The Artificial Harbors That Kept D-Day Moving

The first waves landing in Normandy on June 6, 1944, carried weapons, ammunition, vehicles and enough supplies to begin an invasion. They did not carry everything required to keep that invasion moving.

Every soldier ashore needed food, water, fuel, medical supplies and ammunition. Damaged vehicles had to be replaced. Artillery required a constant flow of shells. Reinforcements needed a reliable route from transport ships to the French coast.

The Allies could not assume that a major port would be captured quickly or returned to service intact. German forces had fortified the Atlantic coastline, and retreating defenders could destroy cranes, locks, railway connections and harbor entrances before abandoning a port.

The solution was one of World War II’s most ambitious engineering projects: take much of a harbor across the English Channel in sections and assemble it off the Normandy beaches.

The temporary ports were known as the Mulberry harbors.

Two were planned. Mulberry A would support American forces off Omaha Beach. Mulberry B would operate off Gold Beach near Arromanches in the British sector.

They were not identical prefabricated structures that simply unfolded when they reached France. Each harbor was a system assembled from specialized components: sunken blockships, floating breakwaters, enormous concrete caissons, mobile pierheads and flexible steel roadways.

Together, those parts attempted to provide something normally created by geography and years of construction—a sheltered place where ships could unload an army.

The Allies assembled Mulberry A off Omaha Beach and Mulberry B off Gold Beach near Arromanches, creating temporary unloading facilities until larger ports could be captured and restored.

Why the Allies Needed Artificial Harbors

A successful landing force can become dangerously vulnerable if its supply system fails.

The forces ashore in Normandy were expected to expand the beachhead, defeat German counterattacks and move toward strategically important roads, towns and ports. That required far more cargo than landing craft could carry during the opening assault.

Pre-invasion planners identified Cherbourg as an important future port. But Cherbourg could not solve the immediate problem. It first had to be captured, its approaches cleared and its heavily damaged facilities restored.

The Allies therefore prepared several overlapping supply methods.

Landing ships and smaller craft would deliver cargo directly to the beaches. Vehicles equipped for shallow-water operation could move supplies ashore. Blockships would create sheltered water. Offshore transfer areas would allow cargo to move from larger vessels into landing craft. The Mulberry harbors would add piers and roadways capable of operating across changing tides.

This layered approach mattered. The Normandy supply system never depended on one structure alone, and the later destruction of Mulberry A proved why redundancy was essential.

The Mulberries were intended to reduce the risks created by open beaches, tides and surf. They did not eliminate those forces. Every part of the system still had to operate in the English Channel, where weather could interrupt unloading and damage equipment faster than engineers could replace it.

A Harbor Made From Separate Machines

A Mulberry harbor was less like one enormous object and more like a connected engineering network.

Its components had different jobs.

Gooseberries were sheltered-water areas formed by deliberately sinking obsolete merchant ships and warships in lines offshore. These blockships reduced the energy of incoming waves and created comparatively protected spaces for smaller vessels.

Phoenix caissons were massive reinforced-concrete structures built in Britain. They were hollow enough to float while being towed. Once positioned off Normandy, valves allowed engineers to flood and sink them onto prepared locations, creating substantial breakwaters.

Bombardons were large floating steel breakwaters anchored farther offshore. They were intended to reduce wave action before it reached the inner harbor. Their effectiveness depended heavily on correct positioning, mooring and sea conditions.

Whale roadways were flexible floating bridges that connected offshore pierheads to the beach. Their steel spans rested on floating pontoons known as Beetles. The articulated design allowed sections of roadway to rise and fall with waves and tides rather than behaving like one rigid bridge.

At the seaward ends of the roads stood Spud pierheads. Long vertical legs extended toward the seabed, holding each pierhead in position while allowing its working platform to move vertically with the tide. Ships could unload onto the platform, and vehicles could carry cargo along the floating roadway to shore.

The complete system involved marine construction, structural engineering, towing, surveying, anchoring, road traffic management and continuous repair.

