You should mark field drain (and/or culvert) locations with canes. Dig down to the drain at a location as close to the wetland edge of the buffer zone as possible, so that the new elevated water level will not affect adjacent cultivated land.
Once located, break the drain and block the lower end with soil or other impervious material. Remove some of the permeable backfill above the down-slope drain and place it in the bottom of the hole above the break.
Seal the drainage backfill on the down-slope side of the hole by placing and compacting soil on this face of the hole. Backfill the hole.
Create a hole with stepped sides around the break. The width of the hole should be 2 times the depth of the hole. Use the spoil from the hole to create a shallow crescent (30 to 50 cm high) down slope (3 to 5 metre) from the break. The crescent will retain a pool of water and will increase the spread of the surface flow down-slope.
Diagram to show how to break a culvert or underdrain