How high tides and gales increase coastal flood risk

You may have read in the media in recent days about the concerns of coastal flooding affecting southern and western parts of the UK.

A combination of high tides and onshore gale-force winds mean that there is a risk of water spilling on to the land along the immediate coastline.

This can cause some places to flood quite quickly – especially if they are low lying and are located in a natural dip in the landscape, making them more prone to coastal flooding effects.

Whilst the causes of coastal flooding seem quite obvious, there’s actually quite a few factors that need to come together to make it happen.

High tides

Tides are the rise and fall of sea level due to the combined effect of gravitational forces exerted by the sun and moon and the rotation of the earth.

Twice a month, the sun, moon and earth line up, increasing the gravitational force exerted on the earth.

This in turn increases the tidal force, which results in the range between high and low tide to be at its greatest, known as a spring tide (or high tide) – something that is happening at the moment.

Onshore gale-force winds

Autumn is a time of year when low pressure systems tend to be most vigorous as the temperature contrast between the poles and equator increases – strengthening the jet stream.

Whilst each low pressure system varies in ferocity, some are active enough to bring gale or severe gale-force winds, reaching in excess of 60mph.

If these strong winds coincide with spring tides and blow onshore, they can have the effect of throwing water onto the coastline, potentially causing coastal flooding.

This can be particularly problematic if it occurs at estuaries because water flowing out to sea from rivers can be blocked or even forced back by the tide. As a result, the coastal flood risk to bordering areas can be exacerbated further.

Low pressure

As well as bringing strong onshore winds, air pressure also influences sea level. Air pressure is effectively the weight of the air above a given point on the earth’s surface.

When air pressure decreases, sea level rises at a rate of around one centimetre for every one millibar fall in pressure.

So when a deep area of low pressure is present of around 970 millibars, sea level can temporarily rise by around 40cm. This, in addition to the rise caused by spring tides, increases the risk of coastal flooding even further.

Where is at greatest risk of coastal flooding?

According to the Environment Agency, during Wednesday into Thursday, southern and western parts of England and Wales have a low to medium risk of coastal flooding.

However, as onshore winds ease and pressure rises at the end of the week, the risk will diminish considerably.

Don’t forget, the latest forecast is available on the Channel 4 Weather website or if you have any reports or pictures of flooding, please get in touch via Twitter – @liamdutton