This morning some of the Physics World team set out from Bristol at 7:00 and by 8:30 we were standing on a muddy riverbank in the pouring rain. Along with a growing crowd of people, we were watching the River Severn rush towards the sea – swollen by this winter’s heavy rains.
While some were sharing flasks of coffee and tea while huddling under umbrellas, the braver in the crowd were launching surfboards and kayaks into the cold river. Most had wetsuits and specialist gear on, but one hardy paddler was out in a T-shirt and tracksuit bottoms. (No Physics World personnel got into the river, we watched safely from the bank).
Then, just after 9:00 and ahead of schedule, a huge wave came roaring up from the sea some 50 km away. This was the Severn’s tidal bore. I first spotted it as it rounded a bend in the river, picking up about half a dozen surfers and kayakers and launching them upstream. While most were just scattered by the wave, two managed to surf several hundred metres past us before being pushed into a tree that was leaning precariously from the opposite bank.
Extreme range
Today’s bore was rated a five-out-of-five, and that’s why we made the trek to watch it. The Severn has one of the highest tides in the world and this morning the tidal range in its estuary (at Avonmouth) was nearly 14 m. This extreme range was caused by the alignment of the Moon and Sun through Earth’s equator – which happens around the equinoxes.
Chasing the Silver Dragon: the physics of tidal bores
The tidal bore is created when the incoming tide enters a shallow, narrowing river. When the rising tide over tops the river flow, a surge of water travels upstream as a series of waves. Indeed, another amazing aspect of this morning was how rapidly the tide rose as the bore passed. Before the event, the level of the river was constant but after the wave passed it had risen about 2 m in what seemed just a few minutes.
There are several other rivers around the world that have tidal bores, and you can read more about them – and the physics behind the phenomenon – in this article by the physicist Michael Berry: “Chasing the Silver Dragon: the physics of tidal bores”.
