Coasts

coastal landscape - what shapes it

The coastal landscape is formed by marine (wave) processes and sub-aerial processes acting on rock over time

Landform type depends on the rock type and structure present

It also depends on the energy of the waves hitting that stretch of coast

Higher energy coasts tend to be more erosional, lower energy coasts more depositional

fetch & factors controlling wave energy

Fetch is the distance of open water that the wind has blown over before reaching the coast

• Wave energy increases with a longer fetch, a stronger wind speed and a longer wind duration

• The UK's south-west coasts have the longest fetch, so they receive the most powerful waves

• A short fetch produces small, low-energy waves

prevailing wind (UK)

The prevailing wind is the most common wind direction at a location

• In the UK the prevailing wind is south-westerly

• This means south-west facing coasts get the longest fetch and therefore the highest energy waves • Coasts facing away from the prevailing wind tend to be more sheltered

wave characteristics - height, length, frequency

Wave height is the vertical distance from the trough to the crest of a wave

• Wavelength is the horizontal distance between two crests

• Wave frequency is the number of waves breaking per minute

• A longer fetch increases both wave height and wavelength

why UK waves are more powerful in winter

Winter storms bring stronger and more frequent winds than in summer

• This means more energy is transferred to the waves, making them more powerful

• Powerful winter waves are usually destructive, eroding the coastline

• Summer waves are generally calmer and more constructive, building up beaches

how a wave breaks; swash & backwash

As a wave approaches the shore, friction with the seabed slows down the base of the wave

• The front of the wave becomes steeper until it topples over and breaks

• Swash is the movement of water rushing up the beach after a wave breaks

• Backwash is the water then flowing back down the beach under gravity

constructive vs destructive waves

Constructive waves are low and long, break around 6 to 8 times a minute, and have a weak backwash that is less powerful than the swash

• This means more material is deposited than removed, gradually building up a beach with a gentle gradient

• Destructive waves are high and steep, break around 13 to 15 times a minute, and have a strong backwash more powerful than the swash

• This erodes material from the beach, creating a steeper profile, and is common in winter

beach profile - berm vs storm beach

A berm is a ridge formed at the high-tide mark by constructive waves depositing material, and it has a gently sloping shape

• A storm beach is a ridge of larger material thrown to the back of the beach by powerful, destructive storm waves

• Storm beaches are steeper than berms because only the strongest waves can move the largest material that far

• Both features show where past wave action has reached on the beach

sediment budget

The sediment budget is the balance between sediment being added to a stretch of coast and sediment being removed from it

• Inputs include material from rivers, eroding cliffs and sediment drifting in from elsewhere

• Outputs include sediment drifting out of the area and material lost permanently offshore

• If more sediment leaves than enters, the coast experiences a net loss and erosion; if more enters than leaves, deposition occurs

concordant vs discordant coastlines

A discordant coastline has alternating bands of hard and soft rock running perpendicular (at an angle) to the coast

• This causes differential erosion, forming headlands where the rock is resistant and bays where it is less resistant

• A concordant coastline has rock bands running parallel to the coast, so erosion is more even and the coastline is straighter

• Lulworth Cove is a well-known example, where a gap in a resistant outer band of rock has exposed much softer rock behind it

headland vs bay - characteristics

A headland is an area of land that juts out into the sea, usually made of more resistant rock such as limestone, with steep cliffs • Wave energy becomes concentrated on headlands through wave refraction, so erosion rates are high

• A bay is an area of coast that curves inland, usually made of less resistant rock such as clay, with gentler slopes

• Wave energy spreads out in bays, so they are more sheltered and often develop sandy beaches

wave refraction

Wave refraction is the bending of waves as they approach an uneven coastline, caused by friction with the seabed

• Around headlands the water is shallower, so waves slow down and bend inwards, concentrating their energy and increasing erosion

• In bays the water stays deeper for longer, so waves keep more of their speed and their energy spreads out over a wider area

