Coastal geography waves, features of erosion and deposition, longshore drift

CSEC GEOGRAPHY - COASTS

1. CSEC GEOGRAPHY
COASTAL PROCESSESS
COAST: WAVES
O.JOHNSON
OCTOBER 2018

What cause waves:
Waves are the most commonly formed by friction as the wind blows over the
surface of the sea.
What cause waves to break?
In deep water the surface waves form part of a circular movement of water. This
explains why there is actually horizontal movement of water in the oceans.
However, look what happens as the waves get nearer to the coast:
 As the sea near the shore is shallow the circular motion of the waves is
interrupted by friction with the seabed
 The water motion becomes more elliptical ( shaped like a rugby ball)
 The wave grows in height and begins to topple forward
 Eventually the waves break on the shore. Water moves up the beach as the
swash and then drains back down the beach as backwash.





When waves near the coast, the bottom of the wave is slowed by friction with the
sea bed. Because the top of the wave is experiencing less friction, it moves faster
and eventually topples over the bottom of the wave and breaks.

2. The Power of the waves when they reach the coast depends on three factors:
The distance of open water over which the wind has blown. This is called the
fetch. The longer the fetch the more the powerful the waves
Strength of the wind- The stronger the wind the more powerful the waves.
Duration of the wind – If strong winds have blown over a long period of time, this
will result in powerful waves.



Wave Terminology
Crest: The top of the wave.
Trough: The low area in between two waves.
Wavelength: The distance between two crests or two troughs.
Wave height: The distance between the crest and the trough.

3. Wave Frequency: The number of waves per minute.
Velocity: The speed that a wave is traveling. It is influenced by the wind, fetch and
depth of water.
Swash: The movement of water up the beach.
Backwash: The movement of water back down the beach after the swash is
completed
Constructive and Destructive waves
It is possible to identify two types of waves:
Constructive Waves:
Constructive waves are low but powerful waves that surge up the beach when
they break. Their swash is much more powerful than their backwash, much of
which percolates through the beach as the water flows back to the sea.
Constructive waves are created by storms many miles away from the coast and
they travel fast across the ocean.
They are called ‘constructive’ waves because they transport beach material to
the top of the beach, thereby ‘ constructing’ it. Because the swash is stronger
than the backwash they tend to deposit material and build beaches up

4. Destructive waves
Destructive waves are essentially the opposite of constructive waves. They are
taller and tend to crash down onto a beach rather than surging up the beach.
There is little swash but the backwash is powerful. This leads to erosion of the
lower beach, hence the term destructive. Destructive waves are usually formed
during local storms that are centred close to the coast.
Because the backwash is stronger than the swash, destructive waves erode and
transport material away from beaches.

5. COASTAL EROSION AND EROSIONAL FEATURES
Abrasion/Corrasion:
When waves approach the coastline they carry material such as sand, shingle,
pebbles and boulders. Abrasion occurs when these materials are hurled against
cliffs as waves hit them, wearing the cliff away. It is the most effective method of
erosion
Hydraulic action- This occurs when water is thrown against the land by breaking
waves, thus compressing the air which is contained in any cracks in the rocks.
When the wave retreats the air expands, thus exerting pressure on the rocks.
When repeated over a long period of time, the expansion of air in the cracks may
cause the rocks to shatter.
Attrition

6. As in the case of material carried by rivers, the material carried by waves also
becomes broken into smaller fragments. This is the result of the pieces of material
hitting against each other and against the land. Beach material is knocked
together in water reducing their size and increasing their roundness &
smoothness. THIS PROCESS IS NOT RESPONSIBLE FOR THE EROSION OF CLIFFS.
Corrosion/solution:
Seawater contains carbonic acid, which is capable of dissolving limestone. The
dissolving of soluble rock, such as limestone.
Coastal Erosion is most effective when the waves are powerful and contain a lot
of energy. These waves need to break at or close to the foot of a cliff if they are to
carry out erosion.

Features of Coastal Erosion
Headlands and Bays
Bays and headlands are formed in a very similar way to rapids (rivers topic). They
are formed when you get alternate layers of hard and soft rock. Alternating
layers of hard and soft rock allow the sea to erode the soft rock faster, forming a
bay but leaving hard rock sticking out, known as a headland. The altering rate of
erosion of hard and soft rock is known as differential erosion.
Bay: An indented area of land normally found between two headlands. Bays are
usually more sheltered so there is less erosive power, meaning you often find
beaches in bays.

