Extents of intermittent water bodies

I visited a retention basin earlier today that hasn’t been mapped yet other than for it’s “dry” use as a park. I’ve mapped the wetlands and missing bridge from imagery, but the information board (see from Geograph picture below) indicates that the earthworks are part of a retention basin.

Presumably this should be mapped as landuse=basin + basin=detention, but I’m not sure where to draw the line (in the literal sense). My pictures of the earthworks are decisively undecipherable, how are the extents of these sorts of features normally determined? From what I could see in person the extent on the map didn’t correspond to any visual features on the landscape and copying other maps is generally forbidden in OSM so what should this be based on? Does the Environment Agency have any data cleared for OSM use?

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Ce sujet de discussion accompagne la publication sur https://community.openstreetmap.org/t/extents-of-intermittent-water-bodies/1572