Sea level and tides
Find times for high and low tides for today and the next six days on the VisitBrighton website. The heights given on tide tables refer to chart datum rather than ordnance datum as used on Ordnance Survey maps. Chart datum varies around the coast, in Brighton it is 3.4m below ordnance datum. Sea level rise is always taken into account when planning new coast defences.
Factors that affect sea levels
Gravitational pull of the moon and the sun
The moon has the most influence, the gravitational force of the sun is about half that of the moon. The highest (and consequently the lowest) tides occur around a full or new moon, ie. the biggest tidal range. These are called spring tides and occur about once a fortnight when the sun and moon are in alignment. The effect of the moon's gravitational pull results in high tides occurring simultaneously on opposite sides of the earth.
Sea levels shown in tide tables are based on average atmospheric pressure: higher than average pressure will result in lower tides and lower pressure higher tides. According to the UK Hydrographic office the actual variation (measured in millibars) is rarely more than plus or minus 0.3m from that predicted. However this can be significant when it coincides with windy weather.
Wind and tidal surges
The strength, direction and duration of winds can have a significant influence on sea level. Onshore winds (winds blowing from the sea onto the land) will 'pile up' the water, whilst off shore winds (those blowing from the land to the sea) will have the opposite effect. Winds blowing parallel to the shoreline will have the effect of advancing or delaying the time of high and low water. In extreme conditions wind and low atmospheric pressure can combine to set up a storm surge. These are like a great bulge in the surface of the sea that can travel over great distances and when combined with a spring tide can cause severe flooding on the coast.
2013 marked the 60th anniversary of the 1953 floods which caused great devastation and loss of life along the east coast of England and the Dutch coast. The floods resulted from a large tidal surge which travelled down the east coast and across to the Netherlands.
The best known example of a seismic event causing a change in sea level was the Boxing Day tsunami which hit the coastline of the Indian Ocean in 2004. The British Isles is not subject to such dramatic seismic activity near its coastline. Following the Indian Ocean event Defra asked the British Geological Survey (BGS) to assess the likely impact of a tsunami type event on the British coastline from the nearest likely source. The nearest origin of a tsunami is out in the Atlantic. The BGS evaluated the residual force of such an event once it had reached our coastline. They concluded that it would represent no more severe a storm event than our defences were designed to deal with and so posed no specific increased risk.
Climate change has many elements, sea level and storm activity are the most significant. Increasing global temperatures are resulting in the melting of the large areas of previously permanent ice. Some of this ice is on land (Greenland and the Antarctic for example) and is effectively keeping large volumes of water out of the sea. The melting process results in this water flowing into the sea increasing its volume and causing sea levels to rise.
Increasing global temperatures are also raising the temperature of the water in the oceans. This increased temperature increases the volume of the sea resulting in sea level rise. These are known as eustatic changes. In the south of England these effects are exacerbated by the sinking of the land - this is as a result of Scotland and Scandinavia continuing to rise following the retreat of the ice sheets at the end of the last ice age and is known as isostatic change.
The latest key findings for sea level rise were published by UK Climate Projections as part of their UKCP09 predictions. For a list of answers to frequently asked questions on climate change please visit the UK Climate Projections website.
The difference in height between high and low tide varies according to the state of the moon and the other influences described above. There are also regional variations, for instance the difference between high and low water during a spring tide in Brighton is about 6m but at Bournemouth is about 2m, whilst in the Bristol Channel and on the French coast at St. Malo it is up to 11m.