Coastal Landforms
Coastal Geomorphology means Coastal features or Coastal landforms. Coastal landforms develop along the coasts and on continental shelves by three main processes. These include:
1. Wave (marine) erosion and deposition.
2. Sea level rise (submergence) and fall (emergence) or (Marine transgression and regression).
3. Coral formation.
Before looking in detail at the ways in which coastal lines develop, we need to define these terms:-
1. Coast: this is a strip of the land, where it meets the sea.
2. Shore: is the strip of land that lies between high water and low
3. Water levels.
4. Coastline: is the margin of the land (sometimes taken to mean the limit
5. at which wave action takes place).
6. Shoreline: refers to the line where shore and water meet.
7. Beach: the deposit of sand or pebbles on some shores is called a beach
FACTORS AFFECTING COASTAL LANDFORMS
The nature of the rocks along the coastline:
Wave erosion is most active where the rocks are weak, jointed or unconsolidated. If a coastline has alternating hard and soft rocks, wave erosion may lead to the formation of bays and hard lands.
Relative movement of sea-level:
The level of the sea may go up or down due to earth movement or due to the development of ice sheets. The movements will cause the waves to work at a different level and, therefore, affect the shape of the coastline.
Special climatic conditions:
Along some coasts special climatic conditions, either past or present, have affected the shape of coastlines. For example, in the tropics where the climate is different from that in temperate regions, the coastlines have mangrove swamps, which affect the shape of the coastline.
The work of people:
People can interfere with the shape of coastlines in several ways. The building of drainage canals, construction of artificial harbours and the dredging of estuaries may affect the shape of the coastline.
Other factors which affect coastal landforms include the shape of the coast,, the effects of glaciers etc.
Features produced by the action of waves:
Waves are the upward and downward movement of water particles resulting from friction caused on the surface of the water by a layer of wind blowing over it. Waves do not involve horizontal movement of water. It is only the shape of the ripples that moves forward while the water only keeps on oscillating up and down.
Most waves are caused by the force of the wind. But other causes include catastrophic events like earthquakes and volcanic eruptions, moving objects such as ships, boats, whales and hippopotami. The strength of the wave partly depends on the strength and fetch of the wind. The longer the fetch, the stronger the wave.
HOW WAVES ARE CAUSED
Most waves are caused by the force of the wind. The surface of the sea exerts frictional drag on the bottom layer of a wind blowing over it, and this layer exerts a frictional drag on the layer above it, and so on. The top layer has the least frictional drag exerted on it which means that the layers of air move forward at different speeds as shown in the diagram below:
The air tumbles forward and finally develops a circular motion as seen in the diagram above. This motion causes downward pressure (D.P) on the surface at its front, and an upward pressure (U.P.) at its rear, and this causes the surface to take on the form of a wave as seen in the diagram below:
WAVE TERMINOLOGY
1. Fetch: the distance of open water, which the wind blows.
2. Crest: is the highest point of a wave
3. Trough: is the lowest point of a wave
4. Wave length (L): is the distance between two successive crusts.
5. Wave height(h): is the distance between the crest and the trough
6. Wave period(T): is the time taken for a wave to travel through one wave length.
7. Wave velocity (c.) is the speed of movement of a crest in a given period of time
8. Wave refraction: Where waves approach an irregular coastline, they are refracted (bent) i.e. they become increasingly parallel to that coastline.
OTHER TERMS ASSOCIATED WITH WAVE ACTION
Tines: is a term applied to the regular periodic alternating rise and fall of the water in the oceans. The raising of the water is termed the Flood tide and the falling of the water is the Ebb tide. Tides are cause by gravitational effect of the Moon.
Tidal range: is the different in the height of the water at high and low tide at a place, varying from day to day.
Current: is a body of water moving vertically or horizontally in a definite direction
WAVES IN DEEP WATER.
Deep water is when the depth of water is greater than half the wavelength.
