Le sujet était : Waves are crucial in altering the morphology of modern sedimentary coastlines. Discuss.
The study of coastal processes has become very important because a large part of the world population lives on the coast, and the recent environmental changes challenge use and management of the shore. The current geomorphology of the coast is the result of different processes: the continental drift, the relative level of the sea and the action of marine or fluvial processes such as erosion or sedimentation. The sedimentary coasts present many specific features and forms in different parts of the world. Generally, sedimentary coastlines are on the trailing-edge coast or passive margin and are present on the edges of sedimentary basin in all latitudes. Numerous factors have a part in the alteration of geomorphology of coastlines but wave action seems to be crucial. Motion of the sea primarily affect changes on the coast. What is the decisive role of waves in generation of modern coastal sedimentary landforms? Firstly, this work will examine the generation of waves and the characteristics indispensable to providing an explanation about wave actions. Secondly, it will establish the prime action of waves on beaches and sandy coasts and, finally, it will identify a range of processes that enables to affect the geomorphology of coastlines.
I. Generation and features of waves
a) Generation of waves
Waves are the prime source of energy on the coast, which is why their study is so important to understand morphological change on the coast.
Firstly waves occur in seas and oceans but it is an atmospheric element which generates them: the wind. In the Earth system, the elements of the oceanic cell and atmospheric cell are very interdependent. Wind is caused by the differential gradient of pressure, and blows from high to low pressure, provoking a mechanical action on the sea surface which creates the disturbance. Nevertheless only the form of the wave is transmitted, not the water. Waves are the prime energy for coastal processes on the shoreline. There are two kinds of waves generated by different sources: the `sea waves΄ are caused by wind and `swell΄ waves that occurs during stormy weather. Swell waves have the longest period and affects the seabed on a deeper surface.
Coastlines are mainly affected by breaking waves that influence specifically physical processes. Waves break because the velocity of water particles in the wave crest is higher than the wave velocity (Masselink, 2005) and by reaching shallow water, wave height decreases and length increases. Moreover, different breaking waves exist depending on local conditions. Spilling waves occur in relatively flat seabeds in the surfing zone; the crest spills down the face of the wave by generating foam and bubbles. In a steeper seabed, the crest of the waves becomes almost vertical and collapses forward by plunging. These are the plunging waves which concentrate their energy instantaneously. Surging waves happen on the steepest beaches with a slow movement of swash and backwash (Galvin, 1968). These different breaking waves do not affect the coastline in the same way owing to energy dissipated specifically. Energy depends on wave height, wind speed, fetch length and wind duration.
b) Modification of waves on the nearshore
Secondly, several processes can explain the differences in energy delivered on the shoreline. The first phenomenon occurs when waves reach the shore, due to coastal orientation and water depth. Refraction of wave energy does not affect uniformly the coastline, although bathymetry changes the angle of the wave crest and modifies water energy. The main consequence is the bending of wave shapes along the coast. Refraction concentrates wave energy on headlands. As a result, high energy and low energy coastlines are characterized respectively by processes of erosion and deposition.
Climatologic phenomenon such as storms, directly influence the power of waves on the coastlines. Extreme events like storms occur periodically in every place in the world. In high latitudes, low pressures system, cyclones, are frequent, whereas in tropical zones these events are rarer but much more powerful. Low pressures provoke a rise in water-level and swell waves transform the coastlines significantly. The east coast of the USA is a good example of low sedimentary coastlines, constituting of barrier islands and deltas. Violent climatologic events such as hurricanes modify the morphology of the shore considerably by breaking the weak balance between erosion and sedimentation. Some barrier islands can disappear after a single event, like those on the Gulf of Mexico after hurricane Gilbert in 1988 (Dingler et al, 1993). Tsunamis and flooding are also episodic events which entail massive consequence on the coast.
II. Actions of waves on the coastlines
a) Action of waves on beaches
Beaches are the most dynamic parts of the coast. They constitute a large amount of deposit-sediment along the coast and are subject to the action of waves and the wind. This deposit sediment constitutes a response to wave energy. Beach profiles can be extremely different in the world; three types of beaches can be identified: the dissipative, reflective and intermediate beaches. Flat beaches with fine sand characterize the dissipative profile. Wave energy on these beaches is generally high with spilling waves. It is the case of long and straight beaches of the Bay of Biscayne in the South-West of France along the Aquitain sedimentary basin. Reflective beaches take place more often in sheltered areas where refraction of the waves is important. They are steeper with coarser-grained sand and mainly associated with surging breakers. Dissipative beaches generate original beach shapes such as the crescent beaches of the South-east coast of the USA (Masselink et al, 1997). Intermediate beaches associate different features of reflective and dissipative beaches and constitute the most unstable profile. Waves bring sediment and energy to sediment transportation. Motion of sediments is function of waves velocity and the size of the grains, they are carried by different water movements along the nearshore. Different local currents occur in the surfing zone and take part in beach morphology changes. Rip currents are a consequence of different level energy points on the nearshore. The water moves from high energy on the front waves to low energy offshore by carrying sediment. [Doc] Sediment entrainment in function of Flow velocity and grain size. (Established by Hjulström, 1935)The longshore currents are parallel to the shore and carry waves after waves sediment in the same way that wave direction (Komar, 1998). However beaches are able to change under several factor actions. State of the beach can be defined as the association of sediment characteristics, wave actions, anterior beach state and the influence of tide and wind.