• This spreading of energy in bays favours deposition rather than erosion

cliff retreat: wave-cut notch & platform

Waves repeatedly erode the base of a cliff at high tide, cutting a wave-cut notch into the rock

• As the notch deepens, the unsupported cliff face above becomes unstable and eventually collapses

• This causes the cliff to retreat further inland over time, leaving behind fallen rock debris

• Repetition of this process leaves a gently sloping, rocky wave-cut platform that is exposed at low tide

cave → arch → stack → stump

Waves exploit a line of weakness, such as a joint or fault, in a headland and gradually erode it into a cave

• Continued erosion, often helped by hydraulic action, eventually erodes right through the headland to form an arch

• The roof of the arch is weakened further until it collapses, leaving an isolated pillar of rock called a stack

• The stack is undercut at its base and eventually collapses too, leaving behind a low stump visible mainly at low tide

beach formation

Beaches form where sand and shingle are deposited, mainly in bays or other low-energy areas of coast

• The material deposited comes from eroded cliffs nearby, sediment carried down by rivers, and material moved in from offshore

• Constructive waves are largely responsible for building beaches, because their backwash is weaker than their swash

• Over time this steady deposition builds beaches up into a recognisable landform

sand vs shingle beaches

Sand beaches are made of fine material and tend to have a flat, gently sloping profile, typically found on lower-energy coasts

• Shingle beaches are made of coarser, larger material and have a much steeper profile

• Shingle is permeable, so the backwash sinks into the beach material rather than dragging sediment back out to sea

• This means shingle beaches lose less material to the sea and are often found on higher-energy coasts

sand dune formation

Sand dunes form when wind, rather than waves, deposits sand on a wide, flat beach above the high-tide line

• An obstacle such as driftwood or debris slows the wind and causes sand to accumulate, forming a small embryo dune

• Pioneer plants such as marram grass then colonise the dune, and their roots help stabilise it and trap further sand

• Over many years this process repeats further inland, forming a series of dune ridges that grow progressively older and more vegetated

spit formation

Longshore drift transports sand and shingle along the coast in the direction of the prevailing wind

• Where the coastline changes direction, for example at a river mouth or a sharp bend, wave energy suddenly decreases

• This drop in energy causes the material being transported to be deposited rather than carried further

• Repeated deposition builds the material up above the level of the sea, forming a long finger of land called a spit extending out from the coast

why does a spit have a hooked/curved end

A spit's end often curves rather than staying straight, and this is caused by a change in wind direction

• A secondary, less dominant wind can approach from a different angle and generate waves travelling in a slightly different direction

• This shifts the direction of longshore drift at the spit's tip, pushing material around into a curve

• Multiple changes in wind direction over time can even produce several small curves along a spit's end

spit, estuary & salt marsh

A spit that grows across a river mouth or estuary can partially block it, creating a sheltered area of water behind it

• Wave energy is very low in this sheltered zone, so fine silt and mud carried by the river are able to settle out

• Over time this build-up of fine sediment forms a salt marsh, a muddy, low-lying habitat

• Salt-tolerant plants called halophytes then colonise the mud, and their roots help stabilise it and trap even more sediment

bar vs offshore bar

A bar is a spit that has grown all the way across a bay or the mouth of a river, completely cutting it off from the open sea

• This traps a body of water behind it, which becomes a lagoon

• An offshore bar is a ridge of sand or shingle deposited parallel to the coastline but some distance out to sea

• Offshore bars are usually submerged, though they may be partly exposed at low tide, and they can protect the coast behind them by absorbing wave energy

why manage the coast

Coastal management is needed to protect people, property and infrastructure from the damage caused by erosion and flooding

• The risk from both erosion and flooding is increasing because of climate change, particularly rising sea levels and more frequent storms

• Not every stretch of coast can be protected, because coastal management schemes are expensive

