Changing coastal environments

Waves and the processes that shape the coast

Waves are created by wind blowing across the sea surface. The distance of open water the wind blows over is the fetch: a long fetch over a strong wind builds large waves with a great deal of energy. As a wave enters shallow water near the coast it slows, its wavelength shortens and its height increases, until it breaks and the water rushes up the beach as swash and drains back as backwash.

The two wave types do opposite work. Constructive waves are low, long and arrive less frequently. They have a strong swash and a weak backwash, so they push more material up the beach than they take away, which builds the beach up. Destructive waves are tall, steep and frequent, with a weak swash and a powerful backwash, so they drag material down the beach and erode it. Destructive waves dominate in stormy weather and in winter, and constructive waves rebuild the beach in calmer conditions.

Wave refraction is the bending of waves as they approach an uneven coastline. Waves travel more slowly in the shallower water near a headland, so the wave line bends and its energy is concentrated on the headland, while the same wave spreads out and loses energy in the sheltered bay. Refraction is the reason headlands are eroded quickly and bays receive material.

The sea erodes the coast in four ways, and the syllabus names all four. Hydraulic action is the force of the water itself: air is trapped and compressed in cracks and joints, and when the wave retreats the air expands explosively and breaks the rock apart. Corrasion is the scraping of the coast by material carried in the water, rather like sandpaper, and it is the process that cuts a notch at the base of a cliff. Attrition is the knocking together of the load in the water, which makes the pebbles smaller and rounder. Corrosion (also called solution) is the chemical dissolving of soluble rock such as chalk and limestone by the slightly acidic sea water.

Transportation along a coast happens in the same four ways as in a river - traction, saltation, suspension and solution - but the movement that matters most on a coastline is longshore drift. Waves approach the beach at an angle because they are driven by the prevailing wind, so the swash carries material diagonally up the beach. Gravity then pulls the backwash straight back down the beach at right angles to the coast. The next wave repeats this, so material moves along the coast in a zigzag. Deposition happens where the sea loses energy: in a sheltered bay, in the shallow water behind a headland, on the inside of a curve, or where the coastline changes direction and the drift continues out into deeper water.

Landforms of erosion: headlands, bays, cliffs, wave-cut platforms, caves, arches, stacks and stumps

On a discordant coastline the bands of hard and soft rock run at right angles to the coast, so the sea meets alternating resistant and weak rock. The hard rock, such as granite or chalk, resists erosion and stands out as a headland; the soft rock, such as clay, is worn away much faster into a bay. Once the bays have formed, wave refraction concentrates wave energy on the headlands and spreads it across the bays, so the headlands are eroded further while the bays receive the eroded material as beaches. Headlands and bays are therefore usually found together, and they are the classic result of a discordant coastline.

On a concordant coastline the bands of rock run parallel to the coast. The sea meets the same rock all along the shore, so the coastline is much straighter and more regular, and a resistant band of rock acts as a barrier protecting the softer rock behind it. Where that barrier is breached - along a fault or a joint, for example - the sea can attack the softer rock inland and a deep bay can form quickly. Concordant coastlines erode slowly but dramatically when the barrier is finally broken.

A cliff is a steep face of rock at the back of the shore. The sea attacks the base of the cliff by hydraulic action and corrasion, cutting a wave-cut notch. As the notch gets deeper the rock above it is left unsupported and collapses, and the cliff retreats inland. Weathering and mass movement also weaken the cliff face: freeze-thaw weathering prises blocks apart in cold weather, and heavy rain can saturate the rock until it slips. As the cliff retreats it leaves behind a gently sloping rocky shelf at the level of low tide, called a wave-cut platform. A cliff of soft rock such as boulder clay retreats quickly and forms a low, sloping profile; a cliff of hard rock retreats slowly and can stand as a vertical face.

