Tidal bore explained

A tidal bore,^[1] often simply given as bore in context, 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, reversing the direction of the river or bay's current. It is a strong tide that pushes up the river, against the current.

Description

Bores occur in relatively few locations worldwide, usually in areas with a large tidal range (typically more than 60NaN0 between high and low tide) 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.

A tidal bore may take on various forms, ranging from a single breaking wavefront with a roller – somewhat like a hydraulic jump^{[3] [4]} – to undular bores, comprising a smooth wavefront followed by a train of secondary waves known as whelps.^[5] Large bores can be particularly unsafe for shipping but also present opportunities for river surfing.^[5]

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.^{[6] [7]} 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.^[8]

Etymology

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

Effects

Tidal bores can be dangerous. 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. 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. 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).

On the other hand, tidal bore-affected estuaries are rich feeding zones and breeding grounds of several forms of wildlife. The estuarine zones are the spawning and breeding grounds of several native fish species, while the aeration induced by the tidal bore contributes to the abundant growth of many species of fish and shrimp (for example in the Rokan River, Indonesia). The tidal bores also provide opportunity for recreational inland surfing, such as the Seven Ghosts bore on the Kampar River, Indonesia.

Scientific studies

Scientific studies have been carried out at the River Dee^[9] in Wales in the United Kingdom, the Garonne^{[10] [11] [12] [13] [14]} and Sélune^[15] in France, the Daly River^[16] in Australia, and the Qiantang River estuary^[17] in China. 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, Rio Mearim, Daly River, and Sélune River.

Rivers and bays with tidal bores

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

Asia

• Ganges–Brahmaputra, India and Bangladesh
• Indus River, India and Pakistan
• Sittaung River, Burma
• Qiantang River, China, which has the world's largest bore, up to 9m (30feet) high, traveling at up to 40km/h
• Batang Lupar or Lupar River, near Sri Aman, Malaysia. The tidal bore is locally known as benak.
• Batang Sadong or Sadong River, Sarawak, Malaysia.
• Bono, Kampar River, at Meranti Bay, Pelalawan, Indonesia. The phenomenon is feared by the locals to sink ships. It is reported to break up to 130km (80miles) inland, but usually up to 40km (30miles) with 6m (20feet) height.^[19]

Oceania

Australia

• Styx River, Queensland
• Daly River, Northern Territory

Papua New Guinea

• Fly River^[20]
• Turama River

Europe

Ireland

• River Shannon, up the Shannon Estuary to Limerick, Ireland: 21 September 2013

United Kingdom

• River Dee, Wales and England
• River Mersey. The second highest tidal bore after the Severn bore, up to 1.70NaN0 high. The bore tends to form around the Manchester Ship Canal.
• The Severn bore on the River Severn, Wales and England, up to 20NaN0 high
• The Trent Aegir on the River Trent, England, up to 1.50NaN0 high. Also other tributaries of the Humber Estuary.
• River Parrett
• River Welland
• The Arnside Bore on the River Kent
• River Great Ouse
• River Ouse, Yorkshire. Like the Trent bore, this is also known as "the Aegir".
• River Eden
• River Esk
• River Nene. This was also known as the Eagre.
• River Nith
• River Lune, Lancashire
• River Ribble, Lancashire
• River Yealm, Devon
• River Leven, Cumbria

Belgium

• Durme, Flanders

France

The phenomenon is generally named un mascaret in French.^[21] but some other local names are preferred.

• Seine had a significant bore until the 1960s, locally named la barre. Since then, it has been practically eliminated by dredging and river training.
• Bay of Mont-Saint-Michel including Couesnon, Sélune, and Sée
• Arguenon
  • Vire
• Sienne
• Vilaine, locally named le mascarin
• Dordogne
• Garonne

North America

United States

• The Turnagain Arm of Cook Inlet, Alaska. Up to 20NaN0 and 20km/h.
• Historically, the Colorado River had a tidal bore up to 6 feet, that extended 47 miles up river.
• The Savannah River up to 100NaN0 inland.
• Small tidal bores, only a few inches in height, have been observed advancing up tidal bayous on the Mississippi Gulf Coast.
• The bay inlet of the Crissy Field Marsh, in San Francisco, California, can exhibit tidal bores near high tide.

