EQ 2 Flashcards
(31 cards)
How are waves created?
Through friction between the wind & water surface, transferring energy from wind into the water. This generates ripples, which grow into waves when wind is sustained.
What are waves?
Transfer of energy from one water particle to its neighbour with indidivudal water particles moving in circular orbit.
- Size of wave-particle orbit decreases with depth.
What is wave height and what is it determined by?
Vertical distance from crest to trough.
What is wavelength?
horizontal distance from crest to crest or trough to trough
What is wave frequency?
Number of waves passing a particular point over a given period of time.
What does wave size depend on?
- Strength of wind
- Wave depth
- Duration the wind blows for
Longer the wind blows, the more energy it transfers - Wave fetch
Larger fetch allows the wind to push against the water for a longer time, transferring more energy
Where are the UK’s largest waves generally experienced?
In Cornwall because of the south-westerly prevailing winds and the fetch from Florida to Cornwall is over 4000km.
Large waves generated by sustained southwesterly wind have a great distance over which to grow.
What are swell waves?
Large waves built over a large fetch,
What are waves affected by?
Wave depth.
As wave approach a shoreline the water shallows and the shape of waves changes significantly. At a water depth of about half the wave length, the internal orbital motion of water within the wave touches the seabed. This creates friction between the wave and the seabed and slows down the wave. As the waves approach the shore, wave length decreases and wave height increases, so waves ‘bunch’ together.
Waves in open water
- Waves are simply energy moving through water
- The water itself only moves up and down, not horizontally
- There is some orbital water particle motion within the wave, but no net forward water particle motion.
What is fetch?
The uninterrupted distance across water over which the wind blows, and therefore the distance waves have to grow in size.
How do waves break?
In shallow water because the crest of the wave begins to move faster tan the wave trough. The trough experiences significant friction with the sediment and rock of the shore. Eventually the wave crest outruns the trough and the wave topples forward - a breaker.
When waves reaching the shore reach a wave depth of 1/2 their wavelength, the internal orbital motion of water within the wave touches the sea bed.
• Friction between the sea bed begins to distort the wave particle orbit from circular to elliptical, and slows down the wave.
o The wave has entered the offshore zone
• The wave depth decreases further, and the wave velocity slows, wavelength shortens, and wave height increases. Waves ‘bunch’ together.
• The wave crest begins to move forwards much faster than the wave trough
• Eventually the wave crest outruns the trough and the wave topples forwards - breaking.
• The wave breaks in the nearshore zone, and water flows up the beach as swash
• The wave then losses energy and gravity pulls the water back down the beach as backwash.
What is swash?
What is backwash?
Swash: flow of water up a beach as a wave breaks
Backwash: When water runs back down the beach to meet the next incoming wave
What are constructive waves?
Other names for them?
a.k.a spilling/surging waves.
- Low wave energy
- Low, flat wave height (<1m)
- Long wavelength (up to 100m)
- Low wave frequency (~6-9 per min)
- Strong swash; pushes sediment up beach
- Weaker backwash unable to transport all particles back down, so deposited it as a ridge of sediment (berm) at the top of the beach.
Gentle beach profile with steep berm means most bachwash percolates into the beach rather than running along the surface.
Long, shallow nearshore, so friction slows down the wave and releases energy.
What are destructive waves?
Other names for them?
a. k.a plunging waves
- High energy waves
- Large wave height (> 1m)
- Short wavelength (~20m)
- High wave frequency (13-15 per minute)
- Common during storms
- Waves with strong backwash that erodes beach material and carries it offshore, creating an offshore ridge/bar
- Weak swash
- Short, steep nearshore zone; little energy loss through friction
What is beach morphology?
The shape of a beach, including width and slope (the beach profile) and features e.g. berms, ridges & runnels. Also includes type of sediment (shingle, sand, mud) found at different locations on the beach.
Decadal variation in beach profiles
- Sediment supply from rivers is reduced e.g. due to construction of dams on rivers that trap sediment upstream
- Interference in sediment supply along the past, often result of coastal management in one place having an effect on processes further along the coast.
- Changes to climate e.g. global warming made UK climate stormier on average, then destructive waves * ‘winter’ beach profiles would become more common.
• Climate change is expected to produce more extreme weather events in the UK.
• Winter profiles may be present for longer time over course of year
• More frequent and more powerful destructive waves may reduce beach size, allowing high tides to reach further inland and increasing rate of coastal erosion in what was backshore zone.
Seasonal variation in beach profiles
Summer = low-energy constructive
steepen beach angle & string particles by size (larger shingle towards back of beach). they build berm ridges typically out of gravel/shingle at high tide mark
Winter = high-energy destructive
lower angle of beach profile & spread shingle over the whole beach. destructive wave erosion and subsequent deposition of sand and shingle offshore forms offshore ridges/bars
Daily variation in beach profiles
- Storm events during summer will produce destructive waves that reshape beach profile in a few hours.
- Calm anticyclonic conditions in winter can produce constructive waves that begin to rebuild beach, steepening profile for few days before storm.
- Destructive waves change to constructive ones as the wind drops.
- Storm beaches, high at the back of the beach, result from high energy deposition of very coarse sediment during the most severe storms
Monthly variation in beach profiles
Tide height varies over course of lunar month, with highest high tide occurring twice a month at spring tide and two very low high tides (neap tides)
• As month progresses from spring down to neap tide, successively lower high tides may produce a series of berms at lower and lower points down the beach.
• Once neap tide passes and move towards next spring tide, berms successively destroyed as material pushed further up beach by rising swash reach.
What are sea waves and swell waves?
- Sea waves are produced by winds currently blowing in the local area, and vary in height and direction
- When the wind drops, wave energy continues to be transferred across the ocean in the form of swell waves
- As swell waves travel away from their origin they may absorb smaller sea waves and gain energy and height
- They can travel long distances before they lose energy and dissipate
- They produce waves at the coast even when there is no wind
- Swell waves can form periodically larger waves amongst smaller, local sea waves at the coast
What are the four erosion processe?
- Hydraulic action (wave quarrying)
- Corrosion (solution)
- Abrasion (corrosion)
- Attrition
What is hydraulic action?
Can occur through
1. Direct impact of water itself
Destructive waves exert a force sufficient to break off material from unconsolidated material, e.g. boulder clay, or weak rocks like clay and shale.
- Force of breaking wave compressing air into cracks/fissures in rocks.
- Wave energy is exhausted, compressed air explodes outwards, causing micro-fractures in rock and expanding the main crack.
Pressure forces cracks open; more air is trapped & greater force is experienced in the next cycle of compression.
Overtime; small fragments of rock become weakened and break away or the main crack can extend until larger slabs of rock fall.
- Process dislodges blocks of rock from the cliff face.
Affect of hydraulic action on lithology
Hydraulic action
Heavily jointed/fissured sedimentary rocks are vulnerable.
In hard, resistant igneous rocks, hydraulic action attacking its cooling joints may be the only effective wave erosion process.
High energy waves with a large wave height are the most effective at erosion through hydraulic action. It is also accentuated when there is no debris at the cliff foot to absorb some of the wave energy and protect the cliff base.
