Wave base, moving sand - and the first underwater feature film on TtS

In addition to my hyper-sophisticated new camera, knowing that there would be snorkelling on our recent trip, I also treated myself to a very modestly priced little camera that is good to 3 meters underwater – and wow, am I glad that I did, it’s amazing. Having overcome the initial sense of horror at jumping into the sea with a camera, I managed to record, albeit amateurishly, in still images and on film, much of the spectacularly beautiful and fascinating world that even a simple snorkel allows you to witness.

Once the ungainly (and abrasive) process of donning flippers in the beach swash zone of the waves is complete, progress seaward reveals the zone of wave-activated sand before depths acceptable for the coral gardens are reached. The variety of types of coral is simply astonishing, never mind the fish, for which the hackneyed word “teeming” is the only appropriate one that comes to mind.

But, of course, the areno-nerd also found that barren sandy zone of some considerable interest. From where the waves break on the beach out to a water depth of around a couple meters, the action is too violent for the corals – but the action is played out by the sand. Ralph Bagnold, having finished all he could do with desert sand (lacking the necessary comprehensive long-term wind data), turned his attention to sand transport in water, and described how he spent many hours simply floating around in shallow waters observing the movement of sand; I understand his fascination. For, even on a calm day, with very modest waves, the sandy bottom is a stage for tempests of grains and what for all the world look like miniature aqueous sandstorms, and yours truly spent some time attempting to document the action.

The bedforms are diverse – large scale ripples and small-scale dunes, one of these looking exactly like a star dune in the desert but on a rather smaller scale:

The action is directed and produced by the waves. The corals stay safely below normal wave base, but here everything is in motion. The movement of water as a wave passes takes on the form of a series of circular orbital paths, their diameter decreasing exponentially with depth, as shown at 2 in this illustration (thanks to the Intro Geology Course at Fort Hays State University):

The scale of water movement is related non-linearly to the wavelength, and essentially that movement becomes trivial at depths greater than half the wavelength – the depth known as the wave base, below which virtually no sediment is moved and corals are happy. But something else goes on as a wave approaches the shallowing sea floor on its way to the beach – the water column becomes squashed, compressed by the lack of room to move, with the result that the orbits become elliptical and the wave crests sharper and higher (3 in the illustration above). This is, of course, particularly dramatic and catastrophic in the event that the water movement is that of a tsunami – but then that is also no ordinary wave. So, off the beach, above wave base, these elliptical orbits energetically pick up sand – and, as the wave passes, move it back and forth, incessantly, and, I found, compellingly. Shape-shifting again, the ripples and mini-dunes constantly destroyed and reformed.

Here are a couple of stills from the movie (I de-saturated them and modified the contrast, since underwater colour is difficult to reproduce accurately), and then – the movie.

This is not a storm, the waves are not exactly pounding (the photo below shows the beach – and yes, the colour is accurate - more of that in a later post); but just look at the scale of sand movement on a normal day, and imagine what this would look like during a hurricane.

But I must end by returning to below wave base and include just a few of the denizens of these wondrous underwater landscapes.