Physical GEOG: Chapter 13 - Coasts


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Physical GEOG: Chapter 13 - Coasts

  1. 1. Coastal Process Description The greater the difference in air pressure, the greater the wind Wind energy speed leading to greater wind energy and the larger the waves. Factors affecting The greater the fetch or distance size of waves Fetch over which the wind blows over the sea, the larger the waves. Other Tsunamis, sea traffic can cause factors the waves to be larger. 1. As a wave approaches the coast, the height between the crest of the waves and the seabed decrease. 2. More friction is exerted and more energy is dissipated. 3. Speed of waves (in front) Swash - decrease but speed of waves (be- hind) are at same speed. Erosion 4. This pushes the front wave forward, forcing it to rise. 5. As height of wave increases, its gradient becomes steeper. 6. It will then collapse and break onto the beach as swash. 1. Due to gravity, water that has crashed onto the coast returns Backwash - back to the sea perpendicularly to the coast. Swash > Backwash Occurs at gentle coasts. Constructive Erosion < Deposition, period is large, 6-7 waves/min. Types of waves Swash < Backwash Occurs at steep coasts. Destructive Erosion > Deposition, period is small, 10-14 waves/min.
  2. 2. Coastal Process Description Waves tend to approach the coast at an angle, depending on prevailing wind direction. Swash carries sediments up on-to the coast at that angle. Backwash carries sediments back to the sea at a right angle dueTransportation Longshore drift - to gravity. This motion is repeated as a zig-zag movement of sediments along the coast. The movement of sediments parallel to the coast is known as longshore drift, which may be expedited by longshore currents. Constructive waves carry out more deposition than erosion. Deposition - - Shallow seabed increases friction and wave has less energy hence it has a weak backwash.
  3. 3. Formation of headlands and bays Some coasts are made of alternate strips of hard and resistant rocks and less resistant rocks. They are arranged at right angles to the coast. Due to the different resistance, different rates of erosion will occur where the less resistant rock is eroded more than the resistant rock. This will form indented coasts with headlands and bays.WAVE REFRACTION: Waves that usually approach the shore in a parallel manner are now refracted or bent as they approach a headland. Due to the uneven depth of the sea floor, waves bend and and their energy is distributed unevenly along the shore line.Formation of beaches As a result of wave refraction, waves approach the shallow sea in front of the headlands first. As energy tends to concentrate and strike at the headlands rather than the bays, erosion takes place at the protruding headlands. Along the bays, waves are diverged and their energy spread out and weaken. Deposition of sediments thus happen along bays and over time, sandy beaches are formed.Formation of cliffs and shore platforms Repeated poundings of waves against a rocky surface undercuts it. Through erosional processes such as hydraulic action and abrasion, rocks are weakened. Lines of weaknesses are formed in the rock face. Over time, the waves will erode the coasts to cut a notch along the lines of weaknesses. The notch may be further deepened inwards to form a cave. The overhanging part of the notch or cave eventually collapses with repeated poundings of the waves. As the process continues, an overhanging cliff is formed. This overhanging cliff will soon collapse and the materials would be deposited at the foot of the cliff.
  4. 4. Some of the materials may be washed away but some of them are picked up by the waves and are thrown against the base of the cliff resulting in further erosion which causes the cliff to retreat further inland. A gently-sloping platform called the shore platform appears at the base where the cliff use to be.Formation of spits Longshore currents transport sediments parallel to the coast. When longshore currents encounter a bay or a bend in the coast with shallow sheltered water, they lose their energy. Materials are deposited in the direction of the longshore drift. Over time, the accumulation of materials will appear above the surface forming a narrow, long ridge of sand called a spit where one end is attached to a mainland while the other extend seawards, it may curve due to changes in wind direction.Formation of tombolos As it extends seawards, it might join up with another offshore island, forming a tombolo. A tombolo may also join 2 islands.
  5. 5. Type of Method Description + - Examplemethod Concrete, Absorbs Very expen- The world’s rocks or wood energy so that sive to largest seawall walls built in coast is not construct and in Japan front of a cliff eroded away, maintain, costing $1.5 bil or along the protected from LDCs cannot to construct but Seawalls coast to absorb strong waves. afford such it failed to energy protection. protect the nuclear plants from the 2011 tsunami. Granite struc- Deposition Very little of In Singapore, tures built occurs in the the coast is they are built parallel to the shallow and protected along ECP as Breakwaters coast to create calm water, hence there well as Siloso a zone of forming would be more Beach, Sentosa shallow and beaches. erosion than and each costs Hard calm water. deposition. S$1 mil. Low wall built Allows On the other at right angles transported side of the to the coast to materials to groyne, beach prevent accumulate on is not repleni- Groynes materials from side of groyne shed and it - being transp- facing LSD. continues to be orted away by eroded away. LSD. Wire cages Absorbs Rusts easily containing energy so that and hence the small rocks to coast is not rocks will fall Gabions - form wall. eroded away, out and it will protected from not work. strong waves. Adding large Ensures that Very expen- Cost of beach amounts of beach is not sive as beach nourishment to sand to an eroded away quality sand is replenish 24km eroded beach. completely very expensive of Miami When LSD, until there is no and replenish- Beach Beach, USA Soft transports sand sand left in ing sand must nourishment costed US$64 away, people some parts of be done every mil. bring in sand the beach. 10 years. from other places to replace it.
  6. 6. Mangroves Reduces Young mang- Malaysia have prop or erosion hence roves are very launched a kneed roots protecting the fragile and project to line that anchor coast from requires co- 4800km of its the trees firm- excessive operation of coastline with Planting ly in the mud- erosion. local people in mangroves mangroves dy soil, roots the area (eg. after realising also bind the farmers not to that it protect- loose soil. let goats feed) ed them during the 2004 Indian Ocean Tsunami. A ridge of Acts as barr- Dunes are sand piled up iers along fragile and by wind on the coast to protect vehicular or Stabilising coast that is human human move- - coastal dunes stabilised by properties ment on it may vegetation (eg. against erosion cause it to will spinifex, and flooding. not be allowed. marram) No building of Safe for Opposed by In England,Soft new proper- people as they people who there is a ties in places do not have to invested on ‘green line known to be at worry about coastal policy’ that risk of coastal their houses properties (eg. does not allow erosion. being affected beach resort Relocation of buildings to be by coastal owners) property built beyond erosion. the line. Properties behind the line is not protected or insured. Coral reefs are Reduces speed Human Malaysia has masses of of waves appr- activities like banned fishing rock-like oaching coast dynamite fish- in certain substances hence most of ing, sand min- areas, built called corals the energy ing and land Growth of waste water growing in would have reclamation coral reefs treatment shallow seas. been lost. destroys coral facilities to reefs. encourage growth of coral reefs.