G21: Coastal Processes
Wave types and characteristics, weathering and mass movement, processes of erosion, transportation including longshore drift, and deposition at the coastline.
🌊 Wave Types
Key Concept: Waves are the primary agent of coastal change. They are generated by wind blowing over the sea surface. The size and energy of waves depend on wind speed, wind duration, and fetch (the distance of open water over which the wind blows).
Constructive Waves
- Low, long waves with a low wave height (typically under 1 metre)
- Long wavelength (distance between crests) - often 30-50 metres
- Swash is more powerful than backwash - water rushes up the beach
- Backwash is weak - less water returns to the sea
- Deposition dominates - builds up beaches with sediment
- Low frequency - typically 6-8 waves per minute
- Associated with calm weather conditions
Destructive Waves
- Tall, steep waves with high wave height (often 1.5 metres+)
- Short wavelength - crests closer together
- Backwash is more powerful than swash - water drags sediment back to sea
- Swash is short and weak
- Erosion dominates - removes beach material
- High frequency - typically 10-14 waves per minute
- Associated with storm conditions and strong winds
| Feature | Constructive Waves | Destructive Waves |
|---|---|---|
| Wave height | Low (under 1 m) | High (over 1 m) |
| Wavelength | Long | Short |
| Swash vs Backwash | Strong swash, weak backwash | Weak swash, strong backwash |
| Process | Deposition | Erosion |
| Frequency | 6-8 per minute | 10-14 per minute |
| Beach shape | Gentle, wide beach | Steep, narrow beach |
| Weather | Calm conditions | Storm conditions |
Fetch: The maximum distance of open water over which wind can blow to generate waves. The UK's west coast has a very long fetch across the Atlantic Ocean, producing larger, more powerful waves than the east coast which has a shorter fetch across the North Sea.
🧪 Weathering at the Coast
Definition: Weathering is the breakdown of rock in situ (in its original place) by physical, chemical or biological processes. It does not involve transport - that is erosion.
Mechanical (Physical) Weathering
- Freeze-thaw weathering: Water enters cracks in the rock, freezes (expanding by about 9%), exerts pressure on the rock, and repeated cycles widen cracks until fragments break off
- Wetting and drying: Clay-rich rocks expand when wet and shrink when dry, causing cracking and disintegration
- Salt weathering: Seawater evaporates in rock pores, leaving salt crystals that grow and exert pressure, breaking the rock apart
- Temperature changes: Daily heating and cooling causes rocks to expand and contract, weakening their structure
Chemical Weathering
- Solution (dissolution): Rainwater (slightly acidic from dissolved CO₂) dissolves soluble rocks like limestone and chalk
- Oxidation: Iron-rich minerals in rocks react with oxygen, causing them to rust and weaken
- Hydrolysis: Water reacts with minerals in granite, breaking down feldspar into clay
Biological Weathering
- Plant roots grow into rock crevices, exerting pressure as they expand
- Burrowing animals loosen and destabilise cliff material
- Seaweed and algae attach to rocks, creating acidic conditions that dissolve rock surfaces
- Lichens secrete weak acids that slowly dissolve rock surfaces
🏔️ Mass Movement
Definition: Mass movement is the downhill movement of material under gravity. It is especially important at the coast where cliffs are unstable. Water plays a key role in making material heavier and reducing friction.
| Type | Description | Conditions | Speed |
|---|---|---|---|
| Rockfall | Fragments of rock break off from steep cliffs and fall vertically | Freeze-thaw weathering loosens rock; steep cliffs | Rapid |
| Landslide (sliding) | Large blocks of rock and soil slide downhill along a slip plane | Saturated soil; heavy rain; undercut cliffs | Rapid |
| Slumping | Curved rotation of saturated soil and weak rock along a curved slip plane | Clay-rich cliffs; heavy rainfall; wave undercutting | Moderate |
| Mudflow | Saturated soil and weathered material flows downhill as a viscous liquid | Very wet conditions; fine-grained material | Rapid |
| Soil creep | Very slow downhill movement of individual soil particles | Gravity on slopes; wetting and drying cycles | Very slow |
Example: Slumping at Barton-on-Sea
The cliffs at Barton-on-Sea in Hampshire are prone to rotational slumping. The cliffs are composed of sandy gravel on top of clay. When rain saturates the permeable gravel, water percolates down to the impermeable clay, making it slippery and heavy. The saturated material then slides along a curved slip plane. Wave undercutting at the cliff base removes support and increases the likelihood of slumping. Coastal management has attempted to stabilise the cliffs with drainage and rock armour.
⛏️ Erosion Processes
Definition: Coastal erosion is the wearing away of the coastline by the sea. Four main processes operate: hydraulic power, abrasion, attrition and solution.
1. Hydraulic Power
Waves crash against the coast, trapping air in cracks and crevices. The compressed air exerts enormous pressure on the rock, forcing cracks apart. During storms, wave pressure can exceed 30 tonnes per square metre.
2. Abrasion (Corrasion)
Waves pick up sediment (sand, pebbles, boulders) and hurl it against the cliff face, acting like sandpaper. This is the most effective form of erosion on cliffs, especially during storms when waves carry larger material. It causes undercutting and creates wave-cut notches.
