What is a tidal bore?

April 1, 2019 | Surfing
Tidal bores: a different type of wave, especially for surfers | Photo: Red Bull

A tidal bore, also known as a surge, is a massive and often single wave caused by the constriction of an incoming spring tide, as it gets funneled up a narrow and shallow river.

Bores are relatively rare and usually only take place in regions with a tidal range of more than 20 feet, i.e., in areas where there are huge water level differences between low and high tide.

As the set of waves funnel into a narrow river, bay or lake, they accumulate energy and accelerate against the natural direction of the stream.

A tidal bore will only occur when the tide is rising (flood tide) and never when the tide is falling (ebb tide). As a result, there are only around 60 tidal bores on the planet.

In most cases, a bore will feature a single-breaking wave that travels at speeds of up to 25 miles per hour (40 km/h). But it can also be comprised of a leading edge wave followed by secondary and smaller rollers.

Tidal bore: a rare and often violent phenomenon | Photo: Shutterstock

A Destructive River Wave

Tidal bores can have powerful and destructive consequences, especially in the margins of the rivers. They can blast roads, smash small houses, and wreck cars and landmarks.

These tide-influenced waves are so violent that they tear vegetation, and bring up sediments and sand from the bottom of the river, changing its colors from green and blue to chocolate-brown.

This turbulent wave phenomenon generates a roaring noise that can be heard far away. Despite all the warnings, the world's most powerful tidal bores have already claimed several lives.

Because they're rare and extremely violent, they are often mistakenly seen as tsunamis. But, unlike tsunami waves, the time, strength and size of tidal bores can be predicted.

The world's largest tidal bore takes place in the Qiantang River, near Hangzhou, in China.

Every year, and halfway through the eighth month of the Chinese lunar calendar, thousands of locals and tourists gather on the margins of the Qiantang River to witness the "Silver Dragon."

"When a full moon occurs, the mismatch in size between the local bay and the much narrower river amplifies the effect of the tide creating a powerful tidal bore wave.

Historians have confirmed that the phenomenon attracts huge crowds for at least 3,000 years.

Tidal bores: these waves can be predicted and surfed | Photo: Creative Commons

Surfing Tidal Bores

Tidal wave events have been attracting a growing number of surfers, who time the passage of the wave and ride it for several miles.

The best tidal bores for surfing are Pororoca (Araguari River, São Domingos do Capim, Brazil), The Bono (Kampar River, Indonesia), Silver Dragon (Qiantang River, Haining Province, China), Severn (River Severn, Gloucester, England), Le Mascaret (Seine River, Le Havre, France), Bay of Fundy (Petitcodiac River, Moncton, Canada), Turnagain Arm (Cook Inlet, Gulf of Alaska, Anchorage, USA), and The Baan (Hooghly River, Kolkata, India).

The Guinness World Record for the longest ride on a river bore belongs to James Cotton, an Australian surfer who rode Indonesia's Kampar River wave for 10.6 miles (17.2 kilometers) and at an average speed of 12.5 miles per hour (20 km/h).

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