AWARENESS OF HIDDEN DANGERS
As the waters ebb to reveal stretches of golden sand, and inviting coves become accessible, it is easy to forget about the threats that tides can pose. “Tidal cut-off is one of the biggest dangers on the coast and a major cause of rescue call-outs,” says Mike Hannam, of the RNLI. “People are often unaware that they are in potential danger and, as such, are not prepared. You should always check tide tables and carry a means of calling for help.”
The most common problems are island causeways and sandbanks, which can quickly be flooded, and isolated bays that are cut off by an incoming tide. In some places, the height of the tide and local topography combine to create particularly dangerous conditions.
“Morecambe Bay has a tidal range of up to 10.5m, and its shifting, very soft sand is exposed for a vast distance,” says Mike. “When the tide comes in, it covers the sand faster than you can walk. A similar problem is experienced at Southend-on-Sea. We have to rescue people by hovercraft, as the sands won’t support trucks, and the water is too shallow for boats.”
Rips, often misleadingly referred to as rip tides, are actually fast currents created when water is pushed from the shore out to sea; often exacerbated by high surf conditions, when waves along the shore are higher than normal. “Rips are found around river mouths, estuaries, exposed beaches and manmade structures, such as piers and groynes,” explains Mike. “They are very dangerous because they can take you from the shallows very quickly and leave you out of your depth.
“People should never try to swim against a rip current: they will quickly become exhausted. It’s best to stand and wade, if possible, or swim parallel to the beach until you are free of the current. The best way to avoid them is to swim between the red and yellow flags on a lifeguarded beach.”
Perhaps the most dramatic of dangerous waters occurs during storm surges, when the sea can breach harbour walls and other defences, sometimes causing widespread flooding.
“During a storm, severe winds can push water onshore,” says Mike. “The severity of the surge will depend on the size and speed of the storm, its orientation to the coastline and local water depth, driven by tidal conditions.”
Between 31 January and 1 February 1953, a storm surge at high spring tide on the North Sea caused devastation down the east coast of Britain, as well as in Belgium and the Netherlands. More than 2,000 people were killed in a night that saw ships overwhelmed by the sea and 6ft (1.8m) waves cascading through coastal towns.
In the UK, 990 miles of coastline were damaged, 160,000 acres of farmland were rendered unusable and more than 25,000 people lost their homes. It remains the UK’s worst natural disaster and led to the creation of today’s storm surge warning service and defences, such as the Thames Barrier.
small seas; for example, the Baltic or Mediterranean, may have little or no tide.
The topography of a coast, whether convoluted with coves and caves or smoothed by shallow beaches, as well as the shape of the seabed and depth of water, will affect the behaviour of the tide. At Avonmouth in Bristol, the funnelled coastline means its exceptionally high tide also comes in quickly as the water is squeezed up a narrow gap; a phenomenon that gives rise to the famous Severn Bore. Tidal waves can be deflected by land, so that even places close together may experience different tides. Cromer and Aldeburgh, for example, nestled approximately 70 miles from one another on the curve of the Norfolk coast, experience high tide several hours apart.
While most places have two tides a day, this is not always the case. “Some places have what is known as double high water; in effect, two high tides, rather than a high and then a low tide,” says Kevin. “This is prevalent around Bournemouth and Southampton, while Weymouth has a double low water. These complex tides occur because local tidal currents and the geometry of the seabed interact with the tidal wave to generate new, higher-frequency waves.”
Tides can also be affected by the weather. Low atmospheric pressure allows the water to rise higher, and high pressure conversely dampens the tide. Strong winds can also push water towards or away from the shoreline.
Predicting risk
The times and heights of tides at more than 700 locations around the coast of the UK are available as tide tables: an indispensable navigational tool for mariners and a potential lifesaver for walkers, surfers and anyone enjoying the many attractions of the sea.
What is thought to be Europe’s first-ever tide table was compiled by the Benedictine monk, John of Wallingford, who died in 1258. His tables predicted the ‘flod at London brigge’, three hours later here than at the mouth of the river and occurring 48 minutes later each day.
“The high tide!” King Alfred cried. “The high tide and the turn! As a tide turns on the tall grey seas, See how they waver in the trees, How stray their spears, how knock their knees, How wild their watchfires burn!”
G K Chesterton, ‘The Ballad of the White Horse’
“Because tides are driven by astronomical forces, they are predictable,” says Kevin. “With a few years’ tidal measurements from any location, we can reconstruct its tides for the next 100 years. We can also work backwards to find out about tides that occurred in the past. We know that the terrible flooding in the South West in 1607 was not caused by a tsunami, as had been suggested, but by high water combined with high winds.
“Tides were initially measured by floats placed in brick tubes, called stilling wells, with machinery that traced the tidal curve onto a rotating cylinder of paper. Big strides in prediction were made after the 1860s, when Lord Kelvin promoted the harmonic analysis of water movements, and the first tide predicting machine was used at Bidston Observatory in the Wirral in 1924. Today, you can get an app on your phone that does much the same.”
But rising sea levels, due to climate change, may make very long-term tide prediction more difficult, as the depth of water and shape of the seabed change. “Any significant change in the average water level is enough to change the tides,” says Kevin. “Building flood defences, such as barriers or other coastal developments, may also have unintended effects, as they can alter the shape of the sea basin; itself affecting water levels.” The jetty at St Annes was abandoned when improvements to the docks at Preston resulted in silting up around St Annes, and steamers could no longer operate.
On a commercial level, knowledge of tides is ever more important as marine commerce expands, and it has been estimated that the UK holds 50 per cent of Europe’s potential for tidal power: a form of hydropower that converts the energy from tides into electricity.
“Tides are one of the most fascinating and important topics in marine science,” says Kevin. “Understanding them is vital for coastal safety, allows us to comprehend a changing coastline and provides the knowledge that we can use for generating clean energy.”
But even as we learn more about the tide, for many, standing on the beach watching the mesmeric power of the sea as it ebbs and flows, shows nature at its most awe-inspiring and, still, mysterious.