Swash is the name given to the waves that rush up the beach after a wave has broken. They are intriguing little waves that inhabit a world of their own.
Most of the waves we see in the sea are known as ‘waves of oscillation’. The water moves up and down in an orbital way, as energy moves from one place to another. But the water itself doesn’t go very far at all and ends up roughly where it started. It is only the energy that travels.
But when waves break on the shore they create a new type of wave, known as a ‘wave of translation’. These waves are totally different, because it is the water itself that is moving from one place to another. These are the waves that rush up and wash foam over our feet. On the beach, these are the waves that are commonly know as ‘swash‘.
The waves get shallower as they run up the shore. Which means that the second wave is in deeper water than the first and the third in deeper water still. Because waves travel faster in deeper water, these waves are always trying to overtake each other, leading to a rather beautiful, never-ending race. As these hungry birds discovered.
To learn more about how waves behave, see the book How to Read Water.