate when there is a macro-tidal range) - therefore wave erosion is less important after formation
100
Q

With the aid of a labelled diagram, describe the main physical features of the landscape shown in Fig. 4.1

A
101
Q

Describe the pattern of land lost and land gained shown in Fig. 4.1 (4)

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102
Q

Suggest two reasons why coastlines lose or gain land (6)

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103
Q

Explain why the rates of erosion and deposition vary along a stretch of coastline (6)

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104
Q

Describe the pattern of mean erosion and mean deposition rates shown in Fig 4.1

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105
Q

Sub aerial processes at cliffs

A

Weathering + mass movement

106
Q

Weathering

A

Loosening and breaking down of rock in situ

107
Q

Where is chemical weathering most common?

A

Generally more common in areas which are warm and wet due to Vant Hoff’s Law and in sedimentary rocks

108
Q

Where carbonation will be the most important chemical weathering process

A

Limestone cliffs eg Southern England

109
Q

Carbonation at coasts

A

Leads to the widening of joints - increases the surface area of joints over which marine erosion may then operate
Relationship between sub-aerial and erosional processes
Most important chemical weathering process affecting limestone cliffs eg Southern England

110
Q

Hydrolysis at coasts

A

Will be common in granite cliffs eg North Devon and Cornish coastlines
Usually takes place in acid conditions

111
Q

Hydration at coasts

A

Alternate wetting and drying of cliff face and wave cut platform will occur as tides ebb and flow. This may further increase effect of salt crystallisation and allow hydration to occur

112
Q

Frost shattering/freeze thaw at coasts

A
  • Repeated melting and freezing weakens joints of rocks to be exploited by wave attack.
  • This may occur on cooler temperate coastlines in winter although freshwater more likely to cause this and this is arguably more relevant to higher latitudes as salt lowers freezing point of water and produces ‘soft ice’ at temperate latitudes
  • More common in well-jointed sedimentary rock eg limestones, sandstones etc
  • Most effective when combined with other weathering types eg salt crystallisation
113
Q

Salt weathering at coasts

A
  • Common along coastlines due to salinity of sea
  • Salt spray enters joints in rock
  • When the water evaporates salt crystals form inside the rock
  • Crystallisation occurs as magnesium and sodium compounds expand causing stresses which weaken rock causing it to decompose and crumble
  • It may then be exploited by wave attack or mass movement
  • Common in the spray zone towards the base of the cliff
  • More common in tropical areas with temperatures around 27 C where temperature fluctuations can cause expansion rates of up to 300%
  • More common in sedimentary rocks
114
Q

Heating and cooling at coasts

A
  • Large diurnal temperature changes can cause expansion and contraction of the surface layers of rock at the cliff face.
  • The stresses caused by these changes weaken the rock and surface layers peel off
  • The availability of moisture at the coastline eg morning fog as the sea is cooler than the land first thing due to a higher specific heat capacity, increases the speed of the process of exfoliation
  • Most common along desert coastlines due to extreme temperature ranges
115
Q

Biological weathering at coasts

A
  • Salt tolerant plants may thrive in well-jointed rock and widen joints and cracks with their roots, increasing the surface area over which erosion may take place by waves
  • Guano from sea birds can actually corrode the cliffs through a chemical reaction
  • Molluscs , sponges and sea urchins can carry out weathering, some through chemicals they release
  • These are important on wave-cut platforms where sub-aerial processes start to become more important in shaping these landforms than waves despite their name
  • These are also prominent on low-energy coasts where these species may thrive
116
Q

Inter-relationship between subaerial processes like weathering and marine erosion

A
  • The efficiency of all erosive processes is controlled to some degree by the amount of prior weathering of the rock
  • Weathering prepares the rock for erosion eg salt weathering weakens and disintegrates rocks so they are attacked or removed more easily by wave erosion
  • eg hydration, makes them more soluble
  • eg carbonation creates bigger surface area over which marine erosion can work and exploit - Sub-aerial weathering will also wear down the vertical cliffs, reducing their height
  • These processes may also help shape cliffs – if the geology is sedimentary consisting of bedding planes, cliffs will tend to have geometric shapes with flat tops as weathering removes material vertically in layers
    – weathering is a very slow process so is a longer term factor in shaping the coastline than wave action.
  • It will play a more significant role in some geographic areas than others due to climate influencing the rate of weathering and in some rock types.
  • It will also be more prevalent at some times of the year due to the temperatures required for freeze thaw cycles or heat required for eg evaporation in salt weathering.
  • It may also play a more significant factor before erosion in weakening the rock and after erosion has occurred eg in shaping wave cut platforms formed after cliff retreat that are exposed at low tide – their influence therefore varies over space and time
117
Q

Explain the importance of rock type, structure and erosional history on the evolution of cliff profiles. (10)

A
118
Q

Describe the variation in rates of salt weathering shown in fig 4.1.