3. Attrition
Sediment particles carried by waves collide with each other, gradually becoming smaller, rounder and smoother. Pebbles on a beach are evidence of attrition - angular rock fragments from cliff falls are progressively worn down into smooth, rounded shapes.
4. Solution (Corrosion)
Dissolved chemicals in seawater (especially carbonic acid) react with and dissolve certain rock types. Limestone and chalk are particularly susceptible - the calcium carbonate reacts with the mild acid in seawater. This is most effective on coastlines with soluble geology.
| Process | How It Works | Most Effective On |
|---|---|---|
| Hydraulic power | Compressed air forces cracks apart | All rock types with cracks/joints |
| Abrasion | Sediment thrown against cliff like sandpaper | Any cliff face; causes undercutting |
| Attrition | Sediment particles collide and break down | Reduces sediment size everywhere |
| Solution | Chemical dissolution by acidic seawater | Limestone and chalk coasts |
Factors Affecting Erosion Rate:
1. Wave energy (fetch, wind speed, storm frequency)
2. Rock resistance (hard rocks erode slowly, soft rocks erode rapidly)
3. Rock structure (joints, cracks and weaknesses allow hydraulic action to exploit)
4. Coastal geometry (headlands concentrate wave energy through refraction)
1. Wave energy (fetch, wind speed, storm frequency)
2. Rock resistance (hard rocks erode slowly, soft rocks erode rapidly)
3. Rock structure (joints, cracks and weaknesses allow hydraulic action to exploit)
4. Coastal geometry (headlands concentrate wave energy through refraction)
🚚 Transportation Processes
Definition: Transportation is the movement of eroded material by the sea. Sediment is moved along the coast and offshore by four processes: traction, saltation, suspension and solution.
| Process | How It Works | Sediment Size |
|---|---|---|
| Traction | Large boulders and pebbles rolled along the seabed by the force of water | Very large (>2 mm) |
| Saltation | Sand-sized particles bounce along the seabed in a hopping motion | Medium (0.06-2 mm) |
| Suspension | Fine particles carried within the water column, making it appear cloudy | Small (<0.06 mm) |
| Solution | Dissolved minerals carried invisibly in the water | Dissolved ions |
Longshore Drift
Key Process: Longshore drift is the main process by which sediment is transported along the coastline. It occurs when waves approach the coast at an angle, carrying sediment diagonally up the beach with the swash, then pulling it straight back down at right angles with the backwash. Over time, this zigzag movement transports sediment along the coast.
The direction of longshore drift is determined by the prevailing wind direction and the angle of wave approach. In the UK, the prevailing south-westerly winds mean that longshore drift generally moves sediment from west to east along southern coasts and from south to north along western coasts.
Longshore Drift Process:
1. Waves approach beach at an angle (determined by prevailing wind)
2. Swash carries sediment up the beach at the same angle
3. Backwash pulls sediment straight back down the beach (perpendicular to coastline under gravity)
4. Net result = sediment moves along the coast in a zigzag pattern
5. This creates distinctive landforms such as spits, bars and tombolos
1. Waves approach beach at an angle (determined by prevailing wind)
2. Swash carries sediment up the beach at the same angle
3. Backwash pulls sediment straight back down the beach (perpendicular to coastline under gravity)
4. Net result = sediment moves along the coast in a zigzag pattern
5. This creates distinctive landforms such as spits, bars and tombolos
Example: Longshore Drift at Holderness
The Holderness Coast in East Yorkshire experiences some of the fastest longshore drift in Europe. Prevailing north-easterly waves transport approximately 500,000 tonnes of sediment southwards each year. This material eroded from the boulder clay cliffs is carried towards Spurn Head, forming the 5.5 km spit at the mouth of the Humber Estuary.
🏖️ Deposition
Definition: Deposition occurs when waves lose energy and can no longer carry their sediment load. It happens in sheltered areas, behind obstacles, or where wave energy is reduced.
When Deposition Occurs
- When waves enter sheltered bays where energy is dissipated
- When wave speed decreases in shallow water
- When there is a lot of sediment available after storms
- When constructive waves dominate
- At river mouths where river sediment meets the sea
- Behind headlands, groynes or other obstacles that reduce wave energy
Sorted Deposition
Deposited sediment becomes sorted by size. The largest, heaviest material is deposited first (close to the source), while the finest material is carried furthest. This is why beaches often have pebbles near the cliff and sand near the waterline.
❓ Practice Questions
Q1: Describe the differences between constructive and destructive waves. (4 marks)
Q2: Explain the process of longshore drift. (4 marks)
Q3: Compare mechanical and chemical weathering at the coast. (6 marks)
Q4: "Mass movement is more important than erosion in shaping coastal cliffs." To what extent do you agree? (9 marks)
✅ Answers
- Constructive waves are low and long with a low wave height and long wavelength (1 mark), with a strong swash and weak backwash that deposits sediment to build up beaches (1 mark). Destructive waves are tall and steep with a short wavelength and high frequency (1 mark), with a weak swash and strong backwash that erodes beaches and removes sediment (1 mark).
