Thanks. That makes sense of the positions of some of them. There's one on a gentle bend in the Somme, just where it widens dramatically as it enters the estuary. Might it discourage formation of a bar at that point?
I did wonder once what they were until I went to Ryde where there is a long channel in from the sea which has a training wall to Port as you enter. You really do have to watch out for them as many are covered towards high water. They often have poles marking the tops but at Ryde many of the poles were broken off.
I would not like to think what would happen if you straddled the wall as the tide was retreating. Sounds painfull!