Waves in the ocean have the ability to reinforce other waves. First the wind starts blowing, producing small waves. Some of these small waves combine with other small waves to form a single, larger wave. This is how you get a choppy sea.
However, anyone who loves the beach can tell you about the distinctive sound of the ocean hitting the sand: Woooosh. That woosh is telling us something. It's telling us that long waves are the ones hitting the beach, not small and choppy ones!
How is it telling us that?
There's an interesting culling process that takes place out in the ocean where waves are being formed. Small waves combine together to form larger waves, but depending on how long the waves are, there's also the possibility of forming a whitecap.
Whitecaps are small waves that grow too tall to support themselves on their short bases. It's kind of like piling up dry sand on the beach. You can make a sand hill only so high before it collapses. If the base is very wide, though, as is the case with sand dunes, collapse is less likely.
Waves that grow too high for their bases collapse quickly and lose their energy. This is what's happening when you see whitish foam being tossed around out on the ocean.
The longer waves, however, can support themselves better and don't collapse. Not only that, longer waves move faster than shorter ones. They get away quickly from the windy region where they were first formed, and roll on smoothly toward the beach.