There is an enclosed park in South Manchester called Fletcher Moss, which we had not really been aware of as it isn't in an obvious place. In fact, it is part of the flood plain for the River Mersey and when the river is dangerously high then Fletcher Moss is deliberately flooded. As this is a formal park, albeit it in effect a valley, that is a bit unusual, but there are warning signs at the entrance and a substantial part of it could be totally immersed. It's also great for photography.