Most people who live near the River Severn pass it without giving it much thought – perhaps only noticing, “The Severn’s looking particularly brown today.” But on certain days of the year, the river offers a surprising spectacle: dozens of surfers riding a seemingly endless wave.
What many don’t realise is that the often-overlooked Severn Estuary is home to one of the UK’s most spectacular natural phenomena – a world-famous tidal surge known as the Severn bore.
What is the Severn bore?
The Severn bore is a large tidal wave that works its way up the Severn Estuary, travelling a distance of over 25 miles. This tidal estuary is where the River Severn meets the Bristol Channel, and this bore is a strong tide that pushes up a river against a current – which results in some pretty astounding waves. The bore can travel at speeds of between 10 and 13mph, with some reaching two metres high.
When the bore is at its best, it attracts kayakers and surfers, who ride the wave down the estuary.
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What causes a tidal bore?
Tidal bores are most common in areas with large tidal ranges and funnel-shaped estuaries. The Severn Estuary meets both of these criteria.
Tidal bores occur where a river joins the sea and are created when the tide rises. Usually, water from a river will flow into the sea – but during a tidal bore, it’s the other way around. In the Severn Estuary, waves are created because the channel gets narrower and shallower – so the water is lifted upwards and forwards, like you see on a beach.
The large tidal range also contributes, as waves are only created when the tide goes over a certain height. With a tidal range of between 12 and 15 metres, the Severn Estuary has the second highest tidal range in the world, after the Bay of Fundy in Canada, which is around 16.3m.
Some areas experience higher tides than others, due to the topology (shape of the land) and bathymetry (shape of the seabed). This is because the flow of water moves differently in these areas.
Most of the Earth has a tidal range of less than 1.3 metres – but the Severn Estuary’s is much larger. This is due to the funnelling effect that takes place around Welsh and Southwest English coastlines that approach the mouth of the River Severn.
Is this tidal range the reason the River Severn is so brown?
Yes – exactly. There is an extraordinary amount of water flowing in and out of the Severn Estuary at all times, which stirs up vast quantities of sediment from the seabed.
What effects does this tidal range have?
The rapid movement of water in the Severn Estuary means that the water here is ecologically rather desolate, with light struggling to penetrate through the sediment. There is also a rapid erosion of coastlines in this area.
However, there are positive benefits too – if the power from this tidal range is harnessed, it can produce tidal power through turbines.
How often does the Severn bore happen?
There are about 260 tidal bores in the Severn Estuary every year, caused by the gravitational pull of the Moon and the Sun. Tides are at their highest in the spring and autumn equinoxes – when the Sun is above the equator and day and night are equal length.
Although there are many tidal bores, they will not all be as majestic as the ones we see in photos, decorated with surfers and kayakers. Bores are rated by a five-star system, so it’s worth following The Severn Bore’s Facebook page for bore reports and updates.
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What’s the deal with surfing the Severn bore then?
The River Severn was first surfed in 1955 by World War II veteran Colonel Jack Churchill – “Mad Jack” – on a homemade 14-foot surfboard. This is thought to be the first instance of someone riding a river’s tidal wave.
Since then, others have followed in his path, with railway engineer Steve King setting a world record in 2006 for the longest surfing ride on a river bore – surfing 12 km on the Severn bore.
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Top image credit: Getty Images
