"Sadly these old drains were not built to cope with todays heavy tractors and machinery, which I think is cause of most of our problems."
Too true - on land we used to farm, was 1 water trough, strange because it was 26 acres, but 1 mains water trough! Cows made a path to it (why do cows and sheep ALWAYS walk on the same path, slightly twisting too? I currently have one on my back field.) Anyhow, mum, donkey years ago, decided to get a contractor in to lay some pipe work to connect that mains trough to several others, that's when trouble began. "Oh, sorry mrs" came the reply as tractor hit a land drain and never repaired it!
I pride myself on my good memory and where we are, on a small field, it's wet, always wet, reeds, everything, put it down to landdrains and septic tanks. A few months ago, I came across some photos taken in the early 90's and that field is NOT wet, NOT 1 REED, a bit of damp but nothing else, so I got thinking, "what happened?" Then I remembered: a neighbour spread some muck with his big tractor/spreader, we told him DON'T go down that side, he knew better! Another neighbour with a bigger tractor had to come to the rescue and we were left with big welts in the ground, waiting for neighbour to come and repair (never happened) we forgot, land now wet.....
I was always told though, if you want to find a land drain or forgotten water pipe, get a contractor and digger in and tell them "Don't hit a drain or water pipe!" We and my sister had to have our septic systems replaced last year, she had new tank and pipes, we had new pipes. Sisters digger hit a water pipe, twice! We told him it was there, wouldn't believe us! Our contractor clipped a land drain (but repaired it).