8.1 Flashcards
Characteristics of a wave:
Crest
Trough
Wave height
Wavelength
Crest: highest point of the wave
Trough: lowest point of the wave
Wave height: vertical distance between crest and trough
Wave length: distance it takes for the wave to repeat (crest to crest)
Wave formation
- in the offshore zone waves have a circular orbit, with constant wave length and speed, as the depth is greater than 1/2 the wavelength, meaning it is unaffected by friction - waves of oscillation
- leads to a circular orbit
- as the seabed becomes shallower in the nearshore zone, the orbit of the water particles becomes more elliptical (waves of transition), leading to more horizontal movement of the waves
- wave height increases, but the wavelength and velocity both decreases (shoaling occurs)
- as the waves move from the breaker to the surf zone, the elliptical orbit increases, until the waves reach the foreshore, where they become waves of transition (plunging, surging or spilling) and eventually break
Factors affecting size/energy of waves
Strength of the wind: stronger wind means that more energy transferred to the water - frictional drag
Size of fetch: longer fetch, more time for wind to transfer its energy to water
Duration of wind: longer duration, more energy transferred
Waves of translation?
When waves break - constructive or destructive
Constructive waves
- depositional waves (spilling waves)
- elliptical orbit
- strong swash, weak backwash
- gentle slope - friction increases gradually, therefore shoaling occurs gradually
- waves therefore small and far apart when they reach shore
- an entire wave cycle is complete before next wave comes in due to long wavelength
- swash runs far up beach, due to low angle, so infiltration rates high and backwash weak
- built up beach
What is shoaling?
Wave shoaling occurs when ocean waves move from deeper water into shallower areas, causing them to slow down, decrease in wavelength, and increase in height.
Destructive waves
- erosional
- circular orbit
- surging/plunging waves
- steep slope = friction increases quickly and shoaling occurs quickly
- waves larger/higher and fall over on themselves (plunging)
- beach material can be eroded (scouring) when waves plunge over
- infiltration rates low, so backwash strong
- waves closer together so backwash often returned in next wave, so material often deposited in offshore bar
3 main types of breaking waves
spilling breakers - constructive
plunging breakers - destructive
surging breakers - destructive
spilling breakers
- constructive
- associated with gentle beach gradients and steep waves
- characterised by gradual peaking of wave until crest becomes unstable, resulting in gentle spilling forward of crest
Plunging wave
- destructive
- tend to occur on steeper beaches than spilling breakers, with waves of intermediate steepness
- distinguished by shore-ward face of wave becoming vertical, curling over and plunging forward and downward as an intact mass of water
Surging waves
- destructive
- found on steep beaches with low steepness of waves
- front face and crest of waves remain relatively smooth and wave slides directly up the beach without breaking
- large proportion of wave energy reflected at the beach
Wave refraction
- wave orthagonals are lines of wave approaching from deep water at right angles tp wave crest
- wave approaching a coastline will firstly encounter shallower water in front of headlands
- this shallower water will slow down the wave - friction
- section of the wave in front of adjacent bays will still be in deeper water and continue to travel, at a faster speed than section encountering headland
- results in wave crest line starting to bend/refract
- wave energy concentrated around headlands and dispersed in the bays
Tides
- regular movements in the sea’s surface, caused by the gravitational pull of the moon and sun on the oceans
- moon accounts for the larger share of the pull, despite being smaller than the sun, because its closer to the earth
semi diurnal: 2 high and 2 low tides every 24 hours
diurnal: one high and low tide every 24 hours
Spring and Neap tides
- tides that are higher/lower than average and occur 2x a month
Spring: - highest/lowest tides of the month
- sun, moon and earth are in alignment and gravitational force is strong
- occur at new moon and full moon
Neap tides:
- low tide or lower than average
- moon is at a right angle to the sun, so graviational force work against each other
- occur at first and last quarter
Implications of tides on the coastline
- less steep the beach the larger the foreshore and so larger the impact of the tidal range
the tidal range will influence the following:
- vertical range of erosion and deposition
- weathering: more time the coastline is exposed the more time there is for weathering such as wetting/drying