The harbor also required people who understood how its separate systems interacted. A displaced caisson could expose another section to heavier waves. A failed mooring could allow one component to strike another. Damage to a roadway could interrupt an entire unloading route even when the pierhead remained usable.

A Mulberry combined concrete Phoenix caissons, sunken blockships, floating breakwaters, mobile Spud pierheads and flexible Whale roadways into one temporary port system.

Building the Components in Secret

The scale of the project created an immediate security problem.

Hundreds of large components had to be designed and manufactured in Britain without revealing where or when they would be used. Construction was distributed among different shipyards, docks and waterside sites. Workers often knew the specifications of the section they were building without knowing the entire operational plan.

The Phoenix caissons were among the most visible elements. Some were several stories high and displaced thousands of tons. Building them consumed concrete, steel, labor and waterfront space already required by other wartime programs.

Completed caissons were temporarily sunk in British waters to conceal and store them. Before the invasion, they had to be raised, inspected and prepared for towing.

The Whale roadways created a different engineering challenge. They needed sufficient strength to carry loaded military vehicles but enough flexibility to survive movement on the water. Their spans also had to be manufactured in sections that could be transported, connected and repaired.

Testing took place at several coastal locations. Engineers examined pierheads, roadway spans, anchor systems and breakwater concepts under realistic tidal and wave conditions. Trials exposed failures and produced design changes, but no test site could perfectly reproduce the exposed Normandy coast during a major Channel storm.

The project was enormous, but its value depended on timing. Harbor components arriving too early could interfere with the assault or become targets. Arriving too late would leave the growing army dependent on open-beach operations for longer than planned.

Crossing the English Channel

The invasion fleet that approached Normandy included warships, transports, landing craft, tugs and specialized vessels. Behind the assault forces came the components of the artificial harbors.

Many Mulberry sections were not designed for independent ocean travel. Tugs had to tow them across the Channel at low speeds. Weather, currents, congestion and enemy action could disrupt the movement.

Once the beachheads were sufficiently secure, engineers began creating the sheltered-water areas. Blockships crossed the Channel under their own power or were towed into position. Crews then sank them in planned lines.

The fate of the crews involved should not disappear behind the engineering language. Men had to place aging vessels accurately, open valves or use demolition charges, abandon the ships and transfer to smaller craft while working close to a recently contested coast.

Tugs and construction teams positioned the Phoenix caissons. Surveyors and naval personnel had to account for depth, tide, seabed conditions and the location of nearby structures. After the caissons were flooded into place, they became part of a fixed barrier.

Other teams installed pierheads and assembled the floating roadways. Connections had to tolerate constant movement while supporting military traffic. Damaged sections required rapid repair because the value of the harbor came from cargo flow, not simply from the amount of equipment assembled offshore.

Mulberry A began operating off Omaha Beach on June 16. Across the Allied sector, the harbor at Arromanches was also taking shape.

Tugs, construction crews and specialized vessels positioned the Mulberry components off Normandy while Allied forces continued fighting inland.

From Ship to Shore

A functioning Mulberry created a controlled route through a difficult coastal environment.

A transport or landing ship approached an offshore pierhead. Cargo and vehicles moved from the vessel onto the platform. From there, drivers followed the Whale roadway toward the beach.

The floating road was not perfectly still. Waves passed beneath its pontoons, and individual sections moved in relation to one another. Drivers had to manage heavy vehicles across a narrow structure whose slope and motion changed with the sea.

Traffic control was essential. A stalled truck could restrict an unloading route. Engineers inspected joints, pontoons and anchor points while vehicles continued moving. Recovery teams needed to respond quickly when equipment failed.

The Spud pierheads helped compensate for the considerable tidal range along the Normandy coast. Their platforms moved vertically around long legs fixed against the seabed, enabling unloading operations to continue through different water levels.

This did not turn the coast into a conventional deep-water port. The Mulberry remained a temporary military installation with exposed components, limited routes and substantial maintenance demands.

Its strength came from combining specialized solutions around one logistical purpose: move the greatest practical volume of men, vehicles and supplies from ships to land.