• Decision-makers use cost-benefit analysis, weighing the value of the land and property at risk against the cost of a scheme, to decide where protection is worthwhile

hard vs soft engineering

Hard engineering uses artificial, man-made structures to directly control natural coastal processes, such as a sea wall, rock armour or groynes

• These structures are usually expensive to build and maintain, can look unnatural, and can sometimes make erosion worse elsewhere along the coast

• Soft engineering works with natural processes rather than against them, such as beach nourishment or dune regeneration

• Soft engineering options are generally cheaper, more sustainable in the long term, and blend more naturally into the environment

sea wall

A sea wall is a concrete or rock wall built at the base of a cliff or along a promenade, designed to reflect wave energy back out to sea

• It is very effective at stopping erosion and often doubles as a walkway or promenade for the public

• However, sea walls are very expensive to build and to maintain over time

• The reflected wave energy can also erode the beach in front of the wall and increase erosion at either end of it, where the wall stops

rock armour (rip rap)

Rock armour consists of large boulders piled up at the base of a cliff, which absorb and break up the energy of incoming waves

• It is considerably cheaper to install than a sea wall, is relatively easy to maintain, and allows some beach material to remain in front of it

• On the other hand, the boulders can look unnatural and out of place, and they can be moved or displaced during severe storms

• Because some wave energy still gets through, rock armour does not completely stop erosion

groynes

Groynes are wooden or rock fences built out from the beach, perpendicular to the shoreline, which trap sediment being moved along the coast by longshore drift

• This builds up a wider beach on the updrift side, which both attracts tourists and acts as an extra buffer against waves

• The main problem is that groynes starve the beaches further along the coast of the sediment they would otherwise have received

• This can lead to increased erosion further downdrift, where the beach has less material to protect it

gabions

Gabions are wire cages filled with rocks, placed at the base of a cliff or embankment to absorb the energy of breaking waves

• They are a relatively cheap option and allow water to drain through them rather than building up pressure behind

• However, gabions have a fairly short lifespan, because the wire mesh can corrode or be damaged and break over time

• They are also considered fairly unattractive and unnatural-looking compared with softer options

revetments

Revetments are sloped structures, made from wood, concrete or rock, built at the base of a cliff to absorb and break up wave energy before it reaches the cliff itself

• They are cheaper to install than a full sea wall while still being reasonably effective at reducing erosion

• A downside is that they require regular maintenance to stay effective, especially after storms

• Revetments can also look unnatural and tend to trap litter and debris underneath and between their slats

cliff stabilisation / drainage

Cliff stabilisation involves draining excess water out of a cliff and planting vegetation on its slope to reduce the risk of mass movement

• Removing water lowers the weight and saturation of the cliff material, making landslides and slumping less likely

• This method is relatively cheap compared with building a hard structure, and it reduces the risk of sudden, large-scale cliff collapse

• Its main limitation is that it does nothing to stop wave erosion at the base of the cliff, since it only tackles one of several causes of instability

beach nourishment

Beach nourishment involves adding extra sand or shingle to an existing beach, usually dredged from the seabed or brought in from elsewhere, to build it up as a buffer against waves

• The result looks natural because it simply adds to the existing beach rather than building a visible structure, and it can boost tourism by creating a wider, more attractive beach

• It is a relatively cheap to medium-cost option compared with hard engineering

• Its main drawback is that it is only temporary, needing to be repeated regularly, and the dredging process can damage marine habitats where the sand is taken from

dune regeneration

Dune regeneration usually involves planting marram grass to help stabilise loose sand dunes, along with fencing and boardwalks to keep people off the most vulnerable areas

• This is a cheap option that looks completely natural and helps maintain the habitat that dunes provide, as well as their role as a natural sea defence

• The main downsides are that it takes a long time for new dunes to establish properly, and they can easily be damaged by storms or by people trampling over them

