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 The Severn Bore

I was introduced to the Severn Bore by Frank King, who described it as an example of a non-linear delay line. In fact any river is a non-linear delay line, because waves in deeper water travel faster than waves in shallow water, so the top of a wave travels a bit faster than the bottom. To observe the effect, you need a wave that is relatively high compared to the depth. Then the top of the wave overtakes the base, making the front face into a wall. That is what happens when waves run up a beach. The depth gets shallow, the top overtakes the bottom and crashes over. If the depth doesn't change much, which it doesn't in a river, then the wave can continue with a wall-like face for a long way.

You don't normally get big enough waves (compared to the depth) on a river to start the process off. That's where the Severn is special (along with a few other rivers round the world), because its estuary and the Bristol Channel, form a huge funnel. As the tide comes in, the funnel effect makes the water rise a huge distance, and it rises fast enough to form a wave that then grows as its top tries to catch up with its bottom, and sets off up the river. It travels for many miles, and takes over an hour and a half to get from Newnham to Over Bridge in Gloucester, where it meets a weir. If you are quick, you can see it at more than on place, since it travels at about 10 miles per hour along the line of the river.

The height of the bore depends on the height of the tide, which varies with the time of year (the biggest are in spring and autumn), the phase of the moon, the prevailing wind and atmospheric pressure. There are two bores a day, one bigger than the other. The height of the bore is also affected by the amount of water already in the river.

There is a timetable, pictures and more information on the Severn Bore website. 

The pictures here were taken on two different occasions. The first four were in September 1971 at Lower Rea. The second four were in 1974, first at Epney, lower down the river, and then at Lower Rea. On both occasions, the Bore was not particularly high.

If a barrage is built across the Severn estuary to harness the huge tidal power that drives the bore, then the bore will be no more. That might be a price we have to pay to reduce our carbon dependency, but it would be yet another bit of the natural world that mankind had killed off, quite apart from the effect it could have on the wildlife that relies on the special conditions in the Severn estuary. In fact there is not just one scheme to build a barrage, but several. Some produce more power, and some produce power that is more continuous. For more information, see the article by Renewable Energy UK . (If that page moves, try the home page .)


See all pictures together  , or click on each to enlarge.

The Bore rounding a bend
Still water ahead of the Bore
Surfer riding the Bore
Turbulent water (and surfer) behind the Bore
The embryo Bore
Close up on the waves
Surfers waiting for the Bore
Surfers riding the Bore

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