- Longshore drift occurs when waves approach the coast at an angle rather than head-on (1 mark). The swash carries sediment up the beach at the angle of wave approach (1 mark). The backwash then pulls sediment straight back down the beach at right angles under gravity (1 mark). This zigzag movement gradually transports sediment along the coastline, with the direction determined by the prevailing wind (1 mark).
- Mechanical weathering physically breaks rock apart without changing its chemical composition - freeze-thaw weathering widens cracks as water freezes and expands (1 mark), while salt weathering grows crystals in pores (1 mark). Chemical weathering changes the rock's composition through reactions - solution dissolves limestone in acidic rainwater (1 mark), and oxidation rusts iron-rich minerals (1 mark). Mechanical weathering is most effective in cold climates with freeze-thaw cycles, while chemical weathering is more effective in warm, wet conditions (1 mark). Both processes weaken rock and make it more susceptible to erosion, but they operate through different mechanisms (1 mark).
- Mass movement is important because it moves large volumes of material downhill rapidly - rockfalls and slumping can remove entire cliff sections in single events (1 mark). It is particularly effective where cliffs are made of weak, permeable rocks overlying impermeable clay, as at Barton-on-Sea (1 mark). However, erosion is arguably more important because wave action undercuts cliffs, removing the base support that triggers mass movement (1 mark). Without erosion undercutting the cliff base, many mass movement events would not occur (1 mark). Hydraulic power and abrasion create wave-cut notches that destabilise cliffs above (1 mark). Additionally, erosion continuously shapes the cliff face, wearing it back, whereas mass movement is intermittent (1 mark). The two processes work together - erosion triggers mass movement, and mass movement delivers material for erosion to remove (1 mark). In conclusion, while mass movement can produce dramatic single events, erosion is the more continuous and ultimately controlling process (1 mark). Both are essential components of coastal retreat, but erosion is the primary driver that sets the conditions for mass movement to occur (1 mark).
🎯 Exam Tips
- Always distinguish between weathering (in situ breakdown) and erosion (wearing away with transport)
- Use the correct terminology: swash and backwash, not "wash up" and "wash back"
- For longshore drift, describe the zigzag movement step by step
- Link erosion processes to the geology they affect most (solution → limestone)
- Use named examples for mass movement (Barton-on-Sea) and longshore drift (Holderness)
- Remember fetch as a key factor in wave energy - not just wind speed
📝 Exam Technique
Geography Exam Tips — Coastal Processes:
1. For Coastal Processes questions, always name specific case studies with factual detail
2. Use geographical terminology precisely (e.g. specific processes, not vague descriptions)
3. Consider social, economic and environmental perspectives in your evaluations
4. Support your points about Coastal Processes with data, statistics or named examples
5. For 'assess' or 'evaluate' questions, reach a clear judgement supported by evidence
1. For Coastal Processes questions, always name specific case studies with factual detail
2. Use geographical terminology precisely (e.g. specific processes, not vague descriptions)
3. Consider social, economic and environmental perspectives in your evaluations
4. Support your points about Coastal Processes with data, statistics or named examples
5. For 'assess' or 'evaluate' questions, reach a clear judgement supported by evidence
⚠️ Common Errors
Watch Out!
Students often write vague answers without specific geographical evidence. Wrong: Writing generalised statements like 'it causes problems' Correct: Using specific data and named examples, e.g. 'the 2010 Haiti earthquake killed over 200,000 people due to poor building quality'
Students often confuse causes and effects. Wrong: Mixing up what caused the event with what resulted from it Correct: Clearly separate causes (why it happened) from effects (what happened as a result)
Students often describe rather than evaluate. Wrong: Listing strategies without assessing their effectiveness Correct: Weighing up strengths and weaknesses of each approach and reaching a supported judgement
✍️ Model Answer
Full-Mark Response
6 marks: Explain the key factors affecting coastal processes.
Coastal Processes involves multiple interconnected factors that geographers must understand. The key concepts include the processes that create and change coastal processes, the impacts on both people and environment, and the strategies used to manage associated challenges. For a comprehensive answer, specific case study evidence should be used throughout, with named examples and data to support each point. Geographical terminology should be used precisely, and the interrelationship between physical and human factors should be demonstrated. Top-level responses evaluate the relative importance of different factors and consider how the situation varies between locations.
Mark scheme: 2 marks for identifying key factors, 2 marks for explaining processes with detail, 2 marks for using specific evidence
📊 AO Deep Dive
Assessment Objective Analysis
AO1 requires knowledge of the key facts and processes related to coastal processes. AO2 demands understanding of how and why these processes operate, and their implications. AO3 asks you to analyse, evaluate and make judgements — this is where grade 9 answers stand out by weighing up competing perspectives and reaching supported conclusions. AO4 may involve interpreting maps, graphs or data related to this topic. To move from grade 5 to grade 9: use precise geographical terminology, support every point with specific case study evidence, and always evaluate rather than just describe.
📝 Exam Questions by Topic
🎬 Video Resources
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