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examBoard: Cambridge
examType: IGCSE
lessonTitle: Wave Refraction
Wave refraction is one of the most important processes shaping our coastlines. It's a bit like how light bends when it passes through water, but with ocean waves instead! When waves approach the shore at an angle, they don't just continue in a straight line. Instead, they bend and change direction, which has huge impacts on how our beaches and headlands develop over time.
Key Definitions:
When waves approach the shore, the parts of the wave in shallower water slow down first, while parts still in deeper water continue at their original speed. This causes the wave to bend or "refract" towards the shallower areas. Imagine a line of people running towards the beach - if the people at one end hit shallow water first, they'll slow down while the others keep running, causing the whole line to curve!
Wave refraction concentrates wave energy on headlands and disperses it in bays. This explains why headlands often experience more erosion than bays and why bays tend to be areas where sediment is deposited. This process is crucial for understanding how our coastlines evolve and helps us predict future coastal changes.
To understand wave refraction properly, we need to look at what happens underwater. Waves travel faster in deeper water and slower in shallow water. This difference in speed is what causes waves to bend as they approach the shore.
When a wave approaches the coast at an angle, the part of the wave closest to the shore enters shallow water first. As this happens:
Waves travel across the open ocean at consistent speeds. Wave crests are roughly parallel to each other.
As waves reach shallower water near the coast, the parts of the wave in shallow water slow down first, while deeper sections maintain speed.
This speed difference causes the wave crest to bend, aligning more closely with the underwater contours of the coastline.
The most visible effect of wave refraction occurs around headlands and bays. This process helps explain why coastlines often develop their characteristic shapes.
When waves approach a headland, they refract around it. The wave orthogonals (lines showing wave energy direction) converge on the headland, concentrating wave energy. This increased energy leads to more powerful erosion, creating features like wave-cut platforms, caves, arches and stacks. Headlands gradually retreat over time due to this concentrated erosion.
In contrast, when waves enter a bay, the wave orthogonals diverge or spread out. This disperses wave energy across a wider area, resulting in less erosion and more deposition. Bays often become sites where beaches form as sediment is deposited in these lower-energy environments.
Geographers and coastal scientists use several methods to study wave refraction:
The Dorset coastline in southern England provides an excellent example of wave refraction in action. The famous Durdle Door arch and Lulworth Cove demonstrate the effects of differential erosion caused by wave refraction. At Durdle Door, wave refraction has concentrated erosion on a headland of resistant limestone, eventually creating the iconic arch. Nearby, Lulworth Cove formed when waves breached a band of limestone and then eroded the softer clays behind it, creating a nearly circular bay. The different rates of erosion between the headlands and bays along this coastline are directly linked to how wave energy is concentrated or dispersed through refraction.
Wave refraction is responsible for creating many distinctive coastal features:
Wave-cut platforms, caves, arches and stacks form primarily at headlands where wave refraction concentrates erosive energy.
Beaches, spits and bars tend to form in bays where wave refraction disperses energy and allows sediment to settle.
Wave refraction influences the direction of longshore drift, which transports sediment along the coast and shapes features like spits.
Understanding wave refraction is crucial for effective coastal management. Engineers and planners need to consider how waves will interact with coastal defences and how energy distribution might change if structures are built.
Coastal engineers must account for wave refraction when designing sea walls, groynes and breakwaters. Placing defences without understanding refraction patterns can lead to unexpected erosion in adjacent areas.
Harbours and marinas are designed with wave refraction in mind to create calm water conditions inside. Breakwaters are positioned to redirect wave energy away from moored vessels.
Chesil Beach in Dorset is an 18-mile long shingle barrier beach that shows clear evidence of wave refraction's effects. The beach material is sorted by size, with pebbles becoming progressively smaller from west to east. This sorting occurs because of wave refraction patterns that affect how wave energy is distributed along the beach. The prevailing south-westerly waves refract around Portland Bill (a headland), creating complex wave patterns that transport and sort the beach material. Coastal managers must understand these patterns when planning any interventions, as disrupting the natural sediment movement could have consequences for the entire beach system.
The study of wave refraction has several practical applications beyond just understanding coastal landforms:
Wave refraction is a fundamental process that shapes our coastlines by determining where erosion and deposition occur. It explains why headlands experience more erosion than bays and influences the development of many coastal landforms. For geographers, understanding wave refraction is essential for interpreting coastal landscapes and predicting how they might change in the future. As climate change leads to rising sea levels and potentially more powerful storms, the patterns of wave refraction may change, creating new challenges for coastal communities worldwide.
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