Bristol Channel Tides

The Bristol Channel has the second largest range in the world (Avonmouth 12.3 mean spring range) there are several factors that have an effect on our tides as follows:


·        The shape of the land

·        The Mass of water to the west (Atlantic ocean)

·        Topography of the land

·        Influential permanent and semi permanent pressure systems


The shape of the land

The Bristol Channel is shaped like a funnel. As the earth rotates and the area makes it’s twice daily passage through the tidal bulges (semidiurnal) the mass of water is squashed causing it to increase in both range and rate.


The Atlantic Ocean

The greater the mass of water that influences the area then the greater the tidal range. As the Atlantic Ocean is directly to the west of the Bristol Channel it has a great effect not only on our tides but also our weather and climate, the effect on the tide is to increase the range.


Topography of the land

The high ground to the south of the Bristol Channel (Exmoor) and the mountains in south Wales have the effect of funnelling the prevailing Westerly winds and increasing the speed, as the wind moves across the Sea surface the wind speed increases slightly due to reduced friction causing another increase in wind speed. Waves are formed and a mass of water will be moved up the channel causing an increase in tidal range


Pressure systems

During the summer the most influential pressure system is the Azores high, once established this high pressure system is relatively constant and the high pressure influence will have the result of a decrease in tidal range. When the Azores high subsides (Autumn) the main influence is from Atlantic and Polar frontal depressions that track from West to east across the Atlantic having the dual effect of reduced air pressure (increasing tidal range) and moving a body of water eastward( storm surges) causing an increase in tidal range.


 Tidal Rates


Spring rates of 6 knots can be experienced in the region of Lavernock Point and the Rannie shoal


The funnelling effect of the land causes the mass of water to accelerate as it travels up the Bristol Channel. At Lavernock the tide splits (main channel and up to Cardiff Bay), as a result the ebb currents also rejoin at this point, squeezing the body of west going water causing an acceleration and the high flow rates.


 Counter Currents



The entrance to Barry experiences strong currents at 90 degrees to the entrance to the harbour. There is a counter current close in to the entrance that has the effect of a SW ‘ly flow even during NE ‘ly flow further out (flood tide). This counter current is a narrower band but stronger during spring tides.


The Wrach Channel


The approaches to and the Wrach Channel itself experience strong counter currents, (SW flow) starting as early as 2 hours before HW.