The Storm That Destroyed Mulberry A

Beginning on June 19, a severe Channel storm struck the Normandy coast and continued for several days.

High winds and waves stopped much unloading activity. Landing craft were driven ashore or damaged. Towing lines and anchors failed. Floating equipment broke loose, and repeated wave action damaged roadway sections and pier facilities.

Mulberry A, the American harbor at Omaha, suffered catastrophic destruction. Its breakwaters and floating components were not sufficiently protected from the direction and power of the storm. By the time conditions improved, the harbor was considered beyond practical repair.

This failure is an important part of the story. Calling both harbors an effortless engineering triumph hides the risks, design compromises and operational differences between their sites.

American forces did not stop receiving supplies because Mulberry A was lost. They expanded direct unloading over Omaha and Utah Beaches, using landing ships, smaller craft, trucks and specialized beach operations. Salvageable components from the damaged American harbor were also used elsewhere.

Mulberry B at Arromanches was damaged but survived. Its location offered better protection, and additional blockships and repaired components strengthened the harbor after the storm.

The storm demonstrated two realities at once: the vulnerability of the artificial-harbor concept and the resilience of the wider Allied logistics system.

The June 19–22 storm destroyed Mulberry A off Omaha Beach, while the better-sheltered harbor at Arromanches survived with damage and continued operating after repairs.

Why Mulberry B Continued Working

Mulberry B—later widely associated with the name Port Winston—continued operating at Arromanches long after its originally expected short-term role.

Its Phoenix caissons and blockships created sheltered water. Floating roadways carried traffic between ships and shore, while repair crews maintained the system through weather, wear and accidents.

The harbor supported British and Allied forces as they fought through Normandy. Its contribution was especially valuable while permanent ports remained unavailable or operated below the capacity required by the campaign.

Yet the success of Arromanches should not erase the work performed directly across the beaches. Enormous quantities of supplies continued to move through beach-maintenance areas using landing ships, landing craft, amphibious vehicles, trucks, cranes and temporary roads.

Nor should every Allied success after D-Day be attributed to the Mulberries. The campaign depended on merchant shipping, naval control, air power, vehicle fleets, fuel pipelines, railway repair, captured ports and thousands of logistics personnel.

Mulberry B was one important part of that network.

Cherbourg was captured in late June, but German demolitions delayed its return to useful operation. Antwerp was captured in September, yet its approaches could not be used safely until Allied forces cleared the Scheldt estuary later in the year.

The longer-than-expected delay in opening major ports increased the value of temporary systems and beach supply.

What the Mulberry Harbors Actually Achieved

The Mulberries are often introduced with an appealing phrase: the Allies built a harbor and carried it across the Channel.

That description is broadly true, but incomplete.

They transported systems capable of performing selected harbor functions. The components reduced wave energy, provided offshore unloading points and connected those points to the shore. They did not reproduce every function of a permanent commercial port.

One harbor was destroyed shortly after becoming operational. The other remained useful for months.

That contrast does not divide the project neatly into failure and success. Mulberry A moved cargo during its short operational life, and lessons from its destruction influenced decisions at Arromanches. Mulberry B benefited from its site, reinforcement, continuous maintenance and components salvaged from elsewhere.

The project also created a form of strategic insurance. Allied planners knew that the invasion could not wait for a captured port. The Mulberries provided an additional way to move cargo during the critical period when the beachhead was expanding but permanent port capacity remained uncertain.

Their greatest achievement may have been organizational. Designers, shipbuilders, civil engineers, naval crews, soldiers, tug operators, surveyors and construction units had to create one working system from components manufactured at different sites.

The harbor existed not only because its structures were innovative, but because those organizations coordinated design, transport, assembly, traffic and repair.

The Engineering Lessons

The Mulberry project offers several lessons that remain relevant beyond World War II.

First, logistics infrastructure can be treated as a deployable system rather than a fixed location. The Allies could not move Cherbourg to the invasion beaches, so they separated essential port functions into transportable parts.

