You are viewing a free preview of this lesson.
Subscribe to unlock all 10 lessons in this course and every other course on LearningBro.
Spec mapping (AQA 7037): Paper 1, §3.1.3 Coastal Systems and Landscapes — marine processes: waves, tides, currents and the sources of energy in coastal environments. This lesson supplies the energy inputs to the open coastal system established in Lesson 1: waves, tides and currents are the transfers that drive every erosional, transport and depositional process in the rest of the topic. It links synoptically to §3.1.1 (energy cascades and flows of energy through a system), to §3.1.5 Hazards (storm surges as coastal hazards, the magnitude–frequency concept) and forward to Lessons 3–6, which are applications of the wave energy delivered here. The dominant Assessment Objectives are AO1 (knowledge of wave generation, anatomy, refraction, tides and currents) and AO2 (applying wave-energy and refraction principles to explain spatial patterns of erosion and deposition). The worked exercise on fetch and the wave-energy equation exercises AO3 (quantitative manipulation and interpretation).
Why this lesson matters. Energy is the currency of the coastal system. If you cannot explain why a headland receives more energy than a bay, or why a 4 m storm wave is so much more destructive than a 1 m wave, you cannot explain the landforms in Lessons 4 and 6. This lesson is where the abstract systems model becomes physical.
Waves are generated by wind blowing over the surface of the sea. The frictional drag of wind on water creates ripples, which grow into waves as energy is transferred from the atmosphere to the ocean surface. The size and energy of waves depend on three factors:
Key Definition: Fetch is the uninterrupted distance of ocean over which the wind blows in a constant direction. The longer the fetch, the greater the wave energy. The maximum fetch for waves reaching the west coast of Britain is approximately 5,000 km across the Atlantic Ocean.
Understanding wave terminology is essential:
| Term | Definition |
|---|---|
| Crest | The highest point of the wave |
| Trough | The lowest point of the wave |
| Wave height | Vertical distance from trough to crest |
| Wavelength | Horizontal distance between two successive crests |
| Wave period | Time taken for two successive crests to pass a fixed point |
| Wave frequency | Number of waves passing a fixed point per minute |
| Wave steepness | Ratio of wave height to wavelength (H/L) |
| Amplitude | Half the wave height (distance from still water level to crest) |
In deep water, waves are oscillatory — water particles move in circular orbits but do not move forward overall. This was demonstrated by George Airy (1841) in his linear wave theory. As a wave passes, a floating object bobs up and down but returns to approximately its original position.
The diameter of the circular orbits decreases with depth. At a depth equal to approximately half the wavelength (the wave base), the orbital motion becomes negligible. This is why submarines below the wave base are unaffected by surface storms.
This orbital model is the key to understanding why waves break, and it rewards careful explanation in exams. Because the orbital diameter at the surface equals the wave height, and because energy must be conserved as the wave slows in shallow water, the crest of a shoaling wave is forced to rise — the same energy is packed into a shorter, steeper wave. Meanwhile the orbits near the bed, no longer circular but flattened into ellipses by the proximity of the sea floor, lose forward symmetry: the water under the crest moves shoreward faster than the water under the trough moves seaward. The wave therefore becomes front-steep and rear-shallow, and when the crest finally outruns the base, the orbit opens and the wave collapses forward as breaking surf. Crucially, it is only at and after breaking that water itself moves bodily shoreward (as swash); before breaking, the wave transferred energy through near-closed orbits while the water stayed put. Candidates who can articulate this transition — from energy-transfer to mass-transfer at the breakpoint — demonstrate exactly the mechanistic understanding that AO1 rewards.
As waves enter shallow water (where water depth is less than half the wavelength), they undergo significant changes:
graph LR
subgraph "Wave Transformation"
A["Deep Water: circular orbits, constant speed"] --> B["Transitional: orbits become elliptical"]
B --> C["Shallow Water: friction slows base"]
C --> D["Wave steepens"]
D --> E["Wave breaks (H/L > 1:7)"]
end
There are three main types of breaking wave, classified by Galvin (1968):
| Type | Beach Gradient | Characteristics |
|---|---|---|
| Spilling | Gentle (< 5°) | Wave crest gradually spills down the front; gentle, long-lasting break |
| Plunging | Moderate (5-15°) | Crest curls over and plunges forward; powerful, dramatic break |
| Surging | Steep (> 15°) | Wave slides up the beach without fully breaking; high energy reflected |
A distinction that elevates exam answers is that between sea (storm) waves and swell waves, because it explains why the same coast can experience constructive and destructive conditions:
The west coast of Britain frequently receives swell generated by mid-Atlantic depressions far to the west, which is why long-period swell can arrive on a calm, sunny day with no local wind — the energy was generated days earlier and a continent away. Recognising that wave conditions reflect distant as well as local weather is a mark of sophisticated understanding.
