"Another wonder is the Duo Rig Habren, that is the two kings of the Severn. When the sea is flooded to The Teared [the bore] within the mouth of the Severn, two waves of spume separately convene and make war between themselves in the manner of sea-rams and each proceeds to the other and they collide at one another and again withdraw one from another and again they proceed on each Teared [bore]. This they do from the initiation of the world all the way to the present day."1,2
This is a watery wonder - one of the three references to the Severn Bore (see the Science page to find out about this) in the Wonders. The Tidal Bore Research Society* suggest that there are/were only two places on the Severn where the Bore wave can collide with itself to produce the Duo Rig [Modern Welsh: Dau Rhi Hafren]. Currently it can be seen in the eastern channel of the Noose, as the leading west-channel tide rebounds off Hock Cliff and flows straight back into collision with the advancing flood tide in the eastern channel. Previously, it could also be seen at Upper Parting where the Western channel wrapped into into the eastern (Maisemore) channel and collided with the tide running up it, however, the weir at Maisemore now stops this.
You may like to read more about the Myth and Science (including when best to see it) of the Severn Bore, or see some photos and Visit Details.