
Basically during the planning leading up to D day, it was realized that the landings would need enormous material support (food, ammunition, vehicles, etc). Capturing some of the heavily fortified harbours was likely to be a lengthy process (and they might not be functional immediately even after capture) - so the allieds did things.... a little differently.
They built a portable harbour.
They built floating units - out of concrete - complete with cranes, antiaircraft, docking faciltities, etc.... and towed the entire thing across the channel to arromanche.
http://www.youtube.com/watch?v=KLMJM_vBtuM
They went from a section of beach to a full fledged harbour with hundreds of ships coming and going in something like 48 hours. (Harbours often take years to build normally.)
Even now, much of the breakwater and several sections of the harbour units are still visible. You can also find out some of the story about the whole thing in the museum there. (Though it is just a wee bit.... touristy.)




There's even a carousel... and almost a "D-day carnival" atmosphere..... a bit surreal really.
