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
- Created by: hannah s1234444444
- Created on: 07-06-14 08:15
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)
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
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
Diagram of Tidal Cycle
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
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
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
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
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
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)
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
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
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
Diagram of Destructive waves
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)
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
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)
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)
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
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)
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
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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