The only sticking point with the identification of the wonder with the Troggy is the lack of historical evidence for the use of the name Linn Liuan for this area. While many features in the Gwent area may have kept the same names for at least 1900 years there are many, particularly on the oft-abandoned coastal plain, that have lost names we know of from historic records (for examples, see Rippon*). However, what is worrying here is that we have two good boundary descriptions for the Troggy area from from the Book of Llan Dâv (See Other Works) which make no mention of it. Here's the entries relating to the Troggy. The text of the translations and parenthesised names are by Evans* (*p.235: c.895 CE*; *p.244: c.980 CE* respectively):
CASTELL CONSCUIT & EC. SANT BREIT [Caldicot Castle and Church]. Their boundary is Aper Taroci (Caldicot Pill) as the Taroci (Troggy) leads along as far as the pant [valley/hollow]. As the pant leads through the middle upwards to its top, to the Crug [hill/tumulus] in the boundary of Tref Peren [The Town of Peren, now possibly Llanvihangel*, though see below]. To the right to the other Crug . From the Crug to the boundary cairns of Tref Peren. From cairn to cairn downwards to the sea. Along the Hafren (Severn) Sea with its weirs and windings with a free mooring for ships at the mouth of the Taroci where the boundary began.
LANN MIHACGEL LICHRIT : Its boundary is: From the ford on the Taroci (Troggy) along the Pont Meiniauc (Meinog) through the meadow to the mouth of the Nant, along the Gupant, upwards to the Allt [height?]. Along the Allt throughout its length as far as opposite to the mouth of the pant [valley/hollow] towards the west, to the spring of the Gwver in the pant. Along the pant till falls into the Taroci . Along the Taroci as far as the ford, where the boundary began.
Later records are no more helpful. Caldicot appears in the Domesday* and Hereford Domesday* under Gloucester, but both have very scant records for the area.
Following silting of Caldicot Pill the River Troggy developed. Formerly it ran to the east of its current position (Map). Manorial bounds from 1613 give the old course as Deepweir, Chark Brook, William Lewis' Mead, Vallis Mead, Dinham's Meadow, Bisditch, the patches of mead made by Portskewett's Ford called Base Half Acre and Alder/Toder-fordes Acre, Sudbrook Mill, Sudbrook Pill, the Severn*. So, again, no mention there.
Portskewett (from "The Port in Gwent IsCoed" - i.e. below the [Went]Wood) is mentioned in the Welsh triads (See Other Works) within the "Names of the Island of Britain" as Porth Ys(g)ewin y Gwent*, noting it as one of the three chief ports of Britain. It is mentioned in poetry which may be as old as the 7th C (in Moliant Cadwallon, c.633 CE, as an example of a place far from Gwynedd*; Rippon* also notes it in a similarly aged but not-identified poem as "beautiful Porth Esgewin") and it may be the port near Caerwent at which St.Tatheus landed at some point in the 'Dark Ages' (Life)*. The location of the port is likely to have been at the mouth of the Troggy near the current village (Map: see "free mooring for ships" above, though St.Pierre's Pill to the east is another possibility) - the sea may have come further inland when the Troggy discharged there, however, in the 17th C Camden*: Brechnockshire simply notes:
Beneath these places upon the Severn Sea, nere unto Wy-mouth, standeth Portskeweth, which Marianus nameth Portscith; who hath recorded that Harald in the yeere 1065 erected a fort there against the Welshmen, which they streightwaies under the conduct of Caradock overthrew. And adjoining to it is Sudbrok the church wherof called Trinity Chappell standeth so neere the sea that the vicinity of so tyrannous a neighbour hath spoiled it of halfe the Church-yarde...
The church was built in the moat of the Roman(?) fort*, and, while now ruined, is still on the coast. The remains are marked on the modern OS map.
Generally, the presence of the port in sources suggests silting had already driven navigability of the pill down to the coast by the time they were written. Conversely, therefore, the absence of the name of Portskewett from the wonder description, while unfortunate, is not unreasonable. Note also that the boundary description above just gives the pill's name as Aper Taroci ["Mouth of the Troggy"]. This name ["Aber Tarogi"] also appears in the Triad "The Three Powerful Swineherds of the Island of Britain" in a number of manuscripts including the oldest [Peniarth 16; possibly based on a text c.1200*], but strangely not in the Llyfr Gwyn Rhydderch ["White book of Rhydderch"] written c.1325*, possibly suggesting the name had been forgotten by then.
Bisditch (Biestediche, 13th C; Bisditch,17th C; Bees Ditch, 19th C*) is first mentioned as a pasture in the late 13th C*, so some reclaimation must have occurred by then. The area to the west of the present pill known as the Common Sea was first mentioned with this name in 1613*, though the degree of reclaimation at this time is not entirely certain.
The new course of the Troggy could have been at least partly flowing at pretty much any time as mills that it may have fed have been mentioned from the Domesday (1086 CE) onwards*. The area immediately north of Caldicot Pill between Deepweir and the Common Sea was just described as a millpond in a sale document of 1734* (a mill is located at the head of Caldicot Pill by a survey of 1613*). It is not clear when the Troggy picked up the name by which it is now generally known - the Nedern.
In short then, historical records other than the Historia are of no great help proving the Troggy was the location. In addition, if Sissam in the description refers to the bore (See Note), we may have to accept that the bore must have started further downstream in the Severn at the time of the wonder than it does now, as currently it starts forming around the Caldicot area, and would be unlikely to enter the Pill.