Canada

With the Bay of Fundy having the highest tidal range in the world, most rivers draining into the upper bay between Nova Scotia and New Brunswick have significant tidal bores. They include:

• The Petitcodiac River formerly had the highest bore in North America at over 2m (07feet) 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.^[22] The restoration of the bore has been sufficient that in July 2013, professional surfers rode a 1m (03feet)-high wave 29km (18miles) up the Petitcodiac River from Belliveau Village to Moncton to establish a new North American record for continuous surfing.^[23]
• The Shubenacadie River in Nova Scotia. When the tidal bore approaches, completely drained riverbeds are filled. It has caused the deaths 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 and Kennetcook rivers in the Minas Basin, and the Salmon River in Truro.^[24]

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. It was once very strong, but diversions of the river for irrigation have weakened the flow of the river to the point the tidal bore has nearly disappeared.

South America

Brazil

• Amazon River in Brazil, up to 40NaN0 high, running at up to 13mph. It is known locally as the pororoca.^[25]
• Mearim River in Brazil
• Araguari River in Brazil. Very strong in the past, it is considered lost since 2015, due to buffaloes farming, irrigation, and dam construction along the river, leading to substantial loss of water flow.

Venezuela

• Orinoco River in Venezuela

Chile

• Southern Channels, ex. Canal de Castro, Chiloé Island (fjord of Castro) in Chile

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
• Tsunami
• Tonlé Sap, a lake and river system in Cambodia where monsoon flooding can cause the river to flow backwards temporarily albeit not as a tidal bore

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. 103–14
• Turbulent Mixing beneath an Undular Bore Front in Journal of Coastal Research, Vol. 24, No. 4, pp. 999–1007
• Tidal bore research (2017) The University of Queensland.