Where a headland is made of hard rock with clear joints, the erosion sequence that examiners ask about most often begins. Hydraulic action and corrasion widen a joint or fault on a weak part of the headland into a cave. If the sea can attack the headland from both sides, two caves may meet to form an arch through it. The arch roof is left unsupported and has no rock beneath it to hold it up, so it eventually collapses under its own weight, leaving a stack, an isolated pillar of rock standing in the sea. The stack is then attacked at the base and by weathering on all sides, and it is reduced to a short, low stump below the level of the waves at high tide.

Landforms of deposition: beaches, spits, bars and sand dunes

A beach is an accumulation of material - sand, shingle or both - between low tide and the highest point the waves reach. The material comes from longshore drift, from eroded cliffs nearby and from rivers depositing sediment at the coast. Beaches are built up by constructive waves and flattened by destructive ones. A beach has a profile: the coarser material is found at the top, nearest the cliff, and the sand is finer towards the sea, and a shingle beach is usually steeper than a sandy one because water drains through it more quickly and the backwash is weaker.

A spit is a narrow ridge of sand or shingle that sticks out into the sea, joined to the land at one end only. It forms where the coastline changes direction sharply, for example at a river mouth or where a bay ends. Longshore drift carries material along the coast in its original direction, and because the coast turns away, the material keeps moving out into the open water and is deposited in the deeper, quieter water offshore. As the ridge grows, the end may curve or hook because waves and wind change direction at the tip. The water behind the spit is sheltered and slow-moving, so mud, silt and salt-tolerant plants accumulate there and a salt marsh develops.

A bar is a ridge of material that stretches across the mouth of a bay or across a river mouth, formed where longshore drift carries material into an area of deeper or quieter water and it is deposited. A bar may completely close off a bay, creating a lagoon of sheltered water behind it, or it may leave a gap through which the river or tide still flows. Where a bar joins the mainland to an island it is called a tombolo, but bars, spits and beaches are the forms you will be asked to describe.

Sand dunes form at the back of a sandy beach. Strong onshore winds pick up dry sand and carry it inland, where it is trapped by an obstacle such as a drift of seaweed or, more importantly, by plants. Marram grass is the key plant: it grows up through the sand, its long roots bind the sand together, and it tolerates being buried. Once plants are established the dune stops moving and grows, and the dunes behind it become older and more stable. A dune system shows a clear succession inland: the young embryo dunes nearest the sea are made of shell sand and are alkaline, with marram grass and sea couch; further inland the dunes are higher, more sheltered and colonised by mosses, lichens and grasses; in the oldest dunes the soil has more humus, holds more water and is more acidic, and trees and shrubs can grow. Dunes protect the land behind them from the sea and from flooding, and they are an important habitat for plants and animals that live nowhere else.

Coasts as opportunities and hazards

Coasts are among the most attractive places in the world to live and work, and a large share of the world's population lives within a short distance of the sea. The opportunities come from the position, the resources and the flat land. Coasts offer tourism and recreation, which bring income, jobs and investment in infrastructure. They support fishing and aquaculture, because the shallow water over the continental shelf and the mixing of nutrients feed large fish stocks. Their ports and harbours handle trade and connect countries, and the flat land beside them is easy to build settlements, industry and transport links on. Coastal areas also provide fertile farmland, sites for power stations that need cooling water, mineral resources such as sand and gravel, wind energy, and habitats and biodiversity that support fishing and tourism.

The hazards come from the same sea. Coastal erosion destroys farmland, undermines buildings, roads and railways, and can force whole communities to move; soft rock cliffs can retreat several metres in a single storm. Flooding from storm surges, high tides and sea level rise puts low-lying coasts at risk, and salt water moving into the ground water makes wells and farmland unusable. Tropical storms bring destructive winds, storm surges and torrential rain to tropical coasts. There are also economic and social hazards: the cost of coastal defences falls on taxpayers, insurance becomes difficult to obtain on an eroding coast, and tourism, fishing and farming can be lost when the beach or the harbour is damaged. Managing the coast creates its own conflict, because protecting one stretch of coast can increase erosion on the next stretch along it.