- velocity of tidal flow is influenced by the tidal range and has an important scouring effect
Different coastal erosion processes
- hydraulic action
- abrasion
- corrosion/solution
- attrition
Hydraulic action
hydraulic action:
- powerful waves (more for destructive) hit the cliffs
- any air sat within cracks put under immense pressure
- as the wave retreats, the pressure is released with explosive force
- pressure release = cavitation
- particularly a problem on well jointed rocks, e.g limestone
- rates of erosion are increased in storm conditions
Abrasion
- as waves approach, wave energy enables material to be picked up and thrown against the foot of the cliff causing pieces of rock to be chipped off
- sand, shingle and boulders hurled against a cliff line will do enormous damage
- apparent on intertidal rock platforms were sediment is drawn back and forth grinding away at the platform
Solution
- sea water is acidic due to organisms such as limpets and barnacles
- acidic water reacts with minerals in the rocks, causing them to dissolve
- prominent on calcium-based rocks such as limestone
- higher levels of acidity may be found in rock pools at low tide
Attrition
- rocks and stones which the sea erodes from the cliff are rounded and broken down as they bump against each other
- eventually the fragments will be reduced to sand or silt
link between mass movement and erosion
- if mass movement results in large boulders, erosional processes such as abrasion are needed to break down the particles
- these particles are then able to be transported elsewhere down the beach and cause further erosion through attrition and hydraulic action
- hydraulic action can result in mass movements, which cause further processes of erosion, such as abrasion and attrition
Energy Factors affecting the rate of erosion
-
Waves:
- wave steepness: steep destructive waves more erosive than less steep constructive
- breaking point: at cliff base cause maximum erosion, whereas waves breaking off shore lose energy - Tides: neap and spring tides vary the zone of wave attack, strong tidal currents can scour estuary channels
- Currents: longshore and rip currents can move large quantities of material
-
Winds:
- onshore winds erode fine beach sand to form dunes
- offshore winds may erode dunes and nourish beach
Material Factors affecting the rate of erosion
- sediment supply: continual supply needed for abrasion, whereas an oversupply can protect the coast
- beach/rock platform width: influence wave energy by absorbing waves (buffer) before they can attack cliffs
- rock resistance: rock type/structure influences rate, more resistant less erosion; erosion rapid where rocks of different resistance overlie one another
How does shore geometry effect erosion?
-
offshore bathymetry: steep seabed creates higher/steeper waves
- longshore bars cause waves to break offshore and lose energy -
orientation of coast:
- headlands with vertical cliffs tend to concentrate wave energy by refraction -
direction of fetch:
- longer the fetch, greater potential for erosion by waves
concordant and discordant coastline
concordant: rock bands lie parallel to the coast
discordant: rock bands lie at right angles
Different rock types
- Metamorphic: e.g marble, resistant/permeable, e.g Cornwall
- Sedimentary rock: e.g chalk, joints make them permeable, e.g Dorset
- Igneous rocks: e.g granite, resistant and impermeable, e.g Portland
- Unconsolidated: e.g boulder clay, not cemented and easy to erode, e.g North norfolk
Sediment sources
- river: bring sediment from land to coast; usually fine-grained silts, clays and sands. May result in salt marshes/deltas
- cliffs: erosion produces large amount of material for beach-building; usually course sand/shingle
- sea: high volumes of sand/clay deposited here in ice age; tide and waves may bring these shore wards to build offshore bars
- mass movement
Physical weathering
Exfoliation:
- outer layers warm faster/cool more rapidly, as outer layers warm, particles expand and contract as they cool
- puts stress on rock layers causing them to peel
Freeze-thaw weathering:
- water seeps into cracks and freezes
- expand by 9% as it freezes
- repeated freezing/thawing causes rocks to split
Salt crystallisation:
- evaporation of salt water in rock leaves salt crystals
- salt crystal perfect cubes but rocks pores irregular shape so crystal exert pressure in the pores of the rock
- lead to granular disintegration
Pressure release
- rocks under great pressure, if rock suddenly released they expand/crack
Chemical weathering
Oxidation:
- oxidation of iron compounds in rocks turns into iron oxide (rust), which weakens/decomposes the rocks on the cliff face
- leads to staining/crumbling
Hydration
- e.g gypsum into anhydrite
- some minerals can absorb water, as they do, they expand/change chemical composition often making them weaker
