A Mulberry was one of either of two artificial harbours designed and constructed by the British in World War II to facilitate the unloading of supply ships off the coast of Normandy, France, immediately following the invasion of Europe on June 6, 1944. One harbour, known as Mulberry "A," was constructed off Saint-Laurent at Omaha Beach in the American sector, and the other, Mulberry "B," was built off Arromanches at Gold Beach in the British sector. Each harbour, when fully operational, had the capacity to move 7,000 tons of vehicles and supplies per day from ship-to-shore.

Each Mulberry harbour consisted of roughly 6 miles (10 kilometres) of flexible steel roadways (code-named Whales) that floated on steel or concrete pontoons (called Beetles). The roadways terminated at great pierheads, called Spuds, that were jacked up and down on legs which rested on the seafloor. These structures were to be sheltered from the sea by lines of massive sunken caissons (called Phoenixes), lines of scuttled ships (called Gooseberries), and a line of floating breakwaters (called Bombardons). It was estimated that construction of the caissons alone required 330,000 cubic yards (252,000 cubic metres) of concrete, 31,000 tons of steel, and 1.5 million yards (1.4 million metres) of steel shuttering.

A "Spud" floating pierhead, as employed in a Mulberry artificial harbour. After the Spud was towed into place, the four legs were lowered to the seafloor, and the pierhead was "jacked" up and down the legs along with the rising and ebbing tide.
The Trustees of the Imperial War Museum, London; neg. no. B 5727
The various parts of the Mulberries were fabricated in secrecy in Britain and floated into position immediately after D-Day. Within 12 days of the landing (D plus 12), both harbours were operational. They were intended to provide the primary means for the movement of goods from ship to shore until the port at Cherbourg was captured and opened. However, on June 19 a violent storm began, and by June 22 the American harbour was destroyed. (Parts of the wreckage were used to repair the British harbour.) The Americans had to return to the old way of doing things: bringing landing ships into shore, grounding them, off-loading the ships, and then refloating them on the next high tide.

Mulberry was conceived after the failed amphibious raid on the French port of Dieppe in August 1942. The German defense of the coast of western Europe was built on the understanding that the Allies would need a port in order to build up sufficient ammunition, armour, food, fuel, and other supplies to support a large invasion force. Both sides understood that an amphibious assault was a race: the assaulting forces had to build up sufficient men and materiel to stay ashore, while the defending forces had to be reinforced with enough men and materiel to push the enemy back into the sea. The Germans therefore constructed formidable defenses around the ports of western Europe and took the added step of sabotaging port facilities. Because of the strength of these defenses, the Allies did not consider it feasible to seize a port in the early stages of an invasion. To win the race, the Allies had to find another means to push large quantities of provisions across the beaches. The British solution to the problem was to bring their own port with them. This solution had the support of Prime Minister Winston Churchill, who in May 1943 wrote the following note: "Piers for use on beaches: They must float up and down with the tide. The anchor problem must be mastered. . . . Let me have the best solution worked out. Don't argue the matter. The difficulties will argue for themselves." With Churchill's support, the artificial harbours received immediate attention, resources, time, and energy.

A line of sunken ships forms a "Gooseberry" breakwater off Utah Beach
National Archives, Washington, D.C.
The British solution was not the only one. Admiral John Leslie Hall, Jr., U.S. Navy commander of the amphibious force that assaulted Omaha Beach, did not believe an artificial harbour could survive the actions of the sea. In a discussion with Admiral Cunningham of the Royal Navy, Hall stated: "I think it's the biggest waste of manpower and equipment that I have ever seen. I can unload a thousand LSTs at a time over the open beaches. Why . . . give me something that anybody who's ever seen the sea act upon 150-ton concrete blocks at Casablanca knows the first storm will destroy? What's the use of building them just to have them destroyed and litter up the beaches?" Admiral Hall was correct in both his assessments--i.e., that the sea would wreck the artificial harbours and that the navy was capable of supplying the army across the beaches using LSTs. (Hall did, however, support the idea of building breakwaters by scuttling old ships.)

The British Mulberry supported the Allied armies for 10 months. Two and a half million men, a half-million vehicles, and 4 million tons of supplies landed in Europe through the artificial harbour at Arromanches. Remains of the structure can be seen to this day near the Musée du Débarquement.