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 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. 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.
Undular bore and whelps near the mouth of Araguari River in northeastern Brazil. The view is oblique towards the mouth from airplane at approximately 30 m (100 ft) altitude.
A tidal bore may take on various forms, ranging from a single breaking wavefront with a roller - somewhat like a hydraulic jump - to undular bores, 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.
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 (Fly and Bamu Rivers), Malaysia (Benak at Batang Lupar), and India (Hoogly 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 shrimps (for example in the Rokan River). The tidal bores also provide opportunity for recreational inland surfing.
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 130 km (81 mi) inland, but usually up to 40 km (25 mi) with 6 m (20 ft) height.
The Petitcodiac River formerly had the highest bore in North America at over 2 metres (6.6 ft) 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. The restoration of the bore has been sufficient that in July 2013, professional surfers rode a 1 metre (3.3 ft)-high wave 29 kilometres (18.0 mi) 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.
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. 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.
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.
^ abSimpson, 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. 60 (4): 619-27. Bibcode:2004ECSS...60..619S. doi: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.CS1 maint: multiple names: authors list (link)
^(in 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 (7.8 mi)."