Managing coastal erosion and flooding: hard and soft engineering

Hard engineering uses built structures to stop the sea reaching the land. A sea wall is a concrete or stone barrier at the back of the beach which reflects wave energy; it protects the land directly and lasts for decades, but it is expensive, needs maintenance, looks artificial and can increase erosion at the ends of the wall as wave energy is reflected along the coast. Groynes are wooden or rock fences built out from the beach at right angles to the coast, which trap material moved by longshore drift and keep the beach wide; a wide beach absorbs wave energy, but the groyne starves the coast downdrift of sediment and causes erosion further along. Rock armour (rip-rap) is a barrier of large boulders placed at the foot of a cliff, which absorbs wave energy and is cheap to maintain, though it is ugly and dangerous to walk on. Revetments are sloping wooden or concrete structures that absorb energy and let water drain through, but they need replacing. Gabions are wire cages filled with rocks, which are cheap and flexible but rust and look poor. Offshore breakwaters break the waves before they reach the beach and create a sheltered area where material is deposited.

Soft engineering works with natural coastal processes, and this is what makes it more sustainable. Beach nourishment means adding sand or shingle from elsewhere to widen the beach, so that the waves break on the beach instead of the cliff; it is cheap and looks natural but the material must be replaced because longshore drift carries it away. Dune regeneration and stabilisation protects the back of the beach by planting marram grass, fencing off the dunes and putting in boardwalks so that people stop trampling the vegetation that holds the sand. Managed realignment (also called managed retreat) deliberately allows the sea to flood land that was previously defended, creating salt marsh and mudflat that absorb wave energy and provide habitat; it is cheap and creates valuable wetland, but farmland is lost and some people will object. Cliff stabilisation, planting vegetation and land-use planning that prevents building in the most vulnerable places all reduce the risk without hard structures. Coastal management is planned for whole stretches of coast rather than one village at a time, so that protecting one place does not make the next one worse.

The judgement to make is that hard engineering gives immediate protection but is expensive, needs constant maintenance and shifts the problem along the coast, so it is difficult to call sustainable in the long term. Soft engineering is cheaper, provides habitats and amenity, and works with the sea rather than against it, but it needs land, takes time to work and may not protect a town in a severe storm. In practice the best answer is usually a managed mix, chosen for the value of what is being protected.

Tropical storms: distribution, impacts and management

Tropical storms are intense low-pressure systems with very strong winds and torrential rain, and they are given different names in different parts of the world: hurricanes in the Atlantic and the eastern Pacific, typhoons in the western Pacific, and cyclones in the Indian Ocean and around Australia. They form over warm tropical seas where the sea surface temperature is high enough to evaporate large amounts of water, and they are carried westwards by the trade winds and then often curve away from the equator. They are found in the tropics but not right on the equator, and they lose their energy once they move over land or over cooler water. Their structure includes a calm eye in the centre surrounded by the most violent winds in the eye wall.

The impacts are the most severe of any natural hazard. The storm surge - a rise in sea level driven by the low pressure and the wind - floods low-lying coasts, and it is usually the surge and the rainfall rather than the wind that kill most people. Torrential rain causes river flooding and landslides. Very strong winds destroy buildings, power lines and crops, block roads with debris and damage ports and airports. In the aftermath, water-borne disease spreads, drinking water and food run out, and the economic cost of rebuilding can set a country back for years. The damage is worst where the coast is low and densely populated, where buildings are not built to withstand wind, and where there is little money for warning systems or emergency services. Coral reefs and mangroves are damaged too, and they are the natural defences that would otherwise have absorbed some of the energy of the surge.

Management works on four fronts: prediction, preparation, planning and protection. Prediction uses satellites, aircraft and computer models to track a storm and forecast where it will make landfall, so warnings can be issued in time. Preparation means educating people about what to do, running evacuation drills, stockpiling food, water and medical supplies, and having emergency services on standby. Planning includes hazard mapping and land-use zoning so that people do not build on the most exposed land, building regulations that make new buildings wind-resistant, insurance and disaster response plans. Protection covers hard defences such as sea walls, embankments and storm drains, and softer measures such as restoring mangroves and coral reefs, which absorb wave energy, and building cyclone shelters on high ground.

