water waves

Water waves


Any disturbance can cause a water wave. A pebble striking the surface, movement of a boat, movement of the earth during an earthquake, or the wind. Here we focus on wind generated waves, although the same principles apply to all water waves.

Fetch is the distance over which the wind interacts with the water surface to creates waves. The longer the fetch the bigger (higher) the waves are. If the shore (green in the diagram) is a hill, there will be a wind shadow which gives protection from the wind, but even if the shore is flat as a pancake and gives no protection, the waves become progressively smaller as you for upwind to the shore. Thus, rowing upwind toward shore is always an escape from waves.

Height and Length of a simple wave (also called a sine wave) are indicated on the left. On the real water the surface often doesn't have this simple shape, rather the surface is the combination of waves with different lengths and heights.

Motion of the water is different than the motion of the wave. Water at each location moves in a circular path, but the motions at different locations are "out of phase", which means that when water at the left of the diagram is moving to the right, water a quarter of a wavelength to the right is moving down, and water next to it is moving to the left, and next to it is moving up, etc. The overall effect is a an "apparent" wave moving to the right. Thus, the velocity (speed) of a wave is not at all the same as the velocity of the water.
The horizontal movement of the water when a wave passes is approximately equal to the up and down movement of the water. If you are on flat water and are parallel to waves made by a passing boat, your boat will move side-to-side as much as up-and--down as the wave passes under you. The side-to-side movement actually creates most of the difficulty in balancing the boat in such a situation. However, under typical conditions in the bay there can be such varied wave action that you can't easily distinguish horizontal and vertical motion