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Whilst
the last few years have seen few impressive Severn Bores, Saturday
morning's Bore (March 12 2005) is expected to be one of the biggest
of the year - a 'four star' bore.
The
Severn Bore is a spectacular natural phenomenon which attracts sightseers
and surfers from near and far. 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 2 metres high, with a speed of 14 knots.
Highly
unusual
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 3 metres higher than the Bristol
Channel.
Channel
At
Cape Split, on the southern side of the Basin, the currents exceed
8 knots and the flow through the 5 kilometre wide channel is said
to equal the combined flow of all the streams and rivers on Earth
(4 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
5 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 7 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.
Location
To
view the bore on Saturday morning's tide you will need to take up
your viewing position about 20 minutes before the forecast times.
Make sure you are not in a location which will flood should the
bore overtop the River.
The
times and recommended viewing locations are as follows:
Minsterworth
at 0921; Stonebench at 0936; and Over Bridge at 0956.
Article
by Richard Angwin
If
you would like to comment on this story get in touch at gloucestershire@bbc.co.uk
or have your say on our
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We're
expecting one of the biggest Severn Bores of the year on Saturday
morning. ÃÛÑ¿´«Ã½ Weatherman Richard Angwin investigates...
