A tidal bore is a type of wave, which may be formed when a rising tide enters a gently sloping river. The bore occurs at the point where the incoming tide meets the downstream flow of the river, dividing the river into sections with opposite directions of flow. Ahead of the bore the river flows downstream, while after the bore has passed the flow is upstream. The bore may have a breaking crest or it may be a smooth wave. Many bores are smooth in mid-channel and breaking near the banks. The character of of a particular bore often changes as the shape and depth of the river changes.
Bores tend to be found in places with large tidal ranges, such as the coasts of Britain and France, the mouth of the Amazon, the Indian Ocean, and the Bay of Fundy. The greatest of all bores forms on the Qiantang river near Hangzhou in China. It is reported to exceed 3m in height and to travel at speeds in excess of 25 km/hour.
In Britain, there are high spring tides on or around the following dates in the second half of 1999:
Saturday 28th August - Monday 30th August
Saturday 25th September - Wednesday 29th September (especially large tidal range Mon 27th)
Sunday 24th October - Thursday 28th October (especially large tidal range Mon 25th - Wed 27th)
Monday 22nd November - Friday 26th November
Wednesday 22nd December - Friday 24th December
More Information
An excellent introduction to the phenomenon may be found in the article:
Tidal Bores
David K Lynch
Scientific American, 247(4), October 1982, pages 134-143
Books
Bores, breakers, waves and wakes
R. A. R. Tricker
1964, Mills and Boon
Two chapters, one largely descriptive, the other covers the
elementary theory of tidal bores.
The Severn bore
F. W. Rowbotham
1983, David & Charles
Tides, surges and mean sea level
David T. Pugh
1987, John Wiley & Sons
ISBN 047191505X
The chapter on shallow water dynamics includes a section about
tides in rivers & bores.
