"When the boar (sic) comes, the stream does not swell by degrees, as at other times, but rolls in with a head...foaming and roaring as though it were enraged by the opposition which it encounter" - Thomas Harrel 1824
A bore is a high wall of water, usually a single wave, which runs against the flow of the river and travels many miles inland. This spectacular event happens in around 60 estuaries around the world. The conditions have to be right: the estuary needs to be a specific funnel shape and the outflow of the river meeting the rising tide need to be of a particular strength. The Severn Estuary provides just these conditions.
The Severn Bore is one of the largest in the world due in the main to the very high tidal range. The water of the rising tide compresses as it pours into the mouth of the 'funnel'. As the width of the Severn decreases, from 5 miles at Avonmouth to less than a hundred yards at Minsterworth, the wave is formed and can be seen travelling along a 15 mile stretch of river. The bore is at its most spectacular between Overton and Maisemore(just north of Gloucester) where it finishes abruptly at the weir .
Surfing the bore has now become a competitive sport and the current record holder has managed to ride the wave for a little over 35minutes. It is also very popular with canoeists.
- This wave can be as high as 3m on a spring tide, but 1m is more usual, and it travels at between 8 and 13km/h.
- There are around 260 bores in a year, ie 130 days with 2 tides a day, and large ones can be seen on about 25 days of the year.
- The very best bores occur with the highest tides: on the 3 days either side of full and new moons and are most spectacular when this coincides with the vernal and autumnal equinoxes between 9.00-11.00am and 9.00-11.00pm.
A Timetable for the Severn Bore can be found at http://www.severn-bore.co.uk