Coastal Processes

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Coastal Processes Compression Air is trapped
between the waves and a cliff. This is compressed
driven into cracks in the rocks causing great
pressure. As the wave retreats it results in
sudden decompression. This activity weakens the
rocks. Longshore drift waves approach the beach
at an angle, driven by the wind. Run up the beach
at an angle and returns at a right angle zig zag
pattern. The swash carries sediment up the beach
and the backwash carries it down the beach. This
causes movement of sediment along the
beach. Abrasion Boulders, pebbles and sand
pounded against coast Hydraulic Action Impact of
waves Attrition Stone rounded off by impact
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Glacial Processes Plucking Meltwater penetrates
cracks in the rock. This freezes expands
weakening and fracturing the rock. The galcier
attaches itself to this rock and as it moves it
pull or plucks the rock away. Assisted by freez
thaw action. Abrasion Rocks attached to side
bed of the galcier scour, scrape polish the
surfaces they come in contact with. These cut
scratches grooves in the bedrock, called
striae, which indicate the direction of movement
of the galcier. Basal slip The sliding
movement of a glacier over its rock flour. (rocks
crushed by the glacier) Freeze-thaw Breaking of
rock due to repeated freezing of water
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Mass Movement

• Soil Creep Movement of soil down slope
• Gravity
• Solifluction swelling of soil, moves soil
  particles
• Frost heave ice crystals move soil
• Earth Flows Saturated Soil
• Lahars or Mud Flows Large and Rapid, triggered
  by ice melts by volcanic eruptions
• Slumping Cliff edge collapses, rotational
  movement

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Coastal Processes

• Coastline line where sea and land meet
• Shore Area between low and high water mark
• Divided into foreshore and backshore
• Waves product of friction between wind and
  water
• Size and power of wave depends on the
  fetch(length of open sea a wave travels over) and
  the power of the wind
• Therefore waves stronger on Atlantic coast than
  Irish sea coast

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• As waves approach shore it gets shallower so
  friction with the bottom increases
• This slows the bottom so the top spills over
  breaking the wave
• Swash rushes up the beach away from the sea
• Backwash comes back towards the sea

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Wave types

• Destructive high energy, 10 p/m
• Break rapidly, plunging vertically, capable of
  breaking of large amounts of rock, up to 25
  tonnes of pressure per metre squared, erodes
  coastline
• Constructive low waves that break gentler,
  -8 p/m, occur on shore with gentle gradient,
  swash spread over large area so backwash is weak,
  leads to more material being deposited than
  eroded

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Wave Refraction

• This causes the waves to bend as they approach
  the shore
• Friction with the bottom near the coast causes
  the wave to slow down
• Wave in deep water stays at original speed
• This causes wave to bend or refract and hit the
  coast line at an angle
• On headlands the wave slows down in front and
  waves swing around to attack the sides eroding
  them
• Little energy left for the bay so deposition
  occurs there

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Coastal Erosion

• Depends on the type of Rock Hard or Soft
• Wave strength destructive v constructive
• Shape of the coastline- wave refraction focuses
  energy on headlands
• Slope of shore gently sloping shores cause
  waves to break further out so they lose energy
• Erosion greater when soft rock lies underneath
  hard rock, where rocks are well jointed or when
  strata dip towards the sea
• Rising sea levels increase erosion expected to
  rise by 20-25 cm in next 30 yrs in Ireland

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Cliffs and Wave Cut Platforms

• Cliff is a vertical slope on the coastline as a
  result of erosion
• E.g. Cliffs of Moher
• Wave energy at its greatest when high steep
  destructive waves hit the land and erode it
  through hydraulic action and abrasion and
  compression
• A notch is cut at high water mark creating an
  overhang
• When overhang develops it will eventually
  collapse and a cliff is formed
• Cliff retreats inland and increases in height and
  material collects at the foot of cliff

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• This is worn down by attrition and some used to
  build a wave built terrace
• As the cliff retreats inland it exposes rock
  underneath with a gentle seaward slope called a
  wave cut platform
• If this platform gets too big the waves lose all
  there energy before reaching the cliff the cliff
  becomes inactive and erosion ceases to occur
• This can be seen by vegetation growing on the
  cliff and debris collecting at the bottom
• Vertical cliffs occur in areas of uniform rock or
  where strata dip inland

