GEO1/6: COASTAL GEOMORPHOLOGY

This unit is about the coastal geomorphology and its terminologies

COASTAL GEOMORPHOLOGY 

Coastal geography is the study of the constantly changing region between the ocean and the land, incorporating both the physical geography and the human geography of the coast.

TERMINOLOGY

Cliff

A cliff is a raised piece of rock found along the coast formed as a result of ware erosion.

A beach

It is a strip of land gently sloping seawards formed as a result of ware deposition.

The shore

Is the area between the high tide and the low tide.

The high tide

Is the furthest point aware can reach on the land? It’s common when there are high tab rants on the sea.

The low tide

It is the point where the waves can reach during calm conditions on the sea.

Tidal Range

Is the difference between the high tide and the low tide.

Waves

Is an oscillation of water caused by moving wind or waves are irregularities in the water surface water of the sea, lake etc.

Offshores drift

These are ocean current moving from the land seaward.

On-shore drift

These are ocean current which are blowing or moving from the sea of the land.

Longshore drift

These are ocean current blowing almost parallel to the coast.

Waves have the ability to crude and deposit materials and this leads to the formation of both erosional and depositional features.

When wares hit the land in most cases it results into erosion a situation which is known as swash and when the waves move back to the sea its known as the backwash.

The swash and the backward effects make the longshore drift effect and this leads to the form of depositional features.

WAVES EROSION

There are different forms of wave erosion and these include:

Corrosion

This is the chemical reaction between the water and the rock which most cases result into the breaking down of the rocks along the coast.

Abrasion

This is a process whereby the waves use the particles and pebbles within the water as tools to break the rocks along the coast.

Hydraulic action

This is a process whereby the strong waves hit the rock and push air through the existing cracks.

Attrition

This is a process whereby the rock pebbles and boulders hit each other and break into smaller particles.

NB

Erosional features are mainly formed long rocky coast or rocky uplands.

Amongst the feature produced we have the cliff wave-cut platforms (cave, arch and stack) blowholes, Geos (inlets) e.t.c

CLIFF

A cliff is a raised piece of rock formed along the coast as a result of ware erosion.

A cliff is formed as a result of waves undercutting the rocks along with the cost especially within the line of weakness or the soft rocks. The hard rocks remain standing in most cases and as a result, a cliff will be formed.

There are two types of cliff and these are differentiated by the dipping of the rocks along the coast which is responsible for the form of a gentle cliff or a steep cliff.

A cliff

When the rock is dipping landwards the cliff formed will be a gentle cliff and this is illustrated in the diagram below.

If the rocks are dipping seawards, steep cliff will be formed as illustrated in the diagram below.

Caves arch and stack (heo)

Through wave erosion along the coast. A series of features are formed and these are the caves, arch and the stack.

Through destructive waves, a headland will be attached form all directions. Caves will be formed on either side of these headlands.

With continuous abrasion, attention and corrosion, caves will be formed and the headland will continue to be eroded until when the two caves meet they form a tunnel and the rock on top is what they call an arch. When an arch collapses their stack will be formed.

A stack is that piece of rock that will be cut off from the headland. If the stack is very big it will form an island. There are very many minor stacks on Lake Victoria especially around Kasenyi there are very many within Malindi areas in Kenya and maim. The diagram below shows the form of the above features.

Aware cut platform

Is a place of rock fronting a cliff?

Aware cut platform is formed by destructive waves during the formation of cliff. When the waves hit the coastal rock they break the rock through Abrasion and Nivation.

When the bottom part of the coastal rock has been affected, the top part will collapse and the remaining open known as the wave-cut platform will be formed. The diagram below shows aware cut platform.

NB

When the ware cut platform are too wide and long they protected the cliff from further erosion by waves (wave action)

Blowholes

Blowholes are small holes found on top of the coastal rock and they are formed as a result of hydraulic action by the waves.

Along the coast there are initial cracks where the air is forced through by the breaking waves by a process known as hydraulic continuous hydraulic will result into the extension of the crack up to the surface forming a hole known as a blowhole.

