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On a day of meagre waves, surfers often say that one should wait for the tide to turn because the waves will get bigger when the tide is 鈥渙n the push鈥�. Is there any reason to believe that waves will increase in height with an incoming tide?

鈥� In brief, yes. Surf arises from swells that may have travelled thousands of kilometres. If swells travel over deeper areas, they gain speed, and thereby stretch out and lose height. Conversely, as depth decreases towards the shore, the shoaling water slows the leading zone of each swell. The water behind piles up until it eventually curls into a breaking wave. The faster the wave, the more energy it has, and the more generously it breaks.

Ebbing water moves away from the beach. This reduces the forward velocity of incoming swells relative to the beach, sapping the surf鈥檚 energy. Similarly, incoming tides increase the waves鈥� velocity towards the beach, increasing the energy.

You might object that tides ebb and flow too slowly to be noticeable, and for swift waves that is true. But for smaller, slower waves like those in the North Sea, the relative difference is greater.

Kinetic energy increases with the square of the velocity, so, in surfing, a difference of just 2 kilometres an hour in wave speed can turn stingy ankle-busters into generous rollers.

Jon Richfield, Somerset West, South Africa

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