Translation for this site. The original is Mommsen*:
Aliud miraculum est, id est Oper Linn Liuan. ostium fluminis illius fluit in Sabrina et quando Sabrina inundatur ad sissam, et mare inundatur similiter in ostio supra dicti fluminis et in stagno ostii recipitur in modum voraginis et mare non vadit sursum et est litus juxta flumen et quamdiu Sabrina inundatur ad sissam, istud litus non tegitur et quando recedit mare et Sabrina, tunc stagnum Liuan eructat omne quod devoravit de mari et litus istud tegitur et instar montis in una unda eructat et rumpit. ćet si fuerit exercitus totius regionis, in qua est, et direxerit faciem contra undam, et exercitum trahit unda per vim humore repletis vestibus et equi similiter trahuntur. si autem exercitus terga versus fuerit contra eam, non nocet ei undać et quando recesserit mare, totum tunc litus, quod unda tegit, retro denudatur et mare recedit ab ipso.
There are three notable problems with the translation. The first is the exact meaning of ad sissam. As with Wonder 2 Morris translates ...Sabrina inundatur ad sissam... as "...the Severn is flooded in the bore...", whereas Guest has "the tide flows into the Severn". Here it has been presumed as scisam "teared" or "splited", and is, presumably, a name for the bore – for more details see Wonder 2 Note on Translation.
The second element that causes difficulty is in modum voraginis. Morris translates in stagno ostii recepitur in modum voraginis as "into the estuary waters like a whirlpool", while Guest has "is received into a pool at its mouth, as into a gulf". It is not clear whether there is a whirlpool, or if the lake is like a whirlpool, or, indeed, if the lake just fills with water. The above hedges its bets with "in the mode of", which could mean it is there, or just something like it. Either way, "whirlpool" is more likely than "gulf".
Thirdly, though it is less importantly for locating the wonder, there is a minor issue concerning the kind of fate that awaits those testing the lake: Morris translates et exercitum trahit unda per vim humore repletis vestibus as "the force of the wave would drag down the army, its clothing filled with water" which seems better than Guest's "it would draw the army to it by force, their clothes being full of moisture". The above doesn't stipulate a direction though. In addition, exercitum may simply be a "multitude", rather than an army as such.