• Dunes also offer limited protection during the most severe storm events

managed retreat

Managed retreat allows a low-value area of coastline to flood or erode naturally, with existing defences removed or simply not replaced or maintained

• It is a relatively cheap option in the long term and can create valuable new habitats, such as salt marsh, as the sea moves inland

• It is also considered a more sustainable long-term approach than constantly rebuilding hard defences

• The main disadvantages are the loss of land, homes or farmland, the cost of compensating those affected, and how unpopular the policy can be with local residents

do nothing (option)

The do nothing option means no coastal defences are built or maintained, allowing natural processes to continue unchecked • Its only real advantage is that it costs nothing to carry out

• The obvious disadvantage is that land, property, businesses and habitats can be lost to erosion and flooding over time

• This option is normally only chosen where the land at risk has a low value and is not considered worth the cost of defending

Shoreline Management Plan (SMP) - 4 options

A Shoreline Management Plan is a long-term plan for sustainably managing the risk of coastal erosion and flooding along a stretch of coast, usually based around a sediment cell

• It considers the whole cell together, rather than protecting one place in a way that might harm another

• There are four possible management options within an SMP: hold the line, which means maintaining the coastline in its current position

• The other three options are advance the line, which means building new defences further out to sea, managed retreat, and do nothing

sediment cell

A sediment cell is a length of coastline, together with its adjoining seabed, within which the movement of sediment is largely self-contained

• There is only a limited exchange of sediment between neighbouring cells, so each cell behaves fairly independently of the ones either side of it

• Because of this, the sediment budget and the management options for a stretch of coast are usually considered on a cell-by-cell basis

• Understanding sediment cells helps planners see how a scheme in one place might affect the supply of sediment elsewhere in the same cell

why coastal management causes conflict

Protecting one stretch of coast, for example by building groynes, can starve beaches further along the coast of the sediment they rely on, increasing erosion there

• Different groups often want different solutions: residents and business owners usually want strong protection, while farmers might prefer a paid managed retreat scheme, and environmentalists often favour soft engineering that preserves natural habitats

• Disagreements also arise over cost versus benefit, especially when the land at risk is low in value

• This mix of competing interests makes it difficult for any single scheme to satisfy everyone affected

climate change & coastal risk

Global warming is causing sea levels to rise, through both the thermal expansion of warming seawater and the melting of glaciers and ice sheets

• Rising sea levels increase the risk of coastal flooding, particularly during storm surges and high tides

• Climate change is also linked to more frequent and more intense storms, which increases wave energy and the rate of coastal erosion

• Together, these changes mean coastal areas need more adaptive and sustainable long-term management strategies than in the past

evaluating hard engineering (overall)

Hard engineering is generally very effective in the short term at protecting high-value areas such as towns, and it reassures residents that their homes and businesses are safe

• However, hard engineering structures are usually expensive both to build and to maintain over their lifetime

• Many hard engineering options look unnatural and can damage the visual appearance of the coastline

• They can also worsen erosion elsewhere through knock-on effects, which makes them a less sustainable long-term solution than softer alternatives

evaluating soft engineering & managed retreat

Soft engineering and managed retreat are usually cheaper than hard engineering and work with natural coastal processes rather than against them

• They tend to be more sustainable in the long term and can create or restore valuable natural habitats along the coast

• The main weakness is that they generally offer less immediate and less complete protection than hard structures

• Soft engineering can also be slow to establish, and managed retreat remains unpopular with people who stand to lose land or property

management example - Mappleton, Holderness coast

Mappleton is a small village on the rapidly eroding Holderness coast in East Yorkshire, where the coastline was retreating at roughly two metres a year before management

• In 1991, two rock groynes and a stretch of rock armour were installed to protect the village and the main coastal road running through it

• The scheme has been successful in protecting Mappleton itself, and the village remains standing today

• However, by trapping sediment moving along the coast, the scheme starved beaches further south of material, accelerating erosion at Cowden Farm and other areas downd