The word bore derives through Old English from the Old Norse word bára, meaning "wave" or "swell". ....
The Severn bore is a tidal bore seen on the tidal reaches of the River Severn in south western England. 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. The bore behaves differently in different stretches of the river; in the lower, wider parts it is more noticeable in the deep channels as a slight roller, while the water creeps across the sand and mudflats. 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.
Bores are present on about 130 days in the year, concentrated on the days immediately following the new and full moon. The size and precise timing of the bore depend on such things as the time of high tide, the barometric pressure, the wind speed and direction, the amount of water coming down the river and how well scoured the main drainage channels are.
Historically, the bore has been of importance to shipping visiting the docks at Gloucester.
The tidal wave starts far out in mid-ocean. It moves towards the continent of Europe at about 700 miles (1,100 km) per hour. When it reaches the continental shelf, its velocity decreases to about 200 miles (320 km) per hour and its amplitude increases. Approaching the Bristol Channel, a segment of the wave has to accommodate to the ever-decreasing width by raising its height. When it reaches the Severn proper, its width has decreased from a 100 miles (160 km) or so to less than 5 miles (8.0 km), and its height is nearly 50 feet (15 m). 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. The front edge of the wave is steep and the trailing edge flatter. The bore consists of three or four sizeable waves followed by a few of diminishing size. As with other waves, the wave tends to break in shallow places and near the bank, and flow smoothly in deep water. The wave travels upstream against the river current at a speed of 8 to 13 miles per hour.
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. Reforming, it runs up-river close to the Overton shore before crossing the estuary towards Box Cliff. As it rounds the Horseshoe Bend it keeps to the outside but it afterwards moves across to the eastern side of the river. Above Langney Sands, the river abruptly narrows to a hundred yards, the sands diminish and the channel occupies the whole of the river. Now the bore is recognisably the spectacular phenomenon that people expect rather than a swelling flood of water. 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. At Lower Parting, close to Gloucester, it splits in two to pass either side of Alney Island. Both branches encounter and overcome weirs and rejoin at Upper Parting, and the much diminished bore continues upstream. In particularly high tides the water may overtop the weir at Tewkesbury, and even the foot of the weir at Worcester may experience a rise in water level of a foot or so.
The largest bores occur around the times of the equinoxes but smaller ones can be seen throughout the year. There are about 260 bores in each year occurring twice a day on 130 days. Because the bores are associated with the phases of the moon, one occurs between 7 a.m. and noon on bore days, and the other between 7 p.m. and midnight GMT, with the largest bores occurring between 9 and 11 in both the morning and evening. Maximum bores occur between one and three days after new and full moons, and smaller ones on the days that precede and follow the maxima.
The bore height is increased by a strong southwest or west wind, low barometric pressure, about 2 ft (0.6 m) of fresh water below Gloucester and well-scoured channels in the estuary. The height is decreased by strong winds from the east or north, high barometric pressure and little fresh water below Gloucester or excessive fresh water. 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. The wind direction out at sea is of more significance than the local air-flow.
Being the onset of the flood tide, the bore is accompanied by a rapid rise in water level which continues for about one and a half hours after the bore has passed. The Severn bore is not a self-reinforcing solitary wave or soliton but rather a shock wave which is formed because the wave is travelling faster than the wave speed in water above the bore. The passing of the bore causes a churning of the water, and the myriads of tiny bubbles popping contributes much of the roaring sound made by the bore. The largest recorded bore was on 15 October 1966, when it reached a height of 9.2 feet (2.8 m) at Stonebench.
Related to the time of the high tide at Sharpness, the bore passes Newnham on Severn one hour before this, Framilode and Arlingham, twenty-five minutes before, Epney twenty minutes before, Minsterworth at the high tide, Stonebench, on the east bank, fifteen minutes after the high tide and Over Bridge thirty-five minutes afterwards.
The River Severn is considered a navigable river and Gloucester used to be an important port, with shipping having to deal with the bore. Vessels can ride the tide up from Sharpness to Gloucester. The tide will outstrip the vessel but by timing the journey correctly, a barge can reach Gloucester at high water. Descending is more difficult, and barges often needed to start their descent with one tide, lay up before crossing Longney Sands, and finish the descent at the next tide.
https://en.wikipedia.org/wiki/Severn_bore
https://en.wikipedia.org/wiki/Tidal_bore
https://www.youtube.com/watch?v=IKA39LQOIck
https://www.youtube.com/watch?v=njY2oOCATR8