Notes and References

1. Sometimes also known as an aegir, eagre, or eygre in the context of specific instances in Britain.
2. Book: Chanson, H. . Tidal Bores, Aegir, Eagre, Mascaret, Pororoca. Theory and Observations . World Scientific, Singapore . 2011 . 978-981-4335-41-6. Hubert Chanson .
3. Chanson, H. . Momentum considerations in hydraulic jumps and bores . Journal of Irrigation and Drainage Engineering. ASCE. 138 . 4 . 382–85 . 2012 . 0372-0187 . 10.1061/(ASCE)IR.1943-4774.0000409. Hubert Chanson .
4. Chanson, H. . Current Knowledge In Hydraulic Jumps And Related Phenomena. A Survey of Experimental Results . European Journal of Mechanics B . 28 . 2 . 191–210 . 10.1016/j.euromechflu.2008.06.004 . 0997-7546 . 2009 . 2009EJMF...28..191C . Hubert Chanson .
5. Book: Chanson, H. . 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 October Editor P.A. Lopez-Jimenez et al., Invited keynote lecture, 20 pp. (CD-ROM) . 2009 . Hubert Chanson .
6. . Turbulent Mixing beneath an Undular Bore Front . Journal of Coastal Research. 24. 4. 999–1007 . 10.2112/06-0688.1 . 2008 . 130530635 .
7. . Turbulence Measurements in Positive Surges and Bores . Journal of Hydraulic Research . 47. 1. 29–40 . 10.3826/jhr.2009.2954 . 2009 . 2009JHydR..47...29K . 124743367 .
8. Chanson, H. . The Rumble Sound Generated by a Tidal Bore Event in the Baie du Mont Saint Michel . Journal of the Acoustical Society of America. 125. 6. 3561–68 . 10.1121/1.3124781. 19507938 . 2009 . 2009ASAJ..125.3561C . Hubert Chanson .
9. Simpson, J.H., Fisher, N.R., and Wiles, P. . Reynolds Stress and TKE Production in an Estuary with a Tidal Bore . Estuarine, Coastal and Shelf Science. 60. 4. 619–27 . 2004 . 10.1016/j.ecss.2004.03.006. 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.. 2004ECSS...60..619S .
10. Book: Chanson, H., Lubin, P., Simon, B., and Reungoat, D. . 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 pp.. 2010 . 978-1-74272-010-4.
11. Book: . 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. 1141–48 . 2011 . 978-0-85825-868-6.
12. Chanson, H., Reungoat, D., Simon, B., Lubin, P. . High-Frequency Turbulence and Suspended Sediment Concentration Measurements in the Garonne River Tidal Bore . Estuarine, Coastal and Shelf Science . 10.1016/j.ecss.2011.09.012 . 2012 . 0272-7714. 2011ECSS...95..298C . 95 . 2–3 . 298–306. 10.1.1.692.2537 .
13. Reungoat, D., Chanson, H., Caplain, C. . Sediment Processes and Flow Reversal in the Undular Tidal Bore of the Garonne River (France) . . 10.1007/s10652-013-9319-y . 2014 . 1567-7419 . 14 . 3 . 591–616. 2014EFM....14..591R . 14357850 .
14. Book: Reungoat, D., Chanson, H., Keevil, C. . Turbulence, Sedimentary Processes and Tidal Bore Collision in the Arcins Channel, Garonne River (October 2013) . Hydraulic Model Report No. CH94/14, School of Civil Engineering, the University of Queensland, Brisbane, Australia, 145 Pp. . 2014 . 9781742721033 .
15. Book: Mouazé, D., Chanson, H., and Simon, B. . 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 pp. . 2010 . 978-1-74272-021-0. 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..
16. . Undular Tidal Bore Dynamics in the Daly Estuary, Northern Australia . Estuarine, Coastal and Shelf Science. 60. 4. 629–36 . 10.1016/j.ecss.2004.03.001 . 2004 . 2004ECSS...60..629W . 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..
17. Li . Ying . Pan . Dong-Zi . Chanson . Hubert . Pan . Cun-Hong . Real-time characteristics of tidal bore propagation in the Qiantang River Estuary, China, recorded by marine radar . Continental Shelf Research . July 2019 . 180 . 48–58 . 10.1016/j.csr.2019.04.012 . Elsevier . 2019CSR...180...48L . 155917795 . The Qiantang River tidal bore was recorded at two different geographical locations. Characteristic flow patterns were derived and analysed, including temporal changes over a relatively large-scale area. The experimental results showed that the radar-derived celerity and calculated height of the tidal bore were consistent with visual observations in this estuarine zone..
18. Book: Chanson, H. . Photographic Observations of Tidal Bores (Mascarets) in France . Hydraulic Model Report No. CH71/08, Univ. of Queensland, Australia, 104 pp. . 2008 . 978-1-86499-930-3 . Hubert Chanson .
19. Web site: Riau to Introduce Bono Wave to International Tourism . Ryan Novitra . February 3, 2017.
20. p. 159, Barrie R. Bolton. 2009. The Fly River, Papua New Guinea: Environmental Studies in an Impacted Tropical River System. Elsevier Science. .
21. definition of mascaret
22. http://www.cbc.ca/canada/new-brunswick/story/2010/06/07/nb-petitcodiac-river-changes-619.html Petitcodiac River changing faster than expected
23. Web site: Surf's Up -- in Canada! Small New Brunswick Town Becomes International Surfing Hotspot . . https://web.archive.org/web/20230401213845/https://abcnews.go.com/Travel/surfs-canada-small-brunswick-town-international-surfing-hotspot/story?id=21450094 . 2023-04-01 . live .
24. http://ojs.library.dal.ca/NSM/article/view/3753 Natural History of Nova Scotia Vol. I, Chap. T "Ocean Currents", p. 109
25. "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 12.51NaN1."