UNIT 5 WAVE EROSION AND DEPOSITION
TOPIC 2 Sustainable Development: Wealth Creation
Key unit competence: By the end of this unit, I should be able to categorize different features resulting
from wave action and their relationships with human activities.
1. What is the type of water body represented on the pictures above?
2. Identify the coastal landforms found on both pictures.
3. Explain the factors of the formation of the coastal landforms identified on
the map at right.
4. According to you, do you think the level of water in ocean is always the
same? Justify your answer.
5. If you need to be a business man / woman at the coastal regions,
explain the business opportunities that you may carry out there and the
challenges you can face.
5.1 Coastal landforms: definition of key terms
Learning activity 5.1
1. Using internet and different books, research and describe the difference
between the following terms related to coastal landforms:
a. Coast
b. Shore
c. Wave
d. Longshore drift
2. Identify the parts of a wave
Definition of key terms:
Below are defined different terms associated with the coastal landforms:
• Coast: A coast refers to the land that borders the sea or the ocean. It is a narrow
zone where the land and the sea overlap and directly interact. Some coasts are
made up of broad sandy beaches, while others form rocky cliffs or low-lying
wetlands. The shape of the coastline is determined by factors such as the typesof rocks present, the forces of erosion, and the changes in sea level.
• Shore: This is the area where land meets the sea or ocean. Different features
are found in this area resulting from erosion and deposition of sediments,
ocean or sea waves, as well as the effects of rivers as they join the sea. It is also
called coastline.
• Waves: Waves are defined as undulations of sea/lake water characterized by
well-developed crests and troughs (see the figure below). Waves are created
by the transfer of energy from the wind blowing over the surface of the sea.
When waves appear with high frequency they demonstrate the short wavelengths.
When they appear with low frequency they demonstrate long wavelengths.
However, there are special waves like Tsunamis that result from submarine
shock waves by earthquakes or volcanic activities.
• Longshore drift: This refers to the movement of sediments along the shore in
a zigzag pattern. Sediments produced by the erosive action of waves or sediments
transported by the river systems are moved by ocean waves and ocean
currents to form beaches. Sediments are as well moved offshore onto
the continental shelf. Most waves reach the shore at an angle of about 10° but this can
change depending on the wind direction. Each successive wave moves sand
at an angle along the beach face. Consequently, currents within the surfacezone flow along the shore as longshore drift (see the figures below).
Application activity 5.1
1. By using a diagram, demonstrate the parts of a wave.
2. Differentiate a shore from a longshore drift.
3. If you find an occasion to visit the Indian ocean coast in East Africa, describe
the coastal features you would be interested to discover.
5.2 Types, factors and action processes of waves
Learning activity 5.2
1. Using internet and other resources, describe the types of waves.
2. A coast may have the steep slope like cliff or gentle slope like beach.
Explain different ways by which waves may hit the coast depending on its
slope.
3. Visit a water body in your local area, observe the waves movement and do
the following:
i. Find out the causes of wave movement
ii. Identify the direction of wind on water surface
iii. Determine the direction of waves in water
iv. Describe the effect of wind on wave4. Explain how waves can cause erosion along the coast.
5.2.1 Types of waves
There are two main types of waves: constructive waves and destructive waves.
1. Constructive waves: These are waves whose swash is more powerful than
backwash. They are depositional in nature.
2. Destructive waves: These are waves whose backwash is more powerful
than swash. They are erosional in nature.
Other types of waves:
• Breaking waves: Formed when the wave collapses on top of itself. They are
of two types:
(i) Plunging breaker: The wave reaches a steeper beach and curls, moving over
a pocket of air. It travels very fast.
(ii) Spilling breaker: The wave reaches a sloping sandy beach, dispersing the
energy over a large area.
• Deep water waves/Swell waves: Are made up of a number of waves of
different lengths superimposed on each other. They are straight and long,
powerful, and travel great distance.
• Inshore waves: These waves drain the beach as a backwash.
• Internal waves: Formed due to the disturbances found between two water
masses of different density. They are high and become turbulent currents
when they hit a landmass.
• Kelvin waves: Formed due to lack of winds in the Pacific Ocean. They are high
and wide waves, warmer than the surrounding water.
• Progressive waves: Move with a steady speed, so they are called Progressive
Waves. They are of two types:
• Capillary waves: Formed when wind creates pressure over capillarity, the
binding force that holds the water molecules of the ocean surface together.
• Orbital progressive waves: Formed at the boundary of two liquids with
different density.
• Refracted waves: Travel in shallow water when they approach the shore. The
shallowness decreases the power of the wave and causes a curve. These are
usually seen near headlands and bays.
• Seiche waves: Caused due to the movement within a confined space. These
have long wavelengths and rarely result in any damage as their height is
generally short.