WAVES IN SHALLOW WATER
Shallow water is when their depth is less than half the wavelength, then friction with the sea bed increases. As the base of the wave begins to slow down then the circular oscillation becomes more elliptical. As the water depth continues to decrease, so does the wavelength. Meanwhile the height and steepness of the wave increase until the upper part spills or plunges over. At this point the depth of water and the height of the wave are equal. The body of forming thrown up the beach by breaking waves is called the Swash, while any water returning down to the sea is the backwash.
A breaking wave produces swash and backwash
TYPES OF WAVES
There are two types of waves:
1. Constructive wave: Wave whose swash is more powerful than the backwash. Such waves usually break at the rate of 10 or less per minute.
2. Destructive wave: Wave whose backwash is more powerful than the swash. Such waves break at the rate of 10 or more per minute.
WAVES AS AGENTS OF EROSION
Ocean waves are very powerful agents of erosion along the coastline. Unsheltered coastlines are mostly affected by erosion. The force at which the breaking wave hits the rock - face at the coastline estimated at 25 tonnes/m3. This force is enough to break up unconsolidated rocks and weak rocks. The following are the main processes of wave erosion:
Corrosion (or abrasion):
caused by boulders, pebbles and sand being hurled against the base of a cliff by breaking waves and resulting in under emitting and rock break-up.
Hydraulic action:
Caused by water, throwing against a cliff by breaking waves, suddenly compressing air in the cracks and crevices of the rocks, which expands when the wave retreats, sometimes explosively. This action cases rocks to shatter as the cracks become enlarged and extended.
Attrition:
The break-up of boulders and rocks as they are dashed against the shore and against each other by breaking waves.
Corrosion (solution):
Some rocks, which make the cliff-face are soluble. Where the coastline is made up of such rocks it will be eroded by the chemical action of the water.
FEATURES PRODUCED BY WAGE EROSION
1.Cliffs and wave-cut platform:
A cliff is a steep rock-face that is more or less vertical. The development of a cliff will depend on the stratification and jointing of the rocks and their resistance to wave attack.
The first stage in the development of a cliff is the cutting of a notch (the undercutting of sea cliffs seen nearing water mark) - on the coastal land. Through hydraulic action and abrasion, waves enlarge the notch until the land above has lost its support and falls into the water. The land above the notch becomes so steep that it forms a cliff.
A wave-cut platform is terrace of fairly even surface left behind at the foot of a cliff. It is formed as a cliff retreats. At low tide it is exposed while at high tide it may be covered by water. The diagram below summarizes the stages of cliff development.
1.
2.
3.
e.g. Likoni Ferry in Mombasa. An example of a wave-cut platform is found at Mama Ngina Drive near the Oceanic Hotel on Mombasa Island.
Headlands and Bays:
If c coastline consists of rocks differing in resistance, the soft rock will be eroded leaving behind a mass of resistant rock referred to as a headland. The eroded rock will give way to a bay as seen in a diagram below:
These features are common along the East African Coast.
3. CAVES, ARCHES, STACKS, STUMPS AND GEOS.
A cave a cylindrical tunnel drilled through the cliff. It is wide at the entrance and narrow at the end. It develops along a line of weakness at the base of a cliff. The weakness is opened up by wave abrasion and hydraulic action.
Where cliffs are of resistant rocks, wave action attacks any lines of weakness such as joints, faults and bedding planes. The impact of breaking waves exert pressure causing air in the joints to be compressed and when the water retreats, the air expands suddenly with shock waves which break rocks along the joints. Corrosion and weathering contribute to the opening up of the joints which become cave. Examples are common on the East African Coast and a few localized ones are found on Entebbe Peninsula. One can be observed at Lutembe beach and small ones can also be observed on the cliffs near Botanical gardens in Entebbe.
A blowhole is an erosional feature associated with caves. It develops when a joint extends from the end of the tunnel to the top of the cliff and becomes enlarged in time and finally open out on the cliff top.
In due course the roof of the cave may eventually collapse producing along narrow inlet referred to as a geo.
