Southend's Mulberry Harbour: How it was destined for key role on D-Day

Just over a mile offshore from Thorpe Bay Yacht Club you can see the remains of a 2,500 tonne concrete Phoenix caisson.

Wreck - It lies partially sunk on a sand bank in the Thames estuary and dates back to D-Day during the Second World War

It lies partially sunk on a sand bank in the Thames estuary and dates back to D-Day during the Second World War.

It’s known locally as the ‘Mulberry Harbour’ but, actually it is just one small section of a Mulberry Harbour which was destined for use once the D-Day landings had taken place.

Operation Overlord (D-Day) - the invasion of Europe by 250,000 allied soldiers, began in the early hours of June 6, 1944.

It was an unprecedented logistical challenge that assumed all major French ports would be unusable.

And of course, there was the task of getting not only thousands of soldiers safely off the boats and onto the beaches of Normandy (under heavy fire from the enemy) but also the colossal amount of equipment including tanks-that needed to somehow get onto dry land too.

This meant the allies needed to take their own harbours with them to re-supply troops once ashore. There was known as Mulberry Harbours and would be floated over to Normandy in sections and assembled just off shore.

There were four major components to Mulberry Harbours. The first comprised of 60 cement-filled blockships' which sailed across the Channel under their own steam then, when in the correct location, lined up bow to stern and scuttled, creating secondary or `Gooseberry' breakwaters for the harbour site.

The second component involved 150 massive `Phoenix' hollow concrete caissons of various sizes, each towed by a tug (like the one sunk off the Southend estuary).

The third component comprised several floating wharves or `Spuds' anchored within the harbour perimeter and the fourth consisted of floating road bridges connecting the Spud wharves with the land, composed of steel and concrete pontoons that supported sections of roadway.

An additional element was positioned farther out to sea in the form of a floating breakwater constructed of hollow tubes or Bombardons' designed to reduce the height of incoming waves.

This whole infrastructure was on a massive scale, designed to cope with large numbers of troop transport and supply ships.

Operation Overlord lasted for little over three weeks and by June 30 had landed over 850,000 men on the invasion beachheads, together with nearly 150,000 vehicles and 570,000 tons of supplies.

As Prime Minister Churchill said, the Mulberry Harbours were `a principal part of the great plan' and were decisive in the first days of the invasion.

Many of the caissons for these portable harbours had been built in the docks on the River Thames, however the one that sits off Southend today was actually made in Goole dry docks on the River Humber. Classes as a C1, it was one of the smallest of the caissons.

Benfleet historian and co-author of the Essex Hundred series of books, Andrew Summers, explained: “The plan was to tow the caissons to the site of the proposed harbour location on the Normandy coast and sink them in position, where they would then serve as supports for the landing bridges.

"Each concrete caisson was hollow and if made watertight would float. It is difficult to imagine these concrete structures, some weighing up to 5,000 tons, bobbing about on the high seas.

“Building the caissons in the first place was a huge task and undertaken in great secrecy. An apprentice, Frank Agar, who worked on the project for over four months, commented that the workers had no idea what they were building.

"Some speculated they were concrete barges. Apart from the 500 people employed on building the six caissons made in Goole another 6,000 worked on them in Essex. The name Mulberry was not significant, simply a codeword. The concrete caissons were coded named ‘Phoenix’.

"It was an accident while towing the caissons to France that led to one of them being stuck here permanently off the coast of Southend.

“One of them, whilst being moved south, sprang a leak off of the River Crouch,” added Andrew.

“It was towed into the Thames Estuary to await inspection but in a squall it broke free. With the concrete shell punctured it flooded and at low tide it settled on a sandbank and broke in two.

"It has remained on the sand bank ever since, generally off limits. In 1954, however, it was used as a viewing platform to witness the return of the Royal Yacht Britannia with the young queen Elizabeth’s aboard returning from an overseas tour.”