• Shallow water waves: Move in shallow waters. They are of two kinds:
• Tidal waves: Formed due to the gravitational pull of the sun and the moon on
the ocean.
• Seismic sea Waves/tsunami: Caused due to earthquakes beneath the ocean.
They travel extremely fast in open water, have significant height in shallow
water, and are very dangerous and devastating.
• Swell waves/Surging waves: Intense waves generating from the center
of a storm where the winds are strong. These expel little energy, travel long
distance, and break on distant shores.
5.2.2 Factors determining the strength of waves
The following are the major factors determining the strength of waves.
- Wind strength: Wind must be moving faster than the wave crests for energy
transfer to continue;
- Wind duration: Winds that blow for a short time will not generate large waves;
- Fetch: The uninterrupted distance over which the wind blows without
changing direction;
- Depth of water or roughness of sea bed: As waves enter shallow water their
speed decreases, wavelength decreases, and height increases. Waves therefore
tend to break in shallow water, for example over a bar at the entrance to a
harbor;
- Direction and speed of tide: If the tide direction is against the wind, this willalso increase wave height and decrease wavelength.
5.2.3 Wave action processes
The wave action includes erosion, transportation and deposition.
• Erosion: Several mechanical and chemical effects produce erosion of rocky
shorelines by waves. Depending on the geology of the coastline, nature
of wave attack, and long -term changes in sea-level as well as tidal ranges,
erosional landforms such as wave-cut, sea cliffs, and even unusual landforms
such as cases, sea arches, and sea stacks can form
• Transportation: Waves are excellent at transporting sand and small rock
fragments. These, in turn, are very good at rubbing and grinding surfaces
below and just above water level in a process known as abrasion. Longshore
drift, longshore currents, and tidal currents in combination determine the net
direction of sediment transport and areas of deposition.
• Deposition: Sediments transported by the waves along the shore are
deposited in areas of low wave energy and produce a variety of landforms,
including spits, tombolo, beaches, bars and barrier islands. Different types
of pediments are deposited along a coast, sometimes in the form of anaccumulation of unconsolidated materials such silt, sand and shingle.
Application activity 5.2
1. Explain the wave action processes.
2. Why do you think in some areas the wave action processes may occur
differently in nature?
3. Carry out a research in your local region to identify if there are wave
actions taking place.
4. Describe the particularities of Tsunami compared to other types of waves.
5. Explain the impact of wind and tides on the strength of the waves.
5.3. Formation of coastal landforms
Learning activity 5.3
1. Observe the following picture showing a coastal landform and answer thequestions that follow
a. What are the major factors for this landform to be formed?
b. Water level on photograph may increase or decrease. What are the causes
of such phenomena?
2. Using the internet and other geographical resources, describe the
following:
i. Wave erosion featuresii. Wave deposition features
5.3.1. Factors influencing the formation of coastal landforms
The following are the major factors influencing the formation of coastal landform:
- Tides: Tides are greatly influencing forces of coastal landforms. They are
commonly semi-diurnal (12-hour cycle). The rise and fall of water levels
produce oscillating currents known as tidal streams. Tidal currents can
transport large quantities of sediments, especially at the mouths of estuaries.
The tidal amplitude also determines the sediments deposition or erosion and
keep redefining the shoreline of coastal landforms.
- Waves: Waves contribute to the erosion of shore. The greater the wave action,
the higher is the erosion and sediment movement. Where the shoreline is long
and flatter, the wave energy gets dispersed. Wherever there are rock formations,
cliffs and short shore area, the wave energy is high. Strong waves can pick up
sediments from deeper waters and make them available for transportation by
the coastal currents. The larger the wave, the larger the particle it can move.
Storm waves can even move boulders. Even small waves can lift the sediments
and deposit along the coastal shoreline.
- Longshore currents: Parallel movement of water is known as longshore current
and it extends up to the zone of breaking waves from the coastal shoreline. As
the long shore currents are formed by refracting waves, the direction of flow
will depend upon the angle of the wave which in turn depends upon the wind
directions. If the wind direction is balanced, the sediment movement is also
balanced. If the wind movement and resultant wave action dominate in one
direction great volumes of sediment may be moved in one direction.
- Weather elements: The elements of climate, such as wind, rainfall and
temperature play an important role in formation of coastal landforms. Winds
are directly related to the intensity of waves. Landforms like coastal dunes are
created by wind action. Temperature is required for physical weathering of
sediments. Rainfalls provide runoff for producing and transporting sediments
from land to seashore.
- Gravity: Gravity is an important factor for the development of coastal
landforms. Gravity is indirectly involved in the movement of wind and waves
as well as in downward movement of sediments.
