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But bores are not unique to the Severn.
The
bore exists in the Severn because the estuary is just the
right shape for funnelling the incoming tides of the Bristol
Channel.
The flow of water that rushes up the Severn becomes too great
for the narrowing, increasingly shallow river to accommodate.
When conditions are favourable - low pressure, a strong westerly
wind and a high tide following a new or full moon - the surge
of water is sufficient to form a wave as much as two metres
high, with a speed of 14 knots.
For topographical and hydrographical conditions to combine
in such a way is highly unusual but they do occur elsewhere.
The
bore on the Severn is the biggest of 8 in the UK but there
are thought to be around 60 throughout the world.
These include the Seine and Gironde in France; the Indus in
India; the Amazon in Brazil and the Knik Arm, Alaska.
Anywhere that has an exceptionally large tidal range is favourable
for the formation of a bore. The highest tides on earth are
to be found in the Bay of Fundy, Nova Scotia. To be more precise,
they occur at the eastern end of the Bay in the Minas Basin.
Here the tidal range can reach 16 metres, about three metres
higher than the Bristol Channel.
At
Cape Split, on the southern side of the Basin, the currents
exceed eight knots and the flow through the five kilometre
wide channel is said to equal the combined flow of all the
streams and rivers on Earth (four cubic kilometres per hour)!
The
flow of seawater into the Minas Basin is so great that it
causes the surrounding land to tilt slightly.
Under such extreme conditions it is not surprising then to
learn that tidal bores are found on five rivers within the
Basin.
The
biggest bore in the world is to be found in Hangzhou Bay south
of Shanghai.
The characteristic narrowing, increasingly shallow Bay and
large tidal range combine to produce a bore that on the highest
tides, can attain a height of over seven metres and a speed
of up to 15 knots.
It is said that the roar of the advancing bore can be heard
at a distance of more than 20 kilometres.
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