Second, interfaces matter as much as individual machines. A strong pierhead was useless without a working road. A roadway required stable pontoons, secure connections and traffic control. Breakwaters had to protect the working area without creating dangerous movement elsewhere.

Third, environmental assumptions can determine whether a design survives. Mulberry A and Mulberry B faced the same regional storm, but differences in exposure, construction progress and protection contributed to different outcomes.

Fourth, redundancy prevented one failure from becoming a strategic disaster. When Mulberry A was lost, American forces increased direct beach operations instead of waiting for the harbor to be rebuilt.

Finally, temporary engineering still requires maintenance. Mulberry B survived because crews inspected, repaired and reinforced it. It was not a structure that could be assembled and forgotten.

What Remains at Arromanches

Visitors to Arromanches today can still see surviving Phoenix caissons offshore. At low tide, remnants of the artificial harbor help reveal the scale of the wartime installation.

Separated from their original roadways and pierheads, the concrete structures can look like isolated ruins. In 1944, however, they formed part of a coordinated machine extending from transport ships to the Normandy shore.

Their survival also provides physical evidence that the Mulberries were not simply an imaginative proposal or a wartime publicity story. Large parts of the harbor were built, transported, positioned and used under operational conditions.

The visible remains do not tell the entire story. Much of the system consisted of steel roadways, floating pontoons, ships, anchors, cables and working platforms that were removed, salvaged, lost or dismantled.

Understanding the harbor therefore requires reconstructing the relationship between the surviving concrete and the missing moving parts.

A Port Built for One Campaign

The Mulberry harbors were created for a narrow purpose and a limited period. They did not need to become permanent commercial ports. They needed to help sustain an invasion while the Allies established access to conventional facilities.

Mulberry A showed how quickly the sea could dismantle even a massive engineering project. Mulberry B showed how a temporary modular system could remain useful well beyond its initial role.

Neither harbor won the Normandy campaign alone.

The invasion continued because Allied logistics did not rely on a single port, beach or technology. When one system failed, others absorbed more of the burden. When permanent ports were delayed, temporary facilities remained in service. When weather interrupted unloading, engineers repaired what they could and reorganized the flow of supplies.

The Mulberries deserve attention not because they were flawless, but because they turned an impossible requirement into a workable—if vulnerable—system.

The troops moving inland rarely saw the full machinery that sustained them. Behind the front, however, an artificial harbor of concrete, steel, ships and floating roads helped connect the armies in France to the factories, depots and ports that supplied them.

That connection kept D-Day from remaining only a successful landing.

It helped turn the landing into a continuing campaign.

HISTORICAL CLARIFICATION

Two Mulberry harbors were assembled in Normandy:

Mulberry A:
Constructed off Omaha Beach for American forces. It became operational in mid-June 1944 but was destroyed beyond practical repair during the severe storm of June 19–22.

Mulberry B:
Constructed off Gold Beach near Arromanches for British forces. It was damaged by the storm, repaired and kept in operation for months.

The Mulberries formed only one part of the Allied supply network. Direct beach unloading and the restoration of captured ports were also essential to the Normandy campaign.

SOURCES AND FURTHER READING

Naval History and Heritage Command. “D-Day: Mulberries.”

Naval History and Heritage Command. “The End of Mulberry A.”

The D-Day Story, Portsmouth. “Mulberry Harbours.”

Imperial War Museums. Collections and historical material relating to the Mulberry harbors and Operation Overlord.

U.S. Army Center of Military History. Ruppenthal, Roland G. Logistical Support of the Armies, Volume I.

Harrison, Gordon A. Cross-Channel Attack. U.S. Army Center of Military History.

UK National Archives. Records concerning Operation Overlord and the construction of the Mulberry harbors.

EDITORIAL AND IMAGE DISCLOSURE

This article was prepared using museum, military-history and archival sources. Operational claims were cross-checked where possible, and approximate descriptions are identified as such.

Some accompanying images may be AI-assisted historical reconstructions or explanatory graphics. They are used to illustrate documented engineering systems and events. They should not be interpreted as original wartime photographs, exact technical blueprints or evidence of an undocumented viewpoint.