- Nature of coastal rocks: Soft rocks are easily eroded hence forming erosional
features like bays while hard or resistant rocks lead to the formation of
headlands.
5.3.2 Landforms produced by wave erosion
The coastal features formed due to marine erosion by sea waves and other currents
and solution processes include cliffs, caves, indented coastline, stacks, chimneys,
arch, inlets, wave-cut platforms.
Geography Senior Six Student Book 117
- Cliffs: A cliff is a steep rocky coast rising almost vertically aabove sea water.
Cliffs are very precipitous with overhanging crest. The steepness of vertical
cliffs depends on the following: lithology of the area, geological structure,
weathering, erosion of cliff face and marine erosion of cliff base.
- Wave-cut platform: Rock-cut flat surfaces in front of cliffs are
called wavecut platforms or simply shore platforms. They are slightly concave upward.
The origin and development of wave-cut platforms is related to cliff recession. The
plat-form is composed of bare rock or it may contain a temporary deposit orrock debris, pebbles or sand.
- Sea caves: A sea cave is a natural cavity or chamber which develops along
the coast due to gradual erosion of weak and strongly jointed rocks by up
rushing breaker waves (surf currents). Sea caves are more frequently formed
in carbonate rocks (limestone and chalks) because they are eroded more
by solution processes. However, sea caves are not permanent as they are
destroyed with time.
• - Blowhole: This is a vertical shaft linking the cave to the surface. It is formed
when wave action attacks the back part of the roof of the cave. At the same
time, weathering by solution acts on the line of weakness from the surface
downwards to form a blowhole.
- Geo: Wave erosion may continue on the roof of the cave along the blowhole.
Hence, the roof of the cave may collapse to form a long and narrow sea inletknown as Geo.
Stack/ Column/Pillar: A stack is an isolated rock monolith or pillar rising
steeply from the sea. It is a former part of the adjoining land that has become
isolated from it by wave erosion, probably after having formed part of a marine
arch.
- Sea arch: A sea arch is a natural opening through a mass of rock limestone or
boulder clay. It is most commonly seen on the sea coast where waves have cut
through a promontory. When the keystone of the marine arch collapses, thefeature will become a stack.
5.3.3 Landforms produced by wave deposition
Sediments transported along the shore are deposited in areas of low wave energy.
They produce a variety of landforms, including spits, tombolo, beaches, bars and
barrier islands. Different types of pediments are deposited along a coast, sometimes
in the form of an accumulation of unconsolidated materials such as silt, sand andshingle.
• Spits: A spit is an embankment composed of sand and shingle attached to the
land on one end and projecting seaward. It may form parallel to the coast and
stretch several kilometers. It may also grow at an angle across an estuary. Spits
are formed when materials are transported and deposited by the long shoredrift, mostly where the orientation of the coast changes.
• Tombolo: It is a spit which grows seawards from the coast and joints to anisland.
• Beaches: A beach is located on a wave-cut platform of solid rock and is generally
of a low gradient with a gently concave platform. Beaches may extend for
hundreds of kilometers. Beaches are classified into: sand beach, shingle beach,and boulder beach.
• Bar: A bar is an elongated deposit of sand, shingle or mud occurring in the sea.
It is more or less parallel to the shoreline and sometimes linked to it. Bars may
be of submerged or emergent embankments of sand and gravel built along
the shore by waves and currents. One of the most common types of bars is thespit.
• Barrier Islands: Barrier Islands are long, offshore islands of sediments tending
parallel to the shore. They form long shorelines adjacent to gently sloping
coastal plains, and they are typically separated from the mainland by a lagoon.Most barrier islands are cut by one or more tidal waves.
• Cuspate foreland: This is a large triangular-shaped deposit of sand, mud and
shingles projecting seaward. It is a rare feature formed when two adjacent spitsgrowing towards each other at an angle join and enclose a shallow lagoon.
• Mud flats: These are platforms of mud, silt and river alluvium kept
by salt tolerant plants to form a swamp or marshland. They are formed when tidesdeposit fine silts along gently sloping coats in bays and estuaries.
• Coastal dunes: These are low-lying mounds of fine sand, deposited further
inland from a wide beach by strong onshore winds. They are common in aridand semi-arid coasts.
Application activity 5.3
1. Describe landforms produced by wave deposition.
2. Explain the factors influencing the formation of coast landforms.
3. Describe the formation of features produced by wave erosion.4. According to you, which landforms are likely to find around lake Kivu?
5.4 Importance of coast landforms produced by wave action
Learning activity 5.4
Observe the following picture and describe the economic activities that can becarried out in this area.
Coastal landforms produced by wave action are very important in different ways as
follows:
• Many of the world’s major cities are located in coastal areas, and a large portion of economic activities, are concentrated in these cities.
