Severn Bore and Trent Aegir
The Severn Bore and Trent Aegir are naturally occurring tidal waves. These natural phenomena occur in the lower reaches of a few rivers throughout the world with large tidal ranges, including the River Severn and the River Trent.
The Severn Bore has been known to reach two metres in height. Its average speed is 16km per hour.The Severn Estuary experiences the second highest tide anywhere in the world, and the difference between the lowest and highest tide in any one day can be more than 14.5 metres. High or spring tides occur on several days in each lunar cycle throughout the year.
The Trent Aegir is named after the god of the seashore or ocean in Norse mythology - and like the Scandinavian sailors in the myths, river people would fear the coming of the aegir as it is very unpredicatable and would sometimes surface to destroy ships. It usually appears during high spring tides, but as with the Severn Bore, its size can be affected by winter floods and the resulting rise in water level.
The size of a bore or aegir can also be affected by opposing winds or high freshwater levels which reduce its height and delay its time of arrival, whereas a following wind can increase its height and advance the time it arrives. There is no true Severn Bore upstream of Gloucester because of weirs on the twin-armed channel near the city.
For details of when and where you can see both, click on the links below: