What Is a Tidal Bore? A Wave That Travels Up a River
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On a few dozen rivers in the world, the incoming tide arrives not as a gradual rise but as a visible wave travelling upstream against the current, sometimes a metre or more high and audible before it is seen. It requires a specific combination of tidal range and river shape, and it is a naturally occurring example of a hydraulic jump.
What has to be true for one to form
A bore requires several conditions simultaneously, which is why so few rivers have them:
- •A large tidal range, generally several metres at minimum, since the bore is driven by the volume of water the flood tide must push into the estuary
- •A funnel-shaped estuary that narrows and shallows upstream, which concentrates that volume into a progressively smaller cross section and forces the water level to rise
- •A shallow gradient and shallow water, so that the wave speed, which depends on depth, is comparable to the speed at which the tide is advancing
- •A river discharge low enough not to overwhelm the incoming tide, which is why bores are frequently strongest in dry seasons and absent during floods
- •A sufficiently smooth channel without obstructions that would break up the front
- •Spring tides, since bores on most rivers appear only around new and full moon when the tidal range is greatest and are absent or negligible at neaps
The physics of the front
A tidal bore is a hydraulic jump, meaning an abrupt transition between two flow states that occurs when fast shallow flow meets slower deeper water. The controlling quantity is the Froude number, comparing the speed of the flow to the speed at which a wave can travel in that depth, and a jump forms when that ratio exceeds one, because the disturbance cannot propagate upstream fast enough to smooth the transition and instead piles up into a front. Below a certain value the bore is undular, appearing as a smooth leading wave followed by a train of smaller waves, which is what most photographed bores look like. Above it the bore becomes breaking, a turbulent foaming wall, which is far more destructive and occurs on the largest examples. The same physics is visible in a kitchen sink, where water from a tap spreads thinly and then jumps abruptly to a deeper slower flow in a circular ring, which is a stationary hydraulic jump of exactly the same kind.
Where they happen
Around eighty rivers worldwide are known to produce bores. The Qiantang River in China has the largest, reaching several metres and travelling at speed, and its arrival has been a spectator event for over a thousand years with poetry and records describing it, alongside a long history of deaths among people watching from riverbanks. The Amazon bore, called pororoca, travels far inland and has become a surfing destination, with rides of extraordinary duration measured in kilometres rather than seconds. The Severn bore in England is the best known in Europe, reliably predicted and published as a timetable, and attracts surfers and spectators. The Petitcodiac and other rivers entering the Bay of Fundy have bores driven by the largest tidal range in the world. The Seine once had a substantial bore, the mascaret, which caused fatalities and was eliminated by dredging and channel engineering in the twentieth century, which is a reminder that the phenomenon depends on channel shape and can be destroyed by altering it.
What they do
Bores are dangerous and useful in roughly equal measure. The front arrives faster than a person can walk out of a channel, and the water level behind it rises rapidly, which traps people on sandbanks and mudflats and has killed spectators, anglers and walkers regularly on several rivers. Small craft can be swamped, and mooring practice on bore rivers accounts for it explicitly. Against that, historical navigation exploited bores to carry vessels upstream without power, and the associated flood tide extends navigable reaches. The turbulence mixes the water column thoroughly, which affects sediment transport, oxygenation and the movement of nutrients and larvae in the estuary, and bore rivers have distinctive ecology as a result. Erosion and deposition patterns are shaped by the repeated passage of the front. The phenomenon is also a natural laboratory for hydraulic jump behaviour at a scale no flume can reproduce, and several bores are instrumented for research.
The takeaway
A tidal bore needs a large tidal range, a funnelling estuary that narrows and shallows, low river flow and usually a spring tide, which is why only around eighty rivers produce one. It is a hydraulic jump, an abrupt transition forming when flow moves faster than a wave can travel in that depth, appearing as a smooth undular wave train or a breaking wall depending on the ratio. The Qiantang has the largest, and dredging eliminated the Seine's entirely.