• There are different activities that take place in coastal zones including coastal
fisheries, aquaculture, industry, and shipping.
• Many of coastal landforms are very favourable for tourism that contributes to
the economic development of countries.
• The peculiar characteristic of coastal environments is their dynamic nature
which results from the transfer of matter, energy and living organisms between
land and sea systems, under the influence of primary driving forces that
include short-term weather, long-term climate, secular changes in sea level
and tides.
• Marine, estuary and coastal wetland areas often benefit from flows of nutrients
from the land and also from ocean upwelling which brings nutrient-rich water
to the surface. They thus tend to have particularly high biological productivity.
• It is estimated that 90 percent of the world’s fish production is dependent on
the nature of coastal landforms.
• The coastal landforms attract people as at present, two-thirds of the world’scities with a population of 2.5 million or more are situated near tidal estuaries.
Application activity 5.4
1. Give five examples of cities located in coastal areas, including at least
two located in East African Community.
2. Describe the main activities that are related to the Lake Kivu.
5.5. Types of coasts
Learning activity 5.5
1. Make your own research using internet or other geography resources and
identify different types of coasts.
2. Describe the areas subjected to fiords coasts.There are two types of coasts: Submerged coasts and Emerged coasts.
5.5.1. Submerged coasts
Submerged coasts fall into two categories: Submerged upland coasts and
submerged lowland coasts.
a. Submerged upland coasts
When the margin of an irregular upland area is submerged, a more or less indented
coastline is produced. It appears with islands and peninsulas representing the
former uplands, and with inlets indicating the former valleys. The following are thethree types of submerged coasts:
i. Ria coasts: Ria is a Spanish term widely used to describe a submerged
coastal valley or estuary resulting from a rise of the sea level. In the case
of a Ria coast, hills and river valleys meet the coastline at right angles. The
rias are characterized by funnel-shaped which decreases width and depth
as they run inland. The head of a stream which is small is responsible forthe formation of the valley at the inlet.
ii. Fiord (Fjord) coasts: A long, narrow inlet of the sea bound by steep mountain
slopes. These slopes are of great height and extend to considerable depths
(in excess of 1,000 m) below sea level. It is formed by the submergence of
glacially over deepened valleys due to a rising sea level after the melting
of the Pleistocene ice sheets. Fiords occur in western Scotland, Norway,
Ireland, Greenland, Labrador, British Columbia, Alaska, Southern Chile and
New Zealand. The main reason for their existence is the submergence of
deep glacial troughs and that is why fiords have many characteristics ofglaciated valleys.
iii. Dalmatian or longitudinal coasts: Dalmatian is a term derived from the
Yugoslavia Adriatic in which the coast runs parallel with the lineament of the
topography and probably with the underlying geological structure. A rise
of sea level (estuary) has drowned the coastal area, resulting in a coastline
of narrow peninsulas, lengthy gulfs and channels and linear islands. TheDalmatian coast tends to be straight and regular.
b. Submerged lowland coasts
These are formed when a rise in the sea level drowns a lowland coast. The sea
penetrates deep inland along rivers to form estuaries. The rise in base level causes
an increase in deposition by rivers leading to formation of mud flats, marshes, andswamps which are visible at low tides.
Emerged coasts comprise emerged highlands coasts and emerged lowland coasts.
a. Emerged upland coasts
The chief feature of an emerged upland coast is a raised beach or cliff-line, found
above the present zone of wave action. The coastlines are revealed as distinct
notches in the slope, backed by a cliff, often with distinct caves. They are fronted by a
wave-cut rock platform covered with each material such as shell banks and shingles.
Many parts of the world show evidence of this emergence. The western coast of
Malta is a typical example of emerged upland coast.
b. Emerged lowland coasts
An emerged lowland coast has been produced by the uplift of part of the neighboring
continental shelf. The landward edge of such coastal plain is found in the southern
of USA. It is formed by the fall-line where rivers descend from the Appalachian in
a series of waterfalls. Other examples of emerged lowland coasts are: the northernshore of the Gulf of Mexico and the southern shore of the Rio-de-la Plata in Argentina.
Application activity 5.5
1. With help of diagrams describe different types of emerged coasts.
2. In a field trip at the lake shore in Rwanda (if any), indicate the type of thevisited submerged coast, and describe its characteristics.
5.6.1 Nature, types and formation of coral reefs
Coral reefs are significant submarine features. They are formed due to the
accumulation and compaction of skeletons of dead marine organisms known as
coral polyps. Coral polyps thrive in the tropical oceans. Numerous coral polyps live
at a place in groups in the form of a colony.