The evaluation is straightforward to write. Prediction and preparation save lives at relatively low cost, but only if warnings reach people and they act on them, which depends on education, trust and money. Building codes and land-use planning reduce damage in the long run, but they do not help existing buildings and are difficult to enforce in poorer countries. Hard defences are effective where they are maintained, but they are very expensive and can fail catastrophically. Restoring mangroves and reefs is cheap, sustainable and gives extra benefits such as fish nurseries, but it takes years to establish. The most successful approach is a combination, and the biggest limitation is always money and the ability of the poorest communities to recover.

Coral reefs and mangroves: importance, threats and management

Coral reefs are found in warm, shallow, clear, sunlit tropical seas, generally between about 30 degrees north and 30 degrees south of the equator, where the water is salty, has plenty of oxygen and carries little sediment. Reef-building corals are animals that live in partnership with microscopic algae inside their tissues, and those algae need sunlight, which is why reefs grow in shallow water. Reefs are the most biodiverse marine ecosystems on Earth, and their importance is both ecological and economic. They are a nursery and feeding ground for a large share of the world's fish, which supports fishing and food supplies; they attract tourists, who bring income and jobs; they protect the coast by absorbing wave energy before it reaches the shore; and they are a source of new medicines and of sand for beaches.

The threats to reefs are almost all linked to people. Rising sea temperatures cause coral bleaching: the coral expels the algae that give it colour and most of its food, and if the water stays too warm the coral dies. Ocean acidification, caused by carbon dioxide dissolving in sea water, makes it harder for coral to build its skeleton. Sediment and nutrients washed off deforested and fertilised land smother the coral and encourage algae to grow over it. Overfishing removes the fish that graze the algae and keep the reef clean, and destructive fishing with explosives or poisons destroys the reef itself. Pollution, damage from anchors, divers and tourists, outbreaks of the crown-of-thorns starfish, coral disease and tropical storms all add to the pressure.

Mangroves are salt-tolerant trees and shrubs that grow in the intertidal mud of sheltered tropical and subtropical coasts, where their stilt and aerial roots cope with being flooded by the tide and with the low oxygen in the mud. Their importance is that they are natural coastal defences: the tangled roots slow waves and hold the sediment, so they absorb the energy of storm surges and reduce erosion and flooding behind them. They are nurseries for fish, prawns and crabs, habitat for birds, crocodiles and other wildlife, and they store large amounts of carbon in their mud and biomass. They also supply timber, fuelwood, honey and food to coastal communities.

The threats to mangroves are mostly about land. They are cleared to make way for shrimp and fish farms, rice fields and oil palm plantations, cut for timber and fuelwood, buried under coastal development, ports and tourism resorts, and polluted by industry and sewage. Upstream dams and deforestation change the flow of fresh water and sediment that mangroves need, and rising sea levels can drown them where the land behind them is already built on so they cannot move inland.

Managing both ecosystems means removing the pressures and then protecting what is left. Marine protected areas and no-take zones allow fish stocks and coral to recover, but they need funding, wardens and the agreement of the people who fish there. Coral restoration uses nurseries where fragments are grown and replanted, and artificial reefs provide new surfaces, but survival rates are low and the cost is high. Reducing pollution and sediment means controlling deforestation, fertiliser use and sewage upstream, and controlling the crown-of-thorns starfish helps locally. On the coast, replanting mangroves works well when the correct species are planted at the right tidal level and the cause of the original loss has been dealt with; community management and giving local people a stake in the resource is usually more effective than a rule imposed from outside. Tourism can help by paying for conservation through park fees and by giving local people an income from keeping the reef healthy, but it has to be limited or it becomes part of the problem. International agreements and conventions provide the framework, but the enforcement always happens locally.