If you’re anything like me, you’re fascinated by water waves on Priest Lake. They wash up upon the shore, they batter our docks during storms, they bounce our canoes, they rattle the bones of kids who are tubing out on the Lake, power boats leave distinctive wakes.

So, how about waves breaking at the shore; what’s that about?

The reason involves conservation of Energy and Momentum, and the fact that the speed of water waves depends upon the depth of the water when the depth of the water is less than or equal to the wavelength of the waves.

It turns out the speed of water waves becomes slower when the water is shallow, and if the wave slows down, the only way for the wave to conserve energy and momentum is for it to get taller. When the wave gets taller, the water in the wave gets accelerated up and down more and more. At some point, the acceleration of the water becomes high enough to exceed the force of gravity --- and that’s when the wave breaks.

In a way, this is similar to traffic jams. When you’re crossing the Columbia Basin on I-90, the traffic flows freely out near Ritzville. You can count the number of cars that pass by some point on I-90 and think of that flow as “momentum of drivers.” When you reach Spokane, however, the traffic will get thicker and slow down. However, if you count the number of cars that pass per second, it might still be about the same: more cars, but slower cars.

But at some point you could encounter a traffic jam, in which the speed of cars is so slow that the number of cars getting through at Division is less than the number of cars at Maple Bridge. We all start muttering, perhaps using our sailor’s language skills … because the momentum of drivers can’t be supported by the lanes of I-90. The density of cars gets thicker, the wave of irritation gets higher …

And that’s basically what happens to water waves when they crash upon the beach. The water gets shallower, there is less room for the water to move, so the wave gets higher, and then … it breaks.


Photo PCox