‘Erosional processes along
coastal margins’
Objective – how does erosion
make the coastline look like it
does?
Erosional processes at the
coast.. What are each?
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•
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Wave pounding
Hydraulic action/cavitation
Abrasion/corrasion
Attrition
Solution/corrosion
Sub-aerial
Human
Past questions
1. Outline the processes by which the
sea erodes a cliff 3
2. Outline the wave characteristics likely
to lead to marine erosion 4
Rates of erosion:
Why do some coastlines erode more rapidly than
others?
Black Ven landslip near Lyme Regis
(sands, clays)
Lands End Cornwall
(granite)
Processes affecting the rate of
erosion:
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•
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•
Atmospheric
Geological
Human
Marine
• Bullet point notes on each…
Sub-aerial
processes
Precipitation directly
Surface run-off and throughflow
Hydration
Salt crystallisation – a form of
corrosion which helps break down weak
layers of rock.
Freeze-thaw – the process whereby
water freezes, expands and degrades
jointed rocks
Biological weathering – carried out by
molluscs, sponges and urchins – this is
very important in low energy coastlines
Bioerosion – e.g. Burrowing or browsing
organisms, or the growth of algae and
other marine plants on rocks.
2.
Geological
•
Rock resistance - The strength of coastal rocks influences rates of erosion with unconsolidated volcanic ash offering the least resistance to wave
attack (see table). In Britain it is coastal areas where glacial till was deposited which are being worn back most rapidly (Holderness coast). When
Surtsey in Iceland first arose out of the sea in 1963, it was composed of unconsolidated volcanic ash. It was only when it was covered and protected
by a lave flow the following year that its survival was guaranteed, at least in the medium term
Rock type and average rates of erosion
Rock type
Location
Rate of erosion
(meters/year)
Volcanic ash
Krakatoa
40
Glacial till
Holderness
2
Chalk
SE England
0.3
Shale
North Yorkshire
0.09
Granite
SW England
0.001
3.
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•
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Geological
LITHOLOGY: Coherent rocks with
interlocking crystals and few lines of
weakness (joints and faults) resist erosion
e.g igneous and metamorphic rocks and chalk,
carboniferous limestone. Incoherent rocks
e.g. clay, sands, till erode easily
PERMEABILITY: incidence of pore spaces.
STRUCTURE: Rocks which are well-jointed or
have been subject to faulting have an
increased vulnerability to erosion. The
steepest cliffs are usually where the rock’s
structure is horizontal or vertical. If dips at
angle blocks may break off and slide
downwards.
Bedding
planes
Joints
4.
Human factors
1. Coastal protection
May slow marine erosion to the point where sub-aerial processes dominate
Groynes: built to increase the width of a beach by limiting longshore drift,
repurcussions further along coast as will lose their supply of beach material and
become more susceptible to erosion e.g. West Bay in Dorset £1.5 million per
groyne (usually 200m apart)
Sea-walls: Deflect the power of the waves back to sea, very effective.
Washes the beach away therefore groynes needed as well. £7000 per metre
Revetments: like a sea wall but cheaper. Break the force of the wave and trap
sand behind them. Don’t protect cliff as well. £1000 per metre
Beach-feeding: replaces sand. Protects land or sea wall behind the beach, looks
natural. Sometimes use mining spoil which isn’t natural to that environment.
£1000 per metre
Cliff-slope angle: to reduce risk of mass movement. Expensive, have spoil
material. Can only lower to certain degree as usually road or houses. ‘Natural’.
Rip-rap (boulders). Aesthetically ugly. ‘foreign’ objects. Expensive. Easily made
into defence.
5.
Human factors
2. Coastal destruction
Mining: removal of sand and shingle,
therefore more prone to wave
attack
Vegetation: binds the sand together
Building: on cliff tops increase
pressure
6.
Marine
1. Wave refraction
High energy waves
(paths of crests converge)
Low energy waves
(paths of crests diverge)
Bay
Headland
Wave approaches irregular coastline
Depth of water decreases rapidly, waves get
higher and steeper, velocity decreases,
shorter wavelength
Marine processes
7.
• Wave refraction: where waves approach an irregular coastline they are refracted i.e. they
become increasingly parallel to that coastline. Best illustrated where a headland seperates 2
bays. We already know that as a wave approaches the shore it loses velocity as the depth of
water decreases. As the sea bed usually shelves more rapidly off a headland than in a bay, the
wave loses velocity more quickly there. The orthogonals (lines drawn at right angles to wave
crests) represent 4 stages in the advance of a particular wave. It is apparent from the
convergence of lines S1 S2 S3 S4 that the wave energy becomes concentrated upon, and so
accentuates erosion at, the headland.
• Breaking point: A wave which breaks as it hits the foot of a cliff releases most energy and
causes maximum erosion. If the wave hits the cliff before it breaks, then much less energy is
transmitted whereas a wave breaking further offshore will have had its energy dissipated as it
travelled across the beach
• Wave steepness: very steep destructive waves formed locally (sea) have more energy and
erosive power than gentle constructive waves formed many kms away (swell)
• Depth of sea: steeply shelving beach creates higher and steeper waves than one with a gentle
gradient
• Length and direction of fetch: longer the fetch, the greater the time available for waves to
collect energy from the wind
• Beach width: cliffs containing a readily available supply of sand will form wider beaches, leading
to a greater loss of energy (waves take longer to pass over it) and the slower the rate of cliff
erosion
8.
