Coastal environments

Waves: fetch, energy and the way a wave behaves at the shore

A wave is energy travelling across the surface of the sea, and almost every process on a coast begins with one. Wind blowing over open water drags the surface into ripples and then into waves, and the energy a wave carries when it reaches the shore depends on three things: how fast the wind blows, how long it blows for, and the fetch. The fetch is the uninterrupted distance of open water over which the wind blows. A long fetch with strong, sustained winds builds high-energy waves; a short fetch, as in a narrow sea or a bay sheltered from the prevailing wind, gives low-energy waves.

You need the vocabulary to describe a wave precisely. The crest is the top and the trough the bottom; the wave height is the vertical distance from trough to crest; the wavelength is the distance between two crests; and the wave period is the time one crest takes to pass a fixed point. Divide height by wavelength and you have the steepness. High-energy waves are tall, long and slow to arrive; low-energy waves are low, short and come in quick succession.

Waves rarely arrive square-on to the shore. As a wave enters water shallower than about half its wavelength, its base touches the sea bed and is slowed by friction while the crest in deeper water keeps its speed, so the crest bends and the wave front turns to become more nearly parallel to the coastline. This is wave refraction, and on an irregular coast it has a striking effect: wave fronts converge on projecting headlands, concentrating energy there, and diverge across bays, spreading the same energy thinly. Headlands are therefore attacked hard while bays receive sediment, which is why so many coasts are scalloped.

A wave breaks when the water is too shallow for it to keep its shape. The base slows, the crest overtakes it and the front of the wave steepens and falls forward. A plunging breaker forms over a steeply shelving shore, curls over and slams down with most of its energy intact, and does the most erosion; a spilling breaker forms over a gently shelving shore and collapses gradually, losing energy as it goes. After the break the water runs up the beach as the swash and drains back as the backwash, and which of the two is stronger decides whether the beach is being built or destroyed.

Where the swash is stronger the beach is constructive: sediment is pushed up the beach and left there, so the beach grows wider and flatter. That happens when the beach is permeable and the water sinks into it, leaving little to return, and when the waves are low-energy with a long period. Where the backwash is stronger the beach is destructive: water that cannot soak away, because the beach is saturated or made of fine sand or clay, drains straight back down the face and drags sediment with it, so the beach becomes narrower and steeper. Storm waves are high, short and steep, and destroy the beach; calm weather afterwards rebuilds it.

Rock, structure, climate and people: the other controls on a coast

Rock type decides how a cliff responds to the same wave. Resistant rocks such as granite, basalt and well-cemented limestone or sandstone produce tall, steep cliffs that retreat slowly and leave only a narrow shore platform, because the rock takes a long time to wear back. Weak rocks such as boulder clay (glacial till), soft shale, loose sand and gravel produce low cliffs that retreat quickly, and their debris slumps and flows instead of standing in a face. Chalk is a special case: it is strong enough to stand as a cliff, but porous and soluble, so it is attacked by solution as well as by waves.

Structure means the lines of weakness in the rock. Bedding planes, joints and faults are gaps that hydraulic action, freeze-thaw and salt crystal growth can exploit, and they decide where caves and geos form. The dip of the strata matters too: beds dipping inland give a steep, stable face, while beds dipping towards the sea are undercut along their bedding planes so that blocks slide off. Permeability changes the behaviour of the whole cliff, because a porous rock absorbs rainfall and is weakened by solution, while an impermeable clay becomes saturated, heavy and prone to mudflows.

Climate works through wind, rain and temperature. The prevailing wind direction and strength set the wave energy reaching the coast and the direction of longshore drift, and the frequency of storms sets how often the big erosional events occur. In cold climates repeated freeze-thaw cycles shatter the cliff face; in warm, humid climates chemical weathering such as hydrolysis is faster and rots the rock deeply. Heavy or prolonged rainfall raises pore water pressure and triggers landslides and mudflows, which is why it is often a wet winter, not a storm, that brings a cliff down.

Human activities now shape coasts as much as physical processes do. Sea walls, groynes and breakwaters interrupt the movement of sediment, so a defended frontage is often paid for by faster erosion further along the coast. Dams and river engineering trap sediment that would have reached a delta or a beach, dredging offshore removes the sand and gravel that would have fed the shore, and clearing dunes, saltmarsh or mangroves removes a natural defence. Urban growth increases surface runoff, tourism tramples dunes, and sand mining and aquaculture take sediment and habitat directly.

