Coasts

Tides 

  • causes: gravitational effects: Moon, partly Sun, rotation of  Earth 
  • also: geomophology of sea basins 

moon: pulls water to side of Earth nearest to it - huge bulge (high tide) & complementaey on opposite side 

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Types of tide

Spring tide

Greatest difference between high & low tides (maximum tidal range) 

when? once every 14/15 days (twice in lunar month)

why? moon & sun are in alignment on same side of Earth 

increase in gravitational attraction produces tide 

Neap tide 

Midway between spring tides

why? sun, earth & moon form right angle with Earth at the apex 

just after first or third quarters of the moon

when? least difference between the high and low tide (minimum tidal range) 

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Effects of Tides

Effects of tides 

  • Morphology (shape/structure) of sea bed/coastline 
  • North Sea: tidal wave travels South, moves into an area where both width & depth of sea decrease, rapid funnelling, higher tidal range 

Why range at Dover several m higher than northern Scotland 

Estuaries 

  • Incoming tides are forced into rapidly narrowing valleys
  • Seven Estuary - 13m 
  • Rance (Brittany) - 11.6m 

Extreme narrowing - concentrate tide so rapidly, tidal bore can travel upriver (e.g. Amazon) 

Med 

  • small, enclosed seas
  • range 0.01m 
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Tidal Cycles during the lunar month

Day 1

  • Sun & mon combine (in alignment) to give spring tides 

Day 7 and 1/2  

  • Sun, moon & earth at right angle to Earth at the apex, giving neap tides 

Day 15 

  • Sun & moon are in alignment again to give spring tides 

Day 22 and 1/2 

  • Sun, moon & earth at right angle to earth at the apex, neap tides 

Day 29 

  • Sun & moon are in alignment again, combine, giving spring tides 
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Diagram of Tidal Cycle

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Storm Surges

Def: rapid rises in sea level due to intense areas of low pressure and tropical cyclones 

Coincide with hurricane-force winds & high tides, surge can be topped by 8m waves 

Case Studies 

1. 1953 (31st January to 1st February) 

Gale force winds, travelling over maximum fetch, produced storm waves over 6m high 

Causing water to pile up on the southern part of the North Sea 

What did it coincide with?

  • spring tides 
  • rivers discharging into seas at flood levels 

Result: a high tide, over 2m in Lincolnshire, over 2.5m in Thames Estuary, over 3m in the Netherlands (1835 died), Thames Barrier & Dutch Delta Scheme constructed 

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Bay of Bengal (Bangladesh)

Autumn 

  • tropical cyclones (low pressure systems)
  • funnel northwards 
  • up Bay of Bengal (becomes increasingly narrower & shallower towards Bangladesh) 
  • producing surge which may exceed 4m 

Surge 1994 

  • Red Cross, 3 days later, over 40,000 drowned 

Climate Change 

  • global warming & rising sea levels (1m, submerge 25%)
  • lowering height of delta  region due to extraction of groundwater for agriculture 
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Waves

How are they created?

  • By the transfer of energy 
  • wind blowing over 
  • surface of the sea 

What happens as the strenghth of wind increases?

  • frictional drag increases 
  • size of wave increases 

What does the energy of the wave depend on?

  • wind velocity 
  • period of time over which wind has blown
  • length of the fetch 

The influence of fetch 

  • places with the greatest fetch experience highest energy waves 
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Wave steepness

What does the steepness of the wave determine?

  • whether the waves will build up or degrade the beach 

What is the average pressure of a wave in winter?

  • 11 tonnes per m2 
  • may be three times greater during storm surge, explains why sea defences are destroyed 
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Waves in shallow water

As the base of the wave slows:

  • friction with seabed increases, slowing base of wave down 
  • circular oscillation becomes more elliptical 
  • wave steepens until it reaches 1:7 ratio (height:length)
  • upper part spills or plunges over

NB: point at which the wave breaks is called the plunge line 

What happens next?

  • swash rises up the beach
  • backwash returns to the sea 
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Wave refraction & beaches

Wave refraction 

  • irregular coastline
  • refracted
  • e.g. headland separating two bays 

Beaches 

  • form a buffer zone between waves & coast, effective, will dissipate the wave energy 
  • gradient dependent on wave energy (constructive vs destructive) & particle size (e.g. shingle beaches are steeper than sand beaches) 
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Types of Wave

High-energy waves 

  • produced by distant storms 
  • large fetch 
  • long wave length
  • travel quickly, lose little energy
  • breaker: 'spilling' breaker 
  • form flat & wide beaches 

Low-energy waves 

  • formed locally 
  • short fethc 
  • short wave length (up to 20m)
  • travel slowly, lost energy quickly 
  • breaker: 'surging' breaker 
  • form steep & narrower beaches 
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Constructive waves

Constructive 

  • form where the fetch distance is long 
  • small waves with a long wavelength (up to 100m)

Approaching the beach:

