A region of extreme tides
The funnel shape of the Bristol Channel traps and concentrates incoming tides, creating a great range between high and low tides. It has the second highest tidal range in the world after the Bay of Fundy in Canada, where the difference between high and low tide ranges up to 15.2 metres. Locally the difference varies considerably with the phases of the moon, weather and along the coast, with the range generally increasing eastwards as far as Hinkley Point, where the highest recorded ranges are up to 13.7 metres. Mean spring tides on the Exmoor coast range from 8.4 metres at Combe Martin to 8.7 metres at Lynmouth, 8.0 metres at Glenthorne, 9.3 metres at Porlock Weir and 9.6 metres at Minehead. The highest tides recorded are greater than these means, especially if backed by strong winds and raised by low atmospheric pressure. The extra height of the highest tides increases roughly in proportion with the range so that the highest tides can reach well over 10 metres in westerly storms.
Why tides happen
The tides in the Bristol Channel relate to the Earth's gravity, the Coriolis force, created by the spin of the Earth, and the moon's gravity. The first force holds the seas to the Earth and the other two try to pull them off into space. The pull of the moon is greater on the side of the Earth nearest to it, and the Coriolis force has more effect on the side away from the moon, so it is at those two points where the high tides occur. As the Earth spins, any point on its surface passes through each of these two high tide zones about once a day. It is not exactly once a day as the moon moves around the Earth once every 28 days, making each high tide a day later in that period, or about 50 minutes later every day.
As the Earth spins from east to west, tides generally come later towards the east. Thus at Combe Martin high tide is about half an hour earlier than at Minehead, which in turn is about half an hour earlier than at Bristol. This pattern is complicated elsewhere by the flow of tides around land masses.
How tide levels change
The pull of the sun's gravity also has an effect on tides. When the sun and moon are in line with the Earth their pulls combine to cause the highest and lowest tides, known as 'springs', and when they are at right angles to the Earth there are low high tides and high lows, known as 'neaps'. This is a 28 day cycle, according to the movement of the moon, with spring tides at full and new moon. There is also a seasonal cycle as parts of the Earth tilt towards or away from the sun and moon.
How far the tide rises depends on the point in the cycle and on local conditions such as the depth of water, slope of the sea bed, weather conditions and restriction of land masses.