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Understanding coastal processes and problems to produce successful management solutions. A case study of Dawlish Warren. . Dr. Chris Spencer University of the West of England, Bristol. Shallow Water Waves : Waves change as they move into shallow water motion of water molecules changes.

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Understanding coastal processes and problems to produce successful management solutions.A case study of Dawlish Warren.

Dr. Chris Spencer

University of the West of England, Bristol

slide4

Shallow Water Waves :

  • Waves change as they move into shallow water
  • motion of water molecules changes

http://www.oceanpix.co.uk/wave-simulator.htm

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Moving into shallow water the sea floor starts to interfere

  • As water shallows wavelength and wave velocity decrease
  • wave height increases
  • wave steepens
  • crest becomes narrower and steeper, can’t continue
  • the wave breaks
  • The point at which this happens varies
  • How does this vary on steep / shallow beaches?
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The wave breaks :

  • water runs up the beach until stopped by gravity – the swash
  • water then flows back down the beach due to gravity – the backwash
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Beaches : Shore Normal Morphology :
  • beach – loose pile of sand and gravel
  • survives in high energy environment…….walls get destroyed??

How do beaches survive?

  • beaches are mobile
  • adapt shape to conditions
  • walls are fixed

Low energy conditions (summer)

    • beach relatively steep - reflective
    • waves break onto the beach, relatively well spaced out
    • backwash returns before the next swash
    • swash not slowed

Lecture 3 : Dr Chris Spencer

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How will this effect sediment movement?

Beach Shape?

  • these are constructive waves
  • give us summer profiles
  • berms at the back of the beach

Under high energy conditions (winter)

  • beach slope is more gentle – absorb / dissipate
  • spilling breakers, breaking over beach
  • waves arrive in rapid succession
  • backwash interferes with the next swash
  • reduced ability to move seidment

Lecture 3 : Dr Chris Spencer

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net seaward movement of sediment
    • destructive waves
    • winter profile
    • wider flatter beach

Beach shape is also influenced by particle size

  • this is due to the variable permeability
  • coarse beaches are more permeable
  • swash dominates, backwash returns through the beach
  • net sediment movement up the beach
    • increases gradient

Lecture 3 : Dr Chris Spencer

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Longshore currents :

  • waves may approach the coast at angle
  • usually <100 due to refraction
  • swash approaches at an angle
  • backwash returns with gravity

Dr Chris Spencer : Lecture 3

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Longshore Drift :
  • wave process leads to characteristics landforms
  • landforms become detached from the coast

Spits

    • narrow, elongate beaches
    • detached from the coast
  • longshore drift transports sediment along the coast
  • where the coast is ‘indented’ some of this sediment is deposited
  • the longshore drift can now extend further

Lecture 3 : Dr Chris Spencer

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The spit then extends across the indentation in the coast
  • can extend right across bay bar
    • if an estuary the high discharge will keep this open
  • spits have a characteristic hook shape at the end, a recurve
  • often a whole series of recurves visible

Example : Dawlish Warren

2 theories

    • waves approach and slow in the shallows
    • curve around the spit
    • angle of the wave varies
    • builds up a recurve

Lecture 3 : Dr Chris Spencer

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Profile 1

Profile 2

Height (m)

Height (m)

Distance along transect (m)

Distance along transect (m)

Profile 3

Profile 4

Height (m)

Height (m)

Distance along transect (m)

Distance along transect (m)