Tidal bores

You would have learned by now that the tide is actually a wave, in fact a shallow water wave, and that the behaviour of the tide near coastline is very much affected by the underwater topography, or bathymetry, and the coastal configuration. The largest tides are generally found in areas with the more complex coastal topographies and gently-sloping continental shelves (like around the UK). The smallest tides are found in the middle of the ocean.

Like ocean waves, tidal waves (I mean here a real tidal wave and not a tsunami) can break and even be surfed. The best example of a tidal wave breaking, which is generally referred to as a tidal bore or river bore, is the Silver (or Black) Dragon in the Qiantang River, China, which can reach a maximum height of 9 m. The Dragon has recently been surfed by some American surfers - see this Utube clip.

Such tidal bores always occur in rivers where the shallowing sea bed and the funnel-shaped river entrance results in enhanced shoaling of the tide, making the tide increasingly asymmetrical and higher. Tidal bores also occur in the UK - the Severn bore is the best known example.