There is a ferry service from Portsmouth to Bilbao, which is on the Nervion River, which empties into the Bay of Biscay (Estuary of Bilbao). The ferry terminus is in the Puerto de Bilbao, which is at Getxo, on the Bay of Biscay.
The port area is protected by several bar-like structures; whether any of these were originally natural features or man-made, I don't know.
These barriers can be seen on google maps of Getxo.