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119
Q

Suggest the reasons for these variations in salt weathering

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120
Q

Describe the coastal landforms shown in the photo and explain the processes of coastal erosion that contribute to their formation

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121
Q

Mass movement at coasts

A

Stress inter-relationship:
- Weathering weakens the rocks at the cliff face, expanding cracks and chemically changing the rock
- This may lead directly to mass movement, or it may lead to greater wave erosion which then undercuts the cliff face leading to rock fall
· Moreover, Rockfalls from weathering lead to cliff retreat and the debris may be used in erosion eg as an agent in abrasion (positive feedback loops which amplifies the impact of wave erosion)
- However, the talus slope at the base of the cliff after rockfall may act as a negative feedback loop for a time as this may protect the base of the cliff from wave erosion
- However, once material at the base of the cliff is removed by waves and currents, the cliff becomes exposed once again
- In this way, mass movement may even slow wave erosion for a short time

122
Q

Importance of water in mass movements at coastlines

A

It is important in the extent to which mass movement plays a role in shaping coastlines:
- Rainwater may add weight to the cliffs, increasing shear stress over shear strength causing mass movement
- Rainwater may also weaken the internal cohesion of the rocks at the cliff face and/or increase pore water pressure in clay slopes. Again the shear stress increases above shear strength and the slope will fail.
- Added water will lubricate slip planes and make slides more common – especially if permeable rock overlies impermeable rock and/or the rock dips downslopes making movements more likely eg sand over clay in Barton-on-Sea
- Marine processes working at the base of the cliff will undercut it and leave it unsupported so mass movement more likely.

123
Q

Combination of weathering + mass movement lead to

A
  • Rock falls along hard rock cliffs eg along chalk cliffs in Dorset on South Coast of UK
  • Rotational slumps and slides in cliffs made of unconsolidated material such as clay or glacial deposits eg boulder clay found on NE English coast eg Holderness coastline
124
Q

Evaluation of impact of mass movement on coastlines

A
  • When evaluating the impact of mass movement on shaping coastlines, you must stress that the impact is episodic rather than continuous and more significant in winter than summer due to increased rainfall and undercutting by high energy wave action
  • The impact on shaping the coastline may also be a temporary reduction in wave erosion due to the rock fall deposited as the base of cliff which may alter angle of cliff
  • This reduced slope angle may in turn protect the cliff from wave action for a while at the base so that mass movement actually becomes the dominant agent shaping the coastline. (Dynamic Equilibrium process)
  • However, the weathered material may be removed and so add material to sediment system and could be transported elsewhere or used as an abrasive agent increasing the impact of erosion in shaping the coastline
125
Q

Describe the rates of change shown in fig 4.1 from North Island to Waites Island

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126
Q

Describe the changes shown in 4.1. (3)

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127
Q

Suggest two reasons for the rates of coastline change (6)

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128
Q

Describe the features of the cliffs shown in fig 4.1 (4)

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129
Q

Suggest how the features described developed (6)

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130
Q

Transportation of coastal sediment including longshore drift

A

Sediment may be moved into the sediment cell by processes of erosion/mass movement/ weathering and even humans. Depending on the sediment budget – ie inputs v outputs, sediment will either be removed or built up (accretion) to form new depositional landforms such as beaches or spits or even saltmarshes and dunes which shape the coastline in low energy environments.

