Hundreds of people gathered to watch a spectacular tidal bore on the Qiantang River in the city of Hangzhou got more than they expected when a massive wave reared up and struck the crowd. Even as they made a mad dash for safety, several people were knocked off their feet and others were washed away into the river. 

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A regular interaction between oceans and rivers, a tidal bore occurs when an incoming high tide meets the out-flow of a river. Under normal circumstances, a bore progresses up the riverbank as a "slow-motion wave" and can flow several miles up a river's course. However, depending on the power of the river, a bore can even be stationary for a time before tidal action wins out. Several rivers, large and small, experience a bore under the right conditions, and are major tourish draws. Amazonian bores are particularly favored by surfers.

The Qiantang River is famous for its record-setting bores, owing to its "trumpet-like" mouth emptying into the East China Sea via Hangzhou Bay. With the banks of the river acting like a funnel, approaching oceanic tides are amplified, resulting in bores over 30 feet in height that can race up the riverbed at 25 miles an hour. The bores are infamous to shipping in the Qiantang, and often hinder vessels docking at Hangzhou.

Every year, on the 18th day of the eighth lunar month, millions of people flock to Hangzhou from all over the world see the wave. It is one of China's oldest tourist sites, drawing spectators since at least 1056.

But then as now, sometimes the experience does not always go as planned. 

In August of last year, more than 30 people were injured by the tidal bore, given extra force by Typhoon Trami.

This year, as well as knocking over spectators and sucking some into the river, the wave dragged off vehicles and flooded a water-treatment plant. No injuries have yet been reported, but photos show several people were left dazed by the incident.