Exercise 3 solution (model answer)

As a wave moves into water shallower than about half its wavelength, the seabed begins to drag on it and its speed falls. The period stays the same, because the waves keep arriving at the same rate, so the wavelength shortens: the crests bunch together and the wave grows taller. The front of the wave is in shallower water than the back, so it slows more, and the wave becomes steeper. When the wave is too steep to be stable, or its height reaches about three-quarters of the water depth, the crest topples forward and the wave breaks. On a gently sloping beach this happens gradually, so the waves tend to spill over a long distance.