Tides and the Bristol Channel
Tidal range in the Bristol Channel is unusually large and one of the largest on Earth. The tide is amplified by narrowing of the channel and in spring tides tidal range can exceed 14 metres in some places. Tidal currents sometimes exceed 10 kilometres per hour. The large tidal range provides an extensive and important habitat in the inter-tidal areas as well as providing an everchanging spectacular coastal landscape. The low lands along the North Somerset coast are, however, vulnerable to flooding and the occurrence of large floods may increase as a consequence of climate change and sea level rise.
Causes of Tides
Tides are caused by the gravitational attraction of the moon and the sun acting on the oceans of the rotating earth. The oceans are pulled towards the moon and sun resulting in the sea moving away from the coast in some places and towards the coast in other places. In a ‘flooding’ tide water flows into a region reaching a maximum called High Water. In an ebbing tide water flows out to a minimum called Low Water. Most locations around the UK experience two high tides and two low tides each day (12 hours 25 minutes between High Waters).
The relative motions of the Earth, Moon and Sun cause the tides to vary in numerous tidal cycles. The spring-neap cycle is a 14.8 day cycle resulting from the gravitational influence of the sun and moon either reinforcing each other (spring tides) or partially cancelling each other (neap tides). The equinoctial cycle is a half yearly cycle caused by the tilt of the earth, and its orbit around the Sun, which leads to larger spring tides around the time of the equinoxes (March and September) and smaller spring tides in June and December. A fuller explanation of how relative motions of the sun, moon and earth work can be found here.
In Spring tides the moon and sun are aligned and pull the ocean away from the Earth while in Neap Tides the moon and sun pull in different directions. This is shown in the yellow and black diagram which accompanies this text.
A typical chart of water-level at a single location showing the two kinds of cycle: the near twice daily cycle (12 hours and 25 minutes) and the near fortnightly cycles dictated by the lunar cycle involving the relative positions of the sun and moon. Spring tides occur at full and new moon.
High tide at Clevedon Pier: a good time for fishing from the Pier and swimming in the sea at Clevedon Beach next to the Pier. The tidal currents are small at high tide when movement of water flows change from up to down the Bristol Channel.
Low tide at Clevedon Pier exposes the concrete bases to the support arches, sea-weed covered rocks near shore and tidal muds. This is a good time to explore rock pools for marine life, study the inter-tidal zones where life flourishes and search for fossils in the rocks and pebbles.
The tidal range varies from place to place around the Bristol Channel. The mean spring tidal range of the Bristol Channel is 12.2 metres. In general the tidal range increases up the Channel from west to east as it narrows. The detailed shape of the coast can make the tidal range slightly higher in enclosed bays. The record tide in the Bristol Channel was recorded at Avonmouth.
The status of the Bristol Channel in the world standings of tidal range is controversial. All agree that the largest range is in the Bay of Fundy, Novia Scotia in Canada. Some sources place the Bristol Channel as second, but others claim Leaf Lake in Ungava Bay in Canada as second but very close to the Bristol Channel. The ranking depends where the measurements are made, since in both places the range varies around the coast.
The Importance of tides
Tides transport energy, moving heat, biology, chemicals, and pollution in our seas. With changing coastline shape, structures (e.g. sea defences) and rising sea-level due to climate change, we want to understand how the tide will change and how this will impact coastal communities and habitats. Floods and storms are major hazards around the Bristol Channel and may be an increasing threat due to climate change and associated sea level rise. Tides are a large potential source of energy, but engineering structures to extract tidal energy can affect the environment, changing habitats and water distribution within the channel.
Measuring tides is not easy everywhere but is crucial. Tide gauges can be placed on structures within the water and measure the sea level precisely by sending sound waves down a tube and recording the reflection. Observations of sea level can be made from satellites which potentially have complete coverage and offer continuous records. Tide gauges can be used to calibrate the satellite measurements.
Diagram illustrating operation and capability of the SWOT satellite launched by NASA to measure ocean and surface water topography across the Earth. Operating 891 km above the Earth the instruments can measure sea surfaces with an accuracy only a few millimetres.
Floods and Storms
Flooding affects people, business and the environment through pollution and physical damage. Over 20 million people in the UK live in low lying areas, vulnerable to sea-level rise and compound flooding events. The 92 estuaries in the UK are home to over 30% of the UK population, host the most diverse natural habitats with significant natural capital, and are also economic corridors that transferred in 2022 over 1 billion tonnes of cargo, with annual trade worth well over £5bn.