DIAGRAMS
An arch is formed when two caves develop from either side of a narrow headland. Waves continue to erode the two retreating caves, until eventually a passage is formed and water begins to aculeate through it. The passage is known as an Arch. An example is Vasco da Gama. Point at Malindi.
When the arch collapses, the end of the headland stands up as a stack. The stack therefore resembles an island except the mode of formation is different. Once a stack is formed, it is continually attacked by wave and agent of weathering and with time it is reduced to a small rock known as a stump.
Coastline arches
On coasts this can form two different types of arch depending on the geology. On discordant coastlines or Atlantic Coastlines, rock types run at 90° to the coast, headlands form. Wave refraction concentrates the wave energy on the headland, and the arch forms when caves break through the headland, e.g., London Bridge in Victoria, Australia. When these eventually collapse they form stacks and stumps. On concordant coastlines or Pacific coastlines, rock types run parallel to the coastline, with weak rock (such as shale) protected by stronger rock (such as limestone) the wave action breaks through the strong rock and then erodes the weak rock very quickly. Good examples of this are at Durdle Door (illustration, above) and Stair Hole near Lulworth Cove on the Dorset Jurassic Coast in south England, although these are on an area of concordant coastline. When Stair hole eventually collapses it will form a cove.
Weather-eroded arches
1. Deep cracks penetrate into a sandstone layer.
2. Erosion wears away exposed rock layers and enlarges the surface cracks, isolating narrow sandstone walls, or fins.
3. Alternating frosts and thawing cause crumbling and flaking of the porous sandstone and eventually cut through some of the fins.
4. The resulting holes become enlarged to arch proportions by rockfalls and weathering. Arches eventually collapse, leaving only buttresses that in time will erode.
Many of these arches are found within Arches National Park in Utah.
Water eroded arches
Some natural bridges may look like arches, but they form in the path of streams that wear away and penetrate the rock. Pothole arches form by chemical weathering as water collects in natural depressions and eventually cuts through to the layer below.
Cave erosion
London Arch, formerly "London Bridge" in Victoria, Australia
Natural bridges can form from natural limestone caves, where paired sinkholes collapse and a ridge of stone is left standing in between, with the cave passageway connecting from sinkhole to sinkhole.
Like all rock formations, natural bridges are subject to continued erosion, and will eventually collapse and disappear. One example of this was the double-arched Victorian coastal rock formation, London Bridge which lost an arch after storms increased erosion. London Bridge before it collapsed
WAVE TRANSPORT
The load transported by waves comes from rivers entering sea, some from landslides on cliffs, and the rest from wave erosion. The load consists of mud, sand and shingles.
The swash and backwash push and drag material up and down the shore. When waves break obliquely to the shore, the swash moves obliquely up the shore but the backwash runs back at right angles to the shore as shown in the diagram below:
The two movements (swash and backwash), which carry materials along the shore, result in long shore drift.
FEATURES PRODUCED BY WAVE DEPOSITION
Materials transported by long shore drift are finally deposited elsewhere along the shore or in the sea where they accumulate to form impressive features. These features include the following:
Beach: is a constructive waves, produced by long shore drift, deposit and sand or pebbles along a coast to form a beach. A beach has a gently sloping surface. Beaches are usually formed between high and low water levels.
A. TYPES OF BEACHES
a) Normal beach: This is the one developing on the shore.
b) Storm beach: is developed by storm waves which throw pebbles, stones and sand well beyond the normal level reached by waves at high tide.
c) Barrier beach: is a long ridge of sand deposits which is approximately parallel to the coast and separated from it by a ...............? A barrier beach develops from an under water off shore bar as the latter moves towards the land. Barriers beaches are common on the East African coast of Madagascar, costs of Benin and Nigeria. Its diagram can be seen below.
Beaches are also distinguished from each other by the type of debris, which is predominant on them. For example, sand beach is predominantly composed of sand, shingle beaches are composed of gravels and shells while those composed of bigger particles are known as stony beaches. Nyali beach on the coast of Kenya, Lido and Kasenyi on the Northern shores of Lake Victoria are example of sand beaches.
