Tidal bore

From Wikipedia, the free encyclopedia

A tidal bore (or just bore, or aegir) is a tidal phenomenon in which the leading edge of the incoming tide forms a wave (or waves) of water that travel up a river or narrow bay against the direction of the current. As such, it is a true tidal wave (not to be confused with a tsunami).

Contents 1 The phenomena 2 Asia 3 Australia 4 Europe 4.1 United Kingdom 4.2 France 4.3 Norway 5 North America 6 South America 7 References 8 See also 9 External links

[edit] The phenomena

Bores occur in relatively few locations worldwide, usually in areas with a large tidal range (typically more than 20 feet (6.1 m) between high and low water), and where incoming tides are funneled into a shallow, narrowing river via a broad bay. The funnel-like shape not only increases the height of the tide, but it can also decrease the duration of the flood tide down to a point where the flood appears as a sudden increase in the water level.

Bores take on various forms, ranging from a single breaking wavefront — somewhat like a shock wave — to ‘undular bores’ comprising a smooth wavefront followed by a train of secondary waves (whelps). Large bores can be particularly dangerous for shipping, but also present opportunities for river surfing.

The word bore derives through Old English from the Old Norse word bára, meaning a wave or swell.

Rivers that have been known to exhibit bores include those listed below. ^[1]

[edit] Asia

• Ganges–Brahmaputra, India, Bangladesh
• Indus River, India, Pakistan
• Qiantang River, China,Tidal bore of Qiantang River which has the world's largest bore, up to 9 meters (30 ft) high, traveling at up to 40 km per hour (25 miles an hour).
• Batang Lupar or Lupar River, near Sri Aman, Malaysia. The tidal bore is locally known as benak.

[edit] Australia

• Styx River, Queensland, Australia
• Daly River, Northern Territory, Australia

[edit] Europe

[edit] United Kingdom

• River Dee, Wales / England
• River Mersey
• River Severn, Wales / England up to 2 metres (6.6 ft) high
• River Trent, (the Aegir) up to 1.5 metres (4.9 ft) high, England. Also other tributaries of the Humber Estuary
• River Parrett
• River Welland
• River Kent
• River Great Ouse
• River Ouse, Yorkshire, like the Trent bore, this is also known as "the Aegir".
• River Eden
• River Esk
• River Nith

[edit] France

The phenomenon is generally named un mascaret in French^[2] but some other local names are preferred.^[1]

• Seine, locally named la barre, had a significant bore until the 1960s. Since then it has been practically eliminated by dredging and river training ^[1].
• Baie du Mont Saint Michel including Couesnon, Sélune, Sée.^[1]
• Arguenon ^[1]
• Baie de la Frênaye ^[1]
• Vire ^[1]
• Sienne ^[1]
• Vilaine, locally named le mascarin
• Dordogne ^[1]
• Garonne ^[1]

[edit] Norway

• Saltstraumen near Bodø, claimed to be the strongest tidal current in the world.

[edit] North America

• Turnagain arm of Cook Inlet, Alaska. Up to 2 metres (6.6 ft) and 20 km per hour.

Most rivers draining into the upper Bay of Fundy between Nova Scotia and New Brunswick have tidal bores. Notable ones include:

• Petitcodiac River formerly the highest bore in North America at over 2 metres (6.6 ft) high. It was reduced to little more than a ripple due to causeway construction and extensive siltation.
• Shubenacadie River, also off the Bay of Fundy in Nova Scotia. When the tidal bore approaches, completely drained riverbeds are filled. It has claimed the lives of several tourists that were in the riverbeds when the bore came in.^{[citation needed]} Tourboat operators offer rafting excursions in summer.
• The bore is fastest and tallest in some of the smaller rivers that connect to the Bay including the River Hebert and Maccan River on Cumberland Basin, the St. Croix, Herbert and Kennetcook Rivers in the Minas Basin, and the Salmon River in Truro.

[edit] South America

• Amazon River in Brazil and Orinoco River in Venezuela, up to 4 metres (13 ft) high, running at up to 20 km/h (13 miles per hour). It is known locally as the pororoca.^[3]
• Mearim River in Brazil.
• Araguari River in Brazil.

[edit] References

[edit] See also

• 1812 New Madrid earthquake (in the US) (causing the Mississippi River to flow backwards)
• Hydraulic jump
• Tidal race
• Tonlé Sap (lake and river system in Cambodia) (where monsoon flooding causes the river to flow backwards)
• Undular bore wave

[edit] External links

Look up tidal bore or eagre in Wiktionary, the free dictionary.

• Quiantang River Tidal Bore in China, USC Tsunami Research Group
• Amateur video of the "Wiggenhall Wave" tidal bore
• link to Proudman Inst. page
• The Tidal Bore of the Seine River, France
• Tidal bores, Mascaret, Pororoca. Myths, Fables and Reality !!!
• Mascaret, Aegir, Pororoca, Tidal Bore. Quid ? Où? Quand? Comment? Pourquoi ? in Journal La Houille Blanche, No. 3, pp. 103-114]
• Turbulent Mixing beneath an Undular Bore Front in Journal of Coastal Research, Vol. 24, No. 4, pp. 999-1007 (DOI: 10.2112/06-0688.1)