The funnelling shape of the Severn estuary means the tide can build to a depth higher than it otherwise might as, broadly, more water enters the estuary than can spread in it at the same level. This is increased again by tidally enhanced resonance in the Bristol channel*. Lynch* notes that globally some 70% of rivers with bores on them have mouths that experience tidally enhanced resonance. Finally, in shallow estuaries waves build in height to keep the amount of energy in the system the same** as the surrounding water shallows. In total then, with a shallow funnelling estuary with tidally-enhanced resonance, the Severn is well set up for a high tidal bore wave.