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Mulberry was the codename given to the artificial, temporary harbours developed to supply the Allies after the D-Day invasion of France during WWII.
The Mulberry Harbour breakwaters were constructed in sections in Britain and floated out for assembly off the French coast. For the first time in history an invading army took its own harbour to the enemy-held shore. Two prefabricated ports were built, one for the British sector and one for the American.
A number of people had ideas about floating harbours. In 1917, Winston Churchill drafted detailed plans for the capture of two islands, Borkum and Sylt, off the Dutch and Danish coasts, using flat-bottomed barges, or caissons. This proposal was never put into practice.
In 1941, Hugh Iorys Hughes, a Civil engineer, had similar ideas but could only attract the attention of the War Office through his brother, a Commander in the Royal Navy. Other accounts credit Professor J. D. Bernal with similar ideas, expanded upon by Brigadier Bruce White, who later helped draw up plans for the final design of Mulberry. He was greatly assisted by Allan Beckett whose "whale" design for the roadway was selected in preference to Hamilton's "Swiss Roll" and Hughes' concrete "Hippo".
Hughes was given the task of proving one of the competing designs by constructing prototypes at Conway Morfa in North Wales, which was turned into a huge construction site housing over 1000 labourers. Oleg Kerensky, son of a former Russian Prime Minister, supervised the construction process. Hughes constructed three "Hippo" caissons, which were towed from the Morfa to the test site at Rigg Bay, on the Solway Firth. When the site was in full production, the caissons were launched into the River Conwy estuary for their journey south
The colossal task of planning and constructing the harbour was carried out between June 1943 and June 1944. Halcrow were joint consulting engineers for the work. Colonel Vassal Steer Webster was in charge of the project for the War Office.
Construction and experiment had to proceed concurrently because of limited time. Two prefabricated ports would be built, one for the British sector and one for the American, each consisting of a breakwater formed from concrete caissons (codename Phoenix). In order to take the enormous shipping traffic necessary to sustain Operation Overlord and the Battle of Normandy each harbour had to be the same size as Dover. This entailed the construction of 150 Phoenix anchorage units, as well as many Whale floating roadway units.
The prefabricated ports were completed on time and towed by a fleet of 85 tugs across the Channel. The caissons, weighing 7000 tons, were sunk accurately and the ports laid out. Fifteen obsolete ships had already been sunk to form a preliminary harbour arm. On D-Day+13 a gale destroyed the American harbour but the British harbour, although damaged, continued to allow men and equipment ashore until the capture of Cherbourg took the strain off the artificial port.
The remains of the harbour in the British sector are still visible today on the beaches of Arromanches.