A spit: is a low, narrow ridge of pebbles or sand joined to the land (main land or island) at one end, with the other end terminating in the sea. It is formed by deposition of materials by long shore drift. Sometimes a spit develops at a headland and if the waves meet the coast obliquely, the end of the spit becomes curved or hooked hence a hooked spit is formed. Its diagram is shown below:
A cuspate Spit: is formed when two spits develop and grow towards the sea and converge in the water., The enclosed water is soon filled with deposits and is soon colonized by vegetation. The area colonized by vegetation is known as Cuspate Foreland. The diagram is drawn below.
A Bar: This is a ridge of material, usually sand, which was parallel, or almost parallel, to the coast. Unlike a spit a bar is not attached to the land.
Offshore Bar: This bar develops on gently sloping seabed away from the shore. They form where sand is thrown up by waves which break before they reach the coast Off shore bars are found on the coast of Kenya at Lamu.
Bay Bars: These are bars which extend across bays
A Tombolo: A Tombolo is a depositional feature resulting from wave-action. When a bar joins an island to the mainland it is called a Tombolo. A Tombolo exists on the North East Somali coast. While several others are found in the cape Verde region. In East Africa, a Tombolo joins Bukakata on the shore of Lake Victoria to Lambu Islands.
Mud Flats: This is a platform built of mud, silt and other types of fine alluring. It develops when rivers and waves deposit materials along genres coasts especially in bays and estuaries between high and low tides. When mud flats are colonized by salt tolerant vegetation, mangrove swamps develop. Mud flats are well developed in the Niger delta and on the coast of Kenya and Tanzania near Tanga and the mouth of river Rufigi.
COASTAL GEOMOLPHOLOGY TYPES OF COASTS AND FEATURES FORMED BY SEA LEVEL CHANGES
B. CLASSIFICATION OF COASTS
Several attempts to classify coasts usefully have been made, based upon a range of criteria. Three types of criterion have been identified.
Changes in sea level (Ecstatic changes) (by D.W. Johnson 1919).
This is the earliest method, which divided coasts into four categories.
1. Emergent Coasts: Those resulting from a fall in sea level and/or uplift of the land. Coast lines formed in this way are basically straight in appearance e.g. Barrier beaches, raised cliffs and raised terraces.
2. Submergent Coasts: Those showing a recent rise in sea level and/or a fall in the land surface. This type is characterized by indented coast line e.g. .....? and estuary coasts.
3. Stable or neutral coasts: These are of two groups.
(a). Those showing no signs of changes in sea level or in the land.
(b). Those produced by non-Marine processes e.g. a river delta, a volcanic Island.
4. Compound coasts: Those with a mixture of at least two of the three previously listed groups.
This classification come to be regarded as too simplistic as new evidence accumulated to show that there had been several changes in sea level (local and Global) and that many coasts exhibited signs both of submergence and emergence.
Relation between coastal and other processes of erosion, and deposition (E.P. Shepard 1963).
This has been shown to be over generalized, having only two categories:
Primary Coast: These are coasts where the influence of the sea has been minimal e.g.F....??(...?), .......and deltas (rivers), and Islands (volcanic or coral).
Secondary coasts: Where marine processes have been dominant e.g. stocks and spits.
Advancing and retreating costs (R. Valentin 1952).
This method also suggested that all coasts could be fitted into a two fold classification.
Advancing: Where Marine deposition or the uplift of the land is dominant
Retreating: Where Marine erosion or the submergence of the land is more significant
Energy - Produced coastline J.L. Davis 1980.
This is the most recent attempt to categories knowledge about coasts, and is gaining credibility because it relates directly to the amount of energy expanded by different types of wave upon a particular stretch of coastline.
