COASTS by qingyunliuliu

VIEWS: 18 PAGES: 35

									                        Waves




The various landforms of coastal areas are almost exclusively the
result of the action of ocean waves. Wave action creates some of
the world's most spectacular erosional landforms. Where wave
energy is reduced depositional landforms, like beaches, are
created.
                 The action of waves
Waves erode or wear away the coast and transport the
eroded material along the coastline - a process called
longshore drift. Eventually the material will be deposited on
a beach or will form a larger feature such as a spit.


Erosion works in four ways:
• Hydraulic action - this results from the force of the water hitting the cliffs.
    Air, trapped in cracks, is compressed under high pressure, which breaks the
    rocks apart.
•   Corrasion - this is caused by the waves picking up stones and hurling
    them at the cliffs.
•   Corrosion [solution] - the dissolving of rocks by sea water. (Chalk and
    limestone)
•    Attrition - any material carried by the waves will become rounder and
    smaller over time.
Headlands and bays
         The shape of a coastline is often
         determined by its geology.(the rocks
         which make up the coastline)

          Some coastlines are very dramatic,
         with tall, vertical cliffs whilst others
         are straight for miles and miles whilst
         others are indented with many
         headlands and bays.

         When there are different rock-types
         along the coast, they may vary in
         their resistance to erosion. Some
         rock is harder and erodes more
         slowly forming headlands. Other
         rock is softer and erodes more easily
         forming bays .
BAYS
                      Beachy Head
              Chalk cliffs on the Kent coast




What are the beach materials ?
Caves, arches and stacks
Erosion can create caves, arches and stacks along a
headland. Again weathering can also help to create
these landforms.
Caves occur when the waves force their way into cracks
in the cliff face. The water contains sand and other
materials that help to grind away at the rock until the
cracks become a cave.
If the cave is formed in a headland, it may eventually
break through forming an arch.
The arch will gradually become bigger and bigger until it
can no longer support the top of the arch. When the arch
collapses, it leaves the headland on one side and a
stack (a tall column of rock) on the other.
                                                          Geography



                      Coastal Processes
Old cliff
  line

                                    stack

                                            stump
            Natural                                 Wave-cut platform
             Arch
            Coastal features
Which is the only feature formed by deposition ?
       What features can you see?




Cave     Arch   Wave cut platform   Stack
     Durdle Door
A natural arch in Dorset
Here the arch has collapsed
      leaving a stack
                                Stacks



Gibson Steps Pillar Australia




                                    Twelve apostles Australia




                                  Old Harry Rocks Dorset
The Needles Isle of Wight




        Chalk stacks
              Longshore Drift
  Beach material almost always moves along the
  beach in a particular direction which is
  determined by the prevailing winds.
If the winds blow the waves parallel to the shore,
sand and pebbles are washed up and back
down the beach. If, however, the wind blows so
that waves come in at a sharp angle to the
coast, the swash carries the pebbles at an
angle up the beach. When the backwash
moves by gravity back down the beach, the
pebbles end up at a different position. This
process is repeated and pebbles gradually shift
along the beach, a process called longshore
drift.
                       Sediment builds up
        Groynes        due to Longshore Drift




Waves approach beach
     at an angle
What is the movement of the wave up the beach called ?
What is the movement of the wave down the beach
called ?
What is a groyne and what is its purpose?
Barton on Sea
                            However, further south the rate of
                            erosion has increased significantly. This
                            is because material which is being
In 1991 two rock groynes    carried south is not being replaced (it is
and a rock revetment were   trapped within the groynes). Therefore
built, as a consequence     there is no beach to protect the cliffs.
there was substantial       Even during a neap tide ( a tide which is
beach accumulated           30% less than the average tidal range)
between the groynes         the sea reaches the base of the soft
halting erosion.            cliffs and erosion occurs.
                                 BARTON ON SEA
Barton on Sea is built on a cliff of clay under sand, and this has eroded rapidly[ 1m p.a.].
                                     This was the result of

1. A high energy wave environment with large waves from the SW. The waves create an
   under tow that removes beach material. The cliff is undercut and slumping occurs.
                            2. Longshore drift from W to E.
                      3. Dredging offfshore starves Barton beach
                  4. Groyne systems to the west starve Barton beach
     5. Sub aerial processes affect the face of the cliff e.g. wind action and run off.
        COASTS
Erosion and deposition on the
      Holderness coast
                     Mappleton
• In 1991, sea defences were built in order to protect the
  village from intense sea erosion that had threatened it.
  The cliffs had been eroded at a rate of 2.9 metres per
  year. The £2million project created 2 large rock groynes
  to halt longshore drift as well as revetments to break the
  waves.
• The success of this project, has created a larger higher
  sandy beach for Mappleton, and the erosion has slowed
  to a point where it is no longer a threat to the settlement.
  It must be noted however, that due to these sea
  defences, the area directly South of the village now
  suffers from an even greater rate of erosion.
•
              Mappleton
       What effect has the Groyne had?
Which direction is long shore drift occurring ?
 Where does the eroded material
from this coastline get transported
                to ?
                   Formation of a Spit
•   Spits are created through the process of deposition. A spit is an extended
    stretch of beach material that projects out to sea and is joined to the
    mainland at one end.
•   Spits are commonly formed where there is a prevailing wind and where
    there is a longshore drift. An example of a spit is Spurn Head, found along
    the Holderness Coast in Humberside.
•   The development of a spit is shown below:
 Look at the two sides of the spit
What are the differences and why?
             SPURN HEAD
• Spurn Head is a spit
  at the end of the
  Humber estuary.
• It has been formed by
  river deposition and
  Longshore drift
 The spit has been here so long there are
permanent buildings at the end and a jetty.




   What do you think the building on
   the jetty is ?
   Notice what shape the spit is




What is happening behind the spit ?
             Another Spit
Hengistbury Head in Dorset at the entrance
            to Poole Harbour
                      Hurst Castle History
Hurst Castle is situated at the seaward end of the shingle spit that extends 1½ mile from Milford-
On-Sea. The end of the spit, only ¾ of a mile from the Isle of Wight, and the views of the big
ships passing through are spectacular.

Hurst Castle was the perfect location to defend the western approach to the Solent.

The castle was built by Henry VIII as one of a chain of coastal fortresses and was completed in
1544. A critical element in the improvement of the defences of the south coast by Henry VIII was
the protection of the Solent, for this stretch of water gave access to the important ports of
Portsmouth and Southampton. Calshot Castle and Hurst Castle were both strategically situated
to provide their protection. Calshot was sited on a shingle spit close to the deep water channel at
the mouth of Southampton Water. Although one of the smaller of Henry VIII's forts, its three-
storey keep and outer curtain wall nevertheless gave it full command of its position. Hurst Castle
was sited at the narrow entrance to the Solent where the ebb and flow of the tides creates strong
currents, putting would-be invaders at its mercy.

Charles I was imprisoned here in 1648 before being taken to London to his trial and execution.

                                                                          s
The castle was modernised during the Napoleonic wars and again in the 1870’ when the
                                                                                           s
enormous armoured wings were constructed. Two of the huge 38-ton guns installed in the 1970’
can still be viewed in their casemates.

During World War II, Hurst was manned with coastal gun batteries and searchlights.
Hurst Castle spit

								
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