COASTAL SYSTEMS AND LANDSCAPES
COASTS AS NATURAL SYSTEMS
Coastal environments are open systems with a range of inputs, stores, flows and outputs.
When there is a balance between inputs and outputs the system is said to be in a state of dynamic equilibrium. Positive
and negative feedback can disrupt equilibrium.
Term Definition Example
Input Material or energy moving into the Precipitation, wind, waves, tides, pollution, defences.
system from the outside.
Flows The relationships between Wind blown sand, mass movement, LSD, weathering,
components of a system. erosion, deposition.
Stores The individual elements of the Beach, sand dunes, cliffs, estuaries.
system.
Output Material or energy from the system Ocean currents, riptides, evaporation.
moving to the outside.
Energy The power behind flows in a Wind, gravity, flowing water.
system, a driving force.
Positive feedback A cyclical sequence of flows leads Coastal management can inadvertently lead to increased
to an increase of growth or erosion along the coast.
something, promoting
environmental instability.
Negative feedback A cyclical sequence of flows leads When there is a storm which removes sediment from a
to decrease of decline of beach. This sediment forms an offshore bar meaning
something (neutralising effect), waves break further from the shore, and the storm calms.
promoting dynamic equilibrium.
Dynamic equilibrium This represents a state of balance Constructive waves deposit material, however as the
within a constantly changing beach profile steepens, waves become more destructive
system. in their nature (plunging rather than surging), removing
material from the beach, making it less steep and
encouraging constructive waves.
COASTAL PROCESSES
SEDIMENT CELLS, SOURCES AND BUDGETS
A sediment cell is a stretch of coastline, often between two prominent headlands, where movement of sediment is more
or less contained. This means processes going on in one cell do not affect another cell.
There are 11 sediment cells around the coastline of England and Wales. These cells can be subdivided into sub-cells.
Shoreline Management Plans are based on such sub-cells.
Sediment sources
Rivers Sediment transported by rivers accounts for around 90% of coastal sediment,
especially in high rainfall environments where active river erosion occurs. Such
sediment will be deposited in river mouths and estuaries where it will be reworked by
waves, tides and currents.
Cliff erosion Although cliff erosion is very active along some coastlines, it only provides around 5%
of beach sediment. In areas of soft, unconsolidated rocks such as the till cliffs of the
, Holderness coast, rates of erosion can be as high as 10m a year. Most common during
winter months due to more frequent storms.
Longshore drift Sediment is transported from one stretch of a coastline (as an output) to another
stretch of the coastline (as an input).
Wind In glacial or hot environments, wind-blown sand can be deposited in coastal regions.
Sand dunes represent stores and possible sources.
Glaciers In some parts of the world like Alaska, Greenland and Antarctica, glaciers calve into the
sea, depositing sediment.
Offshore Sediment is transferred to the coastal zone when waves, tides and currents erode
offshore sediment sinks such as offshore bars. Storm surges or tsunami waves may
also transport sediment.
Volcanoes Some beaches are formed by volcanic ash and may be black in colour.
Artificial beach nourishment Some beaches are completely of partially artificial, using sand imported from
elsewhere.
Sediment budget
- Sediment losses involve deposition in sediment sinks; gains involve sediment from rivers or other sources. In
principal, the sediment budget seeks to achieve a state of dynamic equilibrium, where inputs and outputs are
balances. Human action and natural variation (e.g. storm effects) can effect dynamic equilibrium.
SOURCES OF ENERGY IN A COASTAL ENVIRONMENT
Winds
- Wind is the movement of air from areas of high pressure to areas of low pressure.
- The prevailing wind is the most common direction in which the wind flows (SW in the UK)
Factors affecting the strength of the waves to do with wind:
- The strength of the wind. The stronger the wind the stronger the wave.
- The duration of the wind. The longer the wind blows the stronger the wave.
- The fetch of the wind. This is the distance the wind travels uninterrupted over open water without any obstacles.
The greater the fetch, the bigger the wave. The UK fetch is over can be up to 8,000km from the SW, which
makes our prevailing winds very strong.
Waves
Wave formation: As air moves over water, frictional drag disturbs the surface and forms ripples. In the open sea there
is a circular orbital motion of the water particles. As the water becomes shallower the circular orbit of water
particles becomes elliptical. The wavelength and the velocity both decrease, and the wave height increases -
causing water to back up from behind and rise to a point where it starts to topple over (break). The water rushes up
the beach as swash and flows back as backwash.
, Wave fetch Refers to the distance of open water which a wind blows uninterrupted by major land
obstacles. The length of the fetch helps determine the magnitude and energy of waves (as
well as strength and duration of wind)
Wave crest Tallest point of the wave.
Wave trough Lowest point of the wave
Wave The distance between the crest and the trough.
height/amplitude
Wavelength The distance between successive crests.
Wave period The time takes for one wave to travel the distance of one wavelength.
Swash Water that rushes up the beach.
Backwash Water that rushes down the beach from previous waves.
Constructive Destructive
- Waves with a low wave frequency (fewer than 8 - Waves with a high wave frequency (more than
per minute), low wave height, long wavelength 10 a minute), high wave height, short
(often up to 100m) and long wave period (8-12 wavelength (around 20m) and short wave
seconds). period (3-6 seconds).
- As they approach the beach the wave front - As they approach the beach they rapidly steepen
steepens slowly giving a gentle spill onto the and when breaking plunge down
beach surface. - This creates a powerful backwash as there is little
- This tends to give them a very weak backwash forward movement of water and impedes the
which has insufficient force to pull sediment off the swash of the next wave.
beach or impede swash from the next wave. - Their backwash tends to be stronger than their
- Therefore their swash tends to be stronger than swash so more sediment is removed than added
their backwash. - They are often associated with steeper beach
- As a consequence material is slowly but profiles.
constantly moved up the beach which may leave - The force of each wave may project some
to the formation of ridges. sediment towards the rear of the beach creating a
large ridge known as a storm beach.
Most beaches are subject to an alternating cycle of constructive and destructive waves. Constructive waves build up the
reach resulting in a steeper beach profile. This encourages waves to become more destructive (as destructive waves are
associated with a steeper beach profile). These destructive waves move material back to the sea, reducing beach angle
and encouraging more constructive waves. This is an example of dynamic equilibrium.
(However this is often impossible as other factors such as wind strength and direction are not constant).
Wave refraction
Where waves approach an irregular coastline they are refracted, i.e. they increasingly take on the shape of the coastline.
This is due to coastal shelving. Waves slow down in the shallow water in front of a headland because of friction with the
seabed, so the waves in deeper water move ahead. The result is that the wave crest line will start to bend or “refract”.
This results in wave energy being concentrated around headlands and dispersed in the bays.
Currents
- Rip currents are powerful underwater currents occurring in areas close to the shoreline on some beaches
when plunging waves cause a buildup of water at the top of the beach.
- The backwash is forced under the surface due to resistance from breaking waves, forming an underwater
current.
