A tidal wave which runs up a river is known as a tidal bore, and it forms when the incoming tide collides with the outgoing one. Surfers and kayakers can travel great distances on the wave, as it follows the river along its meandering path.   Photograph: Arnold Price/Wikipedia


By Steff Gaulter

While most people were loathing the weather in the UK this winter, there were a small group of people who were actually enjoying the storms. The severe weather generated immense waves, and the surfers were in heaven.

A buoy just off the coast of Cornwall measured a wave that was a staggering 22.8m (75ft). I’m very glad I wasn’t anywhere near that in a boat!

Choppy seas weren’t the only reason that surfers enjoyed the floods, and it’s certainly not what kept the kayakers entertained. A number of people turned their backs on the sea swells and instead headed to the River Severn, in western England, to ride a tidal wave. The tidal wave is nothing new, it’s seen regularly along the river, but the size of it was far larger than usual.

A tidal wave which runs up a river is known as a tidal bore, and it forms when the incoming tide collides with the outgoing one. Surfers and kayakers can travel great distances on the wave, as it follows the river along its meandering path. It’s not only seen on the Severn; the Brahmaputra in India and the Amazon in Brazil both have their own bore, but relatively few rivers in the world have one. In order to generate a bore, the river has to have a particular shape near its mouth; it has to be shallow and uniform. This allows the water to be funnelled into an increasingly narrow channel as the tide rises, thereby forming the large wave.

The Severn estuary is perfect; at Avonmouth it is approximately 8km (5 miles) wide, but by sharpness, only 30km (20 miles) upstream, it is approximately 1.5km (1 mile) wide.

The shape of the river determines the shape and size of the bore. It can vary dramatically between rivers, from gentle undulations to a powerful single wave that bulldozes everything in its path. The largest bore is believed to be the one on the Qiantang River in China. This can regularly be up to 9m (30ft) in height, but at times it can be much higher.

As a bore is generated by the tides, it makes sense that, because some tides are higher than other, some bores on the same river will also be more spectacular. Tides are caused by the gravitational pull of the sun and the moon on the Earth’s waters.

The moon takes just over 24 hours to go round the earth, and this generates two high tides each day. One is fairly logical, in that the moon pulls on the ocean while it is passing over our heads, and this creates a high tide.

The other high tide is slightly more confusing. It occurs because the earth and the moon are both hurtling through space, with gravity sticking them together. As they spin, the water sloshes outwards at its furthest point, as if it were a speeding car hurtling around a sharp bend, nearly flying off the road.

That’s the reason for the tides, but not all tides are the same. As we know, the moon goes round the earth and the earth goes round the sun. However, it takes the moon longer to go round the earth than it takes for the earth to go round the sun. It’s only at certain times that the sun, the moon, and the earth all line up, and when they do the sun and moon work together to produce the greatest tidal ranges.

This is known as a spring tide, and we know when there is a spring tide simply by looking at the moon. When we can only see a thin sliver of the moon, a new moon, then the moon is between the earth and the sun and therefore we see the side of the moon that is in shadow. When there is a full moon, the sun is on the opposite side to the earth. On both of these occasions there is a spring tide.

However, things get even more complicated because the earth doesn’t go round the sun in an exact circle, and neither does the moon when it orbits the earth. At certain times the moon is closer than at others, and so is the sun. When they are closer, the tides are higher.

Clearly the astronomy behind the tides is rather complicated, but the bore is also affected by the weather. If the winds are blowing upstream, the water will be forced along the river and the bore will be bigger. Conversely, if the wind is blowing downstream, the bore will be smaller. Air pressure also has an effect; if it is high, and the air is pressing down on the earth harder, then the bore will be smaller.

In extreme cases, the weather can make a bore extremely dangerous. In August last year, the bore on the Qiantang River towered more than 20m (65ft) high. It was so powerful that a sea wall gave way in Haining City injuring dozens of people. The reason that it was so large wasn’t only because of the tides, it was also intensified by a typhoon which had just made landfall in eastern China.

The winds within a typhoon usually push water ahead of the storm, forming what’s known as a storm surge. The surge from Typhoon Trami joined forces with the normal tides to generate a huge tidal bore, which roared down the river.

It must have been terrifying for those people next to the riverbank, who would have been surprised at its strength. That is a definitely a valuable lesson we could all learn; never underestimate the power of a typhoon.

 

 

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