An ancient ship could not sail within eight points of the wind (see Smith, 'Voyage of St. Paul'). Thus a S.W. breeze, while permitting Caesar to leave Boulogne, would effectually prevent these vessels from working out of Ambleteuse.
Hence the name Dubris = "the rivers."
The claims of Richborough [Ritupis] to be Caesar's actual landing-place have been advocated by Archdeacon Baddeley, Mr. G. Bowker, and others. But it is almost impossible to make this place square with Caesar's narrative.