A tidal bore
(or just bore
, or eagre
) is a tidal
phenomenon in which the leading edge of the incoming tide forms a wave (or waves) of water that travel up a river or narrow bay against the direction of the current. As such, it is a true tidal wave
(not to be confused with a tsunami
Bores occur in relatively few locations worldwide, usually in areas with a large tidal range (typically more than 20 feet (6 m) between high and low water), and where incoming tides are funneled into a shallow, narrowing river via a broad bay. The funnel-like shape not only increases the height of the tide, 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.
Bores take on various forms, ranging from a single breaking wavefront — effectively a shock wave — to ‘undular bores’ comprising a smooth wavefront followed by a train of solitary waves (solitons). Larger bores can be particularly dangerous for shipping, but also present opportunities for river surfing.
The word bore derives through Old English from the Old Norse word bára, meaning a wave or swell.
Rivers that have been known to exhibit bores include those listed below.
- Turnagain arm of Cook Inlet, Alaska. Up to 2 meters (6 ft) and 20 km per hour.
Most rivers draining into the upper Bay of Fundy between Nova Scotia and New Brunswick have tidal bores. Notable ones include:
- Petitcodiac River formerly the highest bore in North America at over 2 metres (6 ft) high. It was reduced to little more than a ripple due to causeway construction and extensive siltation.
- 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 that were in the riverbeds when the bore came in. Tourboat operators offer rafting excursions in summer.
- The bore is fastest and tallest in some of the smaller rivers that connect to the Bay including the River Hebert and Maccan River on Cumberland Basin, the St. Croix, Herbert and Kennetcook Rivers in the Minas Basin, and the Salmon River in Truro.
The phenomenon is generally named un mascaret
in French but some other local names are preferred.
- Quiantang River Tidal Bore in China, USC Tsunami Research Group
- Amateur video of the "Wiggenhall Wave" tidal bore
- link to Proudman Inst. page
- More than 100, freely available, published research articles on tidal bores, hydraulic jumps and related topics by Professor Hubert Chanson, Department of Civil Engineering, The University of Queensland
- The Tidal Bore of the Seine River, France
- Tidal bores, Mascaret, Pororoca. Myths, Fables and Reality !!!
- Chanson, H. (2005). "Mascaret, Aegir, Pororoca, Tidal Bore. Quid ? Où? Quand? Comment? Pourquoi ?" Journal La Houille Blanche, No. 3, pp. 103-114