  • wave steepens until it gently 'spills' over 
  • swash to move up beach
  • Swash & Backwash

swash: strong, much water is lost through percolation, sand is carried up beach, form berm

backwash: weak, little material is returned down the beach 

smaller, longshore (breakpoint) bar 

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Destructive waves

Destructive waves 

  • Fetch distance is shorter 
  • large waves, steep, short wave length (only 20m)

Approaching the beach

  • steepen rapidly, until they 'plunge' over 
  • near vertical breaking of wave creates strong backwash

Swash & Backwash 

  • weak swash, little water is lost through percolation, most of the material is carried back down beach as backwash 
  • some large material forms a storm beach 

Large, longshore (breakpoint) bar 

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Diagram of Destructive waves

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Shingle and sand beaches

Shingle beaches

  • Steeper gradient due to percolation rate 

NB: water will pass through coarse-grained shingle more rapidly than through fine-grained sand 

Friction: loss of energy resulting from friction on shingle beach due to uneven surface, very little material is move back down beach as backwash, berm & storm beach formed  

Sand Beach 

  • Gentle graident, percolation rate is smaller 
  • Particle size: small, allows sand to become compact when wet 
  • less percolation 
  • large backwash (material carried down beach) 
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Why is the percolation rate on sandy beaches less?

This is due to: 

Structure of sand particles 

  • Storage of water in pore spaces

What role does friction play?

  • Less friction due to smooth surface 

What kind of beach is formed?

  • A wider beach to dissipate energy due to formation of longshore bar at low tide mark 
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Erosional landforms

Headlands and Bays 

  • Alternating resistant and less resistant rock 
  • initally, less resistant, most erosion, developing into bays 
  • leaving more resistant outcrops as headlands 

More resistant will become more vulnerable to erosion as it receives highest-energy waves, than sheltered bays

Wave-cut platform: A wave is at its maximum energy when high, steep wave breaks at foot of cliff, undercutting to form wave-cut notch 

  • contuned undercutting causes stress & tension in the cliff, eventually collapses 
  • cliff retreat, leaving wave-cut platform, gently sloping, exposed at low-tide  
  • slope angle of less than 4 degrees 
  • continued retreat leads to widening 
  • e.g. Flamborough Head (Yorkshire) 
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Caves, blowholes, arches and stacks

Formation

Erosion of areas of weakness e.g. joints or faults 

Marine erosion (abrasion, hydraulic action, solution, wave pounding)

Cracks widen, forming cave 

Where fault lines through headlands, two caves erode backwards into each other, forming arch, or keyhole (small arch) e.g. Durdle door or Stairhole (Dorest)

Wave attack at base & weathering (freezethaw, wind, rain), weakens strucutre & roof collapses 

Stack e.g. The Needles (Isle of Wight) or Old Harry (near Swanage), cut into chalk or The Old Man of Hoy (Orkenys), cut into Old Red Sandstone 

Stump (covered by high tide) 

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Headlands and Bays

E.g. Discordant coastline 

  • Dorset 
  • Portland limestone (Durleston Head),wealden shales (Swanage Bay), Chalk (Studland Head), sands 
  • differential erosion 
  • bands of alternating resistant and less resistant rock 
  • headlands initally experience less erosion, outcrop at right angles to coastline, resistant rock eroded rapidly to form bays 
  • headlands become more vulnerable to erosion by force of destrucitve waves, whereas sheltered adjacent bays 
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Cliff Lithology

Resistant rocks  e.g. Chalk, or limestone cliffs off South Wales 

  • marine erosional proccess will be greater than sub-aerial 
  • will produce wave-cut knotch & platform due to cliff recession 
  • progressively weathered & eroded, salt crystallisation due to spray from sea 

Less resistant rocks 

  • e.g. Naish Farm, Hampshire 
  • soft glacial deposits 

Arrangement of rocks 

  • Brickearth
  • plateau graval (pleistocene, river deposits)
  • Barton sand
  • Barton clay (contains bedding planes)
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Why does Naish Farm experience mass movement?

Mass Movement at Naish Farm 

  • evidence of rotational slumping 
  • heavy rainfall (weathering)
  • multi-benched profiled, 3 main bedding planes, concentration of groundwater encourages slope failire 
  • bench sliding of saturated clays, movement of colluvium (loose, weathered material)
  • spalling 
  • debris & mud slides 
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Other features of Lithology

Resistance of rock 

  • less resistant due to unconsolidated
  • weathered more easily by sub-aerial processes, dominate over marine erosion 
  • e.g. clay cliffs at ChristChurch bay in Hampshire or boulder clay at Mappleton Beach in East Yorkshire) 

Dip of rock strata

  • steep seward dip, rock slabs slide down the cliff along bedding planes, more prone to erosion by waves
  • dip inland - produces steep & stable cliffs 
  • NB:Jurrassic coastline of Dorest 
  • dip inland but with well developed joints at right angles to bedding planes - joints act as slide planes 

Jointing and Faulting 

  • greater mass movement as more easily exploited by weathering e.g. freeze thaw, Chalk cliffs at Beachy Head collapsed, Jan 1999 
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