Tidal bore


Tidal bore
The tidal bore in Upper Cook Inlet, Alaska

A tidal bore (or simply bore in context, or also aegir, eagre, or eygre) 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 river or bay's current. As such, it is a true tidal wave and not to be confused with a tsunami, which is a large ocean wave traveling primarily on the open ocean.

Contents

The phenomenon

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.[1] 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. Note the tidal bore takes place during the flood tide and never during the ebb tide.

A tidal bore may take on various forms, ranging from a single breaking wavefront with a roller — somewhat like a hydraulic jump[2] — to "undular bores", comprising a smooth wavefront followed by a train of secondary waves ("whelps").[3] Large bores can be particularly unsafe for shipping but also present opportunities for river surfing.[3]

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.[4][5] 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.[6]

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

Rivers with tidal bores

Rivers that have been known to exhibit bores include those listed below.[7]

Asia

  • Ganges–Brahmaputra, India and Bangladesh
  • Indus River, Pakistan
  • Qiantang River, China, which has the world's largest bore, up to 9 metres (30 ft) high, traveling at up to 40 kilometres (25 mi) per hour
  • Batang Lupar or Lupar River, near Sri Aman, Malaysia. The tidal bore is locally known as benak.[3]
  • Bono, Kampar River, Indonesia. The phenomenon is feared by the locals to sink ships. It is reported to break up to 130 kilometres (81 mi) inland.

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.[8] but some other local names are preferred.[7]

North America

United States

Tidal bore on the Petitcodiac River
  • The Turnagain arm of Cook Inlet, Alaska. Up to 2 metres (6.6 ft) and 20 km/h.

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 the highest bore in North America at over 2 metres (6.6 ft); however, causeway construction and extensive silting reduced it to little more than a ripple, until 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.[9]
  • 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.[citation needed] 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.

Mexico

There is a tidal bore on the Sea of Cortez in Mexico at the entrance of the Colorado River. It forms in the estuary about Montague Island and propagates 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.[citation needed]

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.

Impact of tidal bores

The tidal bores may be dangerous and some bores have had a sinister reputation. For example, the tidal bores of the Seine River (France), of the Petitcodiac River (Canada) and of the Colorado River (Mexico). In China, despite warning signs erected along the Qiantang River banks, a number of tragic accidents happen each year.[1] 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[1]. 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 of tidal bores

Scientific measurements in tidal bores are challenging because of the force of the tidal bore flow. This is evidenced by a number of field work incidents in the Dee River, Rio Mearim, Daly River and Sélune River: "during this […] deployment, the [ADCP] instrument was repeatedly buried in sediment after the 1st tidal cycle and had to be dug out of the sediment, with considerable difficulty, at the time of recovery" (Dee River)[11]; "About 20 min after the passage of the bore the two aluminium frames at site C were toppled. […] A 3-min-duration patch of macroturbulence was observed. […] This unsteady motion was sufficiently energetic to topple moorings that had survived much higher, quasi-steady currents of 1.8 m/s" (Daly River)[12]; "the field study experienced a number of problems and failures. About 40 s after the passage of the bore, the metallic frame started to move. The ADV support failed completely 10 minutes after the tidal bore." (Sélune River)[13].

Field studies in United Kingdom

Field studies in France

Field studies in Australia

See also

Moore Bridge.jpg UK Waterways portal
  • 1812 New Madrid earthquake, a historic earthquake in the United States that caused the Mississippi River to flow backwards temporarily
  • Hydraulic jump
  • Tidal race
  • Tonlé Sap, a lake and river system in Cambodia where monsoon flooding can cause the river to flow backwards temporarily
  • Undular bore wave

