Editing Severn bore (section)

== Formation ==
The [[Severn Estuary]], which empties into the [[Bristol Channel]], has one of the largest [[tidal range]]s in the world&nbsp;– about {{convert|13|m|ft|abbr=on|sigfig=2}}.<ref>{{cite web | url= http://www.environment-agency.gov.uk/commondata/acrobat/severnpositionmay2006_1508223.pd | format= PDF | work= UK Environment Agency | title= Severn Estuary Barrage | date= 31 May 2006 | access-date=18 August 2016 |archive-url = https://web.archive.org/web/20070930155720/http://www.environment-agency.gov.uk/commondata/acrobat/severnpositionmay2006_1508223.pd <!-- Bot retrieved archive --> |archive-date = 30 September 2007}}</ref><ref>{{cite web | url= http://www.bbc.co.uk/bristol/content/articles/2005/07/04/bristolchannel_feature.shtml | title= Coast: Bristol Channel | work= BBC | access-date= 27 August 2007 | archive-url= https://web.archive.org/web/20130307233742/http://www.bbc.co.uk/bristol/content/articles/2005/07/04/bristolchannel_feature.shtml | archive-date= 7 March 2013 | url-status= live }}</ref> It is exceeded only by the [[Bay of Fundy]], and possibly [[Ungava Bay]], both in Canada.<ref>{{cite conference |last1=O'Reilly |first1=Charles T. |last2=Solvason |first2=Ron |last3=Solomon |first3=Christian |editor1-first=J. A. |editor1-last=Percy |editor2-first=A. J. |editor2-last=Evans |editor3-first=P. G. |editor3-last=Wells |editor4-first=S. J. |editor4-last=Rolston |year=2005 |title=Resolving te World's Largest Tides |conference=The Changing Bay of Fundy—Beyond 400 years: Proceedings of the 6th Bay of Fundy Workshop |publisher=Environment Canada-Atlantic Region |location=Cornwallis, Nova Scotia |volume=Occasional Report No. 23 |url=http://www.bofep.org/PDFfiles/BoFEP6thProceedings.pdf |pages=153–157 |access-date=27 August 2016 |archive-url=https://web.archive.org/web/20160827202033/http://www.bofep.org/PDFfiles/BoFEP6thProceedings.pdf |archive-date=27 August 2016 |url-status=live }}</ref>

The tidal wave starts far out in mid-ocean. It moves towards the continent of Europe at about 700&nbsp;miles (1100&nbsp;km) wide. When it reaches the continental shelf, its width decreases to about 200 miles (320&nbsp;km) wide, 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 {{convert|100|mi|km|order=flip|abbr=on|sigfig=2}} or so to less than {{convert|5|mi|km|order=flip|abbr=on|sigfig=1}}, and its height is nearly {{convert|50|ft|m|order=flip|abbr=on|sigfig=2}}. 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.<ref name=Rowbotham1967/> The wave travels upstream against the river current at a speed of {{convert|8|to|13|mph|km/h|order=flip|abbr=on|sigfig=2}}.<ref name=pretor-pinney>{{cite book|last=Pretor-Pinney|first=Gavin|title=The Wavewatcher's companion |year=2011 |publisher=Bloomsbury |isbn=978-1-4088-0976-1 |pages=239–242}}</ref>

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. Often, the Severn Bore floods the lower village of Pill if flood defences are not closed.  Past [[Sharpness, Gloucestershire|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, Gloucestershire|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 Longney 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, England|Worcester]] may experience a rise in water level of a foot (30&nbsp;cm) or so.<ref name="Rowbotham, Severn Bore">{{cite book |last=Rowbotham |first=Fred |title=Severn Bore |isbn=0-7153-8508-9 |publisher=[[David & Charles]] |year=1983 |orig-year=1964|pages=19–26}}</ref>