Hydrolysis:
- breaking down of rock when it reacts with water
- rocks can break down when they are soluble in water
e.g mineral feldspar found in granite is converted into a white powdery clay called kaolin or China clay
Carbonation:
- when CO2 dissolved in water (carbonic acid) reacts with rocks and breaks them down
- limestone and chalk
- precipitation falling through the atmosphere absorbs CO2 to form a weak carbonic acid
- e.g limestone
- this acidulated rainwater will react with any rock containing calcium carbonate converting it to soluble calcium bicarbonate which then dissolves
biochemical weathering
- seaweed acids
- some seaweed (e.g kelp) contain pockets of sulphuric acid
- when these cells come into contact with rock, the acid will dissolve some rock minerals
different types of mass movement
-
soil creep:
- slowest but most continuous form: movement of soil particles downhill
- particles rise and fall due to wetting and freezing, causing soil to move down the slope and form shallow terracettes -
Mudflows:
- increase in water content of soil reduce friction = reduced shear strength
- lead to earth/mud to flow over underlying bedrock
- water can get trapped within rock increasing pore water pressure, weaking slope -
Rockfall:
- sloped cliffs over 40 degrees when exposed to physical weathering
- can be triggered by earthquakes, lead to scree at base of slope
- chalk/well jointed rocks (Dover cliffs 2012) -
Landslides and Rockslides:
- heavy rainfall leads to water between joints/bedding planes in cliffs can reduce friction/shear strength
- lead to landslide
e.g Aberfan -
Slump:
- section of cliff slumps along line of weakness
- concave plane so material rotated backwards
- clay cliff
Inputs (source of sediment) on the coastline
- cliff erosion: most erosion during winter months due to more frequent storms
- fluvial sediment (alluvium)
- eroding depositional features, e.g beaches, dunes and spits
- offshore bars and sediment
- erosion of wave-cut platforms
Transfers on a coastline
-
long shore drift:
- waves hit the beach at an angle due to prevailing wind
- wave push sediment in this direction and up beach in swash
- perpendicular backwash down the steepest beach gradient moves material laterally downdrift; aided by wave refraction - currents
- saltation: transportation of sand along the shore by the wind
Source, sink and transfer for sediment cells
Source:
- cliff erosion (5% coastal sediment)
- river erosion and transport
- coastal mass movements
- biogenic material: skeleton material of marine organisms
- sea bed erosion - movement onshore
Transfer:
- longshore movement of material
- wind-blown sand
- material moved onshore by post-glacial rise of sea level
- movement of material offshore by currents and wave action under storm conditions
Sink:
- offshore zone storage
- time lag in storage on the sea bed
- wave refraction and reduced energy results in concentrations of sediment in sheltered areas
What are the different kinds of erosion?
-
hydraulic action:
- powerful waves hit the cliffs
- any air sat within cracks is put under immense pressure
- as the wave retreats, the pressure is released (cavitation) with explosive forces
- heavily jointed sedimentary rocks are vulnerable -
abrasion
- as waves approach, wave energy enables material to be picked up and thrown against the foot of the cliff causing pieces of rock to be chipped off
- for this erosion to be effective, suitably loose sediment has to be available for example shingle/pebbles
- softer sedimentary more vulnerable than hard igneous -
attrition:
- rocks/stones the sea erodes from the cliffs are rounded and broken down as they bump against each other
- eventually the fragments will be reduced to sand/silt
- softer rocks very rapidly reduced in size by this -
Solution:
- sea water acidic due to organisms, such as limpets/barnacles
- acidic water reacts with minerals in rocks, causing them to dissolve
- mainly affects limestone, which is vulnerable to break down by weak acids
Human impacts on Californian sediment cell
-
dam built on river that flows into the sea in Southern California:
- 33% less sediment deposited
- less material on beaches along coast, so cliffs more exposed to erosion: (possible economic impact of reduced tourism)
2.building of houses, swimming pool and roads
- destabilising cliffs
- increase cliff erosion
- increased likelihood of mass movements/more sediment movement through LSD, increased deposition further down the coastline
- ec/soc impact of mass movement on homes/infrastructure
-
oceanside harbour blocking southward movement of sediment:
- most sediment is now being diverted to offshore currents and to the La Jolla submarine canyon
- may lead to further erosion along the coastline at La Jolla submarine canyon, due to abrasion/hydraulic action
- ec cost of erosion
