What about: the demand for timber must have rocketd whenever the Roman forces entered an area - building, heating, iron-working. So could rapid deforestation of local watershed have contributed to the flooding?
Yes, Mosaic, it's a thought. The need for masses of timber, and the resultant deforestation prompted the Roman legionaries to dismantle Burrium and float the timbers downriver to what is now Caerleon, for the building of the replacement fortress of Isca Silurum.
Further, I doubt whether the Severn bore would reach so far inland to affect Usk.. Does the bore affect the Severn's tributaries at all ?