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  • of d-day series.

  • When learning the history of D-Day, we are most frequently met with tales of valor of

  • soldiers rushing the beaches. Our collective vision of that day opens with the lowering

  • of Higgins boat ramps on Omaha beach. The fury and violence of battle taking centre

  • stage. In this chaotic vision, it's hard to believe that what was happening was a carefully

  • choreographed plan. One that required hundreds of thousands of people. Labourers in factories,

  • merchant sailors transporting goods across treacherous oceans, engineers dreaming up

  • novel solutions to new problems, and of course the military strategists that conceived the

  • invasion plans. Each played their part in the lead up to D-Day.

  • By the end of July, just a little under 2 months after the first men came ashore, six

  • hundred and seventy five thousand personnel were ashore with one hundred and fifty thousand

  • vehicles, six hundred and ninety thousand tons of supplies and sixty nine thousand tonnes

  • of fuel. The planning and background support required to make this happen was immense.

  • Some of these men and supplies were air dropped into the fields of Normandy, but the vast

  • majority of these supplies were brought ashore by landing craft ranging in size from the

  • personnel landing craft like the iconic Higgins boat, all the way up to massive ships like

  • the LST, or Landing Ship Tank. These behemoths could carry 18 Sherman tanks or 33 trucks

  • with berths for up to 217 troops.

  • These ships would prove invaluable in maintaining the supply chain in Normandy, however the

  • logistics of direct beach supply came with a host of potential and inevitable delays.

  • All of the landing craft, like the Higgins boat, needed shallow draughts to allow them

  • to get close enough to the water's edge to offload their cargo. This made them incredibly

  • unstable and in rough seas many capsized.

  • Some, like the LSTs, had ballast tanks that could be filled or emptied to alter their

  • draught. Filling them to increase their draught for the open ocean, increasing the ships stability,

  • and emptying them to decrease it and come close to shore.

  • Yet, even LSTs faced logistic problems. Arrive at low tide and you are racing the tide while

  • you unload your cargo into deeper and deeper water. Arrive at high tide and you are racing

  • the tide as it leaves you stranded on the beach until the next high tide arrives.

  • On top of all this, the vast majority of war material was coming from the United States

  • aboard Liberty ships. These ships needed deep water harbours to dock. Without a harbour

  • of this kind in France, Liberty ships were forced to dock in Britain and transfer their

  • goods to smaller ships like the LST. If these ships could simply dock in France it would

  • save a great deal of time and effort in this logistical supply chain.

  • Cherbourg was prioritised for this very reason. It's deep water harbour was a key strategic

  • target and the Allies choice of Normandy was largely based on its proximity to the Cotentin

  • Peninsula, a peninsula which could be cut off from reinforcements from Utah beach westward.

  • However, this campaign would take some time. The Allies needed a deep water port as soon

  • as possible. Without a steady stream of supplies and reinforcements, the small Allied beachhead

  • risked being forced back into the ocean.The Allies were faced with a difficult dilemma

  • and their solution is one of the most remarkable feats of the second world war.

  • Two floating harbours were constructed and towed across the English Channel to be placed

  • directly onto the beaches of Normandy.

  • One for the British and one for the Americans, each with an equivalent capacity to the port

  • of Dover. This is the story of the Mulberry Harbours.

  • Harbours serve some key functions to allow ships to unload their cargo in safety. The

  • first role a harbour must fulfill is to act as a safe refuge for the ships. Protecting

  • them from the chaotic weather of the sea. To do this, harbours need large breakwaters

  • and other protection to halt waves in their tracks, leaving the inner harbour relatively

  • calm. Next, they must provide water deep enough

  • to allow ships to moor directly next to the pier and unload their cargo. Regardless of

  • tide levels. Two simple functions that often require an

  • immense engineering effort, dredging of the seafloor to increase the depth of water, construction

  • of large concrete structures and general planning often taking years to complete.

  • Yet the challenge the Allies faced was much greater than a typical harbour. This harbour

  • needed to be built in complete secrecy, floated across the English Channel and assembled in

  • the middle of a war zone. It was an insanely ambitious plan.

  • Let's see how they did it.

  • Before making any detailed plans, the engineers would need detailed surveys of the ocean floor.

  • This information was vital for the design stage.

  • Surveying the ocean floor would inform the designers where to locate deep water offloading

  • points for ships, how high breakwaters needed to be to rest on the ocean floor and sit above

  • the waves at all tide levels, and finally it gave the vital seafloor topology information

  • required to design a floating pier flexible enough to deal with the rough seas and lay

  • on the seafloor without buckling and breaking.

  • Taking soundings while Germans watched and listened from the shore was a job entrusted

  • to the 712th Survey Flotilla.

  • They made a total of 6 trips on moonless nights between November 1943 and January 1944, setting

  • sail from Cowes on the Isle of Wight. Data was gathered in two locations. Here to the

  • east of Omaha beach, the designated location for the American harbour, dubbed Mulberry

  • Harbour A, and here to the west of gold beach, the designated location of the British harbour,

  • dubbed Mulberry Harbour B.

  • Typically, three small LCP landing craft were towed by gun boats across the channel. Once

  • they reached a point, about 48 kilometres off the coast, they slipped their tows and

  • began their stealthy approach to the shore. These LCPs were modified with underwater exhausts

  • to muffle the sound of the engine, tarps to allow the crew to work with light without

  • being seen from shore, and each boat was equipped with echo sounders to survey the ocean floor,

  • along with the necessary navigation equipment to match the soundings with coordinates on

  • the map.