They are generally attached to submarine platforms or islands submerged under
seawater.
a. Types of coral reefs
On the basis of the location of the main types of reefs, we distinguish tropical coral
reefs and marginal belt coral reefs. But, by categorizing on the basis of the nature,
the shape and the mode of occurrence, we have three types of coral reefs which are:
fringing reefs, barrier reefs and atoll.
i. Fringing reefs (Shore Reefs): These are the coral reefs developed along the
continental margins or along the islands. The seaward slope is steep and
vertical while the landward slope is gentle. A fringing reef runs as a narrow
belt which grows from the deep sea bottom sloping steeply seaward side.It is separated from the main land by a narrow and shallow lagoon.
ii. Barrier reefs: Barrier reefs are extensive linear reef complexes that areparallel to a shore and are separated from it by a deep and wide lagoon.
iii. Atoll: An atoll is a roughly circular (annular) oceanic reef system
surrounding a large and often deep central lagoon. There are three types
of atolls, namely, true atolls, island atolls and coral island or atoll islands.
• True atolls are characterized by circular reef enclosing a shallow lagoon but
without an island;
• Island atolls have an island in the central part of the lagoon enclosed by circular
reefs;
• Coral islands or atoll islands do not have islands in the beginning but later on
islands are formed due to erosion and deposition by marine waves
b. Formation of coral reefs
i. The process of coral reefs formation
Coral reefs start to form when the free-swimming coral larvae attach to the
submerged rocks or other hard surfaces along the edges of islands or continents.
This continues to grow under the influence of coral reefs conditions to grow in any
types accordingly. The coral reef formation takes three stages: fringing, barrier andatoll.
Concerning the process, a typical fringing reef is attached to or borders the shore
of a landmass, while a typical barrier reef is separated from the shore by a body of
water and an atoll began as a fringing reef around a volcanic island. Over time, the
volcano stopped erupting, and the island began to sink. Over time, coral growth at
the reef’s outer edge would push the top of the reef above the water. As the originalvolcanic island disappeared beneath the sea, only an atoll would remain.
The general conditions influencing coral formation
• Corals are found mainly in the tropical oceans and seas because they require
high mean annual temperature ranging between 20°C and 21°C for their
survival. They cannot survive in the waters having either very low temperature
or very high temperature.
• Corals do not live in deep waters, that is, not more than 60-77 meters below
the sea level.
• There should be clean sediment-free water because muddy water or turbid
water clogs the mouths of coral polyps resulting into their death.
• Though coral polyps require sediment-free water, fresh water is injurious to
their growth. This is why corals avoid coastal lands and live away from the
areas of river mouths.
• High salinity is injurious to the growth of coral polyps because such waters
contain little amount of calcium carbonates whereas lime is important food of
coral polyps. The oceanic salinity ranging between 27% and 30% is most ideal
for the growth and development of coral polyps.
• Ocean currents and waves are favorable for corals because they bring necessary
food supply for the polyps.
• There should be extensive submarine platforms for the formation of colonies
by the coral polyps. Besides, polyps also grow outward from the submarine
platforms.
• Human activities like deforestation, industrialization cause global warming,
which adversely affects corals in their habitats. Corals are more susceptible to
long-term climatic change. Corals are generally termed as rainforests of the
oceans. These cannot survive in extreme warm environment.
5.6.2 Theories of the origin of coral reefs
There are three main theories about the origin of coral reefs that are:
• The subsidence theory by Darwin,
• Antecedence theory by Murray,
• Glaciated control theory by Daly.
a. Darwin’s Subsidence Theory
• Darwin, a British naturalist developed his theory as follows:
• Darwin’s theory starts with a volcanic island which becomes extinct.
• As the island and ocean floor subside, coral growth builds a fringing reef, often
including a shallow lagoon between the land and the main reef.
• As the subsidence continues, the fringing reef becomes a larger barrier reef
further from the shore with a bigger and deeper lagoon inside.
• Ultimately, the island sinks below the sea, and the barrier reef becomes anatoll enclosing an open lagoon.
b. Murray’s antecedence theory
The Antecedent-Platform or uplift theory for the origin of coral reefs holds that:
• Any bench or bank that is located at a proper depth within the circumequatorial
coral-reef zone is potentially a coral-reef foundation.
• If ecological conditions permit, a reef may grow to the surface from such a
foundation without any change in sea-level.
• Reef foundations, or platforms, are formed by erosion, deposition, volcanic
eruption, or earth movement or by combinations of two or more of these
processes.
• Murray agreed that atoll coral reefs formed when the tops of islands were
undergone wave action resulting to a platform.
• Platforms originating below the depth limit of reef corals were veneered with
tuffaceous limestone and built to the zone of reef-coral growth by organisms
other than corals, chiefly foraminifera and algae.
c. Daly’s glaciated control theory
• Daly studied the coral reefs of Hawaii and he was greatly impressed by two
things:
• the reefs were very narrow and there were marks of glaciations
• there should be a close relationship between the growth of reefs and
temperature.