References

  1. ^ a b c Chanson, H. (2011). Tidal Bores, Aegir, Eagre, Mascaret, Pororoca. Theory and Observations. World Scientific, Singapore. ISBN 978-981-4335-41-6. http://www.worldscibooks.com/engineering/8035.html. 
  2. ^ Chanson, H. (2009). Current Knowledge In Hydraulic Jumps And Related Phenomena. A Survey of Experimental Results. European Journal of Mechanics B/Fluids, Vol. 28, No. 2, pp. 191-210 (DOI: 10.1016/j.euromechflu.2008.06.004 ) (ISSN 0997-7546). http://espace.library.uq.edu.au/view/UQ:162239. 
  3. ^ a b c Chanson, H. (2009). Environmental, Ecological and Cultural Impacts of Tidal Bores, Benaks, Bonos and Burros. Proc. International Workshop on Environmental Hydraulics IWEH09, Theoretical, Experimental and Computational Solutions, Valencia, Spain, 29-30 Oct., Editor P.A. Lopez-Jimenez et al., Invited keynote lecture, 20 pages (CD-ROM). http://espace.library.uq.edu.au/view/UQ:185349. 
  4. ^ Koch, C. and Chanson, H. (2008). Turbulent Mixing beneath an Undular Bore Front. Journal of Coastal Research, Vol. 24, No. 4, pp. 999-1007 (DOI: 10.2112/06-0688.1). http://espace.library.uq.edu.au/view/UQ:151916. 
  5. ^ Koch, C. and Chanson, H. (2009). Turbulence Measurements in Positive Surges and Bores. Journal of Hydraulic Research, IAHR, Vol. 47, No. 1, pp. 29-40 (DOI: 10.3826/jhr.2009.2954). http://espace.library.uq.edu.au/view/UQ:164015. 
  6. ^ Chanson, H. (2009). The Rumble Sound Generated by a Tidal Bore Event in the Baie du Mont Saint Michel. Journal of Acoustical Society of America, Vol. 125, No. 6, pp. 3561-3568 (DOI: 10.1121/1.3124781). http://espace.library.uq.edu.au/view/UQ:178445. 
  7. ^ a b c d e f g h i j Chanson, H. (2008). Photographic Observations of Tidal Bores (Mascarets) in France. Hydraulic Model Report No. CH71/08, Univ. of Queensland, Australia, 104 pages. ISBN 9781864999303. http://espace.library.uq.edu.au/eserv/UQ:158867. 
  8. ^ (French) definition of mascaret
  9. ^ Petitcodiac River changing faster than expected
  10. ^ (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 kilometers."
  11. ^ a b Simpson, J.H., Fisher, N.R., and Wiles, P. (2004). Reynolds Stress and TKE Production in an Estuary with a Tidal Bore. Estuarine, Coastal and Shelf Science, Vol. 60, No. 4, pp. 619-627. 
  12. ^ a b Wolanski, E., Williams, D., Spagnol, S., and Chanson, H. (2004). Undular Tidal Bore Dynamics in the Daly Estuary, Northern Australia. Estuarine, Coastal and Shelf Science, Vol. 60, No. 4, pp. 629-636 (DOI: 10.1016/j.ecss.2004.03.001). http://espace.library.uq.edu.au/view/UQ:74059. 
  13. ^ a b Mouazé, D., Chanson, H., and Simon, B. (2010). Field Measurements in the Tidal Bore of the Sélune River in the Bay of Mont Saint Michel (September 2010). Hydraulic Model Report No. CH81/10, School of Civil Engineering, The University of Queensland, Brisbane, Australia, 72 pages. ISBN 9781742720210. http://espace.library.uq.edu.au/view/UQ:226153. 
  14. ^ Chanson, H., Lubin, P., Simon, B., and Reungoat, D. (2010). Turbulence and Sediment Processes in the Tidal Bore of the Garonne River: First Observations. Hydraulic Model Report No. CH79/10, School of Civil Engineering, The University of Queensland, Brisbane, Australia, 97 pages. ISBN 9781742720104. http://espace.library.uq.edu.au/view/UQ:219711. 
  15. ^ Simon, B., Lubin, P., Reungoat, D., Chanson, H. (2011). Turbulence Measurements in the Garonne River Tidal Bore: First Observations. Proc. 34th IAHR World Congress, Brisbane, Australia, 26 June-1 July, Engineers Australia Publication, Eric Valentine, Colin Apelt, James Ball, Hubert Chanson, Ron Cox, Rob Ettema, George Kuczera, Martin Lambert, Bruce Melville and Jane Sargison Editors, pp. 1141-1148. ISBN 978-0-85825-868-6. http://espace.library.uq.edu.au/view/UQ:243200. 
  16. ^ Chanson, H., Reungoat, D., Simon, B., Lubin, P. (2012). High-Frequency Turbulence and Suspended Sediment Concentration Measurements in the Garonne River Tidal Bore. Estuarine Coastal and Shelf Science (DOI: 10.1016/j.ecss.2011.09.012). ISSN 0272-7714. 

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Look at other dictionaries:

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