9.6: Wave Dissipation

When the force that creates a wave is removed, the wave continues to move across the ocean surface. For example, waves created by a rock thrown into a lake continue long after the rock’s impact, and large waves may crash ashore on days when there is no wind to create them. The reason is that wave energy dissipates (is lost from the wave) very slowly.

Energy is lost from a wave because of viscosity (internal friction) between water molecules moving within the wave and air resistance as the wave moves across the surface, but this energy loss is very small. Frictional loss in a flowing fluid depends on the fluid’s viscosity. The viscosity of water is not high (Chap. 5), so frictional loss is a very small fraction of the total wave energy in all but very short-period (<0.1 s) capillary waves. For capillary waves, viscosity is important as a means of dissipating wave energy because such waves contain only a very small amount of energy. In fact, the energy in capillary waves can be totally dissipated by viscosity in a minute or less after the wind stops. Capillary waves, therefore, disappear quickly when there is no wind, but larger waves do not.

Wave steepness diminishes once the wind ceases. Hence, in the open ocean, waves do not break and lose energy through turbulence unless winds blow and add more energy to the sea. Minor exceptions occur when waves encounter winds that blow, or currents that flow, against the wave’s direction of travel. Waves then slow, and wave steepness increases (Fig. 9-8b).

The waves created by a rock thrown into a lake spread in a circular pattern away from the point of impact. This process does not decrease the total energy of each wave. However, as the wave moves out from the impact point, it spreads over an ever-increasing area (Fig. 9-9), and the wave height is reduced. Storm waves also spread out from where they are generated, but some storms generate their largest waves across a broad storm front. The resulting waves travel out from the storm area in the same general direction, and wave energy tends to be concentrated in that direction (Fig. 9-9b).

Figure 9-9. (a) Waves spread out uniformly in all directions from an impact point, and energy is spread along the increasing length of each wave as it moves away from this point. (b) Waves generated along a storm front travel forward in the direction of the winds in the storm, but they also spread out in a fan shape. This disperses (spreads) the wave energy along a progressively lengthening wave crest with distance from the storm.

Because waves lose very little energy as they travel across the ocean surface, most waves travel until they meet a shore, where their energy is converted to heat or used to erode the shore, transport sand along the coast, or create longshore currents (Chap. 13). Although breaking waves release a considerable amount of heat energy, this process does not cause a measurable change in water temperature, because water has a high heat capacity (Chap. 5, CC5). In addition, heat from breaking waves is dissipated in very large volumes of water.