• According to Daly’s hypothesis, in the last glacial period, an ice sheet had
developed due to the fall in temperature. This caused a withdrawal of water,
equal to the weight of the ice sheet. This withdrawal lowered the sea level by
125-150 m.
• The corals which existed prior to the ice age had to face this fall in temperature
dining this age and they were also exposed to air when the sea level fell. As a
result, the corals were killed and the coral reefs and atolls were planed down
by sea erosion to the falling level of sea in that period.
• When the ice age ended, the temperature started rising and the ice sheet
melted. The water returned to the sea, which started rising. Due to the rise in
temperature and sea level, corals again started growing over the platforms
which were lowered due to marine erosion.
• As the sea level rose, the coral colonies also rose. The coral colonies developed
more on the circumference of the platforms because food and other facilities
were better available there than anywhere else.
• Hence, the shape of coral reefs took the form of the edges of submerged
platforms, a long coral reef developed on the continental shelf situated on
the coast of eastern Australia. Coral reefs and atolls developed on submerged
plateau tops. After the ice age, the surface of platforms was not affected by
any endogenic forces and the crust of the earth remained
5.6.3. Problems facing the development and growth of coral reefs
The following are the major problems facing the development and growth of coral
reefs:
• Overfishing: Increasing demand for food fish and tourism curios has resulted
in over fishing of not only deep-water commercial fish, but key reef species as
well. This affect the reef’s ecological balance and biodiversity.
• Coral disease: coral diseases contribute to the deterioration of coral reef
communities around the globe. Most diseases occur in response to the onset of
bacteria, fungi, and viruses.
• Destructive fishing methods: Fishing with dynamite, cyanide and other
methods that break up the fragile coral reef are highly unsustainable. Dynamite
and cyanide stun the fish, making them easier to catch. Damaging the coral
reef habitat on which the fish rely reduces the productivity of the area.
• Unsustainable tourism: Physical damage to the coral reefs can occur through
contact from careless swimmers, divers, and poorly placed boat anchors.
Hotels and resorts may also discharge untreated sewage and wastewater into
the ocean, polluting the water and encouraging the growth of algae, which
competes with corals for space on the reef.
• Coastal development: The growth of coastal cities and towns generates a
range of threats to nearby coral reefs. Coral reefs are biological assemblages
adapted to waters with low nutrient content, and the addition of nutrients
favours species that disrupt the balance of the reef communities.
• Pollution: Coral reefs need clean water to thrive. From litter to waste oil,
pollution is damaging reefs worldwide. Pollution from human activities inland
can damage coral reefs when transported by rivers into coastal waters.
• Marine debris: It is any solid object that enters coastal and ocean waters.
Debris may arrive directly from a ship or indirectly when washed out to sea
via rivers, streams, and storm drains. Human-made items tend to be the most
harmful such as plastics (from bags to balloons, hard hats to fishing line), glass,
metal, rubber (millions of tires!), and even entire vessels.
• Dredging operations. They are sometimes completed by cutting a path
through a coral reef, directly destroying the reef structure and killing any
organisms that live on it. Operations that directly destroy coral are often
intended to deepen or otherwise enlarge shipping channels or canals, due to
the fact that in many areas, removal of coral requires a permit, making it more
cost-effective and simple to avoid coral reefs if possible.
• Global Aquarium Trade: It is estimated that nearly 2 million people worldwide
keep marine aquariums. The great majority of marine aquaria are stocked
with species caught from the wild. This rapidly developing trade is seeing the
movement of charismatic fish species across borders. Threats from the trade
include the use of cyanide in collection, over-harvesting of target organisms
and high levels of mortality associated with poor husbandry practices and
insensitive shipping. Some regulation is in place to encourage the use of
sustainable collection methods and to raise industry standards.
• Alien invasive species: Species that, as a result of human activity, have been
moved, intentionally or unintentionally, into areas where they do not occur
naturally are called “introduced species” or “alien species”. In some cases where
natural controls such as predators or parasites of an introduced species are
lacking, the species may multiply rapidly, taking over its new environment,
often drastically altering the ecosystem and out-competing local organisms.
• Climate change: Rising sea levels due to climate change requires coral to grow
to stay close enough to the surface to continue photosynthesis. Also, water
temperature changes can induce coral bleaching coral bleaching as happened
during the 1998 and 2004 El Niño years, in which sea surface temperatures
rose well above normal, bleaching or killing many reefs.
• Ocean acidification: results from increases in atmospheric carbon dioxide. The
dissolved gas reacts with the water to form carbonic acid, and thus acidifies
the ocean. This decreasing pH is another issue for coral reefs.
