Sand ripples

On a clear day, off of Makapu'u beach in Waimanalo, you can easily see the ripples in the sand.  I've often wondered about the physics of those ripples - so evenly spaced, and always perpendicular to the direction of the waves.  Coastal engineers think about them, too.  Understanding ripples is important for understanding how much friction, or bottom roughness there is on a beach.  The physics of the ripples gives tell-tale signs about the prevailing currents and waves, and you can tell, to a rough degree, whether or not the sand is being taken from the beach or added to the beach.

There are two different types of ripples - one that is most often seen in the lab, called 'orbital' ripples, and another that is common on real beaches, called 'anorbital' ripples.  These ripples get their names from the fact that in orbital ripples, there is a tiny eddy that forms behind the peak of the ripple that determines the shape and height of the ripple. An anorbital ripple - which is the kind you have likely seen - does not have this feature.  The height of its ripple is determined solely by the grain size - how big the sand is.

Of these sand patches, ripples, can move - and are moving.  This is called 'bedform dynamics'.

Some fun facts: 1)  geologists can use ripples to see what the ocean or bay was likely millions of years ago - since the ripples sometimes become ingrained as sedimentary rocks.   2) Einstein's son became interested in this problem and, of course, tried to solve the problem of bedform motion from a statistical perspective.  Einstein supposedly said it was too hard a problem, and he should do something else.