At a number of spots around the world, not least in the southwest stretches of the UK, you might hear people talking about the surf picking up “on the push of the tide”.
This isn’t an illusion; hundreds of people see it happening at some spots every time there is a swell. It doesn’t seem to matter if the underlying swell is getting bigger, smaller or staying the same size; the incoming tide always seems to give it that extra boost.
But can the incoming tide actually push the wave heights up, like the phrase suggests? Or is there some other factor – perhaps not directly related to the tide itself – that makes the waves bigger when the water is deeper or shallower?
The reasons why it happens probably vary from place to place. At Perranporth on the north coast of Cornwall, according to a unique study carried out a few years ago at the University of Plymouth, it is most likely related to the tidal currents – strong local surface currents driven by the tide, which vary in strength and direction throughout the tidal cycle.
First, they checked to see if wave heights really do increase as the tide comes in at Perranporth. They looked at seven years of buoy data (over 5,000 tides) and found a significant increase in wave height around one hour before high tide. Next, they tested several likely hypotheses as to why the waves might get bigger just before high tide. With the help of mathematical modelling, they concluded that the most likely candidate was the interaction between the swell and the tidal currents.
The modelling was based on the assumption that tidal currents remove energy from the waves. The energy removal is most effective when the current and swell are travelling in opposite directions, and least effective when they are both travelling in the same direction.
Therefore, over a complete tidal cycle, the maximum wave heights should occur when the current and swell are in the same direction which, according to the model, occurred during the incoming tide. This would result in a maximum wave height about an hour before high tide – nicely coinciding with the observations.
At other spots, the shape of the sea floor (the bathymetry), and how its effect on the swell changes throughout the tidal cycle, could also be an important factor. For example, there might be a shallow area just offshore and to one side of the beach in question.
At high tide, the water might be deep enough for the shoal to have no effect on the waves, leaving them to propagate straight in to the beach. But when the tide goes out, and the water is shallower, the shoal might re-direct the swell to one side, causing wave heights on the beach to be much smaller.
Another effect could be due to an isolated shallow area between low and high tide (inter-tidal zone), particularly at places with extremely large tidal ranges such as the Bristol Channel between England and Wales. At low tide, the shoal would be totally dry and the waves would break on the shoreline further out. But when the tide comes in and the shoal is underwater, it could act like a lens, focusing the waves and making them bigger at the shore, which is now much further in.
Cover image of Fistral by Lola Manzanares Gurucharri.