Coastal Processes 8.1 Flashcards
Wave terminology: Crest Trough Height Wavelength Period Velocity
Crest: highest point on a wave
Trough: lowest point on a wave
Height: vertical difference between crest and trough
Wavelength: horizontal difference between 2 identical points on the wave
Period: time taken for wave to travel one wavelength
Velocity: speed of the wave
Name two types of waves
Low-energy (surging breakers, constructive): weak or no wind (little to no energy addition), so height decreases and wavelength increases. Produces gentle swell waves – gently surging up the beach.
High-energy (plunging breakers, destructive): strong winds keep adding energy to produce storm waves. Break vertically with tall wave height, plunging down.
How does open ocean generation occur?
Prevailing wind blows across the surface of the sea water, a frictional force arises between the two
This force causes the water to be ‘dragged’ in the direction of the wind. Waves are parallel to the wind direction
Like tsunamis, waves are not caused by the moon’s effect on Earth. The moon only influences tides
Wave energy only moves forwards, individual particles experience circular motion as the wave passes.
Factors affecting waves:
Wind field: speed, duration and direction
Fetch of wind: distance of sea the wind blows over
Water depth: at the wave generation area
What is a Swell wave?
Occur when factors build up sufficient energy so that the wave can propagate freely in directions independent to the wind.
What causes waves to break?
Wave shoaling
-Near the shallower shore, friction increases with the seabed. Wave energy is compressed into less water volume, so wavelength reduces and wave height increases. A breaker then forms.
What are the three types of Breaking waves?
Breaking = Releasing wave energy in the form of swash, energy reduces due to friction and uphill gradient.
Spilling breakers: gentle beach gradients and steep waves. Wave gradually peaks, then spills forward.
Plunging breakers: steep beach gradients and intermediate wave steepness. Wave from curls over and plunges vertically downwards.
Surging breakers: steep beaches with shallow waves. Wave crest remains smooth and slides up beach without breaking. Wave energy reflected.
What is Swash and Backwash?
Swash: turbulent layer of water that washes up onto the beach once the wave has broken, pushing material.
Backwash: water flow that returns down the beach after the swash freely due to gravity affects.
What are constructive and destructive waves?
Constructive: stronger swash than backwash, long wavelength, breaks forwards, gently sloping wave front and beach, low frequency and height.
Destructive: stronger backwash than swash (average stronger swash than constructive though), short wavelength, breaks vertically downwards, steep wavefront and beach, high frequency and height.
Define these types of marine erosion:
- Hydraulic action
- Cavitation
- Corrasion
- Corrosion
- Attrition
Hydraulic action: air trapped in crack/joints is compressed as the wave (force of the water) hits. Huge pressure increase erodes, causing detachment or crack enlargement
Cavitation: as the wave retreats, pressure drops. Air bubbles form, expand and explode causing a huge pressure release. Air in cracks/joints rapidly expands
Corrasion (abrasion): sand and shingle is hurled by the wave at the cliff, causing rubbing of material
Corrosion: water made acidic by barnacles and limpets erodes weak rocks such as limestone
Attrition: larger particles collide and rub together, causing a mutual erosive action
Collision action
How do you increase erosion potential?
Steep destructive waves (increase the energy)
Have a constant supply of material (constant supply and low resistance rock)
Have a steep seabed and have concentrated waves by refraction.
Define these sub-aerial processes:
- Weathering
- Temperature changes
- Oxidisation
- Biological weathering
- Chemical weathering
- Salt crystal growth
- Mass Movement
Weathering: weaken rocks on the cliff, increasing the likelihood of a mass movement occurring
Temperature changes: extreme expansion/ contraction of rock or water in cracks crumbles rock
Freeze thaw, exfoliation, frost shattering
Oxidisation: rusting of rocks containing iron compounds
Biological weathering: salt bearable (halophilic) plants and animals anchor themselves to the coast, expanding cracks (limpets, salt plant root systems)
Chemical weathering: acids in water and sea bird excrement cause the break down of rocks in-situ
Salt crystal growth: salt water enters cracks, in the coastline and evaporates leaving salt crystals. Crystals dramatically expand and contract with temperature.
Mass movement: triggered by rainwater or undercutting by marine processes cause cliff retreat
How do rock falls affect the coastline?
Weathering processes weaken coastline
Increased crack size allows rainwater to penetrate
Overtime, crack widens to a point where it can no longer be supported – and falls
Dorset Coastline: 300m affected when 4,000 tonnes detached in April 2021 due to dry ground
How do rotational slumps affect the coastline?
Precipitation causes an increase in soil moisture
Moisture penetrates to slip plane by throughflow and baseflow
Lubrication of the slip plane allows for a slump along the rotational slip plane
Christchurch Bay: 42 slumps in 2km stretch in 1900s
What is a sediment cell?
An area of coastline where sediment is in closed system (self contained). Each cell can be broken into smaller sub-cells.
