Dig a deep and wide ditch at the lowest end, water will find the lowest point!
On some of the plots at the side of our allotments a few year ago, nobody could do anything with the one at the lowest point, an older chap took over, and dug such a ditch, now he has no problem with very wet soil!
Ps, and dig a deeper sump at one end of the ditch, you will have water in that!