Natural Phenomenon: Tidal Bore

Tidal Bore

The bore tide is a rush of seawater that returns to a shallow and narrowing inlet from a broad bay. Bore tides come in after extreme minus low tides created by the full or new moon. Tidal bore, also called bore, body of water that, during exceptionally high sea tides, rushes up some rivers and estuaries near a coast where there is a large tidal range and the incoming tide is confined to a narrow channel. Traveling upstream about two or three times as fast as the normal tidal current, a bore usually is characterized by a well-defined front of one or several waves, often breaking, followed by the bore’s main body, which rises higher than the water level at its front. The height of the bore is greater near the banks of a river than at midstream. 

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 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 bores, comprising a smooth wavefront followed by a train of secondary waves known as whelps.

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 tidal bores may be dangerous. Many bores have had a sinister reputation: the River Seine (France), the Petitcodiac River (Canada), and the Colorado River (Mexico), to name a few. 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.^[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 shrimps (for example in the Rokan River). The tidal bores also provide opportunity for recreational inland surfing.

Rivers and bays that have been known to exhibit bores include those listed below

Asia:

 1. Ganges River and Indus River, India

 2. Qiantang River, China (Silver Dragon)

 3. Lupar River and Sadong River, Malaysia

 4. Sittaung River, Burma

 5.  Kampar River, at Meranti Bay, Pelalawan, Indonesia. (Bono)

Australia:

 1. Styx River, Queensland

 2. Daly River, Nothern Territory

Europe:

 1. Shanon River, Ireland

 2. Dee, Mersey, Severn, Trent, Aegir, Parrett, Welland, Kent, and Ouse River,Great Britain.

 3. Seine River (Mascaret), Vire River, and Garonne River, French.

 4. Durme River, Belgium

Papua New Guinea:

 1. Fly River

 2. Turama River

America:

 1. Colorado River, Savannah River, and Turnagain Arm, USA.

 2. Petitcodiac and Shubenacadle River, Canada.

 3. Amazon River (Pororoca), Mearim River, and Araguari River, Brazil.