The tidal bores affect the shipping and navigation in the estuarine zone, for example, in Papua New Guinea (in the Fly and Bamu Rivers), Malaysia (the Benak in the Batang Lupar), and India (the Hooghly River bore). In China, despite warning signs erected along the banks of the Qiantang River, a number of fatalities occur each year by people who take too much risk with the bore. Certain rivers such as the Seine in France, the Petitcodiac River in Canada, and the Colorado River in Mexico to name a few, have had a sinister reputation in association with tidal bores. ![]() The word bore derives through Old English from the Old Norse word bára, meaning "wave" or "swell." The low-frequency sound is a characteristic feature of the advancing roller in which the air bubbles entrapped in the large-scale eddies are acoustically active and play the dominant role in the rumble-sound generation. The bore rumble is heard far away because its low frequencies can travel over long distances. A tidal bore creates a powerful roar that combines the sounds caused by the turbulence in the bore front and whelps, entrained air bubbles in the bore roller, sediment erosion beneath the bore front and of the banks, scouring of shoals and bars, and impacts on obstacles. The velocity observations indicate a rapid deceleration of the flow associated with the passage of the bore as well as large velocity fluctuations. The tidal bore induces a strong turbulent mixing in the estuarine zone, and the effects may be felt along considerable distances. The visual observations of tidal bores highlight the turbulent nature of the surging waters. Two key features of a tidal bore are the intense turbulence and turbulent mixing generated during the bore propagation, as well as its rumbling noise. ![]() Large bores can be particularly unsafe for shipping but also present opportunities for river surfing. Ī tidal bore may take on various forms, ranging from a single breaking wavefront with a roller – somewhat like a hydraulic jump – to undular bores, comprising a smooth wavefront followed by a train of secondary waves known as whelps. The view is oblique towards the mouth from airplane at approximately 30 m (100 ft) altitude. ![]() Undular bore and whelps near the mouth of Araguari River in northeastern Brazil.
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