Factors affecting cliff erosion
Geological structure
If the bands of underlying rocks are set at
right angles to the coast, the rivers cut a
series of deep valleys, often parallel, to reach
the sea. The best known example of this type
of coastline is in south western Ireland. These
are known as discordant coasts.
If the structure of the land
runs parallel to the coast, a
much less indented coastline
can be seen. The best example
of this type of coastline is the
Adriatic coast of former
Yugoslavia, although the
coastline of Dorset around
Lulworth Cove is similar, with
the geology of parallel outcrops
to the coast. These are
concordant coastlines.
9.
Factors affecting cliff erosion
Discordant coastline
Old Harry rocks
Swanage Dorset
10.
Factors affecting cliff erosion
Concordant coastline
Lulworth Cove Dorset
PAST EXAM QUESTIONS
For each of the three locations in the diagram identify
one strategy that may be used to protect the coastline.
[3 marks] Nov 2003
Evaluate the effectiveness of each of the three chosen
strategies. [7 marks] Nov 2003
For each of the three locations in the diagram identify one strategy that may
be used to protect the coastline. [3 marks] Nov 2003
• For each of the three locations shown in the diagram, responses should identify
one appropriate technique, hard or soft, to reduce the impact of erosion.
• Cliff face strategies – cliff pinning, cliff modification, drainage, gabions,
vegetation cover.
• Cliff foot strategies – sea walls, gabions, baffles, boulders, tetrapods.
• Beach management strategies – rip-rap, groynes, beach recycling, nourishment.
• Marks should be allocated on the basis of [3 % 1 mark].
(ii) Evaluate the effectiveness of each of the three chosen strategies. [7
marks] Nov 2003
• Good responses should look at the effectiveness of hard management
techniques and soft management techniques. Techniques like sea walls allow
little movement, whereas boulders or tetrapods and baffles are more absorbent
of wave energy. Beach nourishment is more aesthetic but it is very expensive to
maintain.
• Stronger responses may look at the failures that have occurred with
management strategies. In many areas the impact of many of the protection
methods is felt elsewhere along the coastline. It is an open system and changes
to one component may affect others. The question is open ended and strong
responses will look at the positive and negative impacts and should focus on
appropriate examples.
• Marks should be awarded on the basis of [3 % 2 marks] plus [1 mark] for
additional insight or comment.
State two physical factors which would
need to be considered when designing
the most effective size of revetment.
[2 marks] Nov 2006
Explain how this type of structure can
prevent coastal erosion. [3 marks] Nov
2006
State two physical factors which would need to be
considered when designing the most effective size
of revetment. [2 marks] Nov 2006
• [1 mark] for each of any two relevant factors. These
include wave height, tidal range, the angle of coastal
slope, length of vulnerable coast, wave frequency, and
wave energy.
(ii) Explain how this type of structure can prevent
coastal erosion. [3 marks] Nov 2006
• Revetments prevent coastal erosion by reducing the
effects of wave action
• [1 mark]. Wave energy is absorbed by open armoured
rocks [1 mark]. The final
• [1 mark] may be awarded for any additional relevant
detail.
Referring to examples, examine
how human activity affects
erosional processes and
landforms in coastal areas. [20
marks] May 2006
Human activity often affects the type and rate of erosional processes
on coasts. In general, deliberate, planned actions (such as sea walls,
cliff consolidation and revetments) are often undertaken to reduce the
rate of erosion, protecting constructions (settlements,
communications, industry) and existing land uses. Unplanned
interventions (as a result of such activities as dredging and the
removal of beach material for construction) sometimes result in faster
rates of erosion. The precise area where the unplanned intervention
takes place is not necessarily the same area as where the erosion rate
is altered.
Alternative organization of the material should also be credited, for
example human impacts on dune systems that increase wind erosion.
Stronger responses might discuss both temporal and spatial aspects of
the title, as well as clearly relate human activity to one or more of the
specific erosional processes of abrasion (or corrasion), hydraulic action
and solution (of limited relevance in this context). The strongest
responses may also try to link human activity to rates of attrition, and
thereby to variations in pebble size.
As regards landforms, it is likely that most discussion will focus on
cliffs, but stronger responses may also refer to a variety of other
landforms. Depositional landforms should not be credited.
It is not necessary for the discussion of processes and landforms to
be equal for the awarding of full marks, but those responses that fail
to examine both may not move beyond band E.
Analyse the natural factors
that cause different rates of
coastal erosion. [8 marks]
Nov 2006
Past exam answer
• Analyse the natural factors that cause different rates of
coastal erosion. [8 marks] Nov 2006
• Relevant natural factors include differences in fetch and
exposure to wave action,
• the lengthening of any abrasion (wave-cut) platform (which leads
to a dissipation
• of wave energy), changes in base level, differences in rock type
and structure, cliff
• morphology (angles, height, vegetation cover). The analysis
should be focused
• clearly on the rate of erosion, and not other characteristics.
Credit responses
• which mention variations in time as well as space.
Past A level questions
1. Explain how wave refraction influences the rate of
marine erosion 4 (Jan’01)
2. Explain why rates of marine erosion may very over
time 4 (June’01)
3. Explain how rates of marine erosion may be
influenced by geological factors 3 (Jan’02) 4
(June’02)
4. Explain the role of 2 factors that influences the rate
at which waves erode 4 (June’03)
5. Explain the influence of human factors on rates of
marine erosion 4 (Jan’04)