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.
Bores occur in relatively few locations worldwide, usually in areas with a large tidal range (typically more than 6 meters (20 ft) 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 [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."
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. [2] 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. [2] 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 have been carried out at the River Dee [10] in Wales in the United Kingdom, the Garonne [11] [12] [13] [14] [15] and Sélune [16] in France, the Daly River [17] in Australia, and the Qiantang River estuary [18] 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, [10] Rio Mearim, Daly River, [17] and Sélune River. [16]
Rivers and bays that have been known to exhibit bores include those listed below. [2] [19]
The phenomenon is generally named un mascaret in French. [23] but some other local names are preferred. [19]
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:
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.
Lakes with an ocean inlet can also exhibit tidal bores.[ citation needed ]
The Garonne is a river that flows in southwest France and northern Spain. It flows from the central Spanish Pyrenees to the Gironde estuary at the French port of Bordeaux – a length of 529 km (329 mi), of which 47 km (29 mi) is in Spain ; the total length extends to 602 km (374 mi) if one includes the Gironde estuary between the river and the sea. Its basin area is 56,000 km2 (22,000 sq mi), which increases to 84,811 km2 (32,746 sq mi) if the Dordogne River, which flows from the east and joins the Garonne at Bec d'Ambès to form the Gironde estuary, is included.
The Severn bore is a tidal bore seen on the tidal reaches of the River Severn in south western England. It is formed when the rising tide moves into the funnel-shaped Bristol Channel and Severn Estuary and the surging water forces its way upstream in a series of waves, as far as Gloucester and beyond. The bore behaves differently in different stretches of the river; in the lower, wider parts it is more noticeable in the deep channels as a slight roller, while the water creeps across the sand and mudflats. In the narrower, upper reaches, the river occupies the whole area between its banks and the bore advances in a series of waves that move upstream. Near Gloucester, the advancing water overcomes two weirs, and sometimes one in Tewkesbury, before finally petering out.
An estuary is a partially enclosed coastal body of brackish water with one or more rivers or streams flowing into it, and with a free connection to the open sea. Estuaries form a transition zone between river environments and maritime environments and are an example of an ecotone. Estuaries are subject both to marine influences such as tides, waves, and the influx of saline water, and to fluvial influences such as flows of freshwater and sediment. The mixing of seawater and freshwater provides high levels of nutrients both in the water column and in sediment, making estuaries among the most productive natural habitats in the world.
A hydraulic jump is a phenomenon in the science of hydraulics which is frequently observed in open channel flow such as rivers and spillways. When liquid at high velocity discharges into a zone of lower velocity, a rather abrupt rise occurs in the liquid surface. The rapidly flowing liquid is abruptly slowed and increases in height, converting some of the flow's initial kinetic energy into an increase in potential energy, with some energy irreversibly lost through turbulence to heat. In an open channel flow, this manifests as the fast flow rapidly slowing and piling up on top of itself similar to how a shockwave forms.
Coos Bay is an estuary where the Coos River enters the Pacific Ocean, the estuary is approximately 12 miles long and up to two miles wide. It is the largest estuary completely within Oregon state lines. The Coos Bay watershed covers an area of about 600 square miles and is located in northern Coos County, Oregon, in the United States. The Coos River, which begins in the Oregon Coast Range, enters the bay from the east. From Coos River, the bay forms a sharp loop northward before arching back to the south and out to the Pacific Ocean. Haynes Inlet enters the top of this loop. South Slough branches off from the bay directly before its entrance into the Pacific Ocean. The bay was formed when sea levels rose over 20,000 years ago at the end of the Last Glacial Maximum, flooding the mouth of the Coos River. Coos Bay is Oregon's most important coastal industrial center and international shipping port, with close ties to San Francisco, the Columbia River, Puget Sound and other major ports of the Pacific rim.
Hangzhou Bay is a funnel-shaped inlet of the East China Sea, bordered by the province of Zhejiang and the municipality of Shanghai, which lies north of the Bay. The Bay extends from the East China Sea to its head at the city of Hangzhou, from which its name is derived.
Yaquina Bay is a coastal estuarine community found in Newport, Oregon. Yaquina Bay is a semi-enclosed body of water, approximately 8 km2 (3.2 mi2) in area, with free connection to the Pacific Ocean, but also diluted with freshwater from the Yaquina River land drainage. The Bay is traversed by the Yaquina Bay Bridge.
River surfing is the sport of surfing either standing waves, tidal bores or upstream waves in rivers. Claims for its origins include a 1955 ride of 2.4 km (1.5 mi) along the tidal bore of the River Severn.
In meteorology, an undular bore is a wave disturbance in the Earth's atmosphere and can be seen through unique cloud formations. They normally occur within an area of the atmosphere which is stable in the low levels after an outflow boundary or a cold front moves through.