• Coral mining: Both small scale harvesting by villagers and industrial scale
mining by companies are serious threats. Mining is usually done to produce
construction material which is valued as much as 50% cheaper than other
rocks, such as from quarries. The rocks are ground and mixed with other
materials, like cement to make concrete. Ancient coral used for construction
is known as coral rag. Building directly on the reef also takes its toll, altering
water circulation and the tides which bring the nutrients to the reef.
5.6.4 Impact and problems related to coastal landforms
Coastal landforms have crucial impact in world economic activities. These are:
• Tourist attraction: Coastal features like caves, beaches and arches are tourist
attractions.
• Development of harbors: Rias and fiords favor the development of deep
sheltered harbors.
• Industrial raw materials: Coral limestone provides raw materials for the
manufacture of cement. This is obtained from raised coral reefs.
• Fishing grounds: Fiords contain sheltered waters which are suitable for feeding
and development of fishing ports. Continental shelves contain shallow waters
which favor growth of planktons. This makes them rich fishing grounds.
• Habitat for marine life: Lagoons, mud flats and mangrove swamps are good
habitats for marine life. This has promoted the development of research on
marine life and establishment of marine parks.
• Impact on agriculture: emerged coasts have sand, gravel and bare rock. These
inhibit agriculture, especially crop farming.
• Transport barrier: coastal features such as sandbars and coral reefs inhibitwater transport and development of ports
Application activity 5.6
1. Using illustrative graphics, differentiate the types of coral reefs.
2. Explain the conditions for coral reefs formation.
3. Identify the problems related to coral reefs.
4. Establish the similarities of the subsidence, antecedence and glaciated
control theories of coral reefs formation.5. Describe the economic importance of coral reefs.
5.7 Isostatic and Eustatic changes on the coast
Learning activities 5.7
1. Basing on your knowledge on the concept of isostasy, do you think it can
be related to the isostatic and eustatic changes on the coast? Explain.2. Differentiate isostatic from Eustatic sea level change.
The sea level is not static, which causes the level on coast changing regularly. These
changes are discussed below:
5.7.1 Isostatic change
Isostatic sea level change is the result of an increase or decrease in the height of
the land. When the height of the land increases, the sea level falls and when the
height of the land decreases the sea level rises. Isostatic change is a local sea level
change whereas Eustatic change is a global sea level change.
• During an ice age, isostatic change is caused by the build-up of ice on the land.
As water is stored on the land in glaciers, the weight of the land increases and
the land sinks slightly, causing the sea level to rise slightly. This is referred to
as compression.
• When the ice melts at the end of an ice age, the land begins to rise up again
and the sea level falls. This is referred to decompression or isostatic rebound.
• Isostatic rebound takes place incredibly slowly and to this day, isostatic
rebounding is still taking place from the last ice age.
• Isostatic sea level change can also be caused by tectonic uplift or depression.
As this only takes place along plate boundaries, this sort of isostatic change
only takes place in certain areas of the world.
5.7.2 Eustatic Change
Eustatic change is when the sea level changes due to an alteration in the volume of
water in the oceans or, alternatively, a change in the shape of an ocean basin and
hence a change in the amount of water the sea can hold. Eustatic change is always a
global effect.
During and after an ice age, Eustatic change takes place.
• At the beginning of an ice age, the temperature falls and water is frozen and
stored in glaciers inland, suspending the hydrological cycle. This results in
water being taken out of the sea but not being put back in leading to an overall
fall in sea level. Conversely, as an ice age ends, the temperature begins to rise
and so the water stored in the glaciers will re-enter the hydrological cycle; and
the sea will be replenished, increasing the sea levels.
• Increases in temperature outside of an ice age also affect the sea level since
an increasing temperature causes the ice sheets to melt, putting more water
in the sea.
• The shape of the ocean basins can change due to tectonic movement. If the
ocean basins become larger, the volume of the oceans becomes larger but the
overall sea level will fall since there’s the same amount of water in the ocean.
Conversely, if the ocean basins get smaller, the volume of the oceans decreasesand the sea level rises accordingly
Application activity 5.7
1. Describe the isostatic and eustatic changes on the coast.2. Describe the effects of Eustatic change on the environment.
5.8. Sea level change
5.8.1 Meaning of sea level change and its resulting features
The sea level change is the variation and fluctuation of the sea level throughout
time. On a day to day basis, the sea level changes from the tide action but the sea
level also changes on a much grander time scale too. These changes in sea level
are normally caused by ice ages or other major global events. The sea level change
results from eustatic and isostatic changes. It can contribute to the formation of
submergent landforms such as Ria (a river valley that’s been flooded by the eustatic
rise in sea level), fjords and dalmatian coastline, and emergent landforms such as
raised beaches. These are wave-cut platforms and beaches that are above the current
sea level. There are also some old cliffs (relic cliffs) behind these raised beaches withwave-cut notches, arches and stacks along them.
