British grain for the Rhine army?

[Nathan Ross]

my food was stashed behind the "moat" of the English Channel.

Yes, quite! Aside from the brief disturbance at the fall of Carausian regime, and some Pictish activity in the far north, Britain seems to have been spared the chaos of the 3rd and early 4th centuries. However, it would still be necessary to transport the supplies across the sea and up the lower Rhine, through some very disputed areas on the borders of Frankish territory.

This recent article makes some interesting points:

Guarded River Through No-man's Land

The argument here is that the lower Rhine limes did not fall to barbarian attack c.275, but were abandoned when the surrounding land became too marshy to support a population. Presumably the British supply ships or barges could pass freely through this depopulated frontier zone. Perhaps it was only later, when Frankish groups (and even Saxons, according to Zosimus) began moving southwards into the river area again that the supply route became problematic.

The essay also claims that a pair of large horrea discovered at Valkenburg (Praetorium Agrippinae) were apparently constructed some time after AD316, and repaired c.AD365. This, combined with the late 3rd/4th century-looking fortification of Brittenburg at the Rhine mouth, suggests some sort of partial military reoccupation of the supply corridor, at least where it met the sea, around this time. Maybe a response to barbarians trying to interdict the grain shipments? Or perhaps the larger seagoing vessels could no longer navigate the river for some reason, and this was the place where the grain was loaded onto Zwammerdam-style barges?