A tidal bore, 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 against the direction of the river or bay's current.
Bores occur in relatively few locations worldwide, usually in areas with a large tidal range (typically more than between high and low tide) and where incoming tides are funneled into a shallow, narrowing river or lake 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. 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
– to undular bore
s, comprising a smooth wavefront followed by a train of secondary waves
known as whelps.
Large bores can be particularly unsafe for shipping but also present opportunities for river surfing
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.
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.
The word ''bore'' derives through Old English
from the Old Norse
word ''bára'', meaning "wave" or "swell."
Tidal bores can be dangerous. Certain rivers such as the Seine
, the Petitcodiac River
, and the Colorado River
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
(Fly and Bamu Rivers), Malaysia
(Benak at Batang Lupar), and India
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 shrimps (for example in the Rokan River). The tidal bores also provide opportunity for recreational inland surfing
Scientific studies have been carried out at the River Dee
in Wales in the United Kingdom, the Garonne
in France, the Daly River
in Australia, and the Qiantang River
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.
* Indus River
* Sittaung River
* Qiantang River
, which has the world's largest bore,
up to high, traveling at up to
* Batang Lupar or Lupar River, near Sri Aman
. 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 inland, but usually up to with height.
* Styx River
* Daly River
, Northern Territory
* River Shannon
, up the Shannon Estuary
to Limerick, Ireland: 21 September 2013
* River Dee
* River Mersey
. The second highest tidal bore after the Severn bore, up to high. The bore tends to form around the Manchester Ship Canal
* The Severn bore
on the River Severn
, Wales and England, up to high
* The Trent Aegir
on the River Trent
, England, up to 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 Nith
* River Lune
* River Ribble
* River Yealm
* River Leven, Cumbria
The phenomenon is generally named ''un mascaret'' in French. but some other local names are preferred.
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
, and Sée
* Baie de la Frênaye
, locally named ''le mascarin''
Papua New Guinea
* Fly River
* Turama River
* The Turnagain Arm
of Cook Inlet
. Up to and .
* Historically, the Colorado River
had a tidal bore up to 6 feet, that extended 47 miles up river.
* The Savannah River
up to inland.
* Small tidal bores, only a few inches in height, have been observed advancing up tidal bayous on the Mississippi Gulf Coast.
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 in height, but causeway
construction between Moncton
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. The restoration of the bore has been sufficient that in July 2013, professional surfers rode a -high wave 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
rivers in the Minas Basin
, and the Salmon River
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.
* Amazon River
and Orinoco River
, up to high, running at up to . It is known locally as the ''pororoca
["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 ."
* Mearim River
* Araguari River
in Brazil. Very strong in the past, it's considered lost since 2015, due to buffaloes farming, irrigation, and dam construction along the river, leading to substantial loss of water flow.
Lakes with tidal bores
Lakes with an ocean inlet
can also exhibit tidal bores.
* 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.
''Tidal bore research''
(2017) The University of Queensland.
* 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 albeit not as a tidal bore
Information about The Severn bore, UKAmateur video of the "Wiggenhall Wave" tidal boreMascaret, 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
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