131
Q

Sources of sediment at coasts

A

· RIVERS bring sediment from the land to the coast. Most of the sediment that reaches the coastline comes from rivers
- CLIFF EROSION of cliffs by sea produces large amounts of material for beach-building.
- Material from MASS MOVEMENT
- SEA is a source of sediment brought on and off the shore by waves and tides
- OFFSHORE SEABEDS - Tides and waves may bring these shore-wards to build offshore bars or add to beaches. Especially if waves are spilling as they break and translate into constructive waves with a strong swash.
- HUMANS - artificial beach nourishment – brought from elsewhere
- GLACIAL DEPOSITS – e.g. shingle beaches of southern Britain
- WIND – BLOWN DEPOSITS
- VOLCANIC DEPOSITS – black beaches

132
Q

Effect of sediment size on beach profile

A

Can range in size from silts and sand up to cobbles and boulders. On beaches, finer sand sediment results in very low gradient (~1°) beaches while cobbles may be stacked as steep as 20°

133
Q

Sediment variation reasons

A

Sediment will vary depending on its source and the type and size will also affect if and how the sediment is transported in high energy and low energy coastal environment.

134
Q

Sediment size variations depending on sources

A
  • RIVERS – transport a variety of materials - usually fine-grained silts and clays, but also larger particles of sand and gravel; this is known as alluvium. This sediment may help shape the coastline since when deposited in low energy environments, it may result in formation of salt marshes and deltas.
  • Sandy beaches of Caribbean is largely river sediment that has been reworked by waves.
  • GLACIAL DEPOSITS – shingle beaches of southern Britain are made of shingle derived from glacial and periglacial processes that happened 10000 years ago – showing importance of climate change
  • CLIFFS – usually generate coarse sand and shingle (small rounded pebbles) is produced as waves and longshore currents rework these through attrition and abrasion. These are common in temperate latitudes made up of quartz sand grains
  • SUBAERIAL PROCESSES ON CLIFFS AT HIGHER LATITUDES – physical weathering here may well produce coarser rock fragments and gravel
  • SEA - Huge volumes of sand and clay were deposited here in the ice age and may be brought onshore by waves and tides .
135
Q

Hierarchy of energy to deposit/move for types of sediment

A

Clay, silt, fine sand, medium sand, coarse sand, gravel, pebble, cobble, boulder

136
Q

Methods of transport at coast

A
  • Suspension – fine sediment carried in turbulent water flow, making it look muddy or murky. If permanently in suspension, term = wash loads
  • Traction – larger pebbles and cobbles rolled along sea bed slowly
  • Saltation – bouncing of small pebbles (sets up chain reaction, when one pebble hits another, causing it to bounce)
  • Solution - dissolved material being carried eg calcareous limestones
137
Q

Things that cause sediment movement

A
  • wave action: driven by the wind – swash and backwash up and down the beach.
  • tidal currents: vertical changes in water level, which causes the water to move horizontally and creates currents. When a tidal current moves toward the land and away from the sea, it “floods.” When it moves toward the sea away from the land, it “ebbs.”
  • longshore currents: move parallel to shore caused by waves approaching at an angle determined by the prevailing wind direction - it is caused by large swells sweeping into the shoreline at an angle and pushing water down the length of the beach in one direction – Note that direction may be seasonal and fetch is important
  • Rip currents are narrow, seaward moving currents They are a mechanism for returning the water back out to sea, and a conduit to transport eroded beach sediment out to sea during high seas. 
  • Wind blows sand up beach => moves sediment to form sand dunes or can blow seaward and remove sand from dunes to build up beaches
138
Q

Longshore drift

A
  • Winds and therefore wave fronts/crests arrive at an angle (orthogonally) to shore. Wave angle + direction of swash determined by prevailing wind direction & wave refraction – this sets the LSD currents in motion.
  • Swash carries wave energy and sediment up at an oblique angle
  • Backwash brings it straight down under gravity at a right angle
  • Repeated process which creates a zig zag movement of material along beach causing beach drift
139
Q

Evaluate the role of transportation in shaping the coastline

A
  • Plays an important part in shaping the coastline, transporting material from high energy to low energy environments where it helps to create distinctive depositional coastal features and alters the coastline. These may be at the local, small scale eg microfeatures on a beach such as cusps and ridges and runnels formed by swash and backwash transportation to to large scale landforms like spits formed by LSD or even sediments rolled on shore by longer term sea level rise to form barrier beaches along East coast of USA or bars like Slapton Sands in Devon - Transportation may lead to features which are located further inland eg wind transports sediment beyond the beach to form dunes
  • However, there are limitations on transportation
  • EG LSD does not always take place or is maybe restricted: Eg In areas where there is:
  • Coastal Management - Groynes may interfere with the process, stopping or reducing the amount of sediment transferred along the coast
  • Or If there isn’t enough sediment (e.g. further along from groyne – beach starved of material) or where erosion has been reduced by coastal management plans eg sea walls, where dams have been put in place on rivers feeding into the sea, reducing the transportation of sediment derived from rivers
140
Q

Where and when does deposition occur?