Wearing the coast down: marine erosion and sub-aerial processes

Marine erosion is the work of the sea itself, and the syllabus names five processes. Hydraulic action is the force of the water: waves driven into a joint compress the air trapped inside it, and as the water retreats the air expands explosively and prises the rock apart. Cavitation is the same idea at a smaller scale, in that air bubbles in fast turbulent water collapse under pressure and each collapse sends a tiny shock wave into the rock, pitting and weakening it. Corrasion is the scraping of rock by the sediment the waves carry, as sand and shingle are dragged across the cliff foot, while abrasion is the grinding and polishing of rock by material thrown against it by the breaking wave; the two are so closely related that examiners accept either label provided the mechanism is described. Attrition is the wearing down of the sediment itself, as particles collide in the surf and become smaller, rounder and smoother, which is why shingle on a high-energy beach is rounded.

How fast all this happens depends on the energy arriving, the resistance and structure of the rock, and whether the debris is cleared away. A cliff erodes fastest where high-energy waves attack a jointed, weak rock and the sea is deep enough to reach the base at high tide. It erodes slowest where a wide shore platform or a broad beach absorbs the wave energy before it reaches the rock, which is a negative feedback worth remembering: as the platform widens, the cliff behind it retreats more slowly.

Sub-aerial processes attack the cliff from above and within, and they matter because the sea is at the foot of a cliff for only part of each tidal cycle. Freeze-thaw needs water in a joint, a temperature that falls below 0°C so the water freezes and expands by about a tenth of its volume, and a thaw so that it can refreeze deeper in the crack; repeated cycles wedge the rock apart. Salt crystal growth happens where salt water soaks into pores and the salt crystallises as it dries, expanding and flaking the surface, which is common on spray-soaked cliffs and on warm, arid coasts. The chemical processes are slower but just as important: carbonation is the attack on calcium carbonate by weak carbonic acid formed when carbon dioxide dissolves in rainwater, which is why chalk and limestone cliffs dissolve; solution is the direct dissolving of soluble minerals in slightly acidic water; and hydrolysis is the reaction of water with minerals such as feldspar in granite, turning them into soft clay minerals and leaving the rock crumbly.

Mass movement is the downhill transfer of material under gravity, and on a coast it is the link between weathering and erosion. A rockfall is the sudden fall of fragments from a steep face, typically after freeze-thaw has loosened jointed rock or the sea has undercut the base. A landslide is the sliding of a large block along a slip plane, often a bedding plane or a clay layer, after heavy rain has added weight and lubricated the surface. A mudflow is the flow of saturated, weak material, most common in boulder clay, which turns to slurry after prolonged rain and pours over the beach below. Cliff retreat is the sum of these: the sea cuts a notch at the base, weathering weakens the face above it, the unsupported rock collapses, and the sea clears the debris away.

Moving and depositing sediment: longshore drift and sediment cells

Eroded material is not lost; it is transported, and where it stops it builds landforms. Traction is the rolling of cobbles and pebbles along the sea bed. Saltation is the bouncing of sand grains along the bed in a series of short hops. Suspension is the carrying of fine silt and clay within the water itself, which is why a river mouth or an eroding clay cliff turns the sea brown. The fourth and most important for coasts is longshore drift, the movement of sediment along the shore in a zigzag.

Longshore drift works because waves usually approach the beach at an angle. The swash runs up the beach in the direction of the wave, which is oblique to the shore, but the backwash returns straight down the slope under gravity, at right angles to the shoreline. Sediment is therefore moved along the beach a little with every wave, and the direction is set by the prevailing wave approach. On a coast where the dominant waves come from the south-west, sediment travels north-east, whatever the occasional storm may do.

Deposition happens wherever the energy of the water falls below the amount needed to keep the sediment moving: in the shelter of a headland, across the mouth of a bay, behind a spit or a barrier, in the slack water of an estuary, and during calm weather after a storm. Deposition is selective, so sediment is sorted by size and weight, with the coarsest material dropped first and the finest carried furthest.

