Wave Dynamics and Tidal Processes

Waves are the primary source of energy acting upon the coast. At A-Level, AQA expects a detailed understanding of how waves form, how their energy is distributed, and how tidal processes interact with wave action to shape the coastal landscape.

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Wave Formation and Characteristics

Waves are generated by wind blowing over the surface of the sea. The energy transferred from the wind to the water surface creates oscillatory motion. Three factors determine wave size and energy:

1. Wind speed — stronger winds transfer more energy
2. Wind duration — longer periods of sustained wind build larger waves
3. Fetch — the distance of open water over which the wind blows. The maximum fetch for the southwest coast of England extends across the entire Atlantic Ocean (approximately 5,000 km)

Wave Terminology

Term	Definition
Wave height (H)	Vertical distance from trough to crest
Wavelength (L)	Horizontal distance between two successive crests
Wave period (T)	Time interval between two successive crests passing 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 (H/2)

Wave Energy

The energy of a wave is proportional to the square of its height. The power per unit length of wave front is given by:

P = (ρ × g × H² × L) / (8T)

Where:

• P = wave power (watts per metre)
• ρ = density of seawater (approximately 1,025 kg/m³)
• g = gravitational acceleration (9.81 m/s²)
• H = wave height (m)
• L = wavelength (m)
• T = wave period (s)

This means a doubling of wave height results in a fourfold increase in wave energy — explaining why storm waves are so destructive.

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Wave Behaviour in Shallow Water

As waves approach the shore and enter shallow water (where depth < half the wavelength), their behaviour changes dramatically:

1. The base of the wave is slowed by friction with the seabed
2. The top of the wave continues at its original speed
3. Wavelength decreases and wave height increases
4. The wave becomes steeper until it reaches a critical steepness ratio (H/L ≈ 1/7)
5. The wave breaks

Types of Breaking Wave

Wave Type	Characteristics	Conditions
Spilling	Crest gradually spills down the wave front; gentle, sustained break	Gentle beach gradient; low wave steepness
Plunging	Crest curls over and plunges into the trough; powerful, short break	Moderate to steep beach gradient
Surging	Wave slides up the beach without breaking fully; high energy swash	Very steep beach gradient; low wave steepness

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Wave Refraction

Wave refraction is the bending of waves as they approach an irregular coastline. It occurs because different parts of the wave front reach shallow water at different times.

Process

1. As waves approach a headland, the part of the wave nearest the headland enters shallow water first
2. This section slows down due to friction with the seabed
3. The part of the wave in deeper water (approaching the bay) continues at its original speed
4. The wave front bends, concentrating energy on the headland and dispersing it in the bay

Consequences

• Headlands: Wave energy is concentrated (converged), leading to intense erosion. Orthogonal lines drawn perpendicular to the wave front converge on the headland
• Bays: Wave energy is dispersed (diverged), leading to deposition. Orthogonal lines diverge in the bay
• This differential energy distribution explains why headlands erode while bays accumulate beaches

Exam Technique: When drawing wave refraction diagrams, always include orthogonal lines (arrows perpendicular to the wave crests) to show energy concentration and dispersal. Label areas of convergence (high energy) and divergence (low energy).

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Constructive and Destructive Waves

AQA requires detailed comparison of these two wave types and their geomorphological impacts.

Constructive Waves

• Low wave height (typically < 1 m) and long wavelength
• Low wave frequency — typically 6–8 waves per minute
• Strong swash, weak backwash
• Swash travels further up the beach, depositing sediment
• Associated with calm conditions and long fetches (swell waves)
• Build up berms and widen beaches
• Elliptical orbit of water particles is relatively flat

Destructive Waves

• High wave height and short wavelength
• High wave frequency — typically 10–14 waves per minute
• Weak swash, strong backwash
• Backwash is strong enough to drag sediment down the beach before the next swash arrives
• Associated with storm conditions and short fetches
• Create steep, narrow beaches and form storm berms at the top of the beach
• The steep wave front plunges onto the beach with considerable force

Seasonal Beach Profiles