1. High energy environments: Where destructive storm waves breaking on shingle beaches are most typical.
2. Low Energy environments: Where constructive swell waves breaking upon sandy beaches are more frequent.
3. Protected environments: Where wave action is limited in small, sheltered sea areas.
C. CHANGES IN SEA LEVEL
Sea level has occasionally risen and fallen. Any change in sea level is generally known as Enstatism?. When sea level rises (known as positive Enstatism)? it results in submergence of coastal areas (Marine transgression). When the level falls, known are positive enstatism? and it results in emergence of coasts (Marine regression).
CAUSES OF SEA LEVEL CHANGES
Earth movements: These are the main causes. For example, regional uplift/upward of the coastal area, a fall in sea level is produced. On the other hand down warping/down faulting of coastal land produces a relative rise in sea level.
Glaciations: During a glacial period evaporated water goes to high mountains and polar regions where it freezes into large ice masses. Fall in universal sea level. This is what happened during the Pleistocene glacial period. On the other hand, depreciation (Melting) takes place when global temperatures rise. This is what happened after Pleistocene and today it is still taking place. It has been estimated that if the remaining ice .......As particularly those over Antarctica, were to melt the sea level - would rise by another 30 metres causing submergence of all coastal locations.
Temperature changes: Rise in temperature results in expansion and fall results in contraction of ocean waters. Expansion of water results in increase in sea level while contraction results in decrease.
Sedimentation: Rivers, big and small erode, transport and deposit sediments in ocean basins. This reduces the size of the ocean basin leading to arise in sea level.
CAUSES OF SEA LEVEL CHANGES
D. Increase in rainfall as a result of solar winds
It is believed that hydrogen atoms from the sun are continually entering combined with oxygen in ozone layer to form water molecules. This water finds its way into ocean basins either directly from the sky or indirectly through springs and rivers. The amount of water may be small but over a long period, it accumulates and causes significant sea level rise.
EFFORTS OF THE SEA LEVEL CHANGES
The changes in sea level have affected:
(a). The shape of coastlines and the formation of new features by increased erosion or deposition.
(b) The balance between erosion and deposition by rivers resulting in the drowning of lower sections of valleys or rejuvenation of rivers.
(c.)Migration of plants, animals and humans.
Landforms resulting from submergence (positive movements).
Rias. A ria is a drowned river valley (mouth). It is funnel-shaped and decreases in width and depth inland. It sometimes contains a stream. Originally responsible for the formation of the valley before flooding but after, the stream is too small compared with the size of the drowned inlet. Rias form on coasts where hills and river valleys meet the sea t right angles. Kilindini harbour on which Mombasa Port is situated is a ria, Dar es Salaam Port is also situation on a ria. Similar features have developed along the southern shores of lake Victoria. A ria therefore is a narrow inlet which extends a few kms inland.
Longitudinal (Dalmatian) coastline.
This coastline occurs when river valley runs roughly parallel to the coast, reaching the sea through a gap in the coastal hills. As sea level rises the seawater again enters the lower part of the valley and drowns it. In this case much of the lower land is behind the coastal hills, which may therefore become separated from the mainland to become longitudinal. Islands known as Sounds. Examples of this feature in East Africa are: the Pemba Island in Tanzania. The Smith Sound on the southern shores of lake Victoria at Mwanza is also typical of Dalmatian coast.
The East Coast of Pemba, showing features similar to a Dalmatian coastline.
Fiords: Formed when glaciated hill coasts become submerged and flooded lower parts of the valley are then referred to as Fiords. However, such features in East Africa do not exist.
Rias, Dalmatian and Fiord coastlines are usually found where the coast is a highland coast and so they fall within the category of submerged highland coasts.
But, it is also possible to find submerged low land coasts, In lowlands river valleys have gently sloping sides and so a wide area o the valley bottom is flooded by the rise in sea level. This causes the formation of ......??
Estimates: An estuary different from ria in that an estuary is formed when a lowland coast is submerged while a ria is formed when a highland coast is drowned. Besides an estuary is wider while a ria is narrower and long. However, they both have a funnel shaped structure.
