Coasts
8.1
Coastal processes Wave generation and characteristics: fetch, energy, refraction, breaking
waves, high and low energy waves, swash, and backwash.
Marine erosion: hydraulic action, cavitation, corrasion/abrasion, solution, and attrition.
Sub-aerial processes: weathering and mass movement. Marine transportation and
deposition: sediment sources and characteristics, sediment cells, and longshore drift.
8.2 Characteristics and formation of coastal landforms Erosional landforms: cliffs and
wave-cut platforms, caves, arches and stacks.
Depositional landforms: beaches in cross section (profile) and plan, swash and drift aligned
beaches, simple and compound spits, tombolos, offshore bars, barrier beaches, coastal
dunes, tidal sedimentation in estuaries, coastal saltmarshes, and mangroves.
The role of sea level change in the formation of coastal landforms.
8.3 Coral reefs
Characteristics, distribution and formation of fringing reefs, barrier reefs, and atolls.
Conditions required for coral growth.
Threats to coral reefs (global warming, sea-level rise, pollution, physical damage) and
possible management strategies.
8.4 Sustainable management of coasts
Case study: candidates must study some of the problems of sustainably managing a stretch
or stretches of coastline, and evaluate attempted solutions (including hard engineering and
soft engineering)
Coastal processes Wave generation and characteristics: fetch, energy, refraction,
breaking waves, high and low energy waves, swash, and backwash
Wave generation and characteristics
● Waves result from friction between the wind and the sea surface
● Wave in oscillation are forward surges of energy
● The water particles move in roughly circular orbits within the wave
● The wave orbit is the shape of the wave, it varies between circular and
elliptical
● The orbit diameter decreases with depth, to a length roughly equal to the
wavelength, at which point there is no further movement related to wave
energy - wave base
● Wave height is an indication of wave energy, which is controlled by the
wind strength, fetch and depth of the sea
Fetch: the distance of open water a wave travels through
, ● Waves of up to 12-15 m are formed in the open sea and can travel thousands
of km from the generation area, reaching distant shores as swell waves,
characterised by lower height and longer wavelengths
● In contrast, storm waves are characterised by short wavelengths, high
amplitudes and high wave frequency
● Waves reaching the shore are known as waves of translation - as waves
move further onshore, the wave base comes into contact with the seabed
● Friction slows down the wave advance, forcing the wavefronts to crowd
together
● Wavelengths are reduced and the wave height increases, which is known
as wave shoaling and a breaker is formed
Breaking waves:
● Spilling breaks:
○ Associated with gentle beach gradients and steep waves
○ Gradual breaking of the wave until the crest becomes unstable,
resulting in a gentle spilling forward of the crest
● Plunging breakers:
○ Occurs on more steeper beaches than spilling breaks, with waves
of intermediate steepness
○ The shore-ward face of the wave becomes vertical, curling over and
plunging forward and inwards as an intact mass of water
● Surging breakers:
○ Have low steepness and are found on steep beaches
○ The front face and crest remain smooth and the wave slides directly
up the beach without breaking
○ A large proportion of the wave energy is reflected at the beach
● Once breakers have collapsed, the swash will surge up the beach with its
speed gradually lessened by friction and the uphill gradient
● Then gravity will draw the water back as a backwash
● There are two types of waves in translation: constructive and destructive
waves
Constructive waves
● Tend to occur when wave frequency is low, 6-8 a minute
● Particularly when waves advance over a gently shelving sea floor (formed
with fine material like sand)
● Because of the low frequency, the backwash of each wave will be allowed to
return to the sea before the next wave breaks, which means that the energy of
each wave’s forward motion is not slowed down and remains at the maximum
level
● Adds energy to the shoreline
, ● As they approach the shore, they tend to steepen and increase in height
● The swash is more powerful than the backwash, causing water to surge
up the beach
● Carry significant amounts of sediment into the shoreline, contributing to
beach nourishment and the formation of sandbars
● Tend to create gentle sloped and wide, sandy beaches
● Associated with calm weather conditions and are common during periods
of low wave energy
Destructive waves
● Occurs when wave crests are widely spaced and arrive at the shoreline
irregularly, 12-14 per minute
● As these waves approach the shore, they steepen rapidly, forging a
plunging wave crest
● Have a powerful backwash that is stronger than the swash, causing the
water to quickly retreat from the beach
● These waves erode and remove sediment from the shoreline, resulting in
the gradual loss of sand and the formation of steep beaches
● Associated with stormy weather
● Contribute to coastal erosion and can impact coastal structures and
ecosystems
● Tend to create steeper beach profiles with narrow, rocky beaches
Wave refraction
● As the waves approach the shore, their speed will be reduced as the waves
feel the bottom of the shore
● Usually, wave fronts approach the shore at a diagonal, which causes the
wavefronts to bend and swing around in an attempt to break parallel to the
shore
● The change in speed and distortion of the wave fronts is called wave
reflection
● When waves approach the shoreline at an angle, the part of the wave in
shallower water slows down, causing the wave to bend
● The part of the wave in deeper water continues at a faster speed, resulting in
a change of wave direction
● Wave refraction forces waves to become more parallel to the shoreline
● It can concentrate the wave energy in certain areas, leading to the formation
of headlands and bays
● It also plays a role in erosion and depositions of sediment along the coastlines
● The degree of wave refraction depends on the factors such as wave
frequency, wavelength and the angle at which the waves approach the shore
