Helen Czerski
Helen ponders the beauty of sand ripples at the seaside.
Summer is finally in full swing, and that means it’s time to play outdoors. I spent a recent weekend paddling outrigger canoes along a beach in Dorset, and the lovely thing about a canoe is that you can float and watch the underwater world without disturbing it. The water was beautifully clear, and I started to pay attention to the sandy ripples on the seafloor, and especially how organised they were. When you think about it, a tidy pattern of ridges made of sand is a fairly strange thing, but they’re such a familiar sight that we rarely ask the question: what are they doing there?
The ripples that I could see had their origin in the waves I was bobbing about on. Next time you see a gull parked on the ocean as waves go past, imagine drawing a line that follows the movement of the bird. It doesn’t just go up and down as the wave goes past – it sways forwards and backwards too, following a circle. The water molecules beneath it are doing the same thing, and the circles get smaller as you go downwards through the water column. As the waves travel towards the beach, the circles are all lined up pointing at the shore, like hoops standing on end. But as the sea gets shallower, there’s no room for water to move downwards, so the circles get squished into lines, and right at the seafloor the water just rushes forwards and backwards, towards the beach and then away from it. Looking down from my canoe, I could see tiny fragments of seaweed doing just that, whooshing across the sand and back.
A ripple starts when that oscillating current glides over a bit of a bump so quickly that it can’t flow smoothly over the top and so it generates a small eddy instead – a packet of rotating water just on the downstream side of the bump. The eddy picks up some sand from behind the bump, starting to excavate a trough by moving sand downstream. At the point where this sand settles out of the flow, you’ve got the start of the next ripple crest. And then during the second part of the wave cycle, the water rushes back the other way and the same thing happens on the other side, excavating another trough and building another peak with the same spacing. As the waves travel over the top, the ripples build until the sand rolling off the peaks halts the growth.
This is why the ripples were symmetrical – they were being formed by an oscillating flow that was roughly the same in both directions. The separation of the ripple crests depends on the size of the wave circles and also the sand grain size, which is why the ripples all have the same spacing in any one place. If the tide is going in or coming out quickly, there might be a stronger flow in one direction than the other, making the ripples steeper on one side than the other.
The amazing thing about sand ripples is they’re always being rebuilt. If the water movement stops, the ripples will crumble away, leaving no trace. The ripples you see are an indication of what’s happening today, right here, and tomorrow they’ll look different.
As I paddled along, I watched the waves splashing against the bow of my canoe, and imagined these mini sandsculptures being built beneath me. The ocean is never still, and the building will never stop.