Wave refraction and coastal alignment

Overview

Wave refraction, swash-aligned coast and drift-aligned coast.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   What is wave refraction?

   Model answer

   The bending of wave crests as they approach an irregular coastline, caused by different parts of the same crest entering shallow water at different times and therefore travelling at different speeds. The effect is to swing the crests round so that they become more nearly parallel to the shoreline, and to redistribute the energy of the wave unevenly along the coast.

2. 2

   Explain the mechanism of wave refraction at a headland.

   Model answer

   The water offshore of a headland is shallower than the water off the adjacent bays, because the headland projects seawards. As a wave crest approaches, the section over the headland reaches shallow water first, so friction with the sea bed slows it down while the sections still in the deeper water off the bays continue at full speed. The crest therefore bends round the headland, pivoting about the slowed section, until it is approximately parallel to the coast on each side. Drawing the orthogonals, which are lines at right angles to the crests showing the direction of energy travel, they converge on the headland and diverge into the bay.

3. 3

   What is an orthogonal, and why is it the key to understanding refraction?

   Model answer

   An orthogonal, or wave ray, is a line drawn at right angles to the wave crests showing the direction in which the wave energy is travelling. It is the key because the energy contained between any two adjacent orthogonals stays the same as the wave moves inshore. Where refraction squeezes those two lines closer together the same energy is concentrated into a shorter length of coast, so energy per metre rises; where it spreads them apart the same energy is dissipated over a longer length of coast, so energy per metre falls. Drawing the orthogonals therefore converts the bending of the crests into a statement about where the coast is attacked hardest.

4. 4

   Why does refraction concentrate energy on headlands and disperse it in bays?

   Model answer

   Because refraction bends the crests round towards the headland, so orthogonals from a wide stretch of open sea converge on the relatively short length of coast that the headland presents. The energy that arrived spread over that wide front is delivered onto a much narrower target, so wave height and energy per metre of coast increase and the headland is attacked by powerful, often destructive waves. In the bay the opposite occurs: the crests bend away and the orthogonals diverge, so the energy arriving on a given front is spread along a longer length of shoreline, wave height and energy per metre fall, and the waves reaching the bay head are low and constructive.

5. 5

   What is the long-term effect of wave refraction on the shape of a coastline?

   Model answer

   It tends to straighten it. Energy concentrated on the headlands means they are eroded most rapidly, so they retreat, while the low-energy conditions in the bays encourage deposition, so beaches build out. Over time the projections are cut back and the indentations are filled in, and the coast becomes more regular. The process is self-limiting rather than self-reinforcing: as the headland shrinks the degree of refraction lessens, and as the bay fills the sheltered zone reduces, so the coast tends towards an equilibrium alignment.

6. 6

   What is a swash-aligned coast?

   Model answer

   A coast where the beach has adjusted so that its shoreline is almost parallel to the crests of the incoming waves, because refraction is complete or the waves approach head-on. Swash therefore runs almost straight up the beach and backwash straight down it, so there is very little net movement of sediment along the shore. Sediment stays broadly where it is, and the beach takes on a smooth, gently curved plan, often a crescent between two headlands. Such beaches are relatively stable and are supplied mainly from offshore or from local cliff erosion rather than from along the coast.

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