Coastal Systems and Landscapes
Coastal Systems and Landscapes
Introduction
Coastal environments are dynamic zones where land meets sea, shaped by the interaction of marine, terrestrial, and atmospheric processes. This topic examines the processes of erosion, transportation, and deposition that create distinctive coastal landforms, and evaluates the strategies used to manage coastal systems. Understanding coastal morphology is critical given that approximately 40% of the global population lives within 100 km of a coast.
Key Concepts and Definitions
| Term | Definition |
|---|---|
| Littoral zone | The area between the high and low water marks on a coast |
| Backshore | The area above the high water mark, affected only by storm waves |
| Nearshore | The shallow water zone from the low water mark to where waves begin to break |
| Offshore | The area beyond the nearshore, in deeper water |
| Swash | The upward rush of water up a beach after a wave breaks |
| Backwash | The return flow of water down a beach under gravity |
| Fetch | The maximum distance of open water over which wind can generate waves |
| Wave refraction | The bending of wave crests as they approach shallower water at an angle to the coastline |
| Isostatic change | Vertical land movement due to adjustments in the Earth’s crust (e.g., post-glacial rebound) |
| Eustatic change | Global sea-level change caused by alterations in the volume of ocean water |
| Lithology | The physical and chemical composition of rock, affecting its resistance to erosion |
| Concordant coastline | A coastline where bands of rock run parallel to the shore (e.g., Dorset) |
| Discordant coastline | A coastline where bands of rock run perpendicular to the shore, creating headlands and bays (e.g., Swanage) |
| Sediment cell | A stretch of coastline within which sediment movement is largely self-contained |
| Mass movement | The downslope movement of material under gravity (e.g., landslides, slumping) |
Coastal Processes
Waves
Waves are the primary agent of coastal change. They are generated by wind blowing over the surface of the sea.
Constructive waves (low energy):
- Long wavelength, low wave height (in most cases < 1 m)
- Low frequency (6–8 per minute)
- Strong swash, weak backwash
- Deposit material, building up beaches
- Associated with calm weather conditions
Destructive waves (high energy):
- Short wavelength, high wave height (often > 1 m)
- High frequency (10–14 per minute)
- Weak swash, strong backwash
- Remove material from beaches
- Associated with storm conditions
Wave energy depends on fetch, wind strength, and wind duration. The greatest fetch in the UK is from the south-west (Atlantic Ocean), meaning south-west-facing coasts receive the most powerful waves.
Erosion
Four main processes of erosion operate at the coast:
- Hydraulic action: The force of waves compressing air in cracks and joints in the cliff face. The trapped air exerts enormous pressure, forcing rock fragments apart. Also called wave quarrying when particularly forceful.
- Abrasion (corrasion): Waves pick up sediment (sand, pebbles, boulders) and hurl it against the cliff face, acting like sandpaper. This is often the most effective erosive process.
- Attrition: Eroded material collides with other sediment in the water, gradually becoming smaller, smoother, and more rounded. Does not directly erode the cliff but reduces the size of tools available for abrasion.
- Solution (corrosion): The dissolving of soluble rocks (e.g., limestone, chalk) by weak acids in seawater. A relatively slow process but significant on coasts composed of carbonate rocks.
Factors affecting the rate of erosion:
- Rock type and structure (joints, faults, bedding planes)
- Wave energy (fetch, wind speed, storm frequency)
- Cliff angle and profile
- Beach width (a wide beach absorbs wave energy, protecting the cliff)
- Human intervention (sea walls, groynes, rip-rap)
Transportation
Sediment is transported along the coast by several mechanisms:
- Traction: Large boulders rolled along the seabed by wave action
- Saltation: Pebbles bouncing along the seabed
- Suspension: Fine material carried within the water column
- Solution: Dissolved material carried in solution
Longshore drift is the net movement of sediment along the coast in the direction of prevailing wind and waves. Waves approach at an angle, pushing sediment up the beach in the swash. Backwash returns water and sediment at right angles to the shoreline (under gravity). Over time, sediment zigzags along the coast.
