Mulberry Harbour: the engineering feat that secured D-Day success - History Bombs
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Mulberry Harbour: the engineering feat that secured D-Day success

The Mulberry Harbours were a remarkable engineering achievement during World War II, designed to facilitate the Allied invasion of Normandy in June 1944.

These temporary portable harbours allowed the Allies to efficiently unload troops, vehicles, and supplies onto the beaches, despite the lack of existing port facilities. How and why did the Allies pull off this feat of engineering so fast that they were able to turn the tide of World War Two?

Getting back into mainland Europe

By May 1940, the Allies had been pushed back to the beaches of France and 300,000 troops had to be evacuated out of mainland Europe at Dunkirk. For any chance of winning the war, they needed to secure a foothold in Europe from which to launch a land invasion.

They first attempted to reopen the Western Front by taking the German-occupied harbour at Dieppe in August 1942, sending 6,050 Canadian infantry and a regiment of tanks. This, however, was an epic failure; after 6 hours, the Allies began to retreat, and after 10 hours, more than half of the men who had been landed at the port had been killed, wounded, or taken prisoner.  

The Germans knew that the Allies needed to secure a harbour to be able to supply a D-Day invasion – which they had failed to do. But the Allies thought, if we can’t capture a harbour, why don’t we bring one with us? 

Aerial view of Mulberry B at Arromanches

Development of the Mulberry Harbours

The Mulberry Harbours were temporary portable harbours developed by the British during World War II to facilitate the rapid offloading of cargo onto the beaches during the Allied invasion of Normandy in June 1944.

They were designed to overcome the challenge of establishing supply lines on the French coast, which lacked suitable ports for the large-scale landing of troops, vehicles, and equipment immediately after the initial assault. This also gave the Allies a much larger range of choice of invasion beaches. 

Originally drafted by Churchill himself back in 1917, the plans for the harbour were updated and submitted to the War Office by Welshman Hugh Iorys Hughes. Despite the challenging weather and rough seas, the Mulberry Harbours played a critical role in the success of the D-Day landings by enabling the Allies to quickly establish and maintain supply lines, thereby supporting the advance into occupied Europe.

The harbours incorporated several elements:

  • Breakwaters were made from concrete caissons and block-ships.
  • Pierheads, stabilised on four legs known as Spuds.
  • Roadways featured steel floats called Beetles, 16 km stretches known as Whales, Buffers (approach spans), and Rhinos (power-driven pontoons).

One of the concrete caissons can be seen today off the coast of Hayling Island, after it developed a crack in it’s hull during the manufacturing process and has remained in the water ever since.

Concrete Phoenix caisson, Hayling Island

Preparation for D-Day

The Mulberry Harbours, a pivotal innovation for the D-Day landings, were engineered to withstand severe weather and create areas of sheltered water. Each harbour, an impressive feat of civil engineering, comprised ten primary components. The project enlisted over 55,000 men and began in December 1943.

To determine the optimal sites for these harbours, extensive surveys of the Normandy coastline and seabed were conducted. Mulberry “A” was assigned to the American beaches – Omaha and Utah – while Mulberry “B” supported the British and Canadian beaches – Gold, Juno and Sword.

Although the initial plans didn’t include protective breakwaters, it quickly became clear that calmer waters were essential. The harbours boasted mile-long piers and floating quays set 6.7 metres offshore, allowing vessels to dock. Remarkably, the caissons, crucial to the harbours’ structure, were manufactured in just four days.

“Whale” floating roadway leading onto a “Spud” pier, Mulberry A

Detailed topographical information was critical for the project. The Government even appealed to the public for holiday photos and postcards of the French coast! Teams were sent to the beaches, where they swam ashore to gather sand and mud samples. As a result, two scale models of the landing beaches were able to be built to refine the designs.

Manufacturing the components of the Mulberry Harbours was a massive undertaking, spread across several locations, including the Thames and Clyde, Richborough in Kent, Southsea, Southampton, and Marchwood.

Construction of the Phoenix caissons at Southampton

This colossal effort culminated in harbours capable of transforming the shores of Normandy into structured, efficient hubs, able to land troops and supplies to support the initial success of the D-Day landings.

Operational performance

D-Day was initially planned for June 5th 1944. On June 4th, sections of the Mulberry Harbours began to be towed behind large boats but were held mid-channel due to a sudden delay in the D-Day invasion. As the tension mounted, the harbours awaited their moment. On D+1, the operation resumed, and caissons were towed into position about a mile offshore. Powerful harbour tugs manoeuvred these massive structures into place, laying the groundwork for what would become a lifeline for the Allied forces.

Concrete caisson on its way to France

Each Mulberry Harbour stretched a mile in length, 9 metres above sea level at low tide and 3 metres at high tide. By D+8, the harbours were operational, with 1.2 kilometres of piers and roadways ready to facilitate the influx of men and materials.

However, on June 19th, a fierce storm battered the coast, the worst weather in 20 years, causing significant damage to Mulberry A. Despite this setback, Mulberry B remained in operation, and every day from June 6th until the end of August, facilitated the landing of an astounding 9,000 tons of supplies.


For five months, Mulberry B remained a crucial asset, ensuring the steady flow of reinforcements and resources. Over this period, more than 2 million men, half a million vehicles, and 4 million tons of supplies passed through its harbours. The Mulberry Harbours, with their innovative design and sheer scale, played a pivotal role in sustaining the Allied advance into occupied Europe, underscoring the ingenuity and determination that marked the D-Day operations.

The Mulberry Harbours allowed a steady flow of troops and supplies, which was crucial for the success of the operation and the subsequent advance into occupied Europe. They certainly played a crucial role in allowing the Allies to regain their advantage, turning the tide in their favour at a crucial moment of the Second World War.

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