b) Action of waves on rocky coasts
Sedimentary coasts are not solely constituted of long and straight beaches but also rocky coasts primarily with cliffs. Wave actions have considerable action owing to the relative weak and soft lithology of sedimentary rocks compared to granitic rocks. Waves generate two types of physicochemical weathering on the cliffs. Firstly is a hydraulic action, where waves exert a massive pressure on the rocks due to water movement, increasing lines of weakness and cavities. In the other hand, transportation of sediment provokes the phenomenon of abrasion on the cliff and dislocation of angular blocks, which is why rocks on the coastlines appear smooth. This mechanical phenomenon occurs solely between the wave crests and below still sea-water. Cliff erosion produces spectacular but ephemeral features on the coastline. For example, sea caves, arches and sea stacks are the consequence of differential hardness in rock morphology (Bird, 1993). The coast of Victoria in Australia with twelve sea stacks (called the Twelve Apostles) provides an impressive example of cliff erosion on sedimentary coastlines. A succession of headlands and embayment is the result of differential erosion on the cliffs according to their resistance. Hence, the coastline is the result of erosive force of waves against the resistance of the rocks but cliff recession is a cyclic process, sediments deposited at the feet of the cliff can cut off the action of waves for a while (Savigear, 1952, 1962). However, cliff erosion rate is related to other factors than solely wave actions and, different types of rocks generate different coastlines that are function of other subaerial processes.
III. Other processes
Waves play a considerable role in altering coastlines but a number of other processes influence the morphology of the coast significantly. Some processes primarily affect some parts of the globe and others can be everywhere.
Wave action has massive consequence in the mid-latitude coasts but not in high latitude or tropical zones. These regions are characterized by fairly weak wave action and are affected by other more important processes. Hence the climate determines different types of erosion according to the weather. In high latitudes mechanical weathering is predominant. Ice created in winter is an extremely powerful erosion factor, and ice abrasion generates a large amount of sediment which contributes to erosion of the rocks. Frost weathering is also responsible for large dislodgement and quarrying on rocks along the coast. The tropical zone is characterized by very particular climatic conditions that promote chemical weathering. Chemical reactions increase in warm water and generate very particular erosion shapes. The most significant example is dissolution of limestone that creates original karst features. Ha-long Bay in Viet-Nam or Phangnga Bay in Thailand present impressive tower karst and steep eroded cliffs in their bases.
Other azonal processes occur at all latitudes. Climate and, particularly, rainfalls are powerful factors of disturbance of the rocky coasts. Hillslope processes can be generated by rainfalls and provoke mass movement by gravity. In this case, the conjunction of actions of waves and subaerial processes contribute to the regression of the cliff (Emery and Khun, 1982). Sedimentary coasts of South and East Britain are a good illustration of this phenomenon. Temperature and, primarily, repetition of frost and defrost bring about a strong mechanical constraint on the rocks. Water infiltrates into pores and faults and when temperature goes down below zero Celsius frost leads to fracturing the rocks. Haloclasty is the same action with salt when it grows from crystal to solution. Biological processes may alter coastlines as well (Wellman and Wilson, 1965). Organic activity such as gastropods and some grazing animals, can remote sediments on the seabed. Researcher measured that sponges have been able to remove 6-7 kg of material on one square metre during 100 days on the carbonate coast of Bermuda (Neumann, 1966). Furthermore previous relative sea-levels account for the current coastal landforms; past climates are directly implicated in sea-level changes. Coastlines have changed many times during the past and contributed modifying landforms on the coast by successive regression and transgression periods.
Conclusion
To sum up, waves do not play the same role in altering the geomorphology of modern sedimentary coastlines everywhere in the world. In the mid-latitude they have massive consequences and play a large part in the process of erosion and sediment transportation. Waves are the prime source of energy and contribute to modifying coastal landform on these coasts. However, coastlines are the result of a complex array of processes on a larger scale and it is crucial to consider inheritance of past climate and relative sea-level changes. Numbers of other physicochemical such as wind, temperature, chemical reactions or biological actions processes affect the coasts and the structure and lithology of rocks is also a critical factor in understanding erosion of coastal landforms. Today the main issue concerning coastal environment is the impact of global warming which could affect climate and sea-level. Many uncertainties remains about increase of erosion processes; it constitutes a high problem for populations who live on the coast.