Sediment comes from several sources: the erosion of cliffs, rivers carrying material to the sea, the sea bed offshore, the shells and skeletons of marine organisms, and wind-blown sand from dunes. Its characteristics - the size, shape, sorting and permeability of the grains - decide what kind of beach forms. Shingle is coarse, permeable and rests at a steep angle because water drains through it quickly and the backwash is weak; sand is finer and holds water, so the beach face is gentler; silt and clay settle only in still water and form flat mudflats. A sediment cell is the stretch of coast within which this sediment circulates between its sources, its transfers along the shore and its sinks, largely enclosed by headlands. Each cell is a budget: if more sediment leaves than arrives, the beaches in that cell thin and the coast erodes, which is exactly what happens when a defence traps sediment at one end of a cell.

Erosional landforms: cliffs, platforms, caves, arches and stacks

A cliff in cross-section is a steep rock face with a wave-cut notch cut into its base at about high water mark, a shore platform exposed below it at low tide, and often a weathered and vegetated top. The sea cannot erode rock above the reach of the waves, so it attacks at the base: hydraulic action and corrasion gouge out a notch, the notch deepens until the rock above it is unsupported, and the face shears off along a joint. The fallen debris is then attacked by attrition and carried away, leaving the cliff face further back. Every collapse moves the cliff inland and leaves a little more shore platform under the new notch, so retreat is intermittent rather than steady.

A shore platform is the gently sloping rock surface at the foot of a cliff, exposed at low tide. It is cut by the same processes that cut the notch - hydraulic action, corrasion and abrasion in the surf zone - and it is also lowered by weathering while it is exposed to the air. As the cliff retreats the platform widens, and a wide platform is a sign of long, slow retreat, because it absorbs wave energy and slows everything down.

The smaller erosional features are made by the sea exploiting weaknesses in a headland. A cave forms where hydraulic action and corrasion enlarge a joint, fault or bedding plane at the base of a cliff. A geo is a narrow, steep-sided inlet cut along a vertical joint or fault, either by the sea directly or by the collapse of a cave roof. Where caves are eroded on both sides of a narrow headland they may meet to form an arch, and when the roof of the arch collapses the isolated pillar left standing in the sea is a stack. Further erosion at the base and weathering of the top reduce the stack to a stump below the level of high tide. The sequence is cave -> geo -> arch -> stack -> stump, and its direction is set by the joints in the rock, not by the waves alone.

A coastline is discordant where the rock strata run at right angles to the coast, so hard and soft rocks alternate along the shore. Erosion is fast in the soft rock and slow in the hard rock, giving a series of hard rock headlands separated by wide bays, and wave refraction then concentrates energy on the headlands and spreads it across the bays, exaggerating the pattern. A coastline is concordant where the strata run parallel to the coast, so a single rock type or a band of hard rock runs along the shore. The coast is then straight and cliffed, and a bay is cut only where the sea breaches the hard band at a weakness and quarries out the softer rock behind it.

Depositional landforms: beaches, spits, tombolos and barriers

A beach in cross-section is a slope of loose sediment between low and high water, built up to a ridge at the top by the swash. From the sea it rises gently across the low water terrace, steepens up the beach face where the swash runs up and the backwash drains back, and levels off at the berm or ridge crest, which marks the limit of the ordinary swash. In a storm the profile changes: waves cut into the berm and carry sediment offshore to form a bar, flattening the beach; in calm weather the swash rebuilds the berm. The upper beach is usually coarser than the lower beach, because the finest material is carried furthest, and a shingle ridge at the top is the mark of a high-energy beach.

In plan, a beach is rarely a uniform strip. Its width and alignment depend on the waves reaching it, which gives two types. A swash-aligned beach lies parallel to the crests of the incoming waves, so the swash runs straight up the beach and the backwash straight down it, and there is little net longshore movement. These form where the coast is sheltered, at the head of a bay, or where a wide beach has absorbed the energy of the waves. A drift-aligned beach forms where longshore drift dominates: sediment is carried along the shore faster than it can be returned, so the beach builds out in the direction of drift and lies at an angle to the wave crests, often ending in a curved or hooked feature.