  • This work provided depth charts, like this one, which gave the engineers the information

  • they needed to accurately plan the layout of the harbours.

  • The first components they needed to address were the breakwaters. These would provide

  • shelter to both the ships and floating piers contained within. The breakwaters took several

  • forms. The simplest being codenamedcorncob”.

  • The Allies had a huge inventory of aging ships which were no longer fit for use. Planners

  • sought ships that were at least 30 years old, like the first world war super dreadnought

  • HMS Centurion, a twenty five and a half thousand tonne ship, which had been previously used

  • as a remote control target ship for Naval aiming practice in the nineteen thirties.

  • It was scuttled as part of the breakwater for Mulberry A.

  • This was just one of 14 ships that formed what was termed a Gooseberries at Mulberry

  • Harbour A. One of these break waters were formed at each and every landing beach. Shielding

  • the landing beach from the worst of the waves, and allowing landing craft to unload their

  • wares in the relative calm.

  • The scuttled ships were also ballasted and sunk in a manner that ensured their superstructure

  • remained above the water line. Infact, the crews of these ships had no idea of their

  • fate as they set sail for Normandy. Their useful equipment was removed, two explosive

  • charges were laid and they were then informed of their destination, with no further information.

  • Upon arriving and anchoring off the beaches, a wrecking officer boarded their ship and

  • detonated the charges with the men still aboard. There the ships sank and the crew members

  • remained at action stations. Providing valuable first aid, refueling and repair services for

  • the smaller boats under their protection.

  • The next component of the breakwaters, the Bombardons, were designed for deeper water.

  • The designers of the Mulberry harbours wanted breakwaters placed further from the coast,

  • which would provide some protection to deeper draught ships like Liberty ships, which would

  • unload their wares onto smaller ships to transport to the beaches.

  • These breakwaters underwent several design iterations, namely in the quest to reduce

  • the level of concrete and steel required to build them. Building a structure that lay

  • on the seafloor in water this deep was not feasible, so some innovative designs were

  • trialed.

  • One idea was a pneumatic breakwater. [1] This idea proposed the use of compressed air fed

  • through perforated pipes along the seafloor to create a wall of air bubbles that would

  • block the motion of waves. This sounds like an outlandish idea. How could air bubbles

  • possibly block the immense power of the sea?

  • Two surface currents are created with this method. One opposes the incident waves, the

  • other inner harbour facing current has no effect on the wave action.

  • This was a tried and tested method of creating a breakwater. [2] This is a video of the effect

  • in action. However, it takes an enormous amount of power to pump the volume of air required

  • against the pressure of water bearing down on the pipes. Estimates of the power required

  • for the two mulberry harbour pneumatic breakwaters were placed at an astonishing 3 million horsepower.

  • [3]

  • This being rather obviously unfeasible, left the Allies with one choice. A series of floating

  • breakwaters. Waves are, after all, surface phenomenons, and an adequately designed floating

  • breakwater can provide shelter once anchored sufficiently.

  • Initial designs for these floating breakwaters consisted of 60 metre long flexible rubber

  • hulled breakwaters. These flexible breakwaters however could transmit waves through to the

  • other side if not adequately stiff and were vulnerable to tearing, from gunfire, ship

  • collisions and waves.

  • Rigid floating breakwaters were needed. These cross shaped steel breakwaters were chosen,

  • as they minimized the quantity of steel required, even still each one contained 250 tons of

  • steel. The bottom 3 arms were porous to water, while the top was enclosed to allow it to

  • float. Each was 60 metres long and 7.6 metres in both width and depth.

  • Leaving large gaps between these floating breakwaters was not an option, as the waves

  • would simply pass through the gaps by diffraction and rebuild themselves inside the harbour

  • with most of their energy retained. During the design phase the engineers tested a range

  • of gaps, optimizing the area covered and the wave transmission and found that a 15 metre

  • Two rows of these breakwaters were moored 250 metres apart which provided a reduction

  • in wave height by 70% and reduction in wave energy by 90%. [Reference: Page 77 book] These

  • breakwaters were assembled about 2 kilometres offshore for both Mulberry Harbour A and B

  • in the 6 days after D-Day. Providing Liberty ships some protection as they offloaded cargo

  • to smaller ships capable of offloading directly onto the beaches.

  • The final line of defence was the phoenix Caissons. These provided the brunt of the

  • protection for the harbour. These were essentially boats made of concrete.

  • There were 6 different sizes, each tailored for the depth of water they were designed

  • to be sunk in. The largest being the A1 type, which was 18 metres high, 24 metres wide,

  • 63 metres long [4] and weighed an astonishing 6 thousand tonnes.

  • These larger caissons even had an anti-aircraft gun attached to provide anti-aircraft cover

  • for the harbour, which was not needed in the end.

  • Each end of the phoenix caissons was upswept to allow for easier towing across the channel.

  • The lower level of the concrete boats consisted of a series of watertight compartments, which

  • were flooded to sink them.

  • In fact, once each caisson was complete the Allies sunk them offshore to hide them from

  • German planes, until the time came to refloat them and transport them across the channel.

  • 147 of these phoenix caisson were manufactured on beaches, excavated baisons along the Thames,

  • and in actual dry docks around Britain. With a workforce of over 20,000 people taking 150

  • days to complete the massive breakwaters. The skill of these workers varied greatly

  • and as a result, the concrete was of varying quality.

  • Upon sinking many caissons broke their backs , but at this stage it mattered little. Their

  • purpose was to sit on the seafloor, but removal efforts would prove difficult and to this