There are very any blow holes near and around the Vascodagama pillar in Malindi Kenya.

DEPOSITIONAL FEATURES

There are very many features formed as a result of deposition by waves and amongst these, we have a sandbag, sand spit, peaches, tombolos, lagoons deltas.

SAND BARS

It’s a ridge of sand formed by ware deposition parallel to the coast or off the coast. Its a strip of sand formed by constructive waves.

There are very many sand bar especially along the western coast of Africa. We have the small one along the Zanzibar area.

The big ones are found on the western coast of Africa. The combination is sand, shingle and gravel.

SAND SPIT

It’s a strip of sand formed in the same way as a sand bar but one end of the spit is attached on the coastland and the other and is terminating into the water.

If the depositional waves meet the spit at an oblique angle.

They will form a hook at the end of the part that is terminating into the sea forming a hooked spit.

If depositional continues the hook will meet the land again and form a lagoon. When a spit joins to an island it forms a tombolo.

They are sometimes used as means of transport.

            SEA LEVEL CHANGES

sea level changes refer to relative movements in the level of water in the sea, Ocean, lakes relative to the adjacent land. When the sea level changes on a worldwide, it’s known as eustatism or an eustatic change. water in the sea is never constant.

 

Sometimes the level or water in the sea or ocean can rise relative to the land and this is referred to as submergence or positive change.

At times, the level of water in the sea or ocean Lowers or reduces relative to the land and this is known as emergence or negative sea level changes.

 

Therefore, sea level changes or base level changes involve two processes i.e.

  • submergence or positive base level changes
  • Emergence or negative base level changes

 

Sometimes the change in sea level is Worldwide and uniform which indicates an actual movement of the sea itself. This is known as eustatic sea level changes.

 

EMERGENCE

Emergence refers to where the land has risen relative to the sea or where the sea level has fallen relative to the adjacent land. This results into exposure of features that are formally under water and can be seen.

Causes of emergence

  • Rise in the level of the adjacent land due to isostatic uplift.
  • Fall in the level of the sea level due to drought, glaciation, global warming, widening of the sea floor etc.

 

SUBMERGENCE

Submergence refers to a situation where coastlines fall relative to the adjacent land or where the sea has risen relative to the adjacent land or Coast.

This therefore leads to formally exposed land / features being covered by water (indulated)

 

 

 

CAUSES OF SUBMERGENCE

  • fall in the level of the adjacent land due to relative sinking (Down warping)
  • Rise in the level of the sea because of illuviation, deglaciation, narrowing of the ocean floor etc.

 

CAUSES OF SEA LEVEL CHANGES

 

  1. climactic factors

Temperature changes can also lead to sea level changes. High temperatures result into prolonged droughts hence high rates or excessive evaporation leading to a fall in the sea level relative to the land adjacent.

 

Temperature changes leads to expansion and contraction of the sea water. A rise in temperatures of the water will cause its expansion which later brings about submergence while a fall in temperatures results into contraction of sea water which later brings about emergence.

 

Pluviation i.e. increased precipitation or rainfall leads to a rise in sea level relative to the adjacent land while desiccation i.e. decrease in rainfall totals leads to a fall in sea level relative to the land. E.g. in 1997 – 1998 Uganda experienced el-Mino rains which led to an increase in water in lakes causing submergence. In 2010, there was a prolonged drought period which resulted into emergence due to reduced water in lakes as a result of excessive evaporation.

 

  1. Glaciation and deglaciation

Deglaciation involves the melting of Ice due to a rise in temperatures. The melt waters lead to the release of large quantities of water which flow into the Sea leading to a rise in the sea level.

 

On the other hand, during periods of major glaciation, there is a drop in global temperatures. Water is frozen off into large ice masses in the Polar regions and mountains which cause a universal fall in the sea level.

 

  1. Tectonic movements

These are related to processes of warping, faulting, volcanicity and seismic activity. Up warping of coastal areas and down warping of ocean basins leads to a fall in the sea level resulting into emergence while down warping of coastal areas and down warping of the ocean basins results into the rise of the sea level (submergence).

