Waves and coastal erosion
KEY TERMS: Weathering – the breaking down of rocks. Erosion – the wearing away of rocks.
Mass Movement –
rocks move downhill
due to gravity.
Slumps happen when
the rock (clay) is
saturated with water
and slides downhill.
Sliding happens when
loosened rocks tumble
down the slope.
Constructive waves
have a strong swash and a weak
backwash. Deposition is greater than
erosion. They build up beaches.
Destructive waves
have a weak swash and a strong
backwash. Erosion is greater than
deposition.
Attrition: Material carried by the waves
bump into each other and so are smoothed
and broken down into smaller particles.
Corrosion/solution:
This is the chemical
action of sea water.
The acids in the salt
water slowly
dissolve rocks on the
coast, especially
limestone.
How waves
erode the
coast
Abrasion/Corrasion:
Waves throw sand and
pebbles against the rock.
Hydraulic action: This process
involves the force of water
against the coast. The waves
enter cracks in the rock and
compress the air within the
crack causing a minor
explosion. This process is
repeated continuously.
Landforms of coastal erosion
Headlands and bays
Wave-cut platforms
The erosion of
cliffs can create
wave-cut
platforms – areas
of flat rock at the
base of the cliff.
Caves, arches, stacks and stumps
a) Fault in the gradually gets bigger due to weathering.
b) Erosion by waves widens the weakness in the cliff to
form a cave.
c) Waves cut through the headland to form an arch.
d) Collapse of arch due to gravity to form a stack.
e) The stack is undercut all the way round as wave cut
notches form. It eventually collapses to leave a stump
which is covered at high tide.
Coastal transportation and deposition
How waves
carry material
TRACTION – bigger pebbles
and cobbles and rolled along
the sea bed.
SUSPENSION – fine
sediment is carried as
suspension in the water, this
is what makes water look
dirty.
SALTATION – small
pebbles moved when
one pebble hits another,
making it bounce. This
bouncing sets up a chain
reaction.
SOLUTION –
dissolved material is
carried along and
you can’t see it.
(invisible)
Longshore drift carries material along the coast
1) Waves approach the coast at the angle of the
prevailing (most common) wind direction.
2) Swash pushes sand and gravel up the beach at
the same angle.
3) Backwash carries sand and gravel back down
the beach at 90⁰ under the force of gravity.
4) Sand and gravel move along the beach in a
zigzag fashion.
If longshore drift continues along the
spit, it may join up with the coastline on
the other side to form a bar.
Rising sea levels and coastal habitats
Sea levels are rising because:
- Global warming is warming up the
seas – water expands as it warms so
take up more space.
- Some areas of land are changing
height – some are sinking so the sea
level seems to be rising.
Salt marshes are a special type of coastal
habitat. The provide habitats for birds
and other marine life. They can also
protect against floods.
← red areas show the places at risk
In the UK, impacts of rising
sea levels might include:
Economic:
Loss of farm land in East Anglia.
London might be flooded.
Political:
Some settlements need defending from erosion and
flooding but there will not be enough money to protect
everywhere.
Social:
Storm surges (big coastal floods) could cause loss of life.
Environmental:
Destruction of natural ecosystems.
Conservation of salt marshes: managed retreat
Salt marshes under threat from sea
level rises and development
Managed retreat strategies
encourage salt marshes to develop
The RSPB wants more salt marshes
and is monitoring visitor numbers
and where visitors can go to
protect the areas.
This is an example of sustainable
management – keeping the salt marshes for
future generations to also use the habitats.
Cliff collapse
How fast a coast retreats depends on:
1) rock type – hard rocks erode more slowly than soft rocks.
2) number of joints and faults – rocks with more faults and
joints erode more quickly than rocks with fewer joints and
faults.
3) fetch – if the wind blows over a large area of sea (fetch),
the waves are stronger and will have a greater impact on
the cliffs.
4) cliff exposure – cliffs exposed to string winds and weather
will erode more quickly than sheltered cliffs.
5) storms – cliffs exposed to frequent storms will erode more
quickly.
