Information Page: TidalBoreChina
Tidal Bore in China
|Model name:||Animation model name|
|Where:||Qiantang River in China|
Grade level: High (9-12), Under graduate (13-16), Graduate / Professional
Statement: This movie features the tidal bore in the Qiantang River in China.
Abstract: This movie features the tidal bore in the Qiantang River in China. This tidal bore is the largest in the world can be over 6 metres and travels at 40 km per hour. The bore in the movie is exceptionally high, perhaps due to ocean swells. People gather every year to watch this spectacle. Although a few spectators did get washed of their feet, nobody was killed (as reported on the website of the USC tsunami research group).
The movie usually makes people think this is a 'tsunami', but it is not associated with any earthquakes it is an actual 'tidal wave', but it is not a 'tsunami'!
A tidal bore, also called aegir, is a tidal phenomenon in which the leading edge of the incoming tide forms a wave of water that travel up a river or narrow bay against the river current. As such, it is a true tidal wave.
Bores occur in relatively few locations, but they do occur worldwide, usually in areas with a large tidal range (typically more than 6 m between high and low water).
All these locations are shallow, narrowing rivers or fjords in which water is funneled via a broad bay. The funnel-like shape not only increases the height of the tide, but it can also decrease the duration of the flood tide down to a point where the flood appears as a sudden increase in the water level. The rising tide may force the tidal wave-front to move faster that a shallow water wave can propagate into water of that depth:
T = wave period
L = wave length
c = wave speed
u = speed of current
If the current flows counter the direction of wave propagation, then L will increase and the wave will get shorter and higher (upto the point of breaking). Bore tides come in after extreme minus low tides created by the full or new moon (Chanson, 2004).
Bores take on various forms, ranging from a single breaking wavefront —like a shock wave — to ‘undular bores’ comprising a smooth wavefront followed by a train of secondary waves (whelps). Large bores can be dangerous for shipping. Rivers that do have a tidal bore include the Amazon and Orinoco Rivers, in South America, the Hoogly River in the Ganges-Brahmaputra delta, several rivers in the UK, and rivers draining into the Bay of Fundy. The largest tidal bore occurs in the Qiantang River in China, it is 9m high and travels at 40 km/hr.
Tidal bores have distinct influence on sediment transport. The arrival of the borefront is associated with intense bed shear stress and bed scour. Suspended sediment is advected upwards in the wake of the tidal bore. This phase is associated with turbulent structure. The suspension of sediment is sustained by wave motion for several minutes to half an hour after the bore has passed (Chanson, 2004).
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