 

Enlargement or expansion of the ocean basins due to plate divergence leads to a fall in sea level (emergence) e.g. the Atlantic Ocean is experiencing emergence of its coastlines since it’s getting larger as plate movements cause north and South America to drift away from Europe and Africa respectively.

On the other hand, contraction it ocean basins due to plate convergence leads to a rise in sea level.

  1. Volcanism:

volcanoes at constructive plate boundaries and subduction zones displace water causing rise of sea level relative to the land.

 

  1. Isostatic re-adjustments

The word isostasy is a Greek word meaning equal standing. The structure of the earth is in such a way that lighter rocks (sial) are sitting on denser rocks (sima). Due to isostatic re-adjustments, large amounts of weight e.g. glaciers, buildings, deposited debris etc. may be loaded or unloaded onto a region which may cause the land to sink or to rise and consequently cause emergence or submergence.

 

The addition of materials on continental areas increases weight causing continents to sink slowly hence a rise in the sea level e.g. ice accumulation during the Ice Age. After the melting of the ice sheets, the isostatic uplift of land masses occurred leading to a fall in sea level.

 

Deposited sediments in the sea or ocean basins by in flowing rivers reduce the size and depth of the ocean basins leading to a rise in the sea level hence submergence.

 

  1. Human activities

The pumping of water or oil for the ground can lead to the gradual sinking of the ground which results into emergence.

 

Man can also carryout dredging or desilting of coastlines which results into emergence. Sometimes sand mining at the coast can also bring about emergence. this is because the size of the lake basin is enlarged therefore bringing about lowering or drop or decrease in the water level.

 

Pouring of sewage and other sediments in the lake brings about submergence since the sediments try to fill the ocean basin and causes the rising of water.

in some countries, there is pouring of expired food, items like wheat, rice etc. in the ocean and this brings about submergence.

 

  1. Global warming

The world’s temperatures have been increasing over the years by at least 0.6 degrees Celsius or 1 degree

 

This increase in global temperatures has been due to man’s misuse of the environment involving deforestation, burning of fossil fuels e.g. coal, oil and natural gas etc. which reduces gases that trap heat in the atmosphere e.g. carbon-dioxide and carbon-monoxide which makes the climate to become warmer. This has resulted into melting of snow and ice e.g. at the poles resulting into submergence.

 

In the tropics, global warming has led to the reduction of glaciers which are potential sources of water to rivers that flow into oceans. This has resulted into reduction in the volume of water flowing into the seas and oceans hence bringing about emergence.

 

FEATURES OR LANDFORMS RESULTING FROM SEA LEVEL CHANGES

 

LANDFORMS RESULTING FROM SUBMERGENCE (RISE IN SEA LEVEL)

when the water level in the sea or oceans rises, areas or features which were not formerly in water become submerged.

Submerged features fall into two categories i.e.

  • Highland features
  • Lowland features

 

Submerged highland features

  1. Rias

It’s defined as a long narrow water inlet at the coast. Before the sea level rises, a river flows into the ocean through a Valley. When sea level rises, the valley is flooded or submerged by the rising water levels to form a ria. It’s formed with a shape of a funnel and decreases in width and depth inland. Examples can be found along the coast of east Africa at the mouth of the Mwachi river which forms the Kilindi harbor at Mombasa, the drowned river mouth of the Kombeni river which forms the Mombasa harbor.

Illustration

 

  1. Dalmation coastline / longitudinal coastline.

These are submerged tops of hills or highlands. They are formed in areas where hills and valleys lie parallel to the coast before submergence.

 

When the sea level rises, the valleys become flooded with water and the hills remain as chains of islands within the ocean and run parallel to the coastline. The water within the drowned valleys that separates the chains of islands from the main land is known as a sound.

 

Examples include the Smith sound west of Mwanza on Lake Victoria, the Pemba and Zanzibar coasts etc.

