Many bores occur throughout the world, some even more spectacular than our own Severn Bore. But what is a bore? Quite simply, a large wave travelling upstream at the head of each spring tide. It is taken for granted that the tide will come in and go out again twice a day, every day of the year, forever and ever. During the course of each month, on a two week cycle the tides become higher and lower. Once a month for a period of a few days there occurs the spring tides, where the height of the tide at Sharpness is approximately 9.5 metres. Please note that a bore will happen for all tides above 8 metres at Sharpness, the higher the tide, the more spectacular the bore.
The bore is caused by a large volume of water entering a continual narrowing environment, thereby the decreasing width of the river increasing the height of the water. This action can be likened to pouring water into a funnel. As the tide from the Atlantic enters the Bristol Channel and on into the Severn Estuary, the water has risen nearly 15 metres. At a location near to the Severn bridge is recorded the second highest rise and fall of tide in the world! Where then is the highest recorded tidal range? In the Bay of Fundy, located between Nova Scotia and New Brunswick, Canada in the Petitcodiac River. The width of the Severn narrows very quickly, so the mass of water at the rear of the tide endeavours to overtake the leading wave. With the rising riverbed this causes a unique wave system known as the bore.
Another remarkable feature of this great Severn tide is not only its height in the
estuary, but also its low water mark. Here the water level is lower than
anywhere else in the river! Upstream and downstream the water never falls below
that as at Beachley, but how? As the tide is still coming several miles above
Beachley towards Sharpness, the tide is going out at the mouth of the Bristol
Channel. Likewise, as the tide is ebbing upriver, the tide is turning again at
the mouth of the Bristol Channel. A motion likened to a near full large bowl of
water, being slightly moved at each end. More of this tidal feature later.
Because of the exceptional rise and fall of tide in the Severn Estuary, the
river is a very dangerous environment to be in, especially for the
inexperienced. At low water Lydney Sands may look inviting to walk on. Don't!
The tide coming in isn't like being on the beach at Cornwall. One minute there
is no sign of the tide, the next minute the water is waist high around you, the
current so fast that it would knock a person off their feet and sweep them up
the river. Morbid, maybe, but the truth, for accidents is becoming so common in
the river now, that the Severn Area Rescue Association was formed in 1975 with
rescue boats at Beachley and Sharpness.