The North Field trench has more than its fair share of drains and ditches this year. If I'm reading the analysis correctly, the curving one that starts in the middle of west side and ends in the northeast corner, and the straight one that catches the corner at the south side, are both definitely Roman. Not yet clear from here whether they are from the same period, but there's probably some hints already from pottery and possibly other finds.
There are also at least two annoying Victorian additions present, the straight, stone covered ditches that are probably 19th C in origin. They are not always deep enough to disturb Roman levels, although this year it looks like they are. You may be asking, why they are being left alone, rather than being removed to see what's underneath? The issue is that they are often still working, so if you break them you risk having your trench filled to the brim with water after each rainstorm; this is what happened to us last year. In trying to find the other edge of the ditch the baulk between the Victorian drain and the trench was trimmed down to about a foot, but this gave way one night during another of the many downpours last spring/summer, breaking the drain and producing the pool I blogged about on June 9th. Here's an image from last year's trench, Victorian drain in yellow, now-missing baulk outlined in black, (1st century?) double-ditch in blue, and a 3rd C later drain into the older ditch in red. The black pipe at the end of the Victorian drain was our attempt (90% successful) to reconnect the broken ends.