2.1. Wave, marine and sub-aerial processes Flashcards
Define wave
Waves are oscillation of the water surface
Water does not move forward
Define wave height/amplitude
The distance between the trough and the crest
Indication of wave energy
Define wavelength
The distance between successive crests or troughs
Define wave frequency
The number of waves per minute
Factors affecting the formation and size of a wave
- Wind velocity
- Depth of water
- Fetch - the distance of open water the wave travels over
What are the coastal zones?
- Upper beach/backshore - backed by cliffs or sand dunes
- Foreshore - periodically exposed by tides
- Offshore - covered by water
Define wave orbit
The shape of the wave. Varies between circular and elliptical. The orbit diameter decreases with depth (roughly equal to wavelength
Define wave base
When the orbit diameter decreases to a point there is no further movement related to wind energy
Define swell waves
Waves that are characterised by lower height and longer wavelength
Define wave breaker
Occurs when waves move further onshore, friction slows down the wave advance causing the shortening of wave
What are the 3 main types of breaker?
- Spilling breakers
- Plunging breakers
- Surging breakers
What are spilling breakers?
- are associated with gentle beach gradients and steep waves
- are characterised by gradual peaking of the wave until the crest becomes unstable => gentle pilling forward of the crest
What are plunging breakers?
- tend to occur on steeper beaches than spilling breakers with waves of intermediate steepness
- distinguished by the shore ward face of the wave becoming vertical, curling over and plunging forward
What are surging breakers?
- found on steep beaches with low steepness waves
- the front face and crest of the wave remain relatively smooth
- a large proportion of the wave energy is reflected at the beach
Define swash, backwash
- Swash: forward movement of water up the beach
- Backwash: backward movement of water down the beach
Define constructive waves/swell waves
- Swash is greater than backwash
- Large fetch
- Long wave length
- Low height
- Found on low gradient beaches, low energy waves which deposit materials
- Associated with spilling breakers
Define destructive waves/stom waves
- Backwash is greater than swash
- Short fetch
- Short wave length
- High waves and frequency
- Steeply sloping beaches
- High energy waves which erodes
Define tide
regular movements in the sea’s surface, caused by the gravitational pull of the moon and sun on the oceans
low spring tides occur just after a new moon
high spring tides occur after a full moon
Define tidal range
The difference between high and low tide
3 types of rides coastal areas can be classified into
- Microtidal - <2m
- Mesotidal - 2-4m
- Macrotidal - over 4m
Define storm surges
- changes in the sea level caused by intense low pressure systems and high wind speeds
- for every drop in air pressure of 10mb sea water is raised 10 cm
Define wave refraction
- is the change in speed and distortion of wave
- waves are refracted and energy is concentrated around headlands and more dispersed along beaches located in bays
- headland - converging waves, deeper waters (erosion)
- bay - diverging waves, shallower waters (deposition)
4 types of erosion
- Hydraulic action: water breaks against cliff face, air trapped in cracks and joints => cavitation
- Corrasion/Abrasion: process whereby breaking wave can hurl pebbles and shingle against a coast and thus abrading it
- Attrition
- Solution
Mass movements occur in coasts
- Salt weathering
- Freeze thaw weathering
- Biological weathering
- Solution weathering
- Slaking
Salt weathering
the process by which sodium and magnesium compounds expand in joints and cracks thereby weakening rock structures
Freeze-thaw weathering
The process whereby water freezes, expands and degrades jointed rocks
Biological weathering
carried out by molluscs, sponges and sea urchins that burrow
Solution weathering
the chemical weathering of calcium by acidic water
Slaking
materials disintegrate when exposed to water
Energy factors affecting the rate of erosion
- Waves - wave steepness (steep destructive waves have more erosive power) and wave breaking point (where wave break at cliff base that cause maximum erosion)
- Tides
- Currents
- Winds - onshore winds erode fine beach material, offshore winds erode dunes
Material factors affecting the rate of erosion
- Sediment supply - continual supply -> abrasion. oversupply can protect the coast
- Beach/rock platform width - absorb wave energy
- Rock resistance
- Rock structure - well jointed
Shore geometry factors affecting rate of erosion
- Offshore topography - steep seabased creates higher and steeper waves; longshore bars cause waves to break offshore and lose enerfy
- Orientation of coast - headlands with vertical cliffs tend to concentrate wave energy by refraction
- Direction of fetch - longer the fetc, greater potential for erosion
How is the sediment transport categorised?
2 modes:
- Bedload - grains transported by bedload are moved through continuous traction or discontinuous saltation
- Suspended load - grains are carried by turbulent flow and held up by water. Strong currents
Define sediment cells
A section of the coastal zone here the sediment in the nearshore zone is moved by longshore drift
Sediment cells have clear boundaries to define them such as headlands
Inputs to sediment cell
cliff erosion
offshore bars
river sediment
sand dunes
eroding depositional feature e.g. beach
Transportation in a sediment cell
longshore drift
saltation/traction/suspension
Stores of a sediment cell
temporary features - beaches, dunes and spits bars
permanent stores - estuary
Notes about sediment cell
In reality it is unlikely that sediment cells are fully closed. With variations in wind directions and tidal currents, it is inevitable that some sediment is transferred between cells
Define dynamic equilibrium
The concept states that any system (sediment cell for example) is the result of the inputs and processes that operate within it. Change to one of the inputs will cause a knock on effect on the process and a resulting change in the landforms. e.g. increase in sediment => beach protetion => stabilisation of cliffs
