Input: Agents of coastal changes
Effects of wave erosion on the coast
Effects of wave transportation and deposition on the coast
The coast is the area where the land meets the sea. It includes the beach and the cliff behind the
beach. there are two categories of coasts.
a. Primary coasts, which are created by nonmarine processes. The processes include erosion,
deposition, and tectonic activity. River deltas are an important example.
b. Secondary coasts, which are formed by marine action. The processes include deposition of
sand by waves and currents, and growth of reefs by corals. Barrier islands are an important
The coastline marks the seaward limit of the land which is permanently exposed. It is the
highest point reached by storm waves.
The shore is the area of land which lies between the highest tide level and the lowest tide level. It
is covered by the sea at high tide but exposed at low tide. The shore is divided into three parts:
backshore, foreshore and inshore.
The shoreline is the line demarcating the sea and the shore. The position of the shoreline
fluctuates with the tide.
Waves and Wave Action
Waves are undulations of water when wind blows over the surface of the sea. They form as a
result of the transfer of energy. As the wind blows over the surface of the sea, friction is created
producing a swell in the water. The energy of the wind causes water particles to move in a
circular wave orbit inside the swell. This moves the wave forward.
In the open sea, the motion of the waves is transmitted by the circular movement of the individual
water molecule as the lack of frictional drag with the ocean floor in the deep water allows each
water particle to move in a circular wave orbit.
As the waves approach the land, and the depth of the water decreases to half the wavelength,
the size of the circular wave orbit decreases and the orbital motion of the water, retarded by
friction with the sea floor, forms an elliptical motion. The speed of the wave transmission
decreases, the wavelength decreases and the wave height increases. Eventually the wave
height reaches a point of instability and the wave breaks.
The wave either
a. collapses, at some distance off the shore, to produce a spilling breaker, if the water offshore
is relatively shallow, or
b. topples forward and is forced to break against the land forming a plunging breaker, if the
water remains relatively deep right up to the shore.
In both cases, the water steepens, loses form and become a breaker. The water rushes up the
slope of the shore as the swash. Then the water runs back down the slope of the shore as the
Energy of Waves
The size and energy of a wave is influenced by:
● the length of time that the wind has been blowing (duration)
● the strength of the wind (velocity)
● distance of open water over which the wind blows (fetch)
1. The energy of destructive power of the waves increases with increasing duration,
increasing wind velocity and increasing distance of fetch.
2. The longer the wind blows, the stronger the wind and greater the fetch, the more powerful
Coasts facing open sea are more susceptible to wave erosion while coasts in sheltered
locations with weak wind and short fetch, are more prone to wave deposition.
Type of waves
Long, low waves
Long wavelengths, up to 100m.
Low frequency, a wave period of 6
9 waves per minute
Associated with calm conditions
Short, high wave
Short wavelength, up to 20m.
High wave frequency, a wave period
of 11 15 minute
Associated with stormy conditions.
Waves slide forward in shallow
water due to gentle offshore slope
Also known as spill waves
Waves curve downward in deep
water due to steep offshore slope
Also known as plunge waves
Swash is more powerful than
backwash, so materials are carried
up the beach
Backwash is more powerful than
swahs, so materials are carried
down the beach
Factors Governing the Work of Waves and the Character of Coastal Landforms
● Types of wave action
→ waves that break against the land are associated with wave erosion.
→ waves which break before reaching the shore are associated with wave deposition
→ constructive and destructive waves
● Geology of Coastal Rocks
→ type of rocks
→ resistance of rocks to weathering and erosion
→ direction of dipping of the rock strata
→ presence, absence and arrangement of lines of weakness (e.g. joints, fissures, faults)
● Relief of coastal slope
→ steep coastal slope is associated with wave erosion because waves, especially those driven
by strong winds over long fetch, reach the coast in deep water and break against the land or cliff
→ Gentle coastal slope is associated with wave deposition as waves are retarded by friction with
the bottom of the slope in shallow waters. The waves tend to break before reaching the coast
● Orientation of the coast
→ The position of the coast in contrast to open sea or ocean determines whether the coast is
exposed to oncoming winds that have blown over long fetch or not
→ the aspect of the coast in relation to strong prevailing winds determines whether the coast is
exposed to high energy waves
→ exposed coasts are more susceptible to wave erosion
→ sheltered casts are more favourable to wave deposition
● Relative changes in Sea level
→ may result either
a. from a rise of the mean sea level or submergence/sinking of the coastal land or
b. from a fall of the mean sea level or emergence/uplift of the coastal land
→ Effect of glaciation in coastal areas the drowning of glaciated valleys when sea level rises
→ Effects of Volcanic activity in Coastal Areas occurrence of tsunamis, in relation to volcanic
activity, increases rate of coastal erosion
→ Effects of coastal growth in coastal waters coral growth offshore tends to dissipate energy
before waves reach the coast, thus helping to protect the coastline.
