March | 2015 | Dave Throup

Big tides go in cycles depending on the alignment of the moon and earth, and, to a lesser extent, all the other planets in the solar system. If you want the detail have a look here https://metofficenews.wordpress.com/2015/02/20/super-tides-the-weather-and-coastal-flood-risk/

The cycle is about 18 years and February 2015 saw some of the biggest tides in that cycle. This is big news on the Severn Estuary where the tidal range (the difference in water levels between high and low tide) is huge – up to 15 metres at Avonmouth.

The Severn Estuary is like a huge funnel. Tidal sea water pushes up the Bristol Channel and gets increasingly contrained by the narrowing channel, raising levels dramatically. The bigger the tide, the greater the rise.

The most spectacular result of high tides on the estuary is the world famous Severn Bore. Large tidal bores (which is what the Severn bore is) are actually very unusual and only happen at a handful of locations across the world where there is a large tidal range. The Severn Estuary has the third largest tidal range in the world (behind the Bay of Fundy in North America and Ungava Bay in Canada).

Bores happen when the front edge of the advancing tide becomes contrained by the sides of the estuary, forcing the water up into a wave. It is pushed along by the millions of tonnes of water in the advancing tide behind.

The Severn Bore can usually be seen between Sharpness and Maisemore Weir in Gloucester, a distance of around 20 miles. It travels at a speed of 8-13 miles an hour.

The height of the wave can be anything from a few inches to over six feet. This is largely dependent on the size of the tide but is also influenced by wind, weather systems over the Atlantic and the amount of fresh water coming down the river. Bore forecasts are available well in advance and give them a star rating, the the best (biggest) being five star bores. You can find the forecasts here: http://www.thesevernbore.co.uk/timetable/4586702434

Watching the bore is a great experience. It makes an amazing rumbling sound which you can hear well before you see the wave. The wave itself and the boiling, tumultuous water pushing it seem completely alien on an estuary that is usually fairly calm and placid.

Not surprisingly the spectacle attracts thousands of sightseers and also large numbers of surfers who travel great distances to ride the wave (with varying degrees of success!). Keen surfers try to catch the wave three times during a single pass (usually at Newnham, then Minsterworth and finally Gloucester).

At the Environment Agency we’re more interested in the hour after the bore has passed. This is the time when water levels rise dramatically and very quickly. The rise can be 15 metres at Avonmouth, 10 metres at Sharpness and 6 metres at Minsterworth. This rise is just as spectacular as the bore itself, although the majority of bore watchers do not stay to see it.The impact of the tide is felt much higher up the river than Gloucester. Amazingly, during the February high tides the river at Upton in Worcestershire (about 60 miles from the estuary) was pushed into reverse twice a day. The graph below shows how normal flows of about 100 tonnes of water per second travelling downstream changed to an upstream flow of up to 40 tonnes per second in a matter of minutes. There was also an accompanying rapid rise in water levels of over half a metre. Incredible!

We have miles of flood defences along the length of the Severn Estuary which protect over a thousand homes and businesses. Few people notice most of them as they look like natural grass banks. However, if they weren’t there high tides would routinely cause extensive flooding across the estuary. When high tides are imminent we check all our defences to ensure they are in good working order. This is no small task given the extent of the defences and the remote nature of parts of the estuary.

After the high tides and storms of 2014 some of our defences were damaged. We spent the autumn and early winter carrying out Over £3 million of repairs so they would be ready for the huge tides of 2015. In February they received a good test (like Minsterworth here) and performed perfectly.