Mulberry Harbours: Portable Ports

You've heard of D-Day, but you probably don't know what made it work beyond the beaches. The Allies needed a way to keep half a million troops supplied without a single working port. So they built two from scratch, shipped them across the Channel, and assembled them in days. What they pulled off still defies belief. Keep going — the engineering alone is worth your time.

Key Takeaways

• Mulberry Harbours were portable prefabricated ports towed across the English Channel to supply Allied forces during the D-Day invasion in June 1944.
• Each completed Mulberry harbour was comparable in size to Dover Harbour, constructed by around 200,000 British engineers in under seven months.
• The harbours featured five key components: Bombardons, Phoenix caissons, Gooseberry blockships, Whale floating roadways, and Spud pierheads.
• A devastating nor'easter on 18–19 June 1944 destroyed Mulberry A entirely, while Mulberry B survived for 8–10 months beyond its 3-month design life.
• Mulberry B transported 628,000 tons of supplies, enabling the landing of over 2 million men, 500,000 vehicles, and 4 million tons of supplies.

What Were the Mulberry Harbours?

During World War II, the British Admiralty and War Office developed the Mulberry Harbours—temporary, portable harbours designed to rapidly offload cargo onto Normandy's beaches during the Allied invasion of June 1944. Understanding Mulberry origins requires looking at the failed Dieppe Raid of August 1942, where strong German port defenses made capturing established ports nearly impossible. The Allies needed a portable solution to maintain harbour logistics without relying on seized French ports.

The harbours solved a critical problem: how do you sustain an invasion force before capturing deepwater facilities? By bringing their own port infrastructure across the English Channel, the Allies could push provisions and equipment ashore during the invasion's critical early stages, maintaining momentum until major French ports like Cherbourg could be captured and repaired. When complete, each Mulberry Harbour was comparable in size to Dover Harbour.

When fully operational, each harbour had the capacity to move 7,000 tons of vehicles and supplies per day from ship to shore. The harbours were constructed and towed across the Irish Sea, one of the bodies of water separating the British Isles, before being assembled off the Normandy coastline.

How Britain Built the Mulberry Harbours Without Anyone Noticing

Building an artificial harbour the size of a small city is one thing—doing it without tipping off the enemy is another. Britain pulled it off through covert logistics and strict operational security that would impress any modern intelligence agency.

Workers operated within a compartmentalised workforce, meaning most never understood the full scope of what they were building. Components came from scattered yards across Britain—Southampton, Mitcham, the West India Docks—before assembly points on the south coast brought everything together. Phoenix caissons were deliberately sunk offshore to hide them from Luftwaffe reconnaissance flights.

Over 400 components, totalling 1.5 million tons, departed on 6 June 1944. You'd never guess that fewer than seven months earlier, the project had barely broken ground. The scale of the operation demanded an enormous workforce, with around 200,000 British engineers quietly contributing to the project across sites throughout the country. The urgency behind the entire endeavour stemmed directly from the lessons of the Dieppe raid, where over 3,500 men were killed, wounded, or captured attempting to seize a defended port in 1942.

For those curious about the broader history surrounding this period, online informative tools can help surface concise, categorised facts across subjects like science, politics, and military history at a moment's notice.

The Five Components That Made the Mulberry Harbours Work

Each Mulberry harbour was a feat of modular engineering, built from five interlocking components that together created a functional deep-water port on an open coastline.

Bombardons formed the outermost barrier, cutting wave height by up to 40 percent, though bombardons maintenance was critical to sustaining that protection.

Behind them, Phoenix caissons—concrete giants displacing up to 6,000 tons—formed the inner breakwaters.

Gooseberry blockships filled structural gaps, with scuttled vessels creating sheltered anchorages at all five beaches. Gooseberry storm resilience proved vital during the destructive multi-day storm of June 18–19, as gooseberries remained strong at both locations while much of Mulberry A was destroyed beyond repair.

Whale floating roadways stretched 16 kilometres shoreward, and floating roadway durability determined how consistently cargo moved from ship to sand. The Danube, which flows through ten European countries, demonstrates a similarly vital principle of infrastructure serving as a connective lifeline across vast stretches of territory.

Finally, Spud pierheads anchored the entire system to the beach, letting vessels unload at any tide. Spud pierhead positioning was so effective that Mulberry B at Arromanches averaged an remarkable 6,750 tons of cargo cleared per day between 20 June and 1 September.

Together, these five components transformed an open coastline into a functioning harbour within days.

The Storm That Changed Everything

Nearly two weeks after D-Day, a nor'easter slammed the Normandy coast on June 19, 1944—the worst storm to strike the region in 40 to 80 years. Force 6 to 8 winds reached nearly 60 miles per hour, piling the sea into massive, broken waves that battered both harbors for three days.

The storm impact proved catastrophic for Mulberry A at Omaha Beach. Twenty-one of 28 Phoenix caissons were destroyed, and roadways, piers, and breakwaters were smashed or tossed ashore. Harbor failure was total—engineers abandoned Mulberry A entirely.

