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The tidal bore in Upper Cook Inlet, Alaska

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).

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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 via a broad bay. 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[1] — to ‘undular bores’, comprising a smooth wavefront followed by a train of secondary waves (whelps)[2]. Large bores can be particularly dangerous for shipping, but also present opportunities for river surfing[2].

Two key features of a tidal bore are the intense turbulence and turbulent mixing generated during the bore propagation, as well as its rumble 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 [3] [4]. 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. During his expedition in the Qiantang River mouth, Captain Moore heard the first murmur of the tidal bore one hour before it reached his Pandora ship. 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 [5].

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

Rivers with tidal bores

Rivers that have been known to exhibit bores include those listed below.[6]

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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

France

The phenomenon is generally named un mascaret in French[8] but some other local names are preferred.[6]

North America

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

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 have reduced it to little more than a ripple.
  • 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 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 Cumberland Basin, the St. Croix, Herbert and Kennetcook Rivers in the Minas Basin, and the Salmon River in Truro.

South America

References

  1. ^ 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).  
  2. ^ a b 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).  
  3. ^ 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).  
  4. ^ 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).  
  5. ^ 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).  
  6. ^ 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.  
  7. ^ Chanson, H. (2009). Environmental, Ecological and Cultural Impacts of Tidal Bores, Benaks, Bonos and Burros. Proceedings of the International Workshop on Environmental Hydraulics IWEH09, Theoretical, Experimental and Computational Solutions, Valencia, Spain, 29-30 Oct., P.A. LOPEZ-JIMENEZ, V.S. FUERTES-MIQUEL, P.L. IGLESIAS-REY, G. LOPEZ-PATINO, F.J. MARTINEZ-SOLANO, and G. PALAU-SALVADOR Eds., Invited keynote lecture, 20 pages (CD-ROM).  
  8. ^ (French) definition of mascaret
  9. ^ (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."

See also

External links


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