Summary of wave lectures



Plunging breaker on gravel beach during storm, Cornwall.

Here's an On the Road style summary of the wave lectures. Waves have a 'height', a 'length' and a 'period'. Ocean waves are generated by wind - the stronger the wind and the larger the area over which the wind blows, knows as 'fetch', the higher the waves and the longer their periods. Waves cannot grow beyond a certain point and we speak of a 'fully arisen sea' when waves have attained their maximum height. The behaviour of waves can be decribed by 'linear wave theory'. The universal wave equations are rather complicated, but in deep and shallow water they simplify. In deep water, the wave speed is only a function of the wave period, and such waves are called 'dispersive' waves: the longer the period, the faster they travel. In shallow water, waves are 'non-dispersive' and their travel speed only depends on the local water depth: the shallower the water, the slower the waves travel. Waves represents 'energy', and the amount of energy per wave increases with wave height. In deep water, wave energy travels at half the speed of individual waves; in fact, the wave energy travels in 'wave groups' and while groups retain their identity during propagation, individual waves travel through a group and then lose their identity. In shallow water, individual waves travel at the same speed as the wave groups. When waves enter a water depth of about half their wave length, they start feeling the bed and their behaviour starts to change. Specifically, as the wave travel increasingly in shallower water, their wave length decreases and travel speed also reduces. As a result, three important processes are initiated: 'shoaling', 'refraction' and development of 'wave asymmetry'. In very shallow water, when water depth is only slightly larger than the wave height, wave will 'break' and disintegrate in bubbles and foam. Waves can break in different ways, and the controlling variables are wave height, wave period and beach gradient. After wave breaking, waves continue to lose their energy and this is referred to as 'dissipation'. On gently sloping beaches, waves dissipate practically all their energy and have zero wave height when they reach the shoreline. On steep beaches, waves can bounce off the beach, light light of a mirror, and energy will travel back out to sea - this is referred to as 'reflection'. The type of surf zone and breaker can be predicted using the 'Irribarren Number' and the 'surf scaling parameter'. Finally, at the shoreline, a number of other processes are important. 'Infragravity waves' refers to the motion of the water level at very low frequencies, or long periods (more than 30 seconds). Infragravity waves are especially important under storm conditions, because the amount of energy they represent increases with the ocean wave height and their importance increases towards the shoreline. 'Wave set-up' is the super-elevation of the surf zone water level due to the presence of waves. Near the shoreline the extra rise in water level due to set-up can be 30% of the wave height. Wave set-up and infragravity wave motion together are responsible for beach erosion, dune scarping and overtopping during energetic wave conditions.