Hubert Chanson is a professional engineer and academic in hydraulic engineering and environmental fluid mechanics. Since 1990 he has worked at the University of Queensland.
The Trent Aegir, also known as the Eagre, is a tidal bore on the River Trent in England. At certain times of the year, the lower tidal reaches of the Trent experience a moderately large bore. It is said to take its name from Ægir, a personification of the sea in Norse mythology, although this is disputed. A more likely derivation is from Old English ēagor.
A mouth bar is an element of a deltaic system, which refers to the typically mid-channel deposition of the sediment transported by the river channel at the river mouth.
Eprapah Creek is a sub-tropical stream in Redland City close to Brisbane in South East Queensland, Australia.
A tidal barrage is a dam-like structure used to capture the energy from masses of water moving in and out of a bay or river due to tidal forces.
The Dordogne is a river in south-central and southwest France. It is 483.1 km (300.2 mi) long. The Dordogne and its watershed were designated Biosphere Reserve by UNESCO on 11 July 2012.
The Mont-Saint-Michel Bay is located between Brittany and the Normandy peninsula of Cotentin. The bay was listed as a UNESCO world heritage site in 1979 for its aesthetic quality and its importance to the Christian tradition. Due to the significant tidal movements in this region a large part of the bay is uncovered at low tide. There are two granitic islands in the bay: Tombelaine and the Mont-Saint-Michel. Many birds and harbor seals live in this area.
The Bec d'Ambès is the point of confluence of the rivers Garonne and Dordogne, in the Gironde estuary. Situated 15 miles north of Bordeaux, it has an oil refinery which was destroyed by bombing during World War II, but rebuilt after the war. It is in the Ambès commune.
The Qiantang River, formerly known as the Hangchow River or Tsientang River, is a river in East China. An important commercial artery, it runs for 459 kilometers (285 mi) through Zhejiang, passing through the provincial capital Hangzhou before flowing into the East China Sea via Hangzhou Bay south of Shanghai. Its original name, the "Zhe River" or "Zhe Jiang", is the origin of the name of Zhejiang province. The river is also known, along with Hangzhou Bay, for having what is called by locals as the "Silver Dragon", the world's largest tidal bore, a phenomenon where the leading edge of the incoming tide forms a wave that can rise to a height of 9 meters (30 ft) and travels up the river or narrow bay at top speeds of 40 km/h against the direction of the river or bay's current, and can be seen from miles away.
A river plume is a freshened water mass that is formed in the sea as a result of mixing of river discharge and saline seawater. River plumes are formed in coastal sea areas at many regions in the World. River plumes generally occupy wide-but-shallow sea surface layers bounded by sharp density gradients. The area of a river plume is 3-5 orders of magnitude greater than its depth; therefore, even small rivers with discharge rates ~1–10 m/s form river plumes with horizontal spatial extents ~10–100 m. Areas of river plumes formed by the largest rivers are ~100–1000 km2. Despite the relatively small volume of total freshwater runoff to the World Ocean, river plumes occupy up to 21% of shelf areas of the ocean, i.e., several million square kilometers.
Beaches in estuaries and bays (BEBs) refer to beaches that exist inside estuaries or bays and therefore are partially or fully sheltered from ocean wind waves, which are a typical source of energy to build beaches. Beaches located inside harbours and lagoons are also considered BEBs. BEBs can be unvegetated or partially unvegetated and can be made of sand, gravel or shells. As a consequence of the sheltering, the importance of other sources of wave energy, including locally generated wind waves and infragravity waves, may be more important for BEBs than for those beaches on the open coast. Boat wakes, currents driven by tides, and river inflow can also be important for BEBs. When BEBs receive insufficient wave energy, they can become inactive, and stabilised by vegetation; this may occur through both natural processes and human action. BEBs exist in all latitudes from beaches located in fjords and drowned river valleys (rias) in high latitudes to beaches located in the equatorial zone like, for example, the Amazon estuarine beaches.
{{cite journal}}: CS1 maint: multiple names: authors list (link){{cite journal}}: CS1 maint: multiple names: authors list (link)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.
{{cite journal}}: CS1 maint: multiple names: authors list (link){{cite book}}: CS1 maint: multiple names: authors list (link){{cite book}}: CS1 maint: multiple names: authors list (link){{cite journal}}: CS1 maint: multiple names: authors list (link){{cite journal}}: CS1 maint: multiple names: authors list (link){{cite book}}: |journal= ignored (help)CS1 maint: multiple names: authors list (link)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.
{{cite book}}: CS1 maint: multiple names: authors list (link)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.
{{cite journal}}: CS1 maint: multiple names: authors list (link)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.
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