GEOG 1113: Landform Geography Coastal Processes and Landforms April 9, 2015.
Geography: Coastal Systems
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Transcript of Geography: Coastal Systems
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Coastal SystemsInput: Agents of coastal changesEffects of wave erosion on the coastEffects of wave transportation and deposition on the coastCoastal managementCoastal management
The coast is the area where the land meets the sea. It includes the beach and the cliff behind thebeach. there are two categories of coasts.a. Primary coasts, which are created by nonmarine processes. The processes include erosion,deposition, and tectonic activity. River deltas are an important example.b. Secondary coasts, which are formed by marine action. The processes include deposition ofsand by waves and currents, and growth of reefs by corals. Barrier islands are an importantexample.
The coastline marks the seaward limit of the land which is permanently exposed. It is thehighest point reached by storm waves.The shore is the area of land which lies between the highest tide level and the lowest tide level. Itis covered by the sea at high tide but exposed at low tide. The shore is divided into three parts:backshore, foreshore and inshore.The shoreline is the line demarcating the sea and the shore. The position of the shorelinefluctuates with the tide.
Waves and Wave ActionWaves are undulations of water when wind blows over the surface of the sea. They form as aresult of the transfer of energy. As the wind blows over the surface of the sea, friction is created producing a swell in the water. The energy of the wind causes water particles to move in acircular wave orbit inside the swell. This moves the wave forward.
In the open sea, the motion of the waves is transmitted by the circular movement of the individualwater molecule as the lack of frictional drag with the ocean floor in the deep water allows eachwater particle to move in a circular wave orbit.
As the waves approach the land, and the depth of the water decreases to half the wavelength,the size of the circular wave orbit decreases and the orbital motion of the water, retarded byfriction with the sea floor, forms an elliptical motion. The speed of the wave transmissiondecreases, the wavelength decreases and the wave height increases. Eventually the waveheight reaches a point of instability and the wave breaks.
The wave eithera. collapses, at some distance off the shore, to produce a spilling breaker, if the water offshoreis relatively shallow, or
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b. topples forward and is forced to break against the land forming a plunging breaker, if thewater remains relatively deep right up to the shore.
In both cases, the water steepens, loses form and become a breaker. The water rushes up theslope of the shore as the swash. Then the water runs back down the slope of the shore as thebackwash.
Energy of WavesThe size and energy of a wave is influenced by:
● the length of time that the wind has been blowing (duration)● the strength of the wind (velocity)● distance of open water over which the wind blows (fetch)
1. The energy of destructive power of the waves increases with increasing duration,increasing wind velocity and increasing distance of fetch.
2. The longer the wind blows, the stronger the wind and greater the fetch, the more powerfulthe waves.
Coasts facing open sea are more susceptible to wave erosion while coasts in shelteredlocations with weak wind and short fetch, are more prone to wave deposition.
Type of waves
Constructive Waves Destructive Waves
Frequency Long, low wavesLong wavelengths, up to 100m.Low frequency, a wave period of 6 9 waves per minuteAssociated with calm conditions
Short, high waveShort wavelength, up to 20m.High wave frequency, a wave periodof 11 15 minuteAssociated with stormy conditions.
Breaker Waves slide forward in shallowwater due to gentle offshore slopeAlso known as spill waves
Waves curve downward in deepwater due to steep offshore slopeAlso known as plunge waves
Swash andBackwash
Swash is more powerful thanbackwash, so materials are carriedup the beach
Backwash is more powerful thanswahs, so materials are carrieddown the beach
Process Deposition Erosion
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Factors Governing the Work of Waves and the Character of Coastal Landforms● Types of wave action
→ waves that break against the land are associated with wave erosion.→ waves which break before reaching the shore are associated with wave deposition→ constructive and destructive waves
● Geology of Coastal Rocks→ type of rocks→ resistance of rocks to weathering and erosion→ direction of dipping of the rock strata→ presence, absence and arrangement of lines of weakness (e.g. joints, fissures, faults)
● Relief of coastal slope→ steep coastal slope is associated with wave erosion because waves, especially those drivenby strong winds over long fetch, reach the coast in deep water and break against the land or cliffside→ Gentle coastal slope is associated with wave deposition as waves are retarded by friction withthe bottom of the slope in shallow waters. The waves tend to break before reaching the coast
● Orientation of the coast→ The position of the coast in contrast to open sea or ocean determines whether the coast isexposed to oncoming winds that have blown over long fetch or not→ the aspect of the coast in relation to strong prevailing winds determines whether the coast isexposed to high energy waves→ exposed coasts are more susceptible to wave erosion→ sheltered casts are more favourable to wave deposition
