"Troubles," said William Shakespeare, "never as single spies, but in battalions come." So it is when astronomical factors combine to cause the exceptionally high tides that sometimes cause flooding in vulnerable coastal areas. Many of these factors have been present over recent days.
The main tidal influence is the moon. The lunar gravitation produces two "bulges" on our oceans - one almost directly under the moon and the other on the opposite side of the world. Halfway between these two mounds of water, on either side of the earth, lie two regions where the sea is a good deal shallower than it otherwise ought to be. As the earth completes its single revolution beneath the moon every 24 hours or so, the zones of high and low water retain their position with respect to the moon - so two high tides and two low tides should occur at each point on the earth's surface every day.
But the sun complicates the issue. It too causes tides, slightly less than half the size of the lunar tide in magnitude. Sometimes the two tidal effects work against each other, and at other times they co-operate - notably at the new and full moons, when the earth, the moon and the sun are all more or less directly in line with each other.
On the latter occasions, high tides are significantly higher than normal - a so-called spring tide. Low tides are very much lower.
Even higher waters occur about every 193 days when, as happened yesterday, a spring tide coincides with the moon at its nearest point in orbit to the earth - the perigee.
The tides are the pulse of the world's oceans. They can be thought of as mobile waves with a very long wavelength, a wavelength of half the circumference of the world, in fact. The tides are most obvious at the ocean rim, where their rhythmic ebb and flow brings about that periodic immersion and exposure of the inter-tidal zone which has such profound effects on the indigenous ecosystems.
Like all waves, tides are affected in shallow water by their interaction with the sea floor. In the middle of the oceans the rise and fall of water may be a mere foot or two, but near the coast the "waves" increase in height, as breakers do when they approach the shore. By this means the tidal rise and fall is amplified to an extent depending on the topography of the underwater coastal zone, and produces the large tidal ranges associated with certain bays and estuaries.