The following are approximate times to expect the bore at various locations relative to High Water Avonmouth (HWA) – tidal predictions for Avonmouth can be obtained here. This, together with other influences such as wind, may also alter the timing of the bore by about a quarter of an hour. For example, if there is a lot of extra water in the river due to heavy rainfall the bore is diminished. However, a good bore can't be guaranteed as it is affected by other factors. Hence there are probably just a few occasions in the year at spring tides in the months around the equinoxes in spring and autumn when the bores are at their most spectacular. However it is generally at its best when a very high spring tide is predicted, something in excess of 14 m at Avonmouth. The bore occurs approximately 250 times per year, ranging from a small ripple to the highest waves. It is at its most developed between Minsterworth and Gloucester although it is certainly worth seeing in its estuary form above Newnham. The Severn Bore can be seen from its first small beginnings near Sharpness all the way up to the Maisemore Weir above Gloucester. At Maisemore the sound of the river passing over the weir and the sound of the bore rushing upstream are cancelled instantly as the bore simultaneously drowns the weir and is itself halted. There are many other interesting effects associated with the passage of the bore. Often debris torn from the banks by the bore is caught in the after-rush sometimes including whole trees. The turbulence following the bore stirs up the sediment so the river can appear to change colour. With the passage of the bore the whole character of the river changes instantly from a calm, tranquil, slowly ebbing stream to a turbulent, fast flowing river with a strong upstream current. It can be seen as a wave or series of waves, as much as 2 m in height, that sweep up the River Severn, breaking in the shallows at the side of the river, sometimes splashing onlookers.īeing the onset of the flood tide, there is a rapid rise in water level, which continues for about one and a half hours after the bore has passed. In the UK the most famous and spectacular is the Severn Bore, and it attracts thousands of tourists. However, the world's largest tidal river bore is on the Qiantang River in south-east China. This is a consequence of high tidal ranges occurring in several locations around the British Isles – three rivers with notable bores are the Severn, Dee and Mersey. Of the one hundred or so rivers around the world known to produce bores, around a fifth of these are in the United Kingdom. In a few rivers, the behaviour is remarkably different and the onset of the flood tide is marked by a distinct, sometimes very vigorous wave – a bore. In most tidal rivers the change from ebb to flood is gradual: the ebb current downstream slows, there is a period of slack water, then slowly the flood tide starts flowing upstream.
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