Hydrostatic pressure doesn't read surface slope. We had to drop a perforated collector in a coarse gravel trench half a meter under our swale in Lucerne for the exact same reason—give the rising groundwater a relief path, or your surface grade means nothing.

Did you end up surveying the new trench line to see what fall finally worked against the head pressure, or did you just let the contractor's instincts take over?

Peat has a way of humbling even the cleanest grading model. Did you end up widening the catchment profile, or did dropping the invert by eye get it flowing?

Peat has its own logic and it definitely doesn't read the script. Hope you had a proper pair of wellies with you.