Illustration

  1. Fiords

These are submerged or drowned U-shaped glacial troughs or valleys formed along glaciated coasts. They have steep walls often rising straight from the sea. When the sea level rises, the coast is submerged and the lower parts of the U-shaped valleys or glacial troughs are and filled with water hence forming fiords. They are usually steep sided and very deep. There are no examples in east Africa but there are many at the coast of Norway, British Columbian coast, at the Coast of Chile etc.

llustration

 

SUBMERGED LOWLAND FEATURES

  1. Estuaries

These are submerged or drowned river valleys with a V-shaped cross profile pointing landwards. It is formed when the sea level rises along a low lying Coast causing the sea to penetrate Inland along river valleys eg the rufigi and kibanga estuaries. They are similar to Rias only that Rias form in highland coasts while estuaries form in lowland coasts.

 

  1. Creeks

These are narrow inlets formed by submergence of small streams. They are similar to estuaries only that they are smaller. Best examples of creeks are mfwapa, Tudor, reitz and kilifi all formed due to sinking of small rivers and streams along the East African coast.

 

  1. Mudflats and lagoons.

Mudflats are deposits of fine silt and alluvium of rivers to form plat forms of mud. Sediments are deposited in shallow water either behind shingle, bars, sand spits, or sheltered parts of estuaries and bays. At the Coast, such deposits enclose water and separate it from the rest of the sea to form a lagoon.

 

        LANDFORMS FORMED DUE TO EMERGENCE

 

GENERAL ILLUSTRATION SHOWING THE DIFFERENCE LANDFORMS.

 

 

  1. Raised cliff

Initially a Cliff is formed where the sea is in contact with the land. Through the phases of hydraulic action and abrasion, a notch is created and it becomes deeper due to continued wave erosion. When the above land loses support it collapses and a cliff is formed. when the sea level falls, the cliff is left isolated and it’s no longer in contact with the water and therefore it’s left behind at high tide to form a landform known as a raised cliff.

 

  1. Raised terraces

Initially a wave cut platform develops at the coast as a cliff retreats. When the sea level falls, the Former wave cut platform is no longer in contact with the water and is now known as a raised terrace. An example of a terrace is at Lutembe beach where it was formed when the water level of Lake Victoria fell at one time.

 

  1. Raised beach

This is formed when sea level falls such that the former beach is now left above the new water level which is above the present zone of deposition. This beach which is left suspended above the present water level is referred to as a raised beach. Best examples are found at Bagamyoyo in Tanzania where three layers of raised beaches can be found and at the Mamangina in Mombasa-Kenya.

 

  1. Raised caves

Initially a hole develops in the cliff face where there is a joint in the cliff. Through continued hydraulic action and abrasion, the hole becomes larger and finally the cave forms. When the sea level falls, the cave is left high above the high tide level and with no anymore contact with the sea which is now known as a raised cave. 8

 

  1. A raised geo

It’s formed where the entire roof of a cave collapses through erosion to form a narrow water inlet called a geo. When the sea level falls, it will not be in contact with the sea level and therefore it will have no water in it. It’s now called a raised geo.

 

Economic importance of features resulting from sea level changes

  • Rias and fiords are used in the construction of natural harbours e.g. kilindini and Dar-es-salaam harbors.
  • The landforms formed due to rise and fall in sea level are important tourist attractions and this brings in both local and foreign income in the respective countries
  • The landforms are important sites for education and research.
  • Mudflats are reclaimed for agriculture purposes and this improves on the food security in the country.
  • Rias form natural route ways inland
  • The raised beaches are used for sand mining.

Negative

Settlement is difficult along the sides of a fiord because of lack of level land.

the mudflats, swamps and marshes are breeding places for disease causing vectors eg mosquitoes etc.

 

there is a problem of occasional flooding especially in areas covered by Mudflats and this leads to destruction of property.

 

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ASSIGNMENT : COASTAL GEOMORPHOLOGY Assignment MARKS : 10  DURATION : 1 week, 3 days

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