→ human impact modifies the natural coastal landscape by activities like reclamation,
construction works, dredging, building typhoon shelters and groynes
The Work of Waves and Resulting Coastal Landforms
● Wave refraction is the bending of wave fronts as they approach a shore so as to break
almost parallel with the shore. In deep water, wave fronts are essentially parallel to one
another. As they approach the shallow waters of the shore, the retarding influence of
shallow water or frictional drag with the sea floor, causes the waves to slow down and
the wave fronts to bend. Wave refraction occurs when:
→ along an irregular coast the retarding influence of shallow waters off the headlands before
the shallow waters of the bay causes wave refraction. Wave energy is directed and
concentrated more towards headlands rather than bays. Thus, erosion is more intensive at the
headlands and deposition more common in bays. This combined effect tends to reduce
→ along a straight line with approaching waves from oblique direction
● Wave erosion
wave erosion is accelerated by the following conditions:
→ exposure to strong prevailing winds
→ great wind velocity
→ long fetch
→ large wave size
→ steep coastal slope/deep waters right up to the coast
→ rapid rate of weathering of coastal rocks
→ weak coastal rocks which are less resistant to wave attack
→ presence of lines of weakness (e.g. cracks, fissures, joints, and faults, nonresistant dykes
and bedding planes)
→ large quantity of rock materials carried by waves
The 4 main types of wave action are:
1. hydraulic action
wearing away of coastal rocks when waves striking the cliff face compresses air in cracks on
cliff face. this puts tremendous pressure on the surrounding rock. the air then expands
explosively when the waves retreat resulting in a sudden release of pressure. This process
shatters the rocks, opens up and enlarges the cracks
the mutual wearing down of the materials (e.g. sand, pebbles and boulders) which are
transported by waves. These particles become smoother, rounder in shape and smaller in size.
3. Corrasion (abrasion)
the wearing of the coastal rocks or cliff faces by materials (e.g. sand, pebbles and boulders)
carried and hurled against the coast by waves
4. Corrosion or solution
the dissolving of soluble minerals in coastal rocks by sea water or waves. the dissolved
minerals may crystallize from evaporating sea water spray, and this helps detach mineral grains
from coastal rocks. the dissolved minerals may also be removed by sea water in solution. The
remaining rocks become weakened and are more susceptible to wave erosion by abrasion and
Wave Erosional Features
a. Headlands and Bays
When waves armed with rock debris lash against the shores or along coast that have alternate
bands of resistant (harder) rocks and less resistant (softer) rocks, the process of continued
erosion of rocks of different resistance causes the hard, resistant rocks like limestone and chalk
to resist erosion and persist and the soft, less resistant rocks like clay, sand and gravel to be
worn down easily.This eventually gives rise to an irregular coastline of headlands and bays. As
the headlands become more exposed to the full force of the wind and waves, it will become
more vulnerable to erosion than the sheltered bays.
b. Cliffs, wavecut platforms and offshore terraces
When high energy waves reach land with steep slopes, they erode the weaker arts of the steep
slopes to produce a notch. Continued erosion and undercutting enlarges the notch to form a
steep rock face called a cliff. Undercutting at the base of the cliff, together with the removal of
eroded materials, causes the cliff to retreat landwards, exposing a flat terrace at the foot of the
cliff called a wavecut platform.
After a period of time, the cliff becomes steeper and retreats further landwards while the
wavecut platform becomes wider. When the wavecut platform is buried by deposits, causing a
belt of shallow water which decreases the waveenergy, erosion of the wavecut platform
The eroded materials which are transported away are deposited in the offshore zone to form an
c. Caves, arches, stacks and stumps
Along an irregular coast of headlands and bays, waves converging on headlands, due to wave
refraction, often attack and widen lines of weakness into hollows called caves. When two caves,
on opposite sides of the headland, join to form a complete opening, the cave top remains as an
arch. With further erosion, the arch collapses, leaving behind the seaward pillar of the rock. In
time, it is completely removed by wave erosion.
d. Caves, Blow Holes and Geos
A blowhole is a nearvertical cleft or cylindrical tunnel leading from the rear top of a sea cave
upward to the land surface above.
