High tides and surge tides High tides High tides, often known as ‘spring tides’, are normal and happen twice every month. They are caused by the gravitational pull of the full and new moon on the Earth. High tides alone do not normally cause flooding.
Tidal surges Tidal surges happen as a result of low pressure which causes sea levels to rise – low pressure causes a 1cm water level rise for every 1 millibar drop in pressure. The height of a tidal surge depends on a number of factors such as the size and strength of a storm, high tides and wind direction as it approaches the coast. Strong winds dictate the height of waves and the direction waves are pushed down the east coast. The North Sea acts like a funnel as waves are driven towards the narrow opening of the English Channel. In the UK, the timing of a storm surge coinciding with a high tide is particularly important. The diagram below shows the difference the high tide can make on the overall height of a tidal surge.
Tidal surges and coastal flood risk We forecast tide levels annually to predict when high tide levels will happen for the year ahead. Tidal surges are determined by weather patterns that can only be forecast by the Met Office And our Flood Forecasting Centre about one week in advance. Specific weather conditions which combine to produce a tidal surge and not high tides alone are the dominant factors which influence coastal flood risk. When a tidal surge is forecast in combination with high tides then we start to warn and inform the public about the potential risk of flooding.