For the student of English, two of these words will probably be familiar. Spring could mean that part of the year when nature wakes up again, or a coil of wire which stores kinetic energy. Flowing is a word describing smooth liquid movement, but can also be used to describe any process which moves, or should move, in this way (i.e. work flow, traffic flow etc).

However, put together, these four words are used to describe tides along the coast. Britain is very much a maritime island and tidal behaviour is something we need to think about when building a house or planning land use near the sea, or simply taking a walk under some cliffs.

Careful planning is needed when building close to water.

Careful planning is needed when building close to water. These swans had not read the flood-risk warnings.

Spring tides happen when the sun and moon line up together and have the greatest gravitational pull on the sea. The result is the highest and lowest water levels along coasts and in estuaries, where rivers meet the sea and where tides can travel far inland along the river. High tides can be made even higher by low air pressure (about 1 cm per millibar of pressure), and by strong winds from a certain direction. The result is often then floods. Many communities just tolerate flooding, as the cost of protecting against it can be higher than the benefit.

Flooded garden in Totnes - photo copyright Malcolm Pemberton

Flooded garden in Totnes – photo copyright Malcolm Pemberton

Neap tides happen when the sun and moon are at right angles to each other, viewed from the earth. These tides have the least difference between high and low water levels.

If the earth had no land masses to stop these tidal flows, tides would be a fairly simple one-metre high lump of water travelling around the planet, with a one-metre deep trough following, about six hours later. However, the English Channel is a fine example of how things change when land gets in the way. This lump of water travels west to east across the Atlantic (actually, of course, this is caused by the spinning of the planet in the opposite direction), and then Europe and Africa get in the way.

The result of this is a massive build up of water in the Bay of Biscay (west of France and north of Spain), and a funnelling of water up the English Channel, causing water level changes of over five metres along the coast, not to mention a flow of water travelling up to 5 knots (five nautical miles per hour) eastward up the Channel for just over six hours, and then back westward as the tide ebbs. As you see, tides flow and ebb, rising and falling and travelling up and down channels and estuaries.

As tides depend on the spinning of the earth plus the positions of the sun and moon, they are completely predictable and the working lives of many people are dictated by them. Sailors must wait for there to be sufficient water to enter or leave a harbour, and sailing boats need also to wait for the tidal current to be going their way. Obviously, if your sailing boat can only travel at 5 knots with the available wind and you are sailing against a five-knot current, you will have an unchanging view of the coast!