This is one of the most important distinctions in coastal geography, and it is essential for understanding erosion and deposition:
| Characteristic | Detail |
|---|---|
| Wave frequency | Low — typically 6-8 waves per minute |
| Wave height | Low — usually under 1 m |
| Wavelength | Long |
| Wave steepness | Low |
| Swash | Strong — carries sediment up the beach |
| Backwash | Weak — water percolates into the beach |
| Net effect | Deposition — beach builds up |
| Beach profile | Wide, gently sloping, with well-developed berms |
Constructive waves are typically associated with calm weather conditions and long fetch distances, where swell waves have had time to develop a long wavelength and low height.
| Characteristic | Detail |
|---|---|
| Wave frequency | High — typically 10-14 waves per minute |
| Wave height | High — often over 1 m |
| Wavelength | Short |
| Wave steepness | High |
| Swash | Weak — limited sediment transport up beach |
| Backwash | Strong — drags sediment back down the beach |
| Net effect | Erosion — beach is stripped of material |
| Beach profile | Narrow, steep, with prominent breakpoint bar |
Destructive waves are associated with storm conditions, strong local winds and short fetch distances. They are the dominant wave type during winter in the UK.
Exam Tip: Avoid stating that constructive waves only deposit and destructive waves only erode. In reality, both types of wave both erode and deposit — the distinction is about the net balance of these processes. Using nuanced language like "net deposition" or "predominantly erosive" demonstrates higher-level understanding.
When waves approach a coastline at an angle or encounter an irregular coastline, they undergo refraction — a bending of the wave fronts caused by differential friction with the sea bed.
The result is that headlands experience concentrated erosion while bays experience deposition — creating a self-reinforcing pattern that tends towards an equilibrium coastline shape.
Key Definition: Wave refraction is the process by which wave fronts bend as they approach a coastline, caused by the differential slowing of waves in shallower water. This concentrates wave energy on headlands and disperses it in bays.
This concept was quantified by Munk and Traylor (1947), who demonstrated mathematically how wave energy is focused on promontories, leading to rates of erosion that can be 2-3 times higher at headlands than in adjacent bays.
Refraction is also crucial to transport, not just erosion. Because refraction bends wave crests so that they become more nearly parallel to the shore as they approach, waves that began far offshore at a large angle arrive at the beach at a much smaller angle. This partial alignment limits — but rarely eliminates — the oblique approach that drives longshore drift (Lesson 5). The residual angle that survives refraction is what sets the rate of longshore drift along a given beach. On a strongly refracting, gently shelving coast (such as much of Chesil Beach), waves arrive almost head-on and the beach is swash-aligned with little net drift; on a steeply shelving coast where refraction is weak, a larger oblique angle survives and the beach is drift-aligned with rapid sediment transport. The degree of refraction therefore helps determine whether a beach grows in place or feeds a spit downdrift — a direct bridge from this lesson to the depositional landforms of Lesson 6.
Tides are the periodic rise and fall of sea level caused by the gravitational attraction of the Moon and, to a lesser extent, the Sun on the Earth's ocean water. The scientific understanding of tides was revolutionised by Sir Isaac Newton (1687) in his Principia Mathematica.
| Tide Type | Alignment | Tidal Range | Occurrence |
|---|---|---|---|
| Spring tides | Sun, Moon and Earth aligned (new moon and full moon) | Maximum — high highs and low lows | Every ~14 days |
| Neap tides | Sun and Moon at right angles to Earth (first and third quarter) | Minimum — less extreme highs and lows | Every ~14 days |
The tidal range is the vertical difference between high and low tide. It varies enormously around the world:
| Location | Mean Spring Tidal Range | Classification |
|---|---|---|
| Bay of Fundy, Canada | 16.3 m | Macrotidal |
| Bristol Channel, UK | 14.0 m | Macrotidal |
| Southampton, UK | 4.0 m | Mesotidal |
| Mediterranean Sea | 0.3 m | Microtidal |
Key Definition: Tidal range is the vertical difference in height between consecutive high and low tides. It determines the width of the intertidal zone and influences the types of landforms and ecosystems that develop.
The enormous difference between the Bay of Fundy (16 m) and the Mediterranean (0.3 m) is not explained by the Moon alone — the Moon's pull is essentially identical worldwide. The real controls are the shape and depth of the ocean basin and the phenomenon of resonance. The Atlantic tide travels as a vast wave; where it enters a basin whose natural oscillation period is close to the 12 hour 25 minute semi-diurnal tidal period, the tide resonates and amplifies. The Bay of Fundy has a natural period almost exactly matching the tide, so the range is amplified to the largest on Earth. Funnelling adds further amplification: the Bristol Channel and Severn Estuary taper sharply inland, squeezing the same volume of water into a progressively narrower, shallower channel and forcing the range up to 14 m — the second highest in the world — and generating the famous Severn Bore, a tidal wave that travels upstream against the river current. Recognising that tidal range is a product of basin geometry and resonance, not just lunar gravity, is a discriminator between competent and excellent answers.
Subscribe to continue reading
Get full access to this lesson and all 10 lessons in this course.