A spit is a ridge of sand or shingle projecting from the land into open water, formed where the coastline changes direction sharply, at the mouth of a bay or an estuary. Longshore drift carries sediment past the point where the coast turns, and because the water there is sheltered and the energy falls, the sediment is deposited in a line continuing the original direction of drift. Wave refraction and secondary waves curve the distal end, giving the hooked or recurved tip, and the sheltered water behind the ridge silts up and is colonised by saltmarsh vegetation. Spurn Head, at the mouth of the Humber estuary in East Yorkshire, is a long spit of this kind.

A tombolo is a bar of sediment that joins an island to the mainland or to another island. It forms because the island shelters the water immediately behind it, so the wave energy there is lower and sediment carried along the shore is deposited in the gap until it emerges above the tide. A barrier beach is a long ridge of sand or shingle lying parallel to the coast with a lagoon or marsh behind it, separated from the open sea. Barriers form where there is a plentiful supply of sediment, a gently shelving sea bed and a low tidal range, and where rising sea level after the last glaciation flooded the land behind a beach ridge. Chesil Beach in Dorset is a shingle barrier nearly 30 km long that encloses the Fleet lagoon and links the Isle of Portland to the mainland. Barriers are fragile: a storm can breach one and cut a tidal inlet through it, and a rising sea level makes them migrate landward.

Landforms built with plants: dunes, saltmarsh and mangroves

Where there is a wide, dry beach and an onshore wind, sand is blown inland and trapped by plants, and the result is a dune system that changes from the shore inland in a sequence called a psammosere. The first sand to settle around driftwood or sea couch forms a low embryo dune, home to salt-tolerant species such as sea couch and lyme grass that can survive burial and salt spray. Further inland the foredune or yellow dune is dominated by marram grass, which grows up through fresh sand, holds it with deep roots and rolled leaves, and tolerates burial better than any competitor. As the sand stabilises, organic matter collects, water retention and nutrient levels rise, and the dune becomes a grey dune, fixed and covered with lichens, mosses and short turf. In hollows where the water table reaches the surface, a dune slack carries damp-loving plants, and behind the dune belt scrub and eventually woodland can develop. Each stage changes the conditions so that the next can follow, which is why the dunes nearest the sea are the youngest and the least stable.

Dunes are defences as well as landforms, because a vegetated dune ridge absorbs wave energy during a storm and provides a reservoir of sand for the beach. Break the vegetation and the defence fails: trampling, fire, rabbits or vehicle tracks create a blowout, and the sand is scoured out and blown inland as a migrating parabolic dune. Dune management therefore means fencing off paths, planting marram and keeping people on boardwalks.

A saltmarsh develops in the sheltered, low-energy water behind a spit, a barrier or a dune ridge, or on the sheltered side of an estuary. Fine silt and clay settle out of the still water to form a mudflat, and the mudflat is colonised by pioneer plants such as glasswort, which tolerates being covered by the tide twice a day and can survive in salty, waterlogged mud. These plants slow the water further and trap more sediment, and their decay adds organic matter, so the surface rises until it is flooded only by spring tides. The saltmarsh then matures into an upper marsh of sea aster, sea lavender and sea grasses, drained by creeks, and the whole sequence is a halosere. A saltmarsh is a natural sea defence, because it absorbs wave energy before the waves reach the land, which is why managed realignment schemes deliberately flood farmland to create it.

Mangrove swamps occupy the same sheltered, low-energy niche in the tropics and subtropics, on muddy shores between the tides. Mangroves are trees and shrubs adapted to salt water and to breathing in waterlogged mud: species such as Rhizophora have stilt or prop roots that anchor them in soft mud, others send up vertical pneumatophores to take in air, many exclude or excrete salt, and most are viviparous, so the seedling starts to grow while still attached to the parent. Their dense root networks trap sediment and build land, dissipate wave energy so effectively that they reduce storm surge and tsunami run-up, and act as nurseries for fish, crabs and prawns. They are also cleared for shrimp ponds, fuelwood and coastal development, which is why replanting programmes are now used as cheap coastal defence in countries such as Bangladesh and Vietnam. The Sundarbans, shared by Bangladesh and India, is the largest continuous mangrove forest in the world and protects the Ganges delta from cyclones.

