What are 'super tides' and why are we seeing them in 2015?

Coasts across parts of the UK were battered by a "super tide" this weekend, causing banks to burst in London and other areas.

Dozens of flood alerts were in place, with forecasters predicting waves of up to 50ft along the Bristol Channel. The River Thames burst its banks on Friday, with cars left submerged under several feet of water in west London, reported the Evening Standard.

But what exactly is a super tide and why does it occur?

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What is a super tide?

Tides are governed by the gravitational pull of the moon and sun. When the sun and moon align, their gravitational pull causes larger than average tides, known as spring tides, which occur twice a month. At the same time, a much longer tidal cycle, associated with orbital changes and the tilt of the earth, also causes increases and decreases in the size of the tides over the course of 18.6 years, according to Dr Jason McIlvenny and Dr Philip Gillibrand of the Environmental Research Institute at the University of the Highlands and Islands. This longer astronomical cycle will reach its peak this year. When the peaks of the different cycles combine, a super tide can be seen.

When are the super tides due?

The largest tides were due to occur at the spring equinox (20-21 February) and are expected at the autumnal equinox (29-30 September), say McIlvenny and Gillibrand. The Sunday Times says there are expected to be about six periods of super tides in total this year, with larger tides also predicted on 20 March, 29 August and 27 October. The first one, in January, passed without incident.

Is flooding guaranteed?

No, the high tides will not necessarily cause flooding, unless combined with the effects of storm conditions. The Met Office explained on Friday: "It's important to realise that just because we are expecting big astronomical tides over the next few days, these won't cause the highest sea levels we've seen – even in the last few years. That's because the weather can have a much bigger impact on sea level than the 18-year tidal cycle."

An Environment Agency spokesperson also told the Daily Telegraph earlier this year: "High astronomical high tides do not lead to coastal flooding alone – it requires a combination of low pressure, strong winds and high tides. We will monitor the situation closely, working alongside partners, including the Met Office and local authorities, and will issue alerts and warnings if required."