References
Carter RW, Coastal Environments (1988) London, Academic Press Limited
Schwartz ML, Encyclopedia of Coastal Science (2005), Dordrecht, Springer
Woodroffe CD, Coasts (2002), Cambridge, Cambridge University Press
I. Generation and features of waves
a) Generation of waves
Waves are the prime source of energy on the coast, which is why their study is so important to understand morphological change on the coast.
Firstly waves occur in seas and oceans but it is an atmospheric element which generates them: the wind. In the Earth system, the elements of the oceanic cell and atmospheric cell are very interdependent. Wind is caused by the differential gradient of pressure, and blows from high to low pressure, provoking a mechanical action on the sea surface which creates the disturbance. Nevertheless only the form of the wave is transmitted, not the water. Waves are the prime energy for coastal processes on the shoreline. There are two kinds of waves generated by different sources: the `sea waves΄ are caused by wind and `swell΄ waves that occurs during stormy weather. Swell waves have the longest period and affects the seabed on a deeper surface.
Coastlines are mainly affected by breaking waves that influence specifically physical processes. Waves break because the velocity of water particles in the wave crest is higher than the wave velocity (Masselink, 2005) and by reaching shallow water, wave height decreases and length increases. Moreover, different breaking waves exist depending on local conditions. Spilling waves occur in relatively flat seabeds in the surfing zone; the crest spills down the face of the wave by generating foam and bubbles. In a steeper seabed, the crest of the waves becomes almost vertical and collapses forward by plunging. These are the plunging waves which concentrate their energy instantaneously. Surging waves happen on the steepest beaches with a slow movement of swash and backwash (Galvin, 1968). These different breaking waves do not affect the coastline in the same way owing to energy dissipated specifically. Energy depends on wave height, wind speed, fetch length and wind duration.
b) Modification of waves on the nearshore
Secondly, several processes can explain the differences in energy delivered on the shoreline. The first phenomenon occurs when waves reach the shore, due to coastal orientation and water depth. Refraction of wave energy does not affect uniformly the coastline, although bathymetry changes the angle of the wave crest and modifies water energy. The main consequence is the bending of wave shapes along the coast. Refraction concentrates wave energy on headlands. As a result, high energy and low energy coastlines are characterized respectively by processes of erosion and deposition.
Climatologic phenomenon such as storms, directly influence the power of waves on the coastlines. Extreme events like storms occur periodically in every place in the world. In high latitudes, low pressures system, cyclones, are frequent, whereas in tropical zones these events are rarer but much more powerful. Low pressures provoke a rise in water-level and swell waves transform the coastlines significantly. The east coast of the USA is a good example of low sedimentary coastlines, constituting of barrier islands and deltas. Violent climatologic events such as hurricanes modify the morphology of the shore considerably by breaking the weak balance between erosion and sedimentation. Some barrier islands can disappear after a single event, like those on the Gulf of Mexico after hurricane Gilbert in 1988 (Dingler et al, 1993). Tsunamis and flooding are also episodic events which entail massive consequence on the coast.
II. Actions of waves on the coastlines
a) Action of waves on beaches
Beaches are the most dynamic parts of the coast. They constitute a large amount of deposit-sediment along the coast and are subject to the action of waves and the wind. This deposit sediment constitutes a response to wave energy. Beach profiles can be extremely different in the world; three types of beaches can be identified: the dissipative, reflective and intermediate beaches. Flat beaches with fine sand characterize the dissipative profile. Wave energy on these beaches is generally high with spilling waves. It is the case of long and straight beaches of the Bay of Biscayne in the South-West of France along the Aquitain sedimentary basin. Reflective beaches take place more often in sheltered areas where refraction of the waves is important. They are steeper with coarser-grained sand and mainly associated with surging breakers. Dissipative beaches generate original beach shapes such as the crescent beaches of the South-east coast of the USA (Masselink et al, 1997). Intermediate beaches associate different features of reflective and dissipative beaches and constitute the most unstable profile. Waves bring sediment and energy to sediment transportation. Motion of sediments is function of waves velocity and the size of the grains, they are carried by different water movements along the nearshore. Different local currents occur in the surfing zone and take part in beach morphology changes. Rip currents are a consequence of different level energy points on the nearshore. The water moves from high energy on the front waves to low energy offshore by carrying sediment. [Doc] Sediment entrainment in function of Flow velocity and grain size. (Established by Hjulström, 1935)The longshore currents are parallel to the shore and carry waves after waves sediment in the same way that wave direction (Komar, 1998). However beaches are able to change under several factor actions. State of the beach can be defined as the association of sediment characteristics, wave actions, anterior beach state and the influence of tide and wind.
