7.1.2 What happens beneath the surface?
The surface profile of the ocean is the obvious evidence for the existence of waves, but we also need to understand the sub-surface nature of waves if we are to design schemes to capture energy from them.
As Figure 6 shows, waves are composed of orbiting particles of water. Near the surface, these orbits are the same size as the wave height, but the orbits decrease in size as we go deeper below the surface. The size of orbits decreases exponentially with depth. 95% of the wave energy is contained in the layer between the surface and a depth equal to a quarter of the wavelength.
There are a few areas in the world where the shoreline is formed by a steep cliff that drops into reasonably deep water. These are the most suitable for shore-mounted wave energy converters because the incident waves have a high power density. But, for most coastlines around the world the near-shore water is quite shallow. Due to the frictional coupling between the water particles at the greatest depths with the seabed, deep water waves gradually give up their energy as they move into shallower water and eventually run up the shore to the beach.
This power loss obviously reduces the total wave energy resource. Typically, waves with a power density of 50 kW m–1 in deep water might contain 20 kW m–1 or less when they are closer to shore in shallow water.
A further mechanism for energy loss as waves run up a beach is the formation of breaking waves, which are turbulent and energy-dissipating.
We now move on to discuss the resources available from wave energy.