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Tidal bore

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Tidal bore

Arnside Bore This video size: 360x240 500 kbit/s
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A bore in Morecambe Bay, the United Kingdom.
The tidal bore in Upper Cook Inlet, Alaska

A tidal bore, often simply given as bore in context,[1] is a tidal phenomenon in which the leading edge of the incoming tide forms a wave (or waves) of water that travels up a river or narrow bay against the direction of the river or bay's current.

Contents

  • Description 1
  • Impact 2
    • Scientific studies 2.1
  • Rivers with tidal bores 3
    • Asia 3.1
    • Australia 3.2
    • Europe 3.3
      • United Kingdom 3.3.1
      • France 3.3.2
    • Papua New Guinea 3.4
    • North America 3.5
      • United States 3.5.1
      • Canada 3.5.2
      • Mexico 3.5.3
    • South America 3.6
  • Lakes with tidal bores 4
    • North America 4.1
  • See also 5
  • References 6
  • External links 7

Description

Bores occur in relatively few locations worldwide, usually in areas with a large tidal range (typically more than 6 metres (20 ft) between high and low water) and where incoming tides are funneled into a shallow, narrowing river or lake via a broad bay.[2] The funnel-like shape not only increases the tidal range, 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. A tidal bore takes place during the flood tide and never during the ebb tide.

Undular bore and whelps near the mouth of Araguari River in north-eastern Brazil. View is oblique toward mouth from airplane at approximately 100 ft (30 m) altitude.[3]

A tidal bore may take on various forms, ranging from a single breaking wavefront with a roller – somewhat like a hydraulic jump[4][5] – to undular bores, comprising a smooth wavefront followed by a train of secondary waves known as whelps.[6] Large bores can be particularly unsafe for shipping but also present opportunities for river surfing.[6]

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. The visual observations of tidal bores highlight the turbulent nature of the surging waters. The tidal bore induces a strong turbulent mixing in the estuarine zone, and the effects may be felt along considerable distances. The velocity observations indicate a rapid deceleration of the flow associated with the passage of the bore as well as large velocity fluctuations.[7][8] 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 bore rumble is heard far away because its low frequencies can travel over long distances. 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.[9]

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

Impact

The tidal bores may be dangerous and many bores have had a sinister reputation: the River Seine (France); the Petitcodiac River (Canada); and the Colorado River (Mexico), to name a few. In China, despite warning signs erected along the banks of the Qiantang River, a number of tragic accidents happen each year.[2] The tidal bores affect the shipping and navigation in the estuarine zone, for example, in Papua New Guinea (Fly and Bamu Rivers), Malaysia (Benak at Batang Lupar), and India (Hoogly bore).

On the other hand, the tidal-bore affected estuaries are the rich feeding zones and breeding grounds of several forms of wildlife.[2] The estuarine zones are the spawning and breeding grounds of several native fish species, while the aeration induced by the tidal bore contribute to the abundant growth of many species of fish and shrimps (for example in the Rokan River).

Scientific studies

Scientific studies have been carried out at the River Dee[10] in the United Kingdom, the Garonne[11][12][13][14][15] and Sélune[16] rivers in France, and the Daly River[17] in Australia. The force of the tidal bore flow often poses a challenge to scientific measurements, as evidenced by a number of field work incidents in the River Dee,[10] Rio Mearim, Daly River,[17] and Sélune River.[16]

Rivers with tidal bores

Rivers that have been known to exhibit bores include those listed below.[2][18]

Asia

Australia

Europe

United Kingdom

The Trent Aegir seen from West Stockwith, Nottinghamshire, 20 September 2005
The Trent Aegir at Gainsborough, Lincolnshire, 20 September 2005
A tidal bore wave moves along the River Ribble between the entrances to the Rivers Douglas and Preston.
Tidal bore on the River Ribble

France

The phenomenon is generally named un mascaret in French.[19] but some other local names are preferred.[18]

Papua New Guinea

North America

United States

Tidal bore on the Petitcodiac River
  • The Turnagain arm of Cook Inlet, Alaska. Up to 2 meters (7 ft) and 20 km/h (12 mph).
  • Historically the Colorado River had a tidal bore up to 6 feet, that extended 47 miles up river.
  • The Savannah River up to 10 miles (16 km) inland.
  • Small tidal bores, only a few inches in height, have been observed advancing up tidal bayous on the Mississippi Gulf Coast.

Canada

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

  • The Petitcodiac River, formerly had the highest bore in North America at over 2 meters (6.6 ft) in height but causeway construction between Moncton and Riverview in the 1960s led to subsequent extensive sedimentation which reduced the bore to little more than a ripple. After considerable political controversy, the causeway gates were opened on April 14, 2010, as part of the Petitcodiac River Restoration Project and the tidal bore began to grow again.[21] The restoration of the bore has been sufficient that in July 2013, professional surfers rode a one metre high wave 29 km up the Petitcodiac River from Belliveau Village to Moncton to establish a new North American record for continuous surfing.
  • The 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 who were in the riverbeds when the bore came in. Tour boat operators offer rafting excursions in the summer.
  • The bore is fastest and highest on some of the smaller rivers that connect to the bay including the River Hebert and Maccan River on the Cumberland Basin, the St. Croix, Herbert and Kennetcook Rivers in the Minas Basin, and the Salmon River in Truro.[22]

Mexico

Historically, there was a tidal bore on the Gulf of California in Mexico at the mouth of the Colorado River. It formed in the estuary about Montague Island and propagated upstream. Once very strong, later diversions of the river for irrigation have weakened the flow of the river to the point the tidal bore has nearly disappeared.

South America

Lakes with tidal bores

Lakes with an ocean inlet can also exhibit tidal bores.

North America

  • Nitinat Lake on Vancouver Island has a sometimes dangerous tidal bore at Nitinat Narrows where the lake meets the Pacific Ocean. The lake is popular with windsurfers due to its consistent winds.

See also

  • 1812 New Madrid earthquake, a historic earthquake in the United States that caused the Mississippi River to flow backwards temporarily
  • Tidal race
  • Tonlé Sap, a lake and river system in Cambodia where monsoon flooding can cause the river to flow backwards temporarily

References

  1. ^ Sometimes also known as an aegir, eagre, or eygre in the context of specific instances in Britain.
  2. ^ a b c d
  3. ^ Figure 5 in:
  4. ^
  5. ^
  6. ^ a b c
  7. ^
  8. ^
  9. ^
  10. ^ a b
  11. ^
  12. ^
  13. ^
  14. ^
  15. ^
  16. ^ a b
  17. ^ a b
  18. ^ a b c d e f g h i j
  19. ^ (French) definition of mascaret
  20. ^ p.159, Barrie R. Bolton. 2009. The Fly River, Papua New Guinea: Environmental Studies in an Impacted Tropical River System. Elsevier Science. ISBN 978-0444529640.
  21. ^ Petitcodiac River changing faster than expected
  22. ^ Vol. I, Chap. T^ "Ocean Currents", p. 109Natural History of Nova Scotia
  23. ^ (English) "Pororoca: surfing the Amazon" indicates that "The record that we could find for surfing the longest distance on the Pororoca was set by Picuruta Salazar, a brazilian surfer who, in 2003, managed to ride the wave for 37 minutes and travel 12.5 kilometres [7.8 mi]."

External links

  • Information about The Severn bore, UK
  • Amateur video of the "Wiggenhall Wave" tidal bore
  • link to Proudman Inst. page
  • 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
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