Coasts

 Marine erosional processes

• Hydraulic action – Air is trapped in the joints and cracks of a cliff by a wave. This exerts pressure on the rock wall. As the wave retreats, the pressure is released with a sudden jolt, causing parts of rock to break off.
• Corrosion – Parts of a cliff are eroded by the acid of the sea water which dissolves elements such as limestone or chalk.
• Attrition – Two or more rocks collide and bump into each other, breaking off fragments and making the rocks smaller
• Abrasion – Rocks are thrown against a cliff, breaking off fragments and wearing them away.

Marine transport processes

• Traction– large boulders are rolled along the sea bed
• Saltation– smaller rocks bounce along the sea bed
• Suspension-particles suspended in the water float through the water
• Solution– Transport of dissolved materials in the water

Marine deposition

Sediments may deposit in sheltered areas where land has a lower gradient. This is aided by constructive waves.

Constructive and Destructive waves

Constructive waves:

• strong swash (movement of water up the beach) and weak backwash (movement of water down the beach)
• eliptical orbit
• low
• low frequency
• depositional

Destructive waves:

Image from Lee & Chantelle McArthur, 18 September 2010 on Flickr

• weak swash, strong backwash
• circular orbit
• high
• high frequency (more than 12 per minute)
• erosional

Wave refraction and longshore drift

Waves may approach the coastline at an angle due to the direction of the wind.

Waves are refracted at 90° from the coast under the force of gravity.

If these steps are repeated, particles are transported along the coastline (in a zig-zag fashion). This is known as longshore drift.

Formation of bay and headland

Bays and headlands form in areas of coastline that have alternate bands of hard and soft rock. Hard rock is more resistant to erosional processes, whereas soft rock is more easily worn away. Soft rock forms a bay when it is eroded, whereas the hard rock remains to form a headland.

Formation of a wave-cut platform

1. A wave-cut notch develops at the base of a cliff due to erosional processes.
2. The rock above the notch is weakened and develps cracks and joints.
3. The notch expands and can not support the rock above.
4. The rocks break off and the cliff retreats.
5. The eroded material is transported away (and the previous steps are repeated)
6. A wave-cut platform is developed

Formation of caves, arches and stacks

Caves, arches and stacks are usually found on headlands, as these are eroded by the waves from three sides. They form as cracks/weaknesses in the headland are widened due to hydraulic action, abrasion and solution, leading to the creation of a cave. As the cave widens due to further erosion, eventually it cuts through the whole headland to form an arch. The top of an arch may collapse when it is unsupported, leaving behind a large vertical rock known as stack. When erosion weakens a stack, it eventually collapses, leaving behind a stump.

Formation of a spit

A spit is a sand and shingle ridge that extends from a headland out into the sea.

It is formed as prevailing winds result in wave refraction, causing materials to approach the coast at an oblique angle, while they are refracted at 90°. This causes materials to move in a zig-zag fashion, so sediments are transported along the coast due to longshore drift. Deposition occurs at a bend in the coastline, where materials follow the direction of longshore drift rather than the course of the coast. This leads to the formation of a spit at a headland.

Development of beaches

Powerful swash moves material up the land, weaker backwash does not have sufficient energy to remove it. Therefore beach material builds up over time, and the profile of the beach becomes more gradual. The gentle slope reduces the speed of the waves and causes further sedimentation. Material is also deposited in sheltered bays.

Development of sand dunes

Sand dunes form as onshore winds pick up dry sand and blow it up the blow it up the beach. Sand particles deposit behind obstructions such as a piece of wood or plant where they gradually accumulate to form an embryo dune. An embryo dune may be colonised by Maram grass, then developing into a yellow dune and finally a grey dune.

Salt marshes

Salt marshes are shallow, swampy wetlands colonised by saltwater vegetation. They develop in sheltered areas, eg. behind a hooked spit, and the water is often brackish (mix of freshwater from river or land runoff and sea water)

Coastline management techniques

Coasts can be protected against hazards such as flooding and erosion in a number of ways.

Coastline management techniques can be divided into hard engineering techniques, which are typically high-cost, high technology and relatively invasive into the natural environment, and soft engineering techniques, which may be less effective, low-cost, but tend to be more sustainable and work better alongside nature.

Hard engineering techniques:

• Sea walls: these are large, hard walls that can be built along the coastline to reduce erosion and prevent flooding. Sea walls are a common defensive method to protect coasts, but they can produce a strong backwash in waves which undercuts the sea wall.
• Groynes: groynes are usually wooden structures that extend from the coastline out to sea and trap sediments moving along the coastline. Groynes are an effective coastal management strategy to reduce longshore drift, and thereby limit coastal erosion.
• Breakwaters: breakwaters are offshore concrete walls that can break incoming waves from the sea to reduce the force with which the waves crash upon the coastline. This reduces the erosive force of the waves. Breakwaters are very effective at reducing erosion, however, they can also be easily damaged during a storm.

Soft engineering techniques:

• Beach nourishment: Sand and shingle are added to a beach to make it wider. This increases the resistance waves face when travelling across the beach towards cliffs or other hard structures, and therefore reduces erosion of cliffs and similar hard structures.
• Dune grass planting: This is a land management technique in coastal areas where dunes are common. By planting dune grass, dunes are stabilised and become more resistant against storms. Dune grass planting can create a buffer at the seaward front of existing dunes which can be eroded during storms surges.
• Marshland creation: Marshland can be used to break up the waves and reduce the erosive power of the sea. Encouraging the growth of marshland vegetation also limits the area which waves can reach and can thereby help prevent flooding.

Coral reefs

Coral reefs are living ridges of biochemically deposited magnesium and calcium carbonate structure in the sea formed by the growth and deposit of coral.  They represent a community of organisms including plants, fish and many other species. Only 0.1% of the ocean floor is covered by coral reefs.

There are three main types of coral reef: fringing reef, barrier reef, atoll.

• Fringing reefs are separated from land (typically an island) by a narrow, shallow lagoon. eg. coral reefs on the coast of Fiji.
• Barrier reefs are at least 500 metres from the coast and seperated from land (typically an island) by wide, deep lagoons. eg. the Great Barrier Reef of Australia
• Atolls are narrow circular reefs that surround a large lagoon. Below the coral reef and lagoon is the former island that has been submerged. eg. Suvadiva atoll in the Maldives

Typically, a fringing reef will evolve into a barrier reef which will develop into an atoll.

Conditions required for development of coral reefs:

• clean,unpolluted water
• clear water, that is less than 60m deep so sunlight can penetrate through the water and allow marine plants to photosynthesise
• warm water temperatures (>20°C)
• water with high salinity (high salinity = high salt content)

Benefits of coral reefs:

• Provide habitat for many species
• Provide food (fish & shellfish) for people living along coastline
• Protect shoreline from large waves by absorbing wave energy
• Source of medicine – some cancer drugs and painkillers use substances from coral
• Provide a safe place for fish to spawn

Resources

A nice video that sums it all up.