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The below mentioned article provides a note on tidal bores.
Tidal bores are steep wall of seawater moving upstreams from their mouths when the tidal waves enter the low-lying rivers. “A tidal bore (bore = crest or wave) is a wall of water that moves up certain low- lying rivers due to an incoming tide. Because it is a wave created by the tides, it may be considered to be true tidal wave”.
Tidal bores are formed when tidal waves with great height i.e. high tidal range (here tidal range means height of the crests of tidal waves), enters a narrow and low-lying river, which debauches in the sea or bay, and rushes upstream and is obstructed by the flow of the river. Consequently, the tide water is forced to have a steep wall-like crest.
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The following conditions are conducive for the occurrence of tidal bore:
1. Narrow and low-lying coastal river with gentle channel gradient,
2. A bay with narrow opening and tappering head,
3. Large tidal range (wave height), usually more than 5 meters, and
4. Upward decreasing water depth, meaning thereby decreasing depth of water in upriver section, etc.
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When tide water enters the coastal river with high tidal range, the seaward flow of the river obstructs the incoming tide, with the result the front of tidal wave crest is forced to assume high tidal range. Thus, the tidal wave with steep tidal wall of more than 5 m in height rushes upstream or upbay.
This wall-like high tidal waves are called tidal bores which move upstream with average speed of 20-22 kilometers per hour. As the tidal bores move upstream, their effects continue to decrease. The Amazon River of South America has 800 km long estuary which is travelled by tidal bores.
The estuary of the Chientang river of China is known to have the highest tidal bore, measuring more than 8 meters in height, in the world. Large tidal bores are noticed in Petitcodiac river of New Brunswick of Canada, the Seine River of France etc. Tidal bores also occur in Hooghly river of India.
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It may be mentioned that it is not the tidal range which always determines the height of tidal bores. For example, the Bay of Fundy of Nova Scotia has the largest tidal range in the world (17 meters) but the tidal bore has the height of about one meter or so. This is because the Bay of Fundy has quite wide mouth at its opening. Thus, it is evident that large tidal bore can occur only in those coastal rivers and bays which have narrow and constricted openings in the sea.