Due to the presence of near vertical lines of weakness above the sea cave, waves surging in
during high tides tend to force and compress the air into the lines of weakness. When the waves
retreat suddenly, resulting in the opening and widening of the lines of weakness along the cave.
Ultimately, part of the roof of the cave collapses, producing a blow hole.
Continued wave erosion may widen the blow hole till the entire roof of the sea cave collapses to
form a long, narrow, steepsided inlet, called a geo. A geo may also develop when erosion
extends a sea cave landwards, causing the cliff to be undercut by waves. The top portion of the
cave may collapse, resulting in a geo.
Wave transport is the movement of load (e.g. silt, clay, mud, sand, pebbles, etc.) along the
shore and the seabed. Waves operate as an agent of transport in two ways:
1. Beach drift.
When waves break obliquely at the shore, the swash moves obliquely up the shore/beach but
the backwash runs back at right angles to the shore/beach. Eroded materials are thus gradually
carried along the shore/beach by the combined zigzag movement of the swash and backwash
called beach drift.
2. Longshore drift
Eroded materials moving in a zigzag manner on and off the shore create a net lateral
movement parallel to the coast, called longshore drift.
Features Produced by Wave Deposition
● Bay delta
● Cuspate bar
● Bay mouth bar
● bayhead bar
● complex recurved spit
A gently sloping platform formed by the accumulation of material (e.g. mud, sand, pebbles.
cobbles) deposited by constructive waves on/along the shore, between the highest and lowest
Beaches are classified according to position:
● Bayhead beach.
→ formed at head of a bay between two headlands. The waves which reach the head of a bay
are relatively lowenergy waves and carry finer particles to the shore.
● Bayside beach
→ formed at the sides of a bay. It is usually composed of coarser sand particles, or a mixture of
sand, gravel and pebbles.
● Baymouth beach (or headland beach)
→ formed at the tip of a headland. It consists of coarser particles, gravel and boulders
Beaches are also classified according to type of deposits, for example, sandy beach, shingle
beach and boulder beach.
Spit, Recurved spit and Tombolo
A spit is a narrow ridge of sand or shingle deposited by longshore drift at a sharp or abrupt turn
of the coastline or across the mouth of a river. the occurrence of longshore drift, running parallel
to a relatively straight coast, causes beach materials to be laterally transferred even at the turn of
the coastline. As slack water occurs at the turn, the waves lose energy and deposit the sand or
shingle in the form of a ridge, called a spit.
One end of the spit is attached to the mainland while the other end is free and projects into the
sea. As a spit grows and extends into deeper water, wave action causes the free end to be
curved towards the land, enclosing a water body called the lagoon. Such a spit is called a hook,
or recurved spit.
When the spit extends seawards and joins up an offshore island to another island or the
mainland, a tombolo is formed.
Other features associated with a spit:
i. baymouth bar when two spits extend from opposite sides of the bay (as they longshore drift
follows different directions at different times of the year), and eventually meet and join.
ii. cuspate bar, where two spits develop on the two sides of a headland and they eventually
meet and join each other
iii. cuspate foreland when the water body, enclosed by a cuspate bar, is gradually silted to
form a piece of land.
They are narrow ridges of sand and/or gravel deposited by waves across a bay, usually in a
direction parallel to the shore. It may be continuous or semicontinuous with breaks in between.
When a bar is first formed, both ends are free (i.e. not attached to the land). Subsequent and
continued deposition may cause its end(s) to extend to join the land.
Like beaches, bars are classified according to position:
● Bayhead bar when wave refraction on bay shores sweeps materials towards the head
of a bay to form a ridge of accumulated material rising from the sea floor, above the low
● Baymouth bar When a spit grows form the headland and joins with the headland on the
opposite end, the feature formed is a barmouth bar. The enclosed body of water in the
bar, by the bar, is called a lagoon.
● Offshore bar may develop where waves break offshore, due to gentle slopes under
water, and deposit the material. It is free at both and and runs parallel to the shore.
Silty or muddy platforms deposited by waves and/or by rivers along gently sloping shores. They
may be encroached by saltloving and salttolerant vegetation (e.g. mangrove), to form a swamp