time and tide

Ever noticed how the current around a headland can be rushing like a steam train some times you watch and other times can be slack and even going in the opposite direction? It all depends on the tide, of which there are two cycles: the twice daily high and low tides and the monthly neap and spring tides. At bang on high tide and low tide, as the tide is changing, the water becomes slack. At low tide this slack water period extends for longer because the volume of water in contact with the land is less.

The importance is this: when the tide rushes in or out, as it moves past headlands and estuaries, its direction changes from a 'towards-land-away-from-land' to a rushing current moving parallel to the coast- a tidal stream. Some of these can be ferocious- around Portland Bill or in the Menai Straits for example.

Every month the height of high tide rises and falls. There is an average height of high tide which takes up most days; this slowly lowers to what is called a neap tide and then rises even higher to what is called a spring tide. (A spring tide happens every month not just in spring). So if the average height of high tide is 4 metres the neap tide could be 3 metres and the spring tide 5 metres. Of course tidal range varies depending on where you live- but everywhere experiences this difference between neap and spring tides.  But the difference in speed depends on whether it is high tide or low tide; and on whether it is nearer a spring or neap tide... And when there is a neap tide the speed of the current will be roughly half that of a spring tide. So low water, neap tide, will be the time when the current around any headland or estuary will be least. Spring tide, with the tide running between high and low will be when it is at its fastest- in some places over ten knots.