Wave Processes and Beach Morphology

Waves are the primary agent of change in coastal landscapes. Understanding how waves are generated, how they behave as they approach the shore, and how they shape beach morphology is essential for Edexcel A-Level Geography Paper 1, Topic 2B: Coastal Landscapes and Change and directly supports Enquiry Question 1. This lesson provides detailed coverage of wave generation, wave types, wave refraction, and the relationship between wave processes and beach profiles.

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Wave Generation

Waves are created by the frictional drag of wind across the surface of the ocean. The wind transfers energy to the water surface, creating oscillations that propagate as waves. Three factors control the size and energy of waves:

Factor	Definition	Effect on Wave Energy
Fetch	The distance of open water over which the wind blows	Longer fetch → larger, more powerful waves
Wind speed	The velocity of the wind	Faster wind → greater energy transfer → larger waves
Wind duration	How long the wind blows over the fetch area	Longer duration → more energy transferred → waves reach maximum size for given fetch and speed

In the open ocean, waves are oscillatory — water particles move in circular orbits and there is no net forward movement of water. As the wave approaches shallower water (when the water depth is less than half the wavelength), the circular orbits become elliptical, friction with the seabed slows the base of the wave, the wavelength decreases, the wave height increases, and the wave eventually breaks.

Wave Terminology

Term	Definition
Wave height	Vertical distance from trough to crest
Wavelength	Horizontal distance between two successive crests
Wave frequency	Number of waves passing a fixed point per minute
Wave period	Time (in seconds) between two successive crests passing a fixed point
Wave steepness	Ratio of wave height to wavelength (H/L) — determines whether a wave is constructive or destructive
Swash	The movement of water up the beach after a wave breaks
Backwash	The return flow of water down the beach under gravity

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Constructive and Destructive Waves

The distinction between constructive and destructive waves is fundamental to understanding beach morphology. The classification depends on wave steepness, frequency and the relative strength of swash and backwash.

Property	Constructive Waves	Destructive Waves
Wave height	Low (< 1 m typically)	High (> 1 m typically)
Wavelength	Long	Short
Wave frequency	Low (6–8 per minute)	High (10–14 per minute)
Wave steepness	Low (gentle, spilling breakers)	High (steep, plunging breakers)
Dominant process	Swash > backwash	Backwash > swash
Effect on beach	Deposits sediment; builds up the beach profile	Removes sediment; flattens the beach profile
Breaker type	Spilling — wave crest gently spills forward	Plunging — crest curls over and crashes
Associated with	Calm weather, gentle winds, distant storms (swell waves)	Local storms, strong onshore winds

How Constructive Waves Build Beaches

Constructive waves have a strong swash that carries sediment up the beach face. Because the wave frequency is low, each swash has time to percolate into the beach before the next wave arrives, weakening the backwash (water drains into the permeable beach rather than flowing back down the surface). This means more sediment is deposited than removed, and the beach builds up, forming a steep profile with well-developed berms (ridges of deposited material at the top of the swash zone).

How Destructive Waves Erode Beaches

Destructive waves have a powerful backwash that drags sediment offshore. The high frequency means each wave arrives before the swash from the previous wave has percolated or drained, so the returning water adds to the backwash volume and velocity. The plunging breaker type also impacts the beach surface with force, loosening and suspending sediment. The net effect is sediment removal and a flatter, lower beach profile.

Exam Tip: Avoid the common mistake of saying "constructive waves have a strong swash and no backwash." They have backwash — it is simply weaker than the swash due to percolation. Similarly, destructive waves have swash; it is weaker relative to the backwash. The balance between swash and backwash is what matters.

graph TD
    A["Wave Classification"] --> B["Constructive"]
    A --> C["Destructive"]
    B --> B1["Low frequency (6–8/min)"]
    B --> B2["Low steepness"]
    B --> B3["Swash > Backwash"]
    B --> B4["Deposits sediment<br/>Builds beach profile"]
    C --> C1["High frequency (10–14/min)"]
    C --> C2["High steepness"]
    C --> C3["Backwash > Swash"]
    C --> C4["Removes sediment<br/>Flattens beach profile"]

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Wave Refraction

Wave refraction is the bending of waves as they approach an irregular coastline. It occurs because the part of the wave in shallower water (near a headland) is slowed by friction with the seabed, while the part in deeper water (in the bay) continues at its original speed. This differential slowing causes the wave crest to bend and wrap around the headland.

Consequences of Wave Refraction

1. Energy concentration on headlands: Refracted waves converge on headlands, focusing erosive energy on the projecting rock. This is why headlands are eroded — they receive disproportionately high wave energy.

2. Energy dissipation in bays: Refracted waves diverge in bays, spreading their energy over a wider area. This is why bays are sites of deposition — wave energy is insufficient to erode but sufficient to deposit sediment transported from elsewhere.

3. Headland erosion and bay deposition together: Over time, refraction tends to straighten the coastline by eroding headlands and filling bays — though in practice this process operates over thousands of years and is rarely completed.