14.79 at curiositystream.com/realengineering to watch the other 8 episodes in this logistics

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 codenamed “corncob”.

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

day many of these massive concrete structures remain in Normandy.

The next portion of the Mulberry Harbour design, the piers, required vastly more engineer forethought.

They needed to function at high and low tide and every level of tide in between. That's

a great deal of mechanical movement along the length of the pier and that's not even

accounting for wave action, as the breakwaters couldn't feasible stop all waves from getting

through. These structures needed to deal with these forces and manage to provide a steady

road way for trucks filled with supplies and tanks travelling across. It was a huge engineering

challenge.

The first structure needed was an offloading pier. This couldn't be a floating platform.

It needed to be rigid and stable to accommodate the shift weight of trucks, loading cranes

and supplies.

So, each platform came with 4 legs which could be controlled independently of each other.

Each leg could be lowered to the seafloor, allowing the pier to lie evenly on the surface

even if the seafloor was uneven. The platform was capable of moving up and down these legs

at a rate of about 0.75 metres per minute, more than enough to quickly adjust to tide

levels. The platform could float, but it was typically kept a short distance above the

water to keep wave rocking to a minimum.

Around the 4 edges of these Spud Piers were attachment points for floating pontoons that

could expand the berthing space for ships like this modular slipway designed to allow

LSTs to offload their cargo directly from their front ramp. Each spud pier also had

eight bearing seatings to attach bridge spans to the pier. These bridge spans were the next

challenge. The motion of the sea would place rolling,

pitching and twisting action along the entire length of these bridges. If the bridge was

not capable of moving with the ocean, it would inevitably break apart.