Whenever the tide turns, water heads from one area to another. This sets up a flow known as tidal currents.
Tidal currents moving over a rough seabed make the surface of the sea turbulent. This effect is known as ‘overfalls’.
A shallow sea, fast currents and more dramatic submarine landscape… all make the water rougher.
Overfalls are most common near the coast and nautical charts usually mark these areas with squiggly lines to warn inshore sailors of the hazard.
Many overfalls are particularly dangerous in certain conditions, eg. when the tide is flowing in one direction.
Sensible sailors avoid these areas wherever possible. In my book, How to Read Water, I write about heading deliberately into an area of overfalls very like the ones you can see in these photos. That section of the book is titled, ‘Overfalls and Foolishness’. We live and learn!
You can practice noticing how the depth and speed of water affect the overfalls by watching a river or stream flowing over gravel. Where the water is deep and slow, the surface is calm. But where the water is quick and shallow, the surface becomes agitated.
The picture below was taken at the same time as those above, but facing in the opposite direction. Notice how the sea that day was not especially rough. The waves caused by wind – the normal kind – are not very large. It’s the tidal current moving over the ridges, boulders and caves of the rough seabed off Old Harry’s Rocks that is causing the overfalls – not the wind. The moon and sun take more responsibility for these waves than the wind.