If your local beachbreak works at low tide, almost every time you walk from your car to the surf you will come cross areas where the sand is all rippled, interspersed with areas where the sand is all smooth. These undulations are called sandwaves, and I spent years wondering where they came from.
As it turns out, sandwaves have been studied extensively by coastal scientists. They have been categorised and re-categorised into a bewildering number of different shapes and sizes and wavelengths. The mechanisms responsible for them are very similar to those responsible for water waves and sand dunes.
Debunking Surf Myths: HERE
Sandwaves exist in the intertidal zone, where they are formed by the flow of water near the bed, typically in currents or under unbroken waves. Their shape and size, and whether they are present in the first place, are intimately linked with the near-bed water flows.
They come in various sizes, from less than 10cm to more than 2m high, and can be symmetrical or skewed, just like a water wave. You will often find ones superimposed on top of big ones, like a short-period windsea superimposed on a long-period swell. The two might be orientated in different directions as well as having different sizes and wavelengths.
How exactly are sandwaves formed? As I said, the story is very similar whether you are talking about water waves, sandwaves or sand dunes. In all three cases you have a less-dense fluid transferring energy to an underlying denser fluid: air-to-water, water-to-sand or air-to-sand respectively. As usual, it’s complicated, so I’ll try to be brief.
The interaction between the surface and the overlying flow is the starting point for the growth of the sandwave
The interaction between the surface and the overlying flow is the starting point for the growth of the sandwave. Even before the wave begins to grow, the surface is never exactly flat and the flow is never exactly horizontal. The surface will inevitably contain small bumps and the flow will contain vertical perturbations. If the size of the bumps and the magnitude of the flow perturbations coincide in a certain way they will begin to enhance each other, or resonate. The bumps make the flow perturbations bigger and the perturbations make the bumps bigger – a classic vicious circle or positive-feedback loop.
Most of the time, the system will go out of sync fairly quickly; the bumps will stay small and the surface will remain fairly flat. But sometimes the bumps will become large enough for another, more efficient process to take over. On the upstream side of the bump, the moving fluid accelerates as it is forced upwards towards the crest.
The faster fluid flowing over the top of the loose sediment pushes the sediment up the slope against the force of gravity, which makes the bump grow bigger and bigger until it forms a legitimate sandwave. The stronger the flow, the bigger it grows.
At the same time as the flow is pushing the grains up the upstream slope, the flow at the crest is forcing the grains over onto the other side. In contrast to the upstream side where the flow is ‘squashed up’ and therefore forced to accelerate, on the lee side the flow is ‘stretched out’ and is therefore forced to decelerate.
This means that any grains pushed over the top will settle out on the lee side. An area of circulating flow called an eddy also sometimes forms in the trough, which keeps the grains in a confined area.
The overall result is a migration of the sandwave in the direction of the prevailing flow. The speed at which they migrate seems to be inversely related to their size, with very small sandwaves migrating at up to two metres per day but sandwaves of more than one metre in height never migrating at more than 10 cm per day. Most sandwaves tend to slowly migrate in the direction of the net flow.
However, some types, due to an effect related to the flow speed and water depth, migrate in the opposite direction – something you don’t find with water waves.
So do sandwaves serve some purpose in the grand scheme of things? Of course they do. Just like every natural feature on the planet, they evolved to fit in somewhere. Sandwaves exist at the interface between two fluids where energy is constantly being transferred. ‘Roughness elements’, such as sandwaves, increase the friction and hence make that transfer more efficient. Therefore, the size, shape and frequency of sandwaves help to keep the energy transfer at the right level.
There are many ways in which this is important on large scales. For example, the right amount of energy absorption by the sediments under the breaking waves keeps the coastline in equilibrium. Sediment supply to the coastline is regulated by the up-river sediment pick-up, and this, in turn, depends on the roughness of the river bed.
As a final note, there is now evidence that the sandwave’s closest cousin, the coastal dune, needs to be kept mobile to perform its natural function of coastal protection. Which is good news to many of us who have felt guilty all our surfing lives for ignoring those ‘keep off the dunes’ signs. At some beaches, the ‘dune restoration’ projects are evidently not working. Some dunes are more healthy if they are kept mobile, by not only exposing them to energy input from the wind, but also by allowing people to walk all over them.