7. Headland: A piece of land that sticks out into the sea. Waves refract around
headlands so they experience a lot of erosion forming features like arches and
stacks


Wave cut notch and Wave cut platform
A wave cut notch is simply a small indent at the base of a cliff formed when a cliff
is undercut by the sea. When a wave breaks on a cliff, all of the wave’s energy is
concentrated on one specific point and this section of the cliff experiences more

8. rapid erosion via corrosion and hydraulic action. This eventually leads to the
formation of a wave cut notch, when the cliff has been undercut

Wave cut platforms are made in a similar ways to waterfalls and gorges (rivers
topic). At high tide the power of the sea attacks and erodes the bottom of the
cliff. Over time this erosion creates a wave cut notch (basically an eroded hole at
the base of the cliff). As the wave cut notch gets bigger, the weight of rock above
the notch gets greater. Eventually the cliff cannot support its own weight and it
collapses. The process then starts again, with the erosion of the sea making a new
wave cut notch. As the process continues the cliff starts to move backwards
(retreat). Because the cliff is moving backwards a wave cut platform (an expanse
of bare rock) is created.

Caves, Arches, Stacks and Stumps
Waves are particularly good at exploiting any weakness in a rock, such as a joint.

9. Caves
Caves occur when waves force their way into cracks in the cliff face. An increase
in erosional activity can widen the crack over a period of time until it is large
enough to form a cave. The dominant processes of erosion are
abrasion/corrasion.
Arch
An arch is formed when two caves develop on each side of a headland. With
continuous erosion they will grow in size and eventually meet giving rise to a
feature known as an arch.
Stack
The roof of the arch has no support however and is highly susceptible to
weathering via exfoliation, salt crystallisation and biological weathering. As the
weathering continues, the roof of the arch will collapse leaving a stack, a tall, lone
piece of land sticking out in the sea.
Stump
This stack is exposed to the full force of the water and is weathered and eroded
heavily. The base of the stack receives a lot of erosion from hydraulic action and
corrosion and, eventually, the stack will collapse into the sea leaving behind a
small piece of land called a stump.

12. Blowhole: Cave sometimes develop in the face of cliffs. They develop when there
is a weakness in the rocks, which is enlarged by wave action. Waves pounding
against cliffs can exert great pressure, which causes the air in the cracks to be
suddenly compressed. The alternate compression and expansion of the air in the
cracks enlarges them, and in time they may develop into caves.
If there are vertical joints leading from the roof of the cave to the top of the cliff,
these also may become enlarged. In time a passageway may be opened up,
through which air and water can escape to the surface at the top of the cliff. Such
feature is known as a blowhole.
Caves, arches, stacks and stumps are usually found on headlands, where
wave refraction is causing erosion on three sides. The waves always look for
weaknesses in the headland (cracks and joints). If they find a crack or a joint
they will start attacking it. Hydraulic pressure will be the main type of
erosion. Overtime the crack may turn into a cave. Slowly the cave will get
bigger and cut all the way through the headland, making an arch. As the arch
gets bigger the weight of the arch roof gets too great and it collapses, leaving
a stack. The stack is then eroded by the sea leaving a stump.

14. LONGSHORE DRIFT AND COASTAL DEPOSITON FEATURES
There are many different types of sediment at the coast including beautiful white
coral sand, the more common yellow sand, pebbles (shingle) and mud. Once rock
fragments have been broken off a cliff or brought to the coast by rivers, they
enter the coastal (littoral) transport system. Waves are very effective
transporters of sediment. After a storm, beaches can look very different.
Coastal transportation
Sediment is moved by the waves in a number of ways.
Traction: This is where heavier particles are slowly rolled along the sea bed.
Saltation: Some particles move in a bouncing manner as they are disturbed by
other particles knocking into them. They are too heavy to be carried by the water
but light enough to ‘hop’ along the sea bed.
Suspension: Lighter particles can be picked up and carried within the water. Sand
is most commonly transported in this way.
Solution: Dissolved chemicals will be transported in solution. Limestone (calcium
carbonate) is often transported in this way before precipitating out of solution to
form new limestone deposits on the sea bed. This is a very common process in
the warm Caribbean seas.
Coastal deposition
One important factor affecting the movement of sediment and its deposition at
the coast is the angle at which the waves approach a shoreline. If the waves

