COASTAL ENVIRONMENTS
8.1 Coastal processes
lithology (rock type) – hard rocks such as granite and basalt give rugged landscapes (Giant’s Causeway). Soft rocks such as sands
and gravels produce low, flat landscapes (Nile delta).
geological structure – concordant coastlines occur where the geological strata lie parallel to the coastline. Discordant coastlines
occur where geological strata are at right angles to the shoreline.
processes – erosional landscapes contain many rapidly retreating cliffs, whereas areas of rapid deposition contain many sand
dunes and coastal flats.
sea level changes – advancing coasts due to deposition and/or relative fall in sea level. Retreating coasts due to erosion and/or
drowned by a rise in sea level.
human impacts – some coasts are extensively modified, whereas others are more natural.
ecosystem types – mangrove, coral, sand dune, saltmarsh and rocky shore add variety to the shoreline.
Wave generation and characteristics: fetch, energy, refraction, breaking waves, high and low energy waves, swash, and
backwash.
Waves result from friction between wind and the sea surface. Wave height is an indication of wave energy. It is controlled by wind
strength, depth of sea, and fetch (the distance of open water a wave travels over).
Waves of oscillation (in deep open sea) are forward surges of energy. Although the surface waves appear to move, the water
particles are actually moving in a roughly circular orbit 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
depth roughly equal to wavelength (the distance between neighbouring crests or troughs), at which point there is no further
movement related to wind energy - this point is called the wave energy.
● swell waves are characterised by a lower height and a longer wavelength.
● storm waves are characterised by a short wavelength, greater height and high frequency.
Waves of translation are waves reaching the shore. As waves move further up the onshore, the wave base comes into contact
with the seabed. Friction slows down the wave advance, causing the wavefronts to crowd together. Wavelengths are reduced
and the wave height increases. The shortening of the wave causes an increase in wave height (wave shoaling). Thus a breaker is
formed.
,spilling breakers - associated with gentle beach gradients and steep waves (wave
height relative to wave length). They’re characterised by a gradual peaking of the
wave until the crest becomes unstable, resulting in a gentle spilling forward of the
crest.
plunging breakers - tend to occur on steeper beaches than spilling breakers, with
waves of intermediate steepness. They are distinguished by the shore-ward face
of the wave becoming vertical, curling over and plunging forward and downward
as an intact mass of water.
surging breakers - found on steep beaches with low steepness waves. In surgeon
breakers, the front face and crest of the wave remain relatively smooth and the
wave slides directly up the beach without breaking. In surging breakers, a large
proportion of the wave energy is reflected at the beach.
Once the breaker has collapsed, the wave energy is transmitted onshore. The
swash will surge up the beach, with its speed gradually lessened by friction and the
uphill gradient. Gravity will draw the water back as the backwash.
● constructive waves tend to occur over a gently shelving sea floor (formed from fine material such as sand). These waves
have been generated far offshore.
● destructive waves are the result of locally generated winds. They move onshore up a steeply shelving coastline (formed
from coarse material such as gravel or shingle), creating a rapid increase in friction and thus a steep plunging breaker.
Due to the rapid steepening and curling of breaker, the energy of the wave is transmitted down the beach, breaking
down the beach material.
constructive waves destructive waves
low frequency (6-8 per minute) high frequency (12-14 per minute)
gradual increase (gentle offshore slope) rapid increase (steep shelving coastline)
spilling breaker plunging breaker
strong swash weak swash
weak backwash strong backwash
, TIDES AND THE TIDAL CYCLE
Tides are regular movements in the sea’s surface caused by the gravitational pull of the moon and Sun on the oceans. Tides are
influenced by the size and shape of ocean basins, the characteristics of the shoreline, the Coriolis force and meteorological
conditions. Low spring tides occur just after a new Moon. High spring tides occur after a full Moon when the Sun and the Moon
are aligned.
Neap tides occur when the sun and moons are at right angles to each other, there is least
gravitational pull, meaning the tidal range is at its lowest. The neap tide occurs 7 days after
the spring tide.
Spring tides occur when the sun and the moon align to create the greatest gravitational pull on
the sea.
- tides are greatest in bays and along funnel-shaped coastlines
- in the northern hemisphere, water is deflected to the night of its path
- during low pressure systems, water levels are raised 10 centimetres for every decrease of 10 millibars.
The difference between high and low tide is called the tidal range. Tidal range varies with distance from the amphidromic point
(place where there is no tidal range) and according to the shape of the coast; the strength of tidal currents varies enormously.
The tidal range has important influences on coastal processes:
- controls the vertical range of erosion and deposition
- time between tides affects weathering and biological activity
- influences velocity of tidal flow
A tidal bore is a surge that occurs along a coast where a river empties into an ocean or sea. A
tidal bore is a strong tide that pushes up the river, against the current. It gets stronger as the
channel funnels. The coast’s tidal range must be quite large, usually at least 6 metres. When all
of these conditions are met, a tidal bore is formed. e.g Severn Bore, which occurs in the Severn
Estuary between Wales and England.
Rip currents are important for transporting sediment. They can be caused by tidal motion or
by waves breaking along a shore.
A cellular circulation is caused by differences in wave height parallel to the shore. Water from
the higher sections of the breaker travels further up the shore and returns back through the
points where lower sections have broken. Once rip currents are formed, they modify the
beach by creating cusps, which perpetuate the currents.
STORM SURGES
Storm surges are changes in the sea level caused by intense low-pressure systems and
high wind speeds. For every drop in air pressure of 10 millibars, sea water is raised 10
centimetres. During tropical cyclones, low pressure may be 100 millibars less than normal,
raising sea level by 1 metre. In areas where the coastline is funnel-shaped, this rise in level
