What is a wave?

1 As a wave enters shallower water it encounters friction at the sea bed. The critical point is when the water depth is equal to half the distance between the wave crests, called the wave length. 2 The friction causes the wave length to decrease, but the height of the wave increases. 3 Gradually the wave becomes too steep and unstable, and eventually it overbalances. 4 The crest of the wave advances, breaks, … 5 … and then collapses. 6 The stirred-up water drags sand with it as it washes up onto the foreshore. 7 When its energy is exhausted, some of the water washes back down the slope, taking some of the sand back with it … 8 … while the rest seeps down onto the beach.