Values of Rias/Estuaries
1. They provide deep natural harbours
2. They provide sites for port developments since it is easy to move inland from the head of a ria e.g. Dar es Salaam and Mombasa.
3. They provide natural inland waterways to small vessels.
4. They can serve as good fishing grounds for the coastal fishing villages.
5. They act as tourists' attractions hence earn foreign currency.
6. Settlement however is difficult along the sides of a ria because there is little or not level land.
A Dalmatian coastline:
A. Before the rise in sea level (concordant coast and with rock layers parallel to coast)
B. After the rise in sea level:
A concordant coast drowned to form a Dalmatian coast.
Land forms of Emergence of sea level of highland coast:
Where lowland is raised above sa level or sea-level falls relative to land, it is called an emerged lowland coasts. Emergence lead to the formation of raised cliffs, raised beaches and Marine terraces.
Raised cliffs: These are cliffs which had been in contact with the sea during high tides and are slowly and slowly left higher up beyond the tide level after a fall in the sea-level. Such cliffs are found well above the m\present level of wave action. Example of such cliffs are found around Mombasa, Bagamoyo, Kilifi and Lamu.
Raised beaches: These are former beaches which are now well above the present zone of wave action.
Raised Terraces: These are actually former wave cut platforms, which are no longer in contact with water. They are above the present tide level where new terrace is developing. Sometimes such terraces have materials deposited on them.
Many of the features of emergence are found along Kenyan and Tanzanian coasts.
1. Diagram of an emerged highland coast
Emerged coasts - lowland type: When there is a relative fall in sea level - along a lowland coast, a part of the continental shelf becomes exposed, forming a wide gently sloping coastal plain.
The relative fall in sea level rejuvenates the rivers; and it gives them a new life, and this causes them to deepen their valleys. However, wave action in the shallow offshore water depositional and such features as beaches, spits, mudflats, offshore bars, and barrier beaches develop.
SAMPLE QUESTIONS
1. With reference to East Africa; explain the causes and effects of sea level changes.
- Account for sea level changes
- Identify and examine the formation of features resulting from such changes
- Show understanding of the fact that sea level changes are both positive
(rise) and negative (fall).
2. Give effects of sea level changes in East Africa. How have these affected human activities along the East African Coast?
- Briefly bring out the causes of sea level changes.
- Identify the effects and these should be both positive and negative e.g.
Rias/Estuaries, Dalmatian coasts, raised beaches, raised cliffs and
Marine terraces.
- Explain how the above features were formed with diagrams
- And finally show how these effects have affected human activities e.g.
i) Rias have sheltered water and therefore form good harbours. They have encouraged Port development as clearly seen at Mombasa and Dar es Salaam.
ii) Raised beaches, cliffs, terraces raises and Dalmatian coasts are all tourist attractions and have contributed to an increase in tourist activities along the East African Coast.
iii) Sand of the raised beaches has promoted construction industry. The sand is quarried and used as an ingredient in the various construction industries at the coast and further inland.
iv) The sand of the raised beaches is also an actual and potential source for the manufacture of glass.
v) Steep raised cliffs encourage mass movements of land slides especially rock slide and rock fall are more likely to occur on such steep slopes than on gentle ones.
3. Explain the causes and effects of sea level rise relative to land
- This is an open question. Confine oneself to any specific region.
- The question calls for causes and effects of sea level rise
- Consider only reasons for sea level rise and effects of submergence.
4. Describe features that develop on rivers as a result of changes in sea level.
- Consider both positive and negative sea level changes.
- Causes of the changes are not mandatory but a brief account can be made.
- The student should try to describe the appearance/form of those features
that develop on river valleys as a result of sea level changes. E.g. Rise
in sea level - Fiords, Rias, Dalmatian and Estuaries.
- Fall in sea level - rejuvenated features.
- Other features resulting from changes in sea level but which are not
along river valleys are not called for in this type of question.
If a student examines them will be correct but irrelevant as far as this question is concerned.