Deposition
Deposition occurs when wave energy is insufficient to transport sediment. This happens when:
- Waves enter shallow, sheltered water
- Wave energy is dissipated by a wide beach or offshore bar
- There is a large supply of sediment
- Constructive waves dominate
Coastal Landforms
Erosional Landforms
| Landform | Formation |
|---|---|
| Wave-cut notch | Formed at the base of a cliff by hydraulic action and abrasion at the high water mark. Undercuts the cliff. |
| Wave-cut platform | A gently sloping rock surface exposed at low tide, left as the cliff retreats landward. Formed by the continued erosion of the wave-cut notch, causing cliff collapse and retreat. |
| Cliff and scree slope | Vertical or near-vertical rock face created by wave erosion at the base and mass movement above. |
| Headland and bay | Formed on discordant coastlines: resistant rock (e.g., chalk, limestone) forms headlands; less resistant rock (e.g., clay, sandstone) erodes faster to form bays. |
| Cave, arch, stack, stump | Sequence of landforms on headlands: waves erode weaknesses (joints, faults) to form a cave → cave erodes through headland to form an arch → arch roof collapses leaving a stack → stack eroded to a stump. Example: Old Harry Rocks, Dorset. |
| Geos | Narrow, steep-sided inlets formed by the erosion of caves and subsequent roof collapse along joints and faults. |
Depositional Landforms
| Landform | Formation |
|---|---|
| Beach | Accumulation of sand or shingle between the low and high water marks. Formed by constructive waves depositing material. |
| Spit | A narrow ridge of sand/shingle extending from the mainland into the sea, formed by longshore drift depositing material in deeper water where wave energy decreases. Often has a recurved end due to wave refraction or secondary wind direction. Example: Spurn Head, Yorkshire. |
| Bar | A spit that extends completely across a bay, joining two headlands. Traps a lagoon behind it. Example: Loe Bar, Cornwall; Chesil Beach, Dorset. |
| Tombolo | A spit or bar that connects the mainland to an offshore island. Example: Chesil Beach (connects Portland to mainland) and St Ninian’s Isle tombolo, Shetland. |
| Offshore bar | A ridge of sand parallel to the coast in the nearshore zone, formed by destructive waves depositing material where wave energy decreases. |
| Sand dunes | Mounds of wind-blown sand that form behind the backshore, stabilised by vegetation (marram grass). Pioneer species colonise first, building embryo dunes. |
| Salt marsh | Flat, vegetated area in the upper intertidal zone, formed by the accumulation of fine sediment and colonisation by halophytic (salt-tolerant) plants. |
Sea-Level Change
Relative Sea-Level Change
Relative sea-level change is the combined effect of:
- Eustatic change: Global changes — melting ice sheets and thermal expansion of seawater (currently approximately 3.3 mm per year globally)
- Isostatic change: Local land-level changes — post-glacial rebound (e.g., Scotland rising ~1 mm per year) and subsidence (e.g., southern England sinking ~1–2 mm per year)
In the UK, relative sea-level rise is greatest in southern and eastern England where isostatic subsidence compounds eustatic rise.
Landforms of Emergence and Submergence
Emergence (land rising relative to sea level):
- Raised beaches (e.g., Isle of Arran, Scotland)
- Marine platforms above current sea level
- Abandoned cliffs inland
Submergence (sea rising relative to land):
- Rias (drowned river valleys, e.g., Kingsbridge Estuary, Devon)
- Fjords (drowned glacial valleys, e.g., Milford Sound, New Zealand)
- Dalmatian coast (drowned river valleys running parallel to the coast)
Coastal Management
Hard Engineering
| Strategy | Description | Advantages | Disadvantages |
|---|---|---|---|
| Sea wall | Concrete barrier reflecting wave energy | Effective, long lifespan (50+ years) | Expensive (£5,000–10,000/m), ugly, can increase erosion downdrift |
| Groynes | Wooden/stone barriers built at right angles to the coast | Trap sediment, build up beach | Starve downdrift beaches of sediment, unsightly |
| Rip-rap | Large boulders placed at the base of cliffs | Absorbs wave energy, relatively cheap | Looks unnatural, requires maintenance |
| Revetments | Sloping structures placed along the coast | Absorb wave energy, less intrusive than sea walls | Shorter lifespan, can be damaged by storms |
| Gabions | Wire cages filled with rocks | Cheap, flexible, absorb energy | Wire corrodes, looks industrial |
| Offshore breakwater | Structure built parallel to the coast in the sea | Reduces wave energy before it reaches the shore | Expensive, can interfere with navigation |