5.8.2 Types of sea level changes
There are two types of sea level changes which are:
• Rise of sea level: This is referred to as an increase in global mean sea level as a
result of an increase in the volume of water in the world’s oceans. This leads to
the formation of coastal features of submergence.
• Fall of sea level: This is referred to as decrease in global mean sea level as
a result of a decrease of the world’s oceans. This leads to the production of
emergence coastal landform.
5.8.3 Causes of sea level change
The sea level changes daily because of the following causes:
• Eustatic variations in sea level are the effects of external forces. Most experts
agree that human induced global warming is the force behind the current
global sea-level rise. There are three factors that primarily affect eustatic sea
level change that are: thermal expansion of the ocean, melting of nonpolar
glaciers, and change in the volume of the ice caps of Antarctica and Greenland.
• The changes in global temperature affect the amount of ice stored on land
as water, thus changing the sea levels. A rise in temperatures causes the ice
caps to melt, and sea levels rise, and vice versa.
• The changes in sea levels are also affected by the steric effect. This is where
the density of the water increases or decreases as the temperature rises or falls.
If the temperature rises the water expands and if it falls the water contracts. It
is estimated that sea levels can rise up to 0.4 mm per year.
• The mass of ice adds weight to the earth’s crust causing it to sink lower into
the mantle resulting into relative rise in the sea-level during glacial period.
• Isostatic re-adjustment; at the end of glacial period, the mass of ice melts
and the weight is lost from crust which then rises. When the ice melts the land
begins to rise as the weight is removed. This results in a relative fall in sea-level.
This is called isostatic re-adjustment
• Uplift/mountain building due to plate movements may also result in a
relative fall in sea-level as land rises up.
• Pre-glacial erosion of a coastline causes the coast rise and end-up to the sea
level change.
5.8.4 Evidences of sea level changes
The following are evidences of sea level changes:
• The presence of old coastline high above the present sea level: During the
Ice Age the sea levels fall leaving the old coastline. Since the end of the Ice Age,
sea levels have risen again, but not to their previous levels. The raised beaches
continue to be above the present sea level by quite a distance.
• The estuaries and inlets flooded: Sea level rise after the last Ice Age caused
estuaries and inlets to be flooded. This occurred in South West England,
drowning many river valleys around the coasts of Devon and Cornwall, and
creating Rias. In other, more northern areas, glacial valleys were drowned to
create Fjords.
• Isostatic re-adjustment phenomenon: Some places in Scotland still
undergoing isostatic re-adjustment up to 7 mm per year in some areas.
5.8.5 Effects of the sea level changes
Rising sea level has many impacts on coastal areas. The following are some of them:
• Erosion of beaches and bluffs: Beach erosion is the most common problem
associated with rising sea level. Depending on beach composition, beaches
erode by about 50 to 200 times the rate of sea level rise. That translates a
2-millimeter (0.08-inch) per year increase in sea level eroding from 10 to 40
centimetres (3.9 to 15.6 inches) of coastline per year. Beach erosion has not
only a strong ecological impact, but also a profound economic impact.
• It increases the flooding and storm damage caused by changes in sea level.
• Contamination of drinking water: as the rising sea crawls farther and farther
up the shore, in many places it will seep into the freshwater sources in the
ground that many coastal areas rely on for their drinking water. Saltwater is
unsafe to drink, and while it is possible to remove the salt from water, doing so
is an expensive and complicated process.
• Interference with farming: Those same freshwater sources we use
for drinking also supply the water we use for irrigation. The problems here are the same:
The intruding sea could make these groundwater sources saltier. Saltwater can
stunt or even kill crops, but creating freshwater from saltwater is a costly and
unsustainable practice.
• Change in coastal plant life: more saltwater hitting the shores changes the
soil composition on the coast, meaning the plant life there will most likely
change as well.
• Threating the wildlife population: Many forms of wildlife make their home
on the beach. As the rising ocean erodes the shoreline and floods the areas in
which coastal animals live, animals like shorebirds and sea turtles will suffer
and die and others will migrate.
• Hurting the economy: the tourism and real-estate industries in coastal areas
are likely to take a hit as prime beachfront properties and recreational areas
are washed away by rising waters. This is a fact that some involved in theseindustries are finding hard to swallow.
Application activity 5.8
1. Explain the causes of sea level change
2. Describe the evidences of sea level change
3. Basing on the study of sea level change. Visit a local water body and
identify the evidences of its water level change.
4. According to you, which feature is more attractive to tourism. Defend your
view5. Explain the environmental effects of sea level changes.
