The severn bore happens about 4 times a year (big enough to be rideable) but it is DANGEROUS!.
If you're going to try and catch it, you'd better be a confident surfer already and use an old board that you don't mind getting smashed up.
Wear an old wetsuit because the water is filthy!
You need to be in the water and paddle among about 30 or 40 other surfers for a single wave.
IF you're lucky enough to get on the wave (and it's a big IF) then you have to dodge all sorts of broken trees, shopping trolleys, dead farm animals etc without losing the wave.
It's basically a tidal wave and sweeps all the debris off the fields and banks into the river. If you fall off, you will get churned around in all the cr@p - DON'T OPEN YOUR MOUTH/EYES! There are 4 or 5 other waves that follow the bore, not strong enough to catch, but they bring the debris and will work you over!
That said, it does hold the record for the longest ever ride. Officially I think it still sits at around 4.3 miles, but unofficially it's 6.1 miles.
You need a very long board and be prepared to miss it.!! The wave height and steepness vary along the river. At one point it will be about 1 ft high, then you'll go round a corner, the river will narrow and it'll push up to a 6ft face!! It is not an easy wave to ride, by any means!