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Bays and Headlands

• Coasts rarely worn back evenly due to
  differential erosion as a result to differing
  types of rock resistance
• Less resistant rock forms bays while resistant
  rock forms headlands
• E.g. Galway Bay, Wicklow Head
• Mostly caused by hydraulic action, particularly
  where the rock has joints and bedding planes
• Air compression also adds to the erosive power as
  does abrasion and solution
• Wave refraction erodes the headlands and makes
  them more pronounced while bays are protected
  from high energy waves and are the scene of sea
  deposition forming beaches

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Sea caves, Arches and Stacks

• On headlands wave refraction opens up joints,
  faults or other weaknesses to form sea caves
  mainly through abrasion, Hydraulic action and
  compression
• If the cave is extended all the way through a
  headland a bridge like feature called a sea arch
  is formed this can also happen from caves
  eroding back to back
• If arch walls widen and roof is weathered it can
  collapse forming a sea stack
• If stack is eroded down to sea level its known as
  a sea stump

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Transportation

• Materials broken down by erosion are reduced in
  size by attrition until able to be transported
• Sediments moved up and down beaches by breaking
  waves
• The water that rushes up a beach is known as the
  swash and the water returning to the sea is known
  as the backwash
• Sediments moved by process known as longshore
  drift
• Waves reach beach at an angle and so swash is at
  an angle but backwash returns at right angles
• This process is repeated.

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Beaches

• Result of low energy constructive waves
• Created when amount of material deposited is
  greater then material eroded
• Beaches consist of boulders, pebbles, sand,
  shingle, mud etc
• Some comes from sea erosion but most transported
  by streams
• Beaches are gently sloping between LWM and HWM
  and can vary in length
• Most beaches have a foreshore and backshore
• Backshore has steeper slope, coarser material and
  is only reached in highest tides or storm
  conditions

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• The foreshore is lower, gentle gradient and
  consists of sand, mud and shell particles
• Storm beaches are ridges of large stones thrown
  up by strong waves back beyond reach of normal
  waves
• Berms ridges of coarse material that indicate
  the high tide level of constructive waves
• Ridges and runnels develop on seaward edge of
  foreshore and run parallel to sea where the waves
  break
• Sand dunes develop inland where there is plenty
  of sand and prevailing wind comes from the sea
• Sand blown inland and is trapped to form
  beginning of sand dune
• Dunes unstable and will continually migrate
  unless consolidated by marram grass e.g. Youghal,
  Ventry

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Sandspits and Tombolos

• Sandspits are long narrow ridges of sand and
  shingle that are attached to land on one side
• Formed by longshore drift when coast changes
  direction sharply
• Sediments carried off shore and deposited on sea
  bed building up to form spit
• Continues to grow as long as deposition is
  greater than erosion
• Sand dunes may from on sheltered landward side
  and beach formed on seaward side
• End can often be recurved due to change in wave
  direction
• Salt marsh can also develop on the landward side
• E.g. Strandhill co sligo
• Tombolo is a spit that connects an island to the
  shore e.g. Howth linked to Sutton Island

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Lagoons and Salt Marshes

• Lagoon is small body of water cut off or almost
  cut off from open sea
• Land cutting it off known as a bay mouth bar
• Can be created by sandspit sealing off a entire
  bay or offshore bar migrating towards land
• Offshore bar in parallel ridge of sand that forms
  off shore
• Occurs on shallow gently sloping coasts so waves
  break further out and deposit material
• It is increase in height by constructive waves
  and longshore drift
• Can be moved forwards by waves breaking over it
  to form a lagoon E.g.Our Ladys Island Lake,
  Wexford

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• Salt marshes often develop in these lagoons
• Mud and sand deposited by the sea and alluvium
  deposited by rivers result in infilling and
  lagoon becomes shallower
• Area becomes colonised by salt loving plants
• This increased rate of deposition and builds up
  to form marsh that is exposed permanently above
  normal tide
• E.g. Backstrand in Tramore, and Bull Island in
  Dublin

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