The Flow of Tides

Working on a six hourly cycle, the coast of the UK is continually changing, washed by the tides that run along its shores. Each day the course of the UK coastline is being redrawn, as the falling tide reviles large of areas of a cleansed no-mans land, this only later to be reclaimed by the sea, as a turning tide will rise to recover this temporary extension to the counties surface.

Many people think the tide just goes in and out, which is where the idea of incoming and outgoing tide comes from. However, a better description is that it actually flows along the coast, as a river flows along its banks, reversing its flow twice a day and moving in the opposite direction. The correct technical terms are also different, as a rising tide is known as a Flood tide, while a falling tide is called an Ebb tide.

The phenomenon of tidal flow is controlled by the gravitational pull of the moon and the sun, thus it can be predicted for many years in advance, but these results are only theatrical. Local anomalies which include the weather, and even the rise of sea levels due to climate change and rainfall, all contribute to variations in both timing and height. A low pressure system in the Atlantic will allow the tide to rise above predicted levels, while a high pressure system will suppress even the highest Spring tide. Twice a month the sun and moon line up, so their combined gravitational force will increase the height of the tide, which is called a Spring tide. On the other alternative weeks their gravitational pull will oppose one another and therefore the tidal range is at its lowest; these lower tides are called Neap tides. Tide times are produced for major ports around the coast, which are called standard ports. The UK standard port is Dover and other tide times around the coast are based on high and low water here, expressed in times either before or after Dover. Times for other major ports, such as Liverpool and Avonmouth, are also published to assist in local planning.

A good place to watch tidal flow is on a headland, where its effect can clearly be seen as it accelerates around its corner. Good examples are Flamborough Head on the east coast or Bardsey Sound in North Wales, where the tidal flow is forced between Bardsey Island and the mainland. There are also several places around the UK coastline where the local tides are particularly interesting, such as the Menai Straits or the South Pembrokeshire coast. If you stand on the Menai Bridge and look down you’ll see the water racing through as a river does in spate but if you can be there at slack water, (around 2 hours before high water at Liverpool), you’ll be able to see the flow swirl round and reverse its direction in the space of 10 minutes! Here the water never really stands still. On the South Pembrokeshire coast you can watch the direction of the flow change from east to west, while its height will still continue to rise. This is because of the coastline’s proximity to the Severn Estuary, where the tidal range is the second highest in the world, at 15 metres. For most places the tidal flow runs for approximately 6 hours in one direction and then 6 hours in the other direction. However there are places, such as the NE coast of Anglesey, where it runs in a northerly direction for 9 hours and southerly for only 3 hours. This is caused by the shape of the nearby headland, Point Lynas. For shipping and especially small boats the timing of high water is less important, than the time of change in its direction of flow.

The 1996 shipping disaster of the oil tanker Sea Empress at the entrance to Milford Haven, can be partly contributed to a strong incoming tidal flow that pulled the ship on to rocks. For smaller boats their progress is totally controlled by the tidal flow and the weather. If the wind direction is against the tidal flow then the sea will become rough with short, curly breaking waves, whereas if the wind and tide direction coincide then the sea state will be flatten. Boat users will talk about wind over tide conditions, where the sea can be locally very rough. Another condition can be a tidal race, where changes in the sea bed or obstructions can cause the sea state to build and become confused. The locations of these, have been well know to seafarers for many generations and an example can be seen at South Stack on Anglesey. Another extreme example of the force of tidal flow can be seen at the Corryvrekan on the west coast of Scotland, where the tidal flow can reach 9 knots and innumerable variations of the seabed cause massive waves and whirlpools.

As an island nation with a tradition of seafaring, the ebb and flow of tides have played and will continue to play a central part of all our lives. Often misunderstood, their awesome force should never be underrated but this power has yet to be fully harnessed. Although small scale installations in the UK have shown its potential as a source of energy, the larger scale production that is required, if we are to reduce our dependency on carbon fuels, has yet to materialise.