Coral reefs: what coral needs, what threatens it, and whether management works

Coral is an animal, not a plant or a rock. A reef-building coral is a colony of tiny polyps, each of which secretes a skeleton of calcium carbonate, and the reef is the accumulated skeleton of thousands of years of growth. The polyps live in a symbiotic relationship with single-celled algae called zooxanthellae, which live inside their tissues: the algae photosynthesise and pass sugars and oxygen to the coral, and help it to deposit its skeleton, while the coral gives the algae shelter and the nutrients it excretes. Because the algae need light, reef corals can only grow where the water is clear and shallow. Growth is slow - a massive coral may add only a few millimetres to a centimetre or two a year - so a reef that is destroyed takes decades or centuries to recover.

The importance of a reef follows from what it is. Reefs are among the most biologically diverse ecosystems in the sea: the coral framework and its crevices provide shelter and nursery grounds for a very large number of fish, crustaceans and molluscs, which is why reef fisheries support coastal communities. The reef structure also absorbs wave energy and protects low-lying coasts and islands from storms, and it supports tourism that many tropical economies depend on. Damage to a reef is therefore an economic and social loss as well as an ecological one.

The conditions required for coral growth follow from that partnership. Temperature must stay warm all year, roughly 23°C to 29°C, and corals are sensitive to even a degree or two above the normal summer maximum. Water depth and light matter because the zooxanthellae must photosynthesise, so reefs are largely confined to water less than about 25 m deep and are rare below 50 m. Sediment must be low, because grains smother the polyps and cloud the water; oxygenation must be high, which is why reefs grow best where waves and currents keep the water moving; salinity must be normal marine, so corals are stressed by heavy freshwater runoff or by floods; and the larvae need a hard surface to settle on. Reefs are therefore found in tropical and subtropical seas, roughly between 30°N and 30°S, where all of these conditions are met - the Indo-Pacific, including the Great Barrier Reef and the reefs of Indonesia and the Philippines, the Caribbean, the Red Sea and the Indian Ocean. They are absent where cold currents reach the coast and off the mouths of large rivers that pour out sediment and fresh water.

Global warming is the most serious threat. When the water stays a degree or two above the summer maximum for weeks, the coral expels its zooxanthellae and turns white - coral bleaching - losing both its colour and most of its food supply; it can recover if the water cools quickly, but prolonged heat kills it. Warming also drives marine acidification, because the extra carbon dioxide dissolving in the sea lowers its pH and makes carbonate ions harder to obtain, so corals build weaker skeletons and reefs become more brittle. Increased storm activity breaks branching corals and can smash reef frameworks, though fragments may reattach elsewhere. Changes in salinity from floods or drought stress corals already weakened by heat, and sea-level rise matters because coral can only grow upwards at a few millimetres a year: if the sea rises faster, the reef is left deeper and darker, and eventually drowns.

Pollution and physical damage complete the list. Land-based pollution includes fertiliser and sewage, whose nutrients feed algae that overgrow coral and encourage outbreaks of the crown-of-thorns starfish, and sediment washed off cleared land, which smothers polyps; marine pollution includes oil, antifouling paints and plastics. Physical damage comes from storms and from people: anchors dragged across the reef, divers and snorkellers touching coral, blast and cyanide fishing, coral mined for building lime, and dredging for ports and channels.

Management works at two scales, and the difference between them is the key to any evaluation. Locally, marine protected areas with no-take zones protect fish stocks and allow reefs to recover, mooring buoys stop anchor damage, water quality plans cut fertiliser and sediment runoff, fishing gear and practices can be regulated, crown-of-thorns starfish can be culled, and coral gardening - growing fragments in nurseries and transplanting them - can restore small areas. Tourism can be managed through permits, fees and carrying capacity, and mangrove and seagrass restoration protects the nursery habitats linked to the reef. None of this, however, can stop mass bleaching, because that is driven by ocean temperature. The honest judgement is that local management buys time and can restore reefs where the main pressure is overfishing or pollution, but only global cuts in greenhouse gas emissions can secure reefs in the long term; and even the best-funded and largest protected area, the Great Barrier Reef Marine Park, has still suffered repeated mass bleaching.