15. approach parallel to the coast, sediment will simply be moved up and down the
beach. There will be very little movement along the coast. Under these conditions
beaches will form in bays
If the waves approach a shoreline at an angle, sediment is transported along the
coast in zig-zag fashion. This process is called longshore drift and it results in a pile
up of sediment at one day of a bay.
Coastal deposition takes place in areas where the flow of water slows down.
Sediment can no longer be carried or rolled along and it has to be deposited.
Coastal deposition most commonly occurs in bays where the energy of the waves
is reduced upon entering the bay. This explains the presence of beaches in bays
and accounts for the lack of beaches at headlands where wave energy is much
greater.
Longshore Drift

This is the process of waves moving (transporting) material (load) along a
coastline. When Waves approach the shore at an angle (usually in line with
prevailing wind direction) the swash moves material up the beach in this
direction. Backwash pulls material straight down the beach. The result is that
material is transported in a zig-zag fashion.
Longshore drift only happens when the waves hit the beach at an angle. It is the
process of the swash transporting material up the beach at an angle and the
backwash returning directly under the force of gravity that causes material to be
transported along the beach.

16. Features of Coastal Deposition
Beaches
The best known feature resulting from deposition by waves is the beach. This may
be made up of mud, sand, shingle (small rounded stones) or boulders. Beaches
are the result of longshore drift produced by constructive waves. Beaches usually
have a gently sloping surface. They are generally formed between high and low
tide marks. Sometimes, however, stormy conditions at high tide may throw

17. material up beyond the high tide mark, resulting in what is known as a storm
beach.
Most beaches in the Caribbean are made up of sand. Sandy beaches are most
often found in sheltered bays where they are called bayhead beaches. When
waves enter these bays they are forced to bend to mirror the shape of the coast.
This is called wave refraction. It is caused by the shallowing of the water as the
waves enter the bay. Refraction spreads out and reduces wave energy in a bay,
which is why deposition occurs here.

Spit
A spit is a long, narrow finger of sand or shingle jutting out into the sea from the
land. Spits are very common features across the world. It is formed when

18. sediment transported along the coast by longshore drift is deposited at a bend in
the coastline (usually at a headland). Even though there is a change in shape of
the headland longshore drift will continue and sediments will be continuously
deposited. With continuous longshore drift and deposition the spit will gradually
extend out into the sea further and further away from the mainland. The spit will
become larger over time.
Over time a spit become colonized by grass and bushes, and eventually trees will
grow. On the sheltered landward side of a spit, where the water is very calm,
mudflats and salt marshes will form. These are important habitats for plants and
birds. The end of the spit is usually hooked because of occasional winds and
storms that blow in the opposite direction of the prevailing wind called secondary
winds. The spit will never grow across to the next side of the headland because
the fast flowing river will disturb any deposition.

19. Tombolo
Tombolos are created through the process of longshore drift. A tombolo is
formed where a spit continues to grow until it reaches an island, forming a link
with the mainland, just like bridges. This feature is known as a tombolo. The
island of Scotts Head on the south coast of Dominica has become attached to the
mainland by a tombolo as a result of deposition of sediment by longshore drift
from east to west. The Palisadoes tombolo just south of Kingston, Jamaica is a
highly complex 13km tombolo that connects several offshore cays. It is the site of
Kingston’s Norman Manley International Airport.

20. Bar
A bar is a long narrow deposit of sand or shingle that usually forms parallel to the
coast. There are two main types of bar:
Baymouth Bar
Occasionally a sand or shingle spit extends right across a bay to form a baymouth
bar. There is often water trapped behind the in the form of a fresh water lake or
lagoon.

21. Offshore bar
An offshore barrier bar is a narrow sand or single deposit that runs parallel to the
coast just out to the sea. Along some stretches of coastline tidal currents and
waves combine to cause a build-up of sediment on the sea bed just offshore. This
feature is called a sandbank or offshore bar and it is very often extends for several
kilometers parallel to the coast. The calm. Shallow water trapped between the
offshore bar and the coast is called a seawater lagoon.
Offshore bars may lie just below the surface of the water, combed flat by
breaking waves, or they may emerge over time to form barrier islands. The barrier

22. off the coast of Miami in Florida form the densely urbanized part of the city called
Miami Beach