Only just looked at this thread, golly gosh it is interesting!
I am not a sailor or an expert, but I know that the double tide effect operates in the Solent and in Southampton Water. If you zoom out on the map of Poole Harbour you will see where these features are. I have understood that this occurs because as the water rises for high tide in the English Channel it does so first at the western end of the Channel and then gets progressively later as you move eastward up the Channel, so you get a high tide pushing into the Solent from the west as the tidal effect passes the western arm and then a second body of water from the east as it passes the eastern arm. If this is truly the cause of the double tide, however, I am surprised that it impacts on Poole harbour, and there may some other reason.
When I was a very junior Civil Engineer, more years ago than...., I worked on a project to extend the capability of Poole sewage works. Up to that point they were discharging macerated sewage out to sea. If there were double tides.... well it doesn't bear thinking about really does it...
dxb
