Deterrent

Not only a small notice to discourage one from attempting to traverse a drainage pipe across the North Esk, there are three barriers to prevent such an action:- first a physical barrier, which would be easy to climb, then a narrow vertical plate which has a sharply serrated upper surface, this followed by the standard circular frame - except that it is paired with another a short distance behind.

Get over all that and you might just slide yourself over the pipe into the loving arms of the Military.