If a sea-level canal were dug from east to west across Asia, would the moon have a tidal effect on the water level, with a daily tidal bore in phase with it?
鈥� Tides are raised on Earth by the gravitational pull of the moon and, to a lesser extent, the sun. Even the water in your bathtub is moved as the moon passes overhead, albeit by an immeasurably small amount. The same is true for water in a canal. But the tidal range 鈥� the difference in the water level at high and low tides 鈥� is only significant in huge bodies of water such as oceans, and only noticeable when the bulge of water then encounters a land mass.
Tidal bores occur in some rivers and, more rarely still, on narrow sea inlets, known as fjords in Norway or sea lochs in Scotland. A significant tidal range is required, typically in excess of 6 metres. Entering a wide bay, the incoming tide is funnelled towards the narrow opening of the river or inlet, and the water piles up on top of itself, creating a wave that travels upriver. Bores can be subdivided into hydraulic jumps, where there is a sudden change in water level, and undular bores. The UK鈥檚 Severn bore is undular, characterised by a wavefront followed by a series of solitons, or solitary waves.
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John Scott Russell first described solitons in 1834, when he saw one moving along the Union Canal in Scotland. It was created when a horse-drawn narrowboat suddenly stopped and the water that was being pushed along by the vessel continued for several kilometres, travelling at around 13 kilometres per hour. However, even if there were no locks, the energy of a tidal bore running through a trans-Asia canal would peter out as it worked against friction on the bottom and sides of the canal.
Mike Follows, Sutton Coldfield, West Midlands, UK
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