Soft Engineering
| Strategy | Description | Advantages | Disadvantages |
|---|---|---|---|
| Beach nourishment | Adding sand/shingle to a beach | Natural appearance, maintains tourism | Requires regular maintenance, expensive |
| Beach reprofiling | Reshaping the beach to a gentler gradient | Reduces wave energy, cheap | Temporary, may need repeated after storms |
| Dune regeneration | Planting marram grass and building sand fences | Natural, creates habitats, cheap | Takes time, vulnerable to trampling |
| Marsh creation | Allowing salt marsh to develop without intervention | Excellent wave energy absorber, high biodiversity | Requires land to be surrendered to the sea |
| Managed retreat | Allowing controlled flooding of low-value land | Cheaper than hard engineering, creates habitats | Compensating landowners, loss of farmland |
Shoreline Management Plans (SMPs)
SMPs are strategic documents that set out long-term coastal management policy for each sediment cell in England and Wales. Four policy options:
- Hold the line: Maintain existing defences where the coastline needs to be protected
- Advance the line: Build new defences seaward of existing ones (rarely used)
- Managed realignment: Allow the coast to retreat in a controlled way, managing the transition
- No active intervention: Do not invest in new defences; allow natural processes to continue
Case Studies
Case Study 1: Holderness Coastline, East Yorkshire
The Holderness Coast is one of the fastest-eroding coastlines in Europe, retreating at an average rate of approximately 1.8 m per year (with some sections losing up to 5 m in a single storm event). The coastline extends for approximately 61 km from Flamborough Head in the north to Spurn Head in the south.
Geological vulnerability: The coast is composed primarily of soft boulder clay (glacial till) deposited during the last ice age. This unconsolidated sediment offers very little resistance to wave action. The underlying chalk at Flamborough Head is more resistant, forming a pronounced headland.
Key sites and impacts:
- Mappleton: In 1991, the village was protected by a £2 million scheme including two rock groynes and a rock revetment. This successfully protected the village but starved downdrift beaches of sediment, accelerating erosion south of the defences. The farm at Cowden lost over 30 m of land in the decade after the defences were built.
- Spurn Head: The spit at the southern end of the Holderness Coast is maintained by longshore drift transporting sediment south. Changes in sediment supply and increased storm frequency threaten its long-term stability. The spit was breached by storm surges in 2013.
- Easington: The village and its gas terminal are at significant risk. The SMP policy is “hold the line” near the gas terminal (nationally important infrastructure) but “managed realignment” elsewhere.
Under current rates of erosion and with projected sea-level rise, it is estimated that the Holderness Coast could retreat by a further 200–300 m by 2100, threatening several settlements and approximately 30 km² of agricultural land.
Case Study 2: The Maldives — Sea-Level Rise
The Republic of Maldives is an archipelago of 1,192 coral islands in the Indian Ocean, with 80% of its land area less than 1 metre above sea level. It is among the most vulnerable nations on Earth to sea-level rise.
Physical vulnerability: The islands are low-lying coral atolls formed on the rims of submerged volcanic islands. They lack significant elevation or inland areas to retreat to. Their highest natural point is approximately 2.4 m above sea level (on Villingili island).
Climate change projections: The IPCC projects global sea-level rise of 0.3–1.0 m by 2100 (depending on emissions scenario). Even the lower end of this range would submerge significant portions of the Maldives. Increased storm intensity also threatens coastal erosion and saltwater intrusion into freshwater aquifers.
Adaptation strategies:
- Hulhumalé: An artificial island built by dredging sand from the seafloor, adjacent to the capital Malé. Designed to accommodate approximately 130,000 people at an elevation of approximately 2 m above sea level. Phase 1 was completed in 2002; Phase 2 is ongoing. The project has cost over $400 million.
- Sea walls: A 6 km sea wall surrounds Malé, built with Japanese aid at a cost of approximately $60 million in the 1990s.
- Coral reef protection: Healthy coral reefs provide a natural breakwater, absorbing up to 97% of wave energy. Coral bleaching events (notably 1998 and 2016) have degraded reef systems, reducing their protective function.
