water runoff the sides

Does anyone know a good way to deal with the water at the side of a polytunnel?

I had some potatoes growing quite close to my polytunnel this year and a lot of the deepest rotted. The plants seemed to have grown more roots higher up that were prone to greening even though the plants had been earthed up well.

The advice sought is not about potatoes but about the excessive wet that accumulates along the sides of a polytunnel. What, if anything, do people do to mitigate this?

I've seen photos of "guttering" along the sides of PTs to run the water into Butts.

I’ve seen pictures of guttering also, but I know it wouldn’t work on mine, I think you need a tall straight side for it to work.
My poly tunnel is located on gravel so I don’t have anything growing at the side sorry can’t help in that respect other than to suggest growing bamboo there instead of spuds, the bamboo like lots of water. I had some growing beside a shed in my old house and the water ran off the shed into the bed, the bamboos thrived