Hi North Yorks, welcome to OSM. 

Notes can be a way to communicate errors or possible tagging mistakes to other users. 

Please do not close them without addressing the note and editing the map, or commenting if you are unable to edit the map but can provide useful information.

In this case, the amenity=* tag has a strict set of values that are meaningful. [amenity=flood defence] is not one of them, so I'm asking if adding a tag such as dam=flood_defence is documented on the wiki .

(which it's not):
https://wiki.openstreetmap.org/wiki/Key:dam

