How the tides affect breaking waves

In a previous post, I explained how breaking waves work and how wind interacts with waves at the beach. But what happens to waves as the tide rises and falls?

A quick recap on tides: the reason the water level changes systematically around the world is primarily because of the gravitational pull of the Moon on one side of the Earth, and the centrifugal force of the Earth-Moon rotation on the other side of the Earth. A secondary bulge about half as big occurs for the same reasons due to the Sun. These bulges of water migrate across the planet as the Earth turns, causing rising and falling tides. Spring tides occur when the influence on the ocean from the Moon and Sun are in sync, causing extra-high high tides and low low tides - neap tides, when the difference between high and low tide is more mellow, occur when the Moon and Sun bulges are out of sync.

Tides cause the water level to change baseline elevation at a given beach (and waves are superimposed on this); most beaches experience 2 high and 2 low tides a day. The amplitude (difference in height between high and low tide) varies through the month with the phases of the Moon, and also varies dramatically around the world. The Bay of Fundy in Canada has the largest tides, while the Mediterranean Sea has very small tides.

So how do tides affect the way that waves break? In surfing, there is a popular concept called the "tide push" or "tidal push." The idea is that an incoming tide is pushing more water towards the beach, so this energy should combine with the wave energy to make larger waves. This term is thrown around a lot by surfers, usually to explain why they are not in the water when the waves are very small - they are ostensibly waiting for these better conditions. But is this a real concept?

Researchers Davidson, O'Hare and George from the University of Plymouth decided to find out. Using data from offshore buoys in a location with a large tidal range (7.5 m), they found that indeed, wave energy was higher during incoming tides, with a peak in wave energy just over an hour before high tide. It's pretty cool that surfers figured this out long ago just by observing with their eyes and bodies.

How else can tidal changes affect surfing waves? In locations with very abrupt changes in bottom topography, waves can completely disappear during certain tide heights. Because waves break in water that is approximately 1.25 times as deep as the waves are tall (for example, an 8 foot wave will break in 10 feet of water), if bottom topography consists of a flat shelf of rock that abuts a cliff, waves will only break during the few minutes each day that the water depth is equivalent to 1.25 times the height of incident waves. On higher tides, the waves would just slam into the cliff without breaking, and on lower tides they might break right onto the edge of the rock shelf as a slab wave. On even lower tides, the reef might be fully exposed to the air with no water above it, and waves will just hit the edge of the rock without really breaking.

This is an extreme example - in most places the bottom topography slopes towards the shore, so waves will break regardless of the tide - though the location along the slope will vary based on wave and tide height.

Offshore, tidal changes, and the currents they induce, can affect waves in other ways. For instance, tidal flow moving in the opposite direction to waves might cause waves to pile up on one another and increase in size, which is often seen in strong current systems like the Agulhas. Strong tidal currents can also create exciting boils and whirlpools.

Large incoming tides can also even create surfables waves in river systems and bays, called tidal bores! The Amazon, Severn in the UK, and Qiatang in China are just a few rivers around the world with rideable tidal bores.

Have you been to a shoreline with extreme waves and tides?

References

Davidson, M. A., O'Hare, T. J., & George, K. J. (2008). Tidal modulation of incident wave heights: Fact or Fiction? Journal of Coastal Research, 24(sp2), 151-159.

Scarfe, B. E., Elwany, M. H. S., Mead, S. T., & Black, K. P. (2003). The science of surfing waves and surfing breaks - A review. Scripps Institution of Oceanography.