Weather

Tidal waves hit Haining, China

World’s largest tidal bore sweeps down the Qiantang River.

The surging Qiantang tidal bore has wowed spectators at the Mid-Autumn Festival in Haining, in eastern China's Zhejiang province.

The bore travelled 55km from the mouth of the Qiantang River to Daquekou, one of the best observation points along the river.

The waves towered nearly seven metres high, according to Reuters, before crashing into a dam wall one after the other.

A tidal bore is a series of waves which runs up a river, formed 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.

Bores can only form when the mouth of a river is shallow and uniform, so they are seen in relatively few rivers of the world. However, they can be seen in the River Severn in England, the Brahmaputra in India and the Amazon in Brazil.

As the tide rises, the water is funnelled into an increasingly narrow channel, thereby forming the large waves. The precise dimensions of the rivers determines their exact shape and size.

The tidal bore that forms on the Qiantang River is believed to be the largest one in the world. It can regularly be up to nine metres (30ft) in height, but at times it can be much higher.

As a bore is generated by the tide, just as some tides are bigger than others, some bores are also more spectacular than others on the same river.

The size of the tidal wave is also affected by the weather. If the winds are blowing upstream, the water will be forced along the river creating a larger bore. Conversely, if the wind is blowing downstream, the bore will be smaller.

In extreme cases, the weather can make a bore extremely dangerous. In August 2013, a typhoon made landfall in eastern China, causing the Qiantang bore to swell more than 20 metres. It was so powerful that a seawall gave way in Haining injuring dozens of people.

Additional reporting by Steff Gaulter.

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