Several major coastal floods have affected the low-lying regions adjacent to the Bristol Channel in the last few centuries. Although the flood of 26th January 1607 has received the most attention there have been several major floods since then, with those on 19th October 1883, 14th December 1981 and 26th February 1991 being well documented. Flood risk in North Somerset has increased significantly in the last few decades due to the rapidly increasing population with many new building developments well within the high hazard zones from past floods and in modelling of future floods. Coastal floods happen by a combination of a high tide and a major storm which generates a large wave of water (storm surge) pushed up the Bristol Channel by high winds.
Some have claimed that huge flood of 1607 was due to a tsunami. However, the most recent research suggests that it can be explained by a large storm surge coinciding with a spring tide. There are detailed accounts around Clevedon of the great flood of 19th October 1883. Water poured over the sea defenses at Saltfields flooding Church Lane, while breaches in the sea wall flooded meadows for miles to the south of Clevedon according to reports in the Bristol Mercury.
Wood engraving of 1607 flood in the Bristol Channel
Clevedon beachfront on 26th February 1990. High tide and large storm form dangerous waves and coastal flooding.
Damage and debris along the marine front from the 14th December 1981 storm. Flying debris and huge waves threaten life and property.
Coastal flooding on the Salt House Fields from 14th December 1981 at Clevedon seafront.
Flooding around Kingston Seymour from 14th December 1981.
Climate change, sea level and risk mitigation
Observations indicate that sea level in the Bristol Channel is increasing by about 2.4 mm per year over the last couple of decades. This does not sound very much but sea level is projected to increase by about 30 cm by 2050. However, there is no evidence that extreme high-water tides have been increasing, which is not fully understood but reflects the complexities in the weather systems interacting with tides. Flood risk is increasing with population growth and housing developments in the areas prone to flooding. The North Somerset coast is only partly protected by sea wall defenses. Efforts to manage this risk can be found at the Severn Estuary Shoreline Management Plan and in the Severn Estuary Flood Risk Management Strategy of the Environment Agency.
Tidal energy could provide a low-carbon future
The Severn Estuary and Bristol Channel offers great potential to provide the UK with clean, low carbon energy. It is estimated that 8 to 13 GW of power could be harvested from the tide of the Bristol Channel, which could be up to 20% of the UK’s electricity needs. Although marine renewable energies are at the forefront of engineering and research skills, this is nothing new and plans were drawn up in the 19th Century to dam the Bristol Channel to create a harbour.
Did you know? The tidal currents of the Bristol Channel have the equivalent power to a hurricane force wind. Imagine having to cope with a hurricane four times a day!
There are at least three concepts for harvesting tidal power. One is to build a large barrage across the Severn estuary in which water is discharged through turbines. In a second variation tidal lagoons are formed by building barriers across the bays along the Bristol Channel (e.g. Swansea Bay) to form ponds of water at high tide which are discharged through turbines during low tide. A third concept is to place arrays of underwater turbines in the channel which generate energy from tidal currents. The turbines can be placed so that they are exposed and can be maintained at low tide.
Historical and modern plans have not yet been advanced due to cost and impact concerns. Soaring costs of energy and the drive to reduce dependence on fossil fuels may solve the former concern soon. To minimise or avoid adverse environmental impacts such as destruction or harming habitats, will require more careful consideration of how schemes can enhance rather harm the environment.
Painting by Thomas Fulljames in 1849 of his idea to build a barrage across the Severn Estuary to create a shipping harbour (Newport & Gwent Gallery).
Acknowledgements
Matt Lewis, Rosemary Smith and Brian Gribben are thanked for providing information and some illustrations used in these web pages.
Further reading and sources of information.
Read about the SWOT project to map sea level by satellite:
https://projects.noc.ac.uk/swot-uk/
Watch this video to learn more about the highest tides because of the complex changes in the Moon’s orbit around the Earth:
Watch this video to explain more about tides and tidal currents around Bardsey Island in North Wales through the adventures of endurance swimmers:
Watch this video which explains the causes of tides in more detail:
This a rather more technical article explaining about the modelling of tides. Rather more for experts with mathematical, physics or engineering background:
