what is it?

I had to get home from work in the middle of the day for a hospital appointment (yay not a stay) and after running for a train found it was cancelled...nooo! So I had to re-route myself and call on the dadster to help my situation out (car at other station and so on). It gave me a wander in a direction I've not been in a while - and I noticed they've knocked down lots of the old Falkirk and District Royal Infirmary- and this has appeared just at the grounds. I can't work out what it is- the water running through looks a bit minging but on the whole it looks really nice. But you can't get into it, its fenced off. What is it?