A

Deposition is governed by sediment size and shape and wave/wind/river estuary energy. In some cases, sediment will flocculate (eg clay) and so become heavier and get deposited more easily.
This will determine the nature of sediment on beaches and their gradient, will determine how far the sediment is transported and where and when it gets deposited and the depositional landforms that are created as a result.

141
Q

Landforms created by deposition

A

Deposition will occur when there is a drop in energy – caused by change in alignment of the coastline, river estuary as it reaches the sea s mixing of fresh and saline water ie slack water, sheltered bays, between islands and mainland where refraction has occurred.
Sand may be deposited to form sand dunes and river silts may be deposited to form mudflats and salt marshes
Beaches are main stores of sediment that is deposited but these are dynamic and will change seasonally etc. Other landforms include spits, bars, barrier beaches, tombolos, cuspate forelands.

142
Q

Evaluation of the role of deposition at coasts

A

Plays an important role in shaping low energy environments so will not affect all stretches of coastlines.
Will therefore depend on wave energy and sediment type and how much sediment
May be modified by other processes eg subaerial , wave erosion , wind etc
Will take a long time to shape large scale change on the coastline eg formation of a spit but may affect a beach profile over a shorter time frame.

143
Q

Describe the pattern of marine processes in fig 4.1 (4)

A
144
Q

Suggest two reasons for variations in rates of marine deposition (6)

A
145
Q

Explain the roles of marine transportation and deposition in the formation of the coastal landforms shown here

A
146
Q

Sediment cell history

A

Since 1994 the Coastline of England & Wales has been divided into 11 Shoreline Management Plan (SMPS) major sediment cells, with smaller sub-cells within them.

147
Q

Sediment cell characteristics

A
  • Boundaries of each cell tend to be at headlands and estuaries.
  • Closed system – relatively little transfer between sediment cells.
  • Dynamic equilibrium – change to one part of system (e.g. input like cliff collapse) affects rest.
  • System: Sediment is gained (From erosion), transported (by waves, tides & currents), stored (e.g as a deposit) and lost (to sea)
  • Inputs of sediment – see before (rivers, estuaries, erosion of dunes or marine or cliffs)
  • Movements of sediment – longshore drift & longshore currents (driven by wave energy) ; tidal currents (driven by gravitational pull); offshore and onshore currents; rip currents. The direction of these is determined by the prevailing wind direction which may change seasonally.
  • Outputs of sediment (e.g. loss of sediment to deeper water) – loss does not cross cell boundaries (that is the point of the cell boundaries!). Tidal currents are very important in loss of sediment and carry large amounts of sediment (they surge up an estuary as tide comes in and surge out again when tide falls). The seaward tidal currents also enhanced by river flow so greater chance of loss of sediment. Tidal currents remove large amounts of mud, silt, sand (esp. by suspension) into deeper water so sediment is lost from the cell
  • Shoreline Management Plans (SMPs) are based on sediment cells – a type of integrated coastal management whereby coastal management strategies can be devised to protect one stretch of coastline without having an adverse effect on another stretch of coastline further downdrift
  • Humans disrupt sediment cells – groynes, breakwater, dredging, sediment removal, beach replenishment, dam construction, destabilisation of cliffs or stabilization of them. This means less sediment transported and deposited elsewhere so puts those areas at risk from greater erosion
148
Q

Sediment cell evaluation

A

Important therefore to recognise transportation and deposition of sediment and sediment cells when weighing up the factors that shape the coastline as will determine depositional landforms but will also affect rates of erosion along the coast if the input of sediment is reduced in some way and so those places are starved of sediment which would usually create a barrier to erosion by absorbing wave energy as a first line of defence eg beaches or offshore bar

149
Q

Southern California Oceanside littoral cell

A

200,000m ³ of sediment gets transported southwards every year. Irregular and variable river discharge and supply of sediment has been compounded by construction of dams which have reduced river sediments by 33%. The harbour has blocked the southward movement of sediment and so most is now diverted to offshore currents and so is lost out to sea to the La Jolla submarine canyon

Physical Geography (9 decks)

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