Severn bore

It is formed when the rising tide moves into the funnel-shaped Bristol Channel and Severn Estuary and the surging water forces its way upstream in a series of waves, as far as Gloucester and beyond.

In the narrower, upper reaches, the river occupies the whole area between its banks and the bore advances in a series of waves that move upstream.

As the bed of the estuary starts to rise and the sides continue to converge, the bore forms and begins to surge up the river in a tidal stream.

[5] In the lower, broader part of the estuary near Avonmouth, the tidal surge advances as a slight roll in the deepwater channels and the water rapidly spreads across the sands and mudbanks.

Past Sharpness, the bore begins to form and when it encounters the large left-handed bend at Hock Cliff, it crashes headlong into the rocks.

From Minsterworth to Gloucester, the width of the river varies little and the bore continues unhindered, climbing the banks on the outer side of bends and breaking over shallow places.

The bore height is increased by a strong southwest or west wind, low barometric pressure, about 0.6 m (2 ft) of fresh water below Gloucester and well-scoured channels in the estuary.

[4] The bore is made earlier by strong southwest or west wind, low barometric pressure, between two and five feet of fresh water, and shorter and well-scoured channels in the estuary.

Conversely, it is delayed by strong winds from the east or north, high barometric pressure, little fresh water and more meandering, poorly scoured channels.

[6] In March 2006, Steve King, a railway engineer from Gloucestershire, set a world record for the longest surfing ride on a river bore while riding the Severn bore; King surfed up the river for a distance of 12 km (7.6 mi), a Guinness World Record.

[14][15] On days when a large bore is expected, hundreds of surfing enthusiasts may accumulate, waiting for the waves to arrive.

Hazards in high water conditions can include floating trees, collapsing portions of river bank, overhanging branches and even dead farm animals.

[17] An eighteenth century account of the bore that has been attributed to Charles Blagden (died 1820), secretary of the Royal Society, is in the collection of the Gloucestershire Archives under reference Glos.

In the account, the author described it rising to eight feet and how he rode on horseback between bends in the river in order to "overtake" the bore as observers still attempt to do.