The Severn bore — one of the most spectacular natural phenomena in the United Kingdom — has combined with inclement weather and an influential astronomical alignment to trigger high flooding alerts across the west of the country this weekend. The warnings, issued by the Environment Agency, indicate that flooding is “expected” and “immediate action is required.”
The Severn bore is a large tidal surge that occurs in the Severn Estuary — the tidal basin of the longest river in the U.K — several times a year. Running at 220 miles in length, the River Severn becomes an estuary in Gloucestershire, before discharging into the Bristol Channel, the Celtic Sea, and ultimately, the Atlantic Ocean. The vast body of water has the third-highest tidal range in the world, behind the Bay of Fundy and Ungava Bay — both in Canada — and the bore phenomenon is characterized by a large wave travelling upstream, against the current. The geography of the estuary, as reported by Wales Online, means that a funnel effect is created as water is forced through the narrowing channel during a rising tide. The Severn bore has been known to travel at speeds of up to 13 miles per hour, with spring time generating the largest surges. The largest recorded was on October 15, 1966, reaching a height of 2.8 metres.
The phenomenon is popular with surfers, who watch for the Severn bore predictions and gather each year to ride the wave. In March 2006, surfer Steve King set a world record by surfing the phenomenon in a standing position for 7.6 miles in one hour and 17 minutes. The Telegraph reports that predictions of a particularly large bore drew large numbers of surfers on this occasion.
Though the bore itself has now peaked, the situation is complicated by bad weather — with strong winds expected to generate more waves and increase sea levels. Adding further complexity is the presence of a particular astronomical alignment that is expected to exaggerate tidal surges. Tides are caused by the combination of gravitational forces from the sun, the moon, and the rotation of the earth. It is understood that an 18-year cycle can be seen through these processes, finding that the effects of gravitational forces — when occurring at the same time as this specific planetary alignment — can exacerbate the effects of high tides and associated coastal flooding.
Responding to the predicted surges and environmental complications, the Environment Agency of the U.K have issued flood warnings for western coastal areas — particularly Bristol — suggesting that the surge could reach up to 20 miles inland. A statement released jointly by the Environment Agency and the Met Office (which deals with atmospheric conditions), as reported by The Western Daily Press, sought to manage expectations, however.
“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 levels than the 18 year cycle.
“Strong winds can pile water up on coastlines, and low pressure systems can also cause a localised rise in sea level. Typically, the difference in water level caused by the weather can be between 20 to 30 centimetres, but it can be much bigger.”
The flood warnings are expected to remain in force for the weekend, due to bad weather, as reported by The Western Daily Press, with Devon and Cornwall also particularly affected.
[Image: Oli Scarff/Getty Images]