It is difficult to create a mental picture of your situation, do I take it as a ditch which overflows after rainfall, or a puddle/pond that appears/dissapears days after rain?
If the former, head downstream and check out obstructions.Do this in survey mode, go empty handed but a camera may help.
If the latter, I suspect that you may have a springline, ie two differing ground conditions and artesian water is emerging as a result of the watertable being at /above the surface. sometimes this occurs several weeks after a period of rainfall due to the rate at which groundwater percolates If it hits a non porous,or heavily saturated layer, it finds a route sideways
Probably the best approach, since drainage in either case would be problematic as it affects beyond your domain, is to proceed with raising beds, planting mindfull of the situation and possibly seeking early transfer to higher ground if a better plot becomes available.It is not unheard of for allotment secretaries to test the resolve of "newbies", with the "lemon" plot first. Remember it was available, until you took it!!!