Mulberry B survived, partly because of better anchoring and protection. Workers salvaged wreckage from Mulberry A to repair it, and the harbor operated for 8 to 10 months beyond its original 3-month design life. During its operation, Mulberry B handled the landing of two and a half million men, half a million vehicles, and four million tons of supplies.

Engineer Leon DeLong walked among the ruins of Omaha Beach after the storm, and the devastation he witnessed inspired a vision for a jack-up pier design that could be elevated to allow waves to pass beneath it.

Mulberry A vs Mulberry B: Why One Survived

Though both harbors shared the same design blueprint, Mulberry A and Mulberry B couldn't have ended more differently. Engineering differences and operational decisions shaped each harbor's fate after the brutal June storm.

Here's why Mulberry B survived:

1. Reinforced stability – Engineers pumped Mulberry B's Phoenixes full of sand, giving them greater resistance against waves and harsh winds.
2. Salvaged reinforcements – Crews strengthened Mulberry B using units recovered directly from the destroyed American harbor.
3. Resilient Gooseberries – Scuttled ships held firm at both locations, but only Mulberry B could capitalize on that protection long-term.

Mulberry A's damage proved too severe to justify repairs, so it was abandoned. Mulberry B went on to land 628,000 tons of supplies, proving that smart engineering decisions truly determine survival. At its peak, the harbor achieved a single-day unloading record of 11,491 tons on 29 July 1944.

Over its five months of operation, Mulberry B facilitated the movement of over 2 million men, 500,000 vehicles, and 4 million tons of supplies, making it one of the most consequential engineering achievements of the entire war.

How the Mulberry Harbours Kept the Allied Invasion Supplied

Without a single deepwater port under Allied control, the Mulberry Harbours had to do the impossible—keep an invasion army fed, fueled, and armed across open beaches. That logistics innovation paid off fast. Within the first days, Omaha and Utah discharged 6,614 tons of cargo. One month later, you're looking at 9,200 tons daily. Two months in, that figure doubled to 16,000 tons.

Mulberry B at Arromanches ran for ten months, landing over two million men, four million tons of supplies, and half a million vehicles. Eleven pipelines across the Channel added 3,100 tons of fuel daily by April 1945. That supply resilience meant Allied forces never stalled for lack of resources—even after German forces sabotaged France's major ports during their retreat. The steady flow of materiel through Port Winston ultimately supported the Allied push that led to the liberation of Paris on 25 August 1944.

The harbours themselves were feats of engineering on an almost incomprehensible scale, built by a workforce of 40,000 across special construction sites opened at ports throughout the UK, with the largest caissons standing as tall as a five-storey building.

The Tonnage and Throughput Behind the Mulberry Harbours

Those supply numbers only tell part of the story—the raw tonnage targets and actual throughput reveal just how hard the Mulberry Harbours worked to meet Allied demand.

Planners set ambitious logistical capacity targets for each harbour:

1. 6,000 tons of stores and 1,250 vehicles daily by D+14
2. Combined Chiefs estimated 12,000 tons per day, excluding motor transport
3. Mulberry B averaged 6,750 tons daily, peaking at 11,491 tons on 29 July

These supply metrics show a mixed picture. Mulberry B handled 628,000 tons of supplies, 40,000 vehicles, and 220,000 troops between D+4 and October 1944.

It represented 35% of British stores landed. On good weather days, open beaches initially outperformed the harbours, but Mulberry B consistently proved its value across ten operational months. Each harbour was originally calculated to handle 7,000 tons of supplies per day, making the peak throughput figures a remarkable testament to the engineering ambition behind the entire project. The two harbours were strategically deployed across American and British sectors, ensuring both Allied forces maintained reliable supply lines throughout the Normandy campaign.

What You Can Still See of the Mulberry Harbours Today

Scattered across beaches and seabeds along the English Channel, remnants of the Mulberry Harbours still stand as tangible links to the D-Day invasion. At Arromanches-les-Bains, you can walk directly onto the beach and examine Phoenix caissons sitting on the sand, with more visible offshore. The D-Day museum nearby enriches your visit with historical context. Remains tours at Arromanches let you appreciate the scale of these massive reinforced concrete structures up close.

Off Pagham Harbour in West Sussex, two Phoenix caissons—the "Near Mulberry" and "Far Mulberry"—offer tidal visibility at low tide, revealing caissons that sank before D-Day even began. Both were scheduled for protection in 2019.

At Rigg Bay, you can still spot a collapsed "Hippo" structure and abandoned brick buildings that supported the wider harbor network. The harbour at Arromanches was so effective that it remained operational for 10 months, facilitating the unloading of an extraordinary volume of men, vehicles, and supplies throughout that period.

Portland Harbour in the United Kingdom also holds visible remnants of the wartime effort, where Phoenix Units or caissons can still be seen today, serving as a reminder of the enormous logistical infrastructure assembled for the Normandy campaign.