Autumn spring tides rise metres in minutes
There will be tidal bores on UK rivers later this week. Where and when? The Severn Bore is the best known, especially as a challenge for surfers and kayakers. For Nature UK, guest blogger Ian Garman set out to get a different perspective on the phenomenon.
'Spring tides' are large tides that in fact occur all year round. But the spring and autumn equinoxes do roughly coincide with the largest (ie highest and lowest) tides of the year. The variation in tide height across the year is mainly due to the gravity of the Sun and Moon - but the rate at which it rises and falls is also influenced by the coastline.
On estuary rivers that narrow down - such as the Severn, Trent and Ouse - instead of the tide rising and falling at the same rate, it comes in much more abruptly than it goes out.
A significant tidal wave (known as a bore or eagre/aegir) builds up, because the incoming seawater must squeeze into a channel that's getting shallower and narrower. The wave heads upriver, growing in height and gathering speed.
So if you go to watch a bore, you may think the spectacle is all done and dusted in just a minute or so, as the wave goes past.
Here's a time-lapse of the Severn Bore and the hour that follows.
You can tell when the bore goes by (about four seconds into the video). That's when the disappointed surfers start to trudge back to the warmth of their cars. It wasn't their day.
Stick around though and you can witness nature's might to the full, as the river doggedly swells from no tide to flood tide. Might be worth checking where you've parked your car first, mind.
On the day and at the spot we took this time-lapse, the river height rose around 10 feet in the 90 minutes after the bore passed.
This regular flooding helps to make the Severn Estuary a valuable wildlife ecosystem. (The Slimbridge nature reserve is just nearby.)
And it also leads to much debate about the pros and cons of building a Severn Barrage to generate renewable electricity.
You can watch BBC Gloucestershire's video showing a more traditional view of the same Severn Bore.
Or investigate the 'shape' of the tide rise and fall around the coast of the UK on the BBC Weather tide table pages. Compare sites on an open coast, for example Newlyn in Cornwall, with river channel locations like Sharpness in Gloucestershire or Perth on the Tay.