"There is another wonder: it is the confluence of Linn Liuan; the mouth of that river flows into the Severn, and when both the Severn is flooded to The Teared [the bore], and the sea is flooded similarly into the aforementioned mouth of the river, both it is received into the lake/pool of the mouth in the mode of a whirlpool and the sea does not advance up. And a bank/shore exists near the river, and so long as the Severn is flooded to The Teared [the bore] that bank/shore is not covered, and when the sea and Severn ebbs, at that time lake Liuan vomits all that it has devoured from the sea and both that bank/shore is covered and in the likeness of a mountain in one wave it spews and bursts. And if there was the army of the whole region, in the midst of where it is, and it directed its face against the wave, even the army the wave carries off through the force, by fluid full clothes. If, on the other hand, the backs of the army were turned against it, the same wave doesn’t harm, and when the sea may have ebbed, then the entire bank, which the wave covers, backwards is bared and the sea recedes from it.”" 1,2
This confusing account apparently describes an inlet3 on the Severn which swallows the tide as it rises, possibly in a whirlpool, only to disgorge the water in a giant wave as the tide retreats4. Linn Liuan is probably a variant of the Welsh "Llyn [Lake] Lliwan"5 talked of in other works. However, this doesn't help us much, as, despite being mentioned over many years6, no one now knows where this is either! One work that features it is a Welsh story "Culhwch and Olwen", which gives it as near the Wye and downstream of Gloucester (see Myth). While there have been many suggestions as to where the lake was7, a very likely spot has been identified from the description above by John Nettleship and the Caerwent Historic Trust (CHT) as the Nedern and Troggy Brook8, which was once a much larger inlet and until recently had two whirlpools on it9,10. In addition, there was an alternative whirlpool near Southbrook Farm at Sudbrook, also near the Troggy11.
You may like to read about the Myth of Linn Liuan, the Science of the Troggy and its whirlpools, or some Visit Details. You may alternatively like to read a more detailed analysis in the Paper we've written12,.