The Severn Bore
This is a large surge wave that forms in the River Severn. It is caused by the shape of the estuary being like a funnel. The width of the river narrows going upstream and so an incoming tidal wave is formed by the rising water. At the same time the depth of the river decreases and so a tidal bore is the result. The bore can be between 1 and 3 metres high in midstream and greater at the banks, particularly on bends.
I haven’t seen one yet, but I will post a photo of one on this blog when I do.
This website seems to have some more useful facts & figures.