● Relative changes in Sea level→ may result eithera. from a rise of the mean sea level or submergence/sinking of the coastal land orb. from a fall of the mean sea level or emergence/uplift of the coastal land→ Effect of glaciation in coastal areas the drowning of glaciated valleys when sea level rises→ Effects of Volcanic activity in Coastal Areas occurrence of tsunamis, in relation to volcanicactivity, increases rate of coastal erosion→ Effects of coastal growth in coastal waters coral growth offshore tends to dissipate energybefore waves reach the coast, thus helping to protect the coastline.→ human impact modifies the natural coastal landscape by activities like reclamation,construction works, dredging, building typhoon shelters and groynes
The Work of Waves and Resulting Coastal Landforms● Wave refraction is the bending of wave fronts as they approach a shore so as to break
almost parallel with the shore. In deep water, wave fronts are essentially parallel to oneanother. As they approach the shallow waters of the shore, the retarding influence ofshallow water or frictional drag with the sea floor, causes the waves to slow down andthe wave fronts to bend. Wave refraction occurs when:
→ along an irregular coast the retarding influence of shallow waters off the headlands before
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the shallow waters of the bay causes wave refraction. Wave energy is directed andconcentrated more towards headlands rather than bays. Thus, erosion is more intensive at theheadlands and deposition more common in bays. This combined effect tends to reduceshoreline irregularity.→ along a straight line with approaching waves from oblique direction
● Wave erosionwave erosion is accelerated by the following conditions:→ exposure to strong prevailing winds→ great wind velocity→ long fetch→ large wave size→ steep coastal slope/deep waters right up to the coast→ rapid rate of weathering of coastal rocks→ weak coastal rocks which are less resistant to wave attack→ presence of lines of weakness (e.g. cracks, fissures, joints, and faults, nonresistant dykesand bedding planes)→ large quantity of rock materials carried by wavesThe 4 main types of wave action are:
1. hydraulic actionwearing away of coastal rocks when waves striking the cliff face compresses air in cracks oncliff face. this puts tremendous pressure on the surrounding rock. the air then expandsexplosively when the waves retreat resulting in a sudden release of pressure. This processshatters the rocks, opens up and enlarges the cracks
2. Attrition the mutual wearing down of the materials (e.g. sand, pebbles and boulders) which aretransported by waves. These particles become smoother, rounder in shape and smaller in size.
3. Corrasion (abrasion) the wearing of the coastal rocks or cliff faces by materials (e.g. sand, pebbles and boulders)carried and hurled against the coast by waves
4. Corrosion or solution the dissolving of soluble minerals in coastal rocks by sea water or waves. the dissolvedminerals may crystallize from evaporating sea water spray, and this helps detach mineral grainsfrom coastal rocks. the dissolved minerals may also be removed by sea water in solution. Theremaining rocks become weakened and are more susceptible to wave erosion by abrasion andhydraulic action.
Wave Erosional Featuresa. Headlands and Bays
When waves armed with rock debris lash against the shores or along coast that have alternatebands of resistant (harder) rocks and less resistant (softer) rocks, the process of continuederosion of rocks of different resistance causes the hard, resistant rocks like limestone and chalk
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to resist erosion and persist and the soft, less resistant rocks like clay, sand and gravel to beworn down easily.This eventually gives rise to an irregular coastline of headlands and bays. Asthe headlands become more exposed to the full force of the wind and waves, it will becomemore vulnerable to erosion than the sheltered bays.
b. Cliffs, wavecut platforms and offshore terraces
When high energy waves reach land with steep slopes, they erode the weaker arts of the steepslopes to produce a notch. Continued erosion and undercutting enlarges the notch to form asteep rock face called a cliff. Undercutting at the base of the cliff, together with the removal oferoded materials, causes the cliff to retreat landwards, exposing a flat terrace at the foot of thecliff called a wavecut platform.
After a period of time, the cliff becomes steeper and retreats further landwards while thewavecut platform becomes wider. When the wavecut platform is buried by deposits, causing abelt of shallow water which decreases the waveenergy, erosion of the wavecut platformceases.
The eroded materials which are transported away are deposited in the offshore zone to form anoffshore terrace.
c. Caves, arches, stacks and stumps
Along an irregular coast of headlands and bays, waves converging on headlands, due to waverefraction, often attack and widen lines of weakness into hollows called caves. When two caves,on opposite sides of the headland, join to form a complete opening, the cave top remains as anarch. With further erosion, the arch collapses, leaving behind the seaward pillar of the rock. Intime, it is completely removed by wave erosion.
d. Caves, Blow Holes and Geos
A blowhole is a nearvertical cleft or cylindrical tunnel leading from the rear top of a sea caveupward to the land surface above.
Due to the presence of near vertical lines of weakness above the sea cave, waves surging induring high tides tend to force and compress the air into the lines of weakness. When the wavesretreat suddenly, resulting in the opening and widening of the lines of weakness along the cave.Ultimately, part of the roof of the cave collapses, producing a blow hole.
Continued wave erosion may widen the blow hole till the entire roof of the sea cave collapses toform a long, narrow, steepsided inlet, called a geo. A geo may also develop when erosionextends a sea cave landwards, causing the cliff to be undercut by waves. The top portion of the
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cave may collapse, resulting in a geo.
Wave TransportWave transport is the movement of load (e.g. silt, clay, mud, sand, pebbles, etc.) along theshore and the seabed. Waves operate as an agent of transport in two ways:
1. Beach drift.When waves break obliquely at the shore, the swash moves obliquely up the shore/beach butthe backwash runs back at right angles to the shore/beach. Eroded materials are thus graduallycarried along the shore/beach by the combined zigzag movement of the swash and backwashcalled beach drift.
2. Longshore driftEroded materials moving in a zigzag manner on and off the shore create a net lateralmovement parallel to the coast, called longshore drift.
Wave DepositionFeatures Produced by Wave Deposition
● Bay delta● Bay● Cuspate bar● Tombolo● Bay mouth bar● bayhead bar● lagoon● hook● complex recurved spit
BeachA gently sloping platform formed by the accumulation of material (e.g. mud, sand, pebbles.cobbles) deposited by constructive waves on/along the shore, between the highest and lowestwater levelsBeaches are classified according to position:
● Bayhead beach.→ formed at head of a bay between two headlands. The waves which reach the head of a bayare relatively lowenergy waves and carry finer particles to the shore.
● Bayside beach→ formed at the sides of a bay. It is usually composed of coarser sand particles, or a mixture ofsand, gravel and pebbles.
● Baymouth beach (or headland beach)→ formed at the tip of a headland. It consists of coarser particles, gravel and boulders
Beaches are also classified according to type of deposits, for example, sandy beach, shingle
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beach and boulder beach.
Spit, Recurved spit and TomboloA spit is a narrow ridge of sand or shingle deposited by longshore drift at a sharp or abrupt turnof the coastline or across the mouth of a river. the occurrence of longshore drift, running parallelto a relatively straight coast, causes beach materials to be laterally transferred even at the turn ofthe coastline. As slack water occurs at the turn, the waves lose energy and deposit the sand orshingle in the form of a ridge, called a spit.
One end of the spit is attached to the mainland while the other end is free and projects into thesea. As a spit grows and extends into deeper water, wave action causes the free end to becurved towards the land, enclosing a water body called the lagoon. Such a spit is called a hook,or recurved spit.
When the spit extends seawards and joins up an offshore island to another island or themainland, a tombolo is formed.
Other features associated with a spit:i. baymouth bar when two spits extend from opposite sides of the bay (as they longshore driftfollows different directions at different times of the year), and eventually meet and join.ii. cuspate bar, where two spits develop on the two sides of a headland and they eventuallymeet and join each otheriii. cuspate foreland when the water body, enclosed by a cuspate bar, is gradually silted toform a piece of land.
BarsThey are narrow ridges of sand and/or gravel deposited by waves across a bay, usually in adirection parallel to the shore. It may be continuous or semicontinuous with breaks in between.
When a bar is first formed, both ends are free (i.e. not attached to the land). Subsequent andcontinued deposition may cause its end(s) to extend to join the land.
Like beaches, bars are classified according to position:● Bayhead bar when wave refraction on bay shores sweeps materials towards the head
of a bay to form a ridge of accumulated material rising from the sea floor, above the lowtide level.
● Baymouth bar When a spit grows form the headland and joins with the headland on theopposite end, the feature formed is a barmouth bar. The enclosed body of water in thebar, by the bar, is called a lagoon.
● Offshore bar may develop where waves break offshore, due to gentle slopes underwater, and deposit the material. It is free at both and and runs parallel to the shore.
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MudflatsSilty or muddy platforms deposited by waves and/or by rivers along gently sloping shores. Theymay be encroached by saltloving and salttolerant vegetation (e.g. mangrove), to form a swampor marsh.