Artificial harbour at Arromanches

Port Mulberryis an artificial harbor built on the Normandy coast during the World War II to enable Allied supplies in the days following the Normandy landings. In fact, two ports were assembled off the beaches of Calvados, but only the one at Arromanches was operational, the second in front of Omaha Beach having been destroyed by a storm on June 19, 1944. The construction of this port saved the Allies from having to take a deepwater port directly.
The Mulberry harbor was made up of various large elements prefabricated in England, transported and assembled on the Normandy coast. This artificial port was often credited with the success of the Battle of Normandy, and presented as an example of military engineering and logistics. However, historians now relativize its role somewhat, and until the port of Cherbourg was reopened, the majority of Allied supplies would be made by landing directly on the beaches or using the smaller ports on the Normandy coast.(Source Wikipedia)