Constructive and Destructive Waves

Waves are the primary force that affects a coastline’s shape and depth. Waves are created by winds and a wave’s strength depends on a few factors, which in turn defines its impact on the coast. These factors are

  • The wave’s fetch (the distance over water that the wind has blown)
  • The strength of the wind
  • How long the wind has blown for

The fetch, or the distance over water that the wind blows, plays a significant role in determining the size and energy of waves. A shorter fetch typically produces smaller, less energetic waves, while a longer fetch can lead to the formation of larger waves with greater energy. However, it’s the wave’s overall energy and the manner in which it breaks upon the shore that categorises it as constructive or destructive.

Waves that break with higher energy are destructive waves and waves that break with lower energy are constructive waves.

Remember, it’s the energy and not the fetch that defines the wave. For example, waves with a long fetch can still be constructive if they break gently on the shore.

Comparing the Wave Types

The two types of waves have different effects on the coast, as you may be able to tell by their names—constructive and destructive.

  • Constructive Waves tend to have a long wavelength and lower height, breaking gently on the shore. They carry material up the beach with a strong swash and have a weaker backwash, which leaves sediment on the beach, effectively building it up.
  • Destructive Waves are characterized by a shorter wavelength and higher height, breaking with more force. They tend to have a stronger backwash than swash, pulling materials away from the beach and contributing to erosion.

The key to differentiating between the two is the wave’s energy at the point of breaking on the shore. Constructive waves have enough energy to carry materials up onto the beach but not enough to take them back into the ocean. In contrast, the high energy of destructive waves not only brings material up the beach but also has the power to drag it back, leading to erosion.

Here’s a table that shows some key features of constructive and destructive waves:

Constructive waves (the beach builders)Destructive waves (the beach destroyers)
Long wavelength with low heightShort wavelength with high height
Lower frequency, with 6-8 per minuteHigher frequency, with 10-12 per minute
Strong swash and weak backwashWeak swash and strong backwash

Key Term Table

FetchThe distance that the wave has travelled 
SwashMovement of water up the beach
BackwashMovement of water down the beach, back towards the ocean.
Destructive WavesWaves that result from a long fetch and high levels of energy. They erode beaches, removing material due to their strong backwash
Constructive WavesWaves that result from a shorter fetch and lower levels of energy. They deposit material onto the beach due to their strong swash
WavelengthThe horizontal distance between two successive crests or troughs of a wave