6) sea defences – some cliffs are protected by sea defences
which will slow retreat down.
The consequences of cliff erosion
Some places decide to spend money on
protecting cliffs so they do not erode so quickly. If
they decide not to do this (as coastal
management is often very expensive), cliff
erosion can have a range of effects.
Hotels and caravan parks abandoned.
Loss of roads
Houses destroyed
Farmland lost
Loss of income
Case study: Holderness Coast
• 60km of coast in East Yorkshire.
• The cliffs are eroding at 2m per year, the fastest rate in Europe.
• The coast is very exposed, and waves have a long fetch over the North Sea.
• The cliffs are made of soft, easily eroded boulder clay.
• The beaches are thin and narrow as most material is washed out to sea or moved along
the coast by longshore drift.
• The waves are mainly destructive waves.
• As sea level rises, more and more of the cliffs come under attack from the waves.
• Dozens of villages have been washed away; property prices have slumped.
Managing the coast 1
Where coasts are being rapidly eroded, there are 3 options.
1.Do nothing – allow erosion to occur and spend the money developing towns and
communities instead
2.Hold the line – maintain existing defences but don’t develop more
3.Advance the line – invest money in more defences along the coast, protecting the
homes and businesses of those along the coast
Once the management strategy has been decided, they choose which hard of soft
engineering techniques should be used.
Hard Engineering
Methods:
Hard Engineering
usually involves heavy
construction and use
of concrete and steel
to reduce erosion.
Method
Benefits
Costs
Sea walls
Protection is generally good
Land and buildings protected
Wave energy is reflected as the wall is
curved
Expensive to build - £10000 per m
Not attractive to look at
Restrict access to beach
Do not protect against large storms
Groynes
Beaches are protected as groynes trap
sediment/sand from longshore drift so
cliffs behind are protected
Expensive to build - £5000 per m
Need rebuilding every 10 years
Trapping sand in one place means there is less sand
elsewhere so more erosion
Revetments
Not unslightly
Allow longshore drift to take place
Can be £1000 per m
Need regular maintenance
Rip rap
Absorbs wave energy
Looks natural
Can be £500 per m
Can obstruct access to shoreline
Offshore reef
Create new habitat
Only seen at low tide
Can be £600 per m
Can cause problems for boats
High upkeep costs
Managing the coast 2
Soft Engineering Methods: Soft Engineering techniques are more environmentally friendly
and are considerably cheaper to implement.
Benefits
Costs
Beach
replenishment
Acts as shock absorber protecting any
structures behind
Natural looking and good for tourism
High cost depending on source of sand
Sand needs to be replenished every few years
Managed retreat
Relatively cheap
Allows natural processes to occur
People still want compensation or insurance
Some businesses will be lost or damaged
Cliff regrading
Prevents rapid retreat by slumping or rockfall
Makes beaches and shorelines safer
Can be expensive
Still requires other management techniques to
prevent erosion at base
Cliff drainage
Removes water from soft rocks making them
harder to erode
Can be expensive
Still requires other management techniques to
prevent erosion at base
Case study: What
have the one on the
Holderness Coast?
Method
PLACE
Hornsea (pop. 7,500) Holiday resort with bathing
beach of EU quality standard.
Mappleton (pop. 100) Small village (on the B1242
coast road) in a farming area
Spurn Head
Sand and shingle spit with a lifeboat station, the
Humber Pilots base and a nature reserve. It has
Heritage Coast status.
EXISTING COASTAL DEFENCES
Wooden groynes, and concrete sea wall. Built
in early 1900s and repaired in 1980s.
Rock revetment and rock groynes, with graded
cliffs. Built in 1991 at a cost of £1.9m.
Abandoned sea defences.
Test yourself….
What are the 4
different ways
that waves erode?
How are stacks
formed?
What is the
difference
between a bar
and a spit?
What are the
advantages and
disadvantages of
hard engineering?
How is material
transported along
the coast?
What are the
possible impacts
of rising sea levels
on the UK?