Waves, Tides, and Currents

Waves, tides and currents are the principal sources of energy in the coastal system. They drive erosion, transport sediment and create the landforms that define our coastlines. A thorough understanding of these processes is essential for A-Level Geography, as they underpin virtually every other topic in Coastal Systems and Landscapes.

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Wave Formation and Characteristics

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:

1. Wind speed — stronger winds transfer more energy, producing larger waves
2. Wind duration — winds must blow consistently to build up wave height
3. Fetch — the distance of open water over which the wind blows

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.

Wave Anatomy

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)

Wave Motion

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.

Wave Shoaling and Breaking

As waves enter shallow water (where water depth is less than half the wavelength), they undergo significant changes:

1. The sea bed interferes with the circular orbits of water particles — the orbits become elliptical
2. Friction with the sea bed causes the base of the wave to slow down
3. The top of the wave continues at its original speed, causing the wave to steepen
4. When the wave steepness (H/L) exceeds approximately 1:7 (about 0.14), the wave becomes unstable and breaks

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

Types of Breaking Waves

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

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Constructive and Destructive Waves

This is one of the most important distinctions in coastal geography, and it is essential for understanding erosion and deposition:

Constructive Waves

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.

Destructive Waves

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.

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Wave Refraction

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.

Refraction at Headlands and Bays

1. Waves approaching a headland-bay coastline travel faster in deeper water (in front of bays) and slower in shallower water (near headlands)
2. This causes the wave fronts to bend and concentrate their energy on the headlands
3. Wave energy is dispersed in the bays, where waves arrive at a lower angle and with less energy

The result is that headlands experience concentrated erosion while bays experience deposition — creating a self-reinforcing pattern that tends towards an equilibrium coastline shape.