Coasts under pressure: the challenges and how they are managed

The physical challenges on a coast are erosion, flooding and the loss of the habitats that depend on the shore. Erosion removes land, property, roads and farmland, and it is rarely steady: a single storm can take several metres of a soft cliff and years of calm can do nothing. Flooding comes from storm surges, when low pressure and strong onshore winds pile the sea up against the coast, and the risk is rising because sea level is rising as the ocean warms and expands and as ice sheets and glaciers melt. A higher sea level also means that the same storm reaches further inland and that waves break closer to the cliff foot. Underneath all of this is sediment starvation: defences, dams, dredging and the stabilisation of cliffs all reduce the supply of sediment to the beaches that absorb wave energy. These physical problems are made harder by human ones - expensive land behind the coast, competing claims from homeowners, farmers, tourists and conservationists, and the fact that defending one frontage usually damages another.

Hard engineering uses structures to hold the coastline in place. A sea wall is a concrete or masonry wall at the back of the beach that reflects wave energy and prevents flooding and erosion; it is effective where it is maintained, but it is expensive, it can scour the beach away in front of it, and once it is built the coast behind it must be defended for ever. A groyne is a wall built out across the beach to trap sediment moving by longshore drift; it keeps the beach wide and therefore protects the cliff behind it, but it starves the coast downdrift, and the last groyne in a series usually produces the worst erosion beyond it. Riprap (rock armour) is a pile of large imported boulders that absorbs wave energy and is cheap and flexible, though it is overtopped by big waves and can make the coast inaccessible. A revetment is a sloping structure of concrete, timber or rock at the foot of a cliff which absorbs energy, and an offshore breakwater is a detached ridge of rock offshore that forces waves to break early, reducing energy at the shore and often building a beach behind it.

Soft engineering works with natural processes instead of against them. Beach nourishment adds sand or shingle dredged from elsewhere to widen the beach, so that the beach itself absorbs the wave energy; it looks natural and provides habitat, but it is expensive, it has to be repeated every few years because the new sediment drifts away, and the dredging may damage the seabed where the material is taken from. Dune stabilisation uses fencing, boardwalks and marram planting to keep the sand in place and the dune ridge intact. Cliff drainage and regrading reduce pore water pressure and slope angle so that mass movement is less likely. Managed realignment (coastal retreat) deliberately breaches a defence or moves it inland, allowing the sea to flood low-value farmland and create saltmarsh, which absorbs wave energy and is far cheaper than holding the line; the cost is that land is lost. Alongside these sit planning measures: shoreline management plans that decide, cell by cell, where to hold the line, where to advance it and where to retreat; setback lines that keep new building away from the shore; and building regulations for flood risk.

Evaluating any of these means asking four questions: what is being protected, and is it worth more than the defence; how long will the defence last and who pays for its maintenance; what does it do to the sediment cell and therefore to the coast downdrift; and what does it do to the beach, the dunes and the habitats. Hard defences protect specific assets well and are politically popular because they are visible, but they are costly, they need constant maintenance, they alter the sediment budget and they can make the coast less natural. Soft defences are cheaper and more sustainable in the long run and can create habitat, but they need space, they need repeating, and they cannot protect a town against a large storm surge. The most defensible judgement is that hard engineering should be reserved for settlements and infrastructure that cannot be moved, soft engineering used where there is space and time, and realignment accepted where the land is of low value - which is roughly the logic of a shoreline management plan.

The Holderness coast in East Yorkshire shows all of this in one stretch of about 60 km. Its cliffs are boulder clay left by the last glaciation, weak, impermeable and prone to slumping; its beaches are narrow; and the waves arrive from the north-east across the long fetch of the North Sea. It is one of the fastest-eroding coastlines in Europe, retreating by roughly one to two metres a year on average and much more in a storm. Sea walls and groynes protect the towns of Hornsea and Withernsea and the village of Mappleton, where rock groynes were built in the early 1990s together with beach nourishment to save the village and the coast road behind it. The evaluation is the interesting part: the defences have worked where they were built, but they trap sediment that would have fed the beaches further south, so the unprotected boulder clay between and beyond the defended frontages erodes faster, and the beach in front of the sea walls has narrowed as the walls reflect energy and prevent the natural exchange of sediment. Spurn Head, the spit at the mouth of the Humber, is the sink at the end of the same sediment cell and is itself dynamic, having been breached by the storm surge of December 2013.