b) Action of waves on rocky coasts
Sedimentary coasts are not solely constituted of long and straight beaches but also rocky coasts primarily with cliffs. Wave actions have considerable action owing to the relative weak and soft lithology of sedimentary rocks compared to granitic rocks. Waves generate two types of physicochemical weathering on the cliffs. Firstly is a hydraulic action, where waves exert a massive pressure on the rocks due to water movement, increasing lines of weakness and cavities. In the other hand, transportation of sediment provokes the phenomenon of abrasion on the cliff and dislocation of angular blocks, which is why rocks on the coastlines appear smooth. This mechanical phenomenon occurs solely between the wave crests and below still sea-water. Cliff erosion produces spectacular but ephemeral features on the coastline. For example, sea caves, arches and sea stacks are the consequence of differential hardness in rock morphology (Bird, 1993). The coast of Victoria in Australia with twelve sea stacks (called the Twelve Apostles) provides an impressive example of cliff erosion on sedimentary coastlines. A succession of headlands and embayment is the result of differential erosion on the cliffs according to their resistance. Hence, the coastline is the result of erosive force of waves against the resistance of the rocks but cliff recession is a cyclic process, sediments deposited at the feet of the cliff can cut off the action of waves for a while (Savigear, 1952, 1962). However, cliff erosion rate is related to other factors than solely wave actions and, different types of rocks generate different coastlines that are function of other subaerial processes.
III. Other processes
Waves play a considerable role in altering coastlines but a number of other processes influence the morphology of the coast significantly. Some processes primarily affect some parts of the globe and others can be everywhere.
Wave action has massive consequence in the mid-latitude coasts but not in high latitude or tropical zones. These regions are characterized by fairly weak wave action and are affected by other more important processes. Hence the climate determines different types of erosion according to the weather. In high latitudes mechanical weathering is predominant. Ice created in winter is an extremely powerful erosion factor, and ice abrasion generates a large amount of sediment which contributes to erosion of the rocks. Frost weathering is also responsible for large dislodgement and quarrying on rocks along the coast. The tropical zone is characterized by very particular climatic conditions that promote chemical weathering. Chemical reactions increase in warm water and generate very particular erosion shapes. The most significant example is dissolution of limestone that creates original karst features. Ha-long Bay in Viet-Nam or Phangnga Bay in Thailand present impressive tower karst and steep eroded cliffs in their bases.
Other azonal processes occur at all latitudes. Climate and, particularly, rainfalls are powerful factors of disturbance of the rocky coasts. Hillslope processes can be generated by rainfalls and provoke mass movement by gravity. In this case, the conjunction of actions of waves and subaerial processes contribute to the regression of the cliff (Emery and Khun, 1982). Sedimentary coasts of South and East Britain are a good illustration of this phenomenon. Temperature and, primarily, repetition of frost and defrost bring about a strong mechanical constraint on the rocks. Water infiltrates into pores and faults and when temperature goes down below zero Celsius frost leads to fracturing the rocks. Haloclasty is the same action with salt when it grows from crystal to solution. Biological processes may alter coastlines as well (Wellman and Wilson, 1965). Organic activity such as gastropods and some grazing animals, can remote sediments on the seabed. Researcher measured that sponges have been able to remove 6-7 kg of material on one square metre during 100 days on the carbonate coast of Bermuda (Neumann, 1966). Furthermore previous relative sea-levels account for the current coastal landforms; past climates are directly implicated in sea-level changes. Coastlines have changed many times during the past and contributed modifying landforms on the coast by successive regression and transgression periods.
Conclusion
To sum up, waves do not play the same role in altering the geomorphology of modern sedimentary coastlines everywhere in the world. In the mid-latitude they have massive consequences and play a large part in the process of erosion and sediment transportation. Waves are the prime source of energy and contribute to modifying coastal landform on these coasts. However, coastlines are the result of a complex array of processes on a larger scale and it is crucial to consider inheritance of past climate and relative sea-level changes. Numbers of other physicochemical such as wind, temperature, chemical reactions or biological actions processes affect the coasts and the structure and lithology of rocks is also a critical factor in understanding erosion of coastal landforms. Today the main issue concerning coastal environment is the impact of global warming which could affect climate and sea-level. Many uncertainties remains about increase of erosion processes; it constitutes a high problem for populations who live on the coast.
References
Carter RW, Coastal Environments (1988) London, Academic Press Limited
Schwartz ML, Encyclopedia of Coastal Science (2005), Dordrecht, Springer
Woodroffe CD, Coasts (2002), Cambridge, Cambridge University Press
2 commentaires:
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