High Spring Tides Threaten Flooding

When are the high spring tides?
This weekend sees the last in a series of three 'exceptionally high' tides which have occurred over the past few months. The severity of these tides will be determined by the weather conditions that occur during the weekend. Wind strength and atmospheric pressure play key roles in the making of tide heights.

On 11 - 13 August and 7 - 11 September the UK saw high spring tides, which didn't bring vast flooding. Although no properties were flooded in September, there was some localised coastal flooding at locations exposed to the east such as Mevagissey and Looe in Cornwall.

In Somerset, the tide reached a height of 7.3 metres in Bridgewater and 8.12 metres in Avonmouth. Flood watches were issued for the Somerset coast from Porlock to Avonmouth and Pill with flood warnings issued at Porlock and Uphill.

Some flooding was experienced at Muddlebridge in North Devon closing the B3233, but no flooding to property occured. Strong easterly winds caused spray overtopping at Dawlish, Teignmouth and Torbay and there was also minor overtopping along the seafront at Preston Sands, and the southern part of Paignton Beach.

Who is at risk

The Environment Agency's message is "people should be aware, but not worried by the tidal event".

During the high tides this weekend it looks as if the high risk areas transfer from the east to the west and then the south, with strong wind expected to drive large waves into these area, giving more height to already high seas. Northern parts of Devon and Cornwall, along with western parts of Wales, such as Cardigan Bay seem to be at higher risk.

Southern Wales and some coastal parts of Somerset could be at risk from flooding when we see a 4 star bore sweeping its way up the Bristol Channel and the Severn. As the river narrows through Gloucestershire, some localised flooding is a risk here too. The south coast looks as though it could bear the brunt of the high waters on Sunday as the wind switches from a westerly to a southwesterly.

In Scotland the Western Isles (Nan h-Eileann An Iar) have begun preparations for the high tides as it looks like they may take the brunt of the strong to gale force westerly winds during Friday and the early hours of Saturday.

The Highland Council have continued placing sandbags along the banks of the River Ness, which runs right through Inverness city centre. They have also been handing out sandbags to the people living in the low-lying areas and have warned that the most likely areas to be affected are Stornoway Town Centre and the Southern Isles. The council has assured people living close to the river that this is just being done as a precautionary measure. Orkney and Shetland could also see some very large waves crashing onto their western shores during Friday and Saturday.

What can we expect from this year's spring tides?

The Met Office and the Environment Agency are in close contact and will continue to monitor any developments which may increase tide heights.

The Met office have warned that gale force winds in the west on Friday could coincide with high tides and forecasters are raising awareness of possible coastal flooding, especially around western coasts. Winds with gusts to 60 mph could lead to large waves over-topping some coasts, with possible flooding. Areas most at risk this evening are western coasts of England, Wales and Scotland and also the north coast of Northern Ireland.

After a calmer period of weather on Saturday, a change in wind direction may lead to further flood risk for southern coasts of England on Sunday evening.

Forecasters at the Met Office work closely with the Environment Agency to provide early warnings and up to date advice in these situations. David Rooke, Head of Flood Defence at the Environment Agency, explains:

"We will continue to monitor the situation along the coastline. Flood watches or warnings will be issued where appropriate, to give people as much notice as possible. High tides are expected for this weekend, so anyone out near the coast should be aware of any changes in weather conditions."

People are advised to follow latest weather forecasts from the Met Office and see the Environment Agency website for the latest flood information.

How spring tides can lead to flooding
Tide heights are controlled by the gravitational pull of the sun and the moon on the surface of the planet, and vary throughout the year.

'Spring' tides are higher than normal and occur twice a month. Severe flooding can occur during these tides if coupled with low pressure in the North Sea and a strong northerly wind, which pushes water towards the coasts.

Low pressure also 'domes' the sea upwards, rather like what happens under a tropical storm (an intense area of low pressure). If coupled with already high waters (spring tides), a strong wind and heavy rain, severe coastal flooding can occur.

High spring tides themselves are unlikely to flood coastal areas as they will only increase the tide heights by around 4cm above normal. But add in low pressure and we find the tide height increasing by 1cm for every drop in pressure of 1millibar.