- International advocacy: The Maldives has been a vocal advocate for climate action on the global stage, hosting an underwater cabinet meeting in 2009 to highlight its vulnerability.
Common Pitfalls
-
Describing landforms without explaining the process: Merely naming a stack is insufficient. You must explain the sequence: weakness → cave → arch → roof collapse → stack → stump, with named processes (hydraulic action, abrasion) at each stage.
-
Confusing isostatic and eustatic change: Isostatic change relates to vertical land movement (e.g., post-glacial rebound in Scotland). Eustatic change is global sea-level change (e.g., from ice melt). The relative sea-level change is the combination of both. Students often attribute all sea-level change to one or the other.
-
Ignoring sediment cells: Coastal management in one area affects downdrift areas within the same sediment cell. Groynes that trap sediment at one location starve beaches downdrift, accelerating erosion elsewhere. Always consider the sediment cell as a system.
Worked Examples
Example 1: 9-Mark Question
“Assess the effectiveness of hard engineering strategies in managing coastal erosion.”
Answer:
Hard engineering strategies aim to resist coastal erosion through the construction of artificial structures. Sea walls, in most cases built from reinforced concrete, reflect wave energy and prevent erosion of the cliff behind. They are effective at protecting high-value assets in the short to medium term — for example, the sea wall at Scarborough has protected the town’s spa and gardens for over a century. However, sea walls are extremely expensive (up to £10,000 per metre) and can create a false sense of security. Reflected wave energy can also scour the beach in front of the wall, ultimately undermining it.
Groynes trap sediment moving by longshore drift, building up a wider beach that inherently absorbs wave energy. This can be highly effective locally — the groynes at Mappleton on the Holderness Coast have successfully protected the village. However, they starve downdrift beaches of sediment, accelerating erosion elsewhere within the sediment cell. This is a significant limitation, as coastal management must consider the entire system rather than individual locations.
Rip-rap and revetments are cheaper alternatives that absorb rather than reflect wave energy, reducing scour. However, they have shorter lifespans and may need frequent maintenance after storm events.
Overall, hard engineering is most effective for protecting high-value infrastructure in the short term but is financially and environmentally unsustainable over large stretches of coastline. A combination of hard and soft engineering, guided by Shoreline Management Plans, is generally the most effective long-term approach.
Example 2: 6-Mark Question
“Explain the formation of a spit.”
Answer:
A spit is a narrow ridge of sand or shingle that extends from the mainland into the open sea. It forms where longshore drift transports sediment along the coast in the direction of the prevailing wind. When the coastline changes direction — for example, at a river mouth or a bay — longshore drift continues to transport sediment into deeper, open water.
As the sediment extends into deeper water, wave energy decreases because the waves are no longer being refracted and focused by the headland. This reduction in energy causes deposition. The spit gradually builds up above sea level. Over time, vegetation may colonise the sheltered side of the spit.
Wave refraction often causes the tip of the spit to curve landward (a recurved end), as waves approaching from a secondary direction push sediment back towards the coast. Behind the spit, sheltered water accumulates fine sediment, potentially forming salt marsh. Spurn Head on the Holderness Coast is a well-known example, extending approximately 5.5 km across the Humber estuary.
Summary
- Coastal systems are shaped by erosion (hydraulic action, abrasion, attrition, solution), transportation (longshore drift), and deposition.
- Erosional landforms include cliffs, wave-cut platforms, caves, arches, stacks, and stumps; depositional landforms include beaches, spits, bars, tombolos, dunes, and salt marshes.
- Sea-level change results from the interaction of eustatic (global) and isostatic (land-level) factors.
- Hard engineering provides direct protection but is expensive and can transfer erosion downdrift.
- Soft engineering and managed realignment work with natural processes and are more sustainable but require acceptance of land loss.
- Shoreline Management Plans provide strategic frameworks for coastal management based on cost-benefit analysis and environmental considerations.
Sources: AQA Geography (7037) specification; Masselink and Hughes, Introduction to Coastal Processes and Geomorphology (2003); East Riding of Yorkshire Council coastal erosion data; IPCC AR6; Government of Maldives climate reports.