The Mulberry Harbours

UKHO and Netsurvey teamed up to record detailed 3D data over the remains of Mulberry B.

When planning the invasion of France the allies were extremely worried that they would be unable to supply the troops who were ashore.  An army on the offensive consumes vast quantities of stores. The only reliable method of landing such quantities was through a major port.

However, the Germans held all ports and had fortified them to such an extent that they were expected to hold out for at least 60 days, ample time for an invading army to be starved of stores and hence defeated.

If a steady stream of supplies could not be maintained the German forces in France would be able to overwhelm the invaders and force them back into the sea, effectively ending any chance of liberating France for several years.

The allies had attempted, and failed, to capture a French port in 1942 with the disastrous commando raid against Dieppe.  As a result of this they knew that to capture a port in the early days was impractical so another means had to be found.

The solution was the Mulberry harbours.  Two temporary harbours, each the size of the port of Dover, were built of prefabricated sections in Britain then towed to Normandy in the wake of the invading armies.  In a ferocious building program the pieces were put together on the beaches of Normandy, creating two operational harbours within two weeks.  Mulberry A, located off St. Laurent on Omaha beach was allocated to support the American forces.  Mulberry B was located at Arromanches les Bains, adjacent to Gold beach, and intended to support the British and Canadian forces.  Mulberry A was all but destroyed by a massive storm that hit the Normandy area between 19 and 21 June.  However, Mulberry B, although badly damaged by the storm, was repaired and continued in operation until late November greatly exceeding the original 90 day design period.


Mulberry B Survey Data
DVD Offer

If you want to explore the remains of Mulberry B further you may wish to receive a DVD containing the survey data and other images.

For more information about this offer, please see the details at the end of this article.







F.1287 'Approaches to Arromanches les Bains'.  A chart produced by the Hydrographic Department in July 1944 showing the layout of the harbour and soundings collected by HM Motor Launch 1001, the inshore survey vessel assigned to Mulberry B.

A photo mosaic from August 1944 showing an overview of Mulberry B as it was when in service.

An overview of the survey data.  This view shows the multibeam data overlaid on the current Admiralty chart of the area.  The multibeam data is coloured by depth with blue being deep (about 12m) and orange/red being shallow (about 1m).

Phoenix caissons

The main breakwaters for the Mulberry harbour were made of giant concrete caissons codenamed Phoenix.  The caissons were designed to float so they could be towed from Britain to Normandy. They had numerous scuttling valves built into the lower hull which could be opened to sink them once in the correct position.  To cater for different water depths the caissons were made in six different sizes.  The largest (Type A1) were 200 ft long, 56 ft wide, 60ft tall and weighed 6,044 tons.  The smallest (Type D) were 174 ft long, 28 ft wide, 25 ft tall and weighed 1,672 tons.  The caissons were built with a boat hull (to enable towing) above which were a walkway and then the upper walls which acted as the actual breakwater.  The original units were open topped.  To enable sufficient numbers to be built in time, 146 caissons had to be built in a mere nine months, they were built at numerous sites around Britain, wherever there was space and a means of launching them.  After being completed the Phoenixes were towed to storage sites at Pagham or Dungeness where they were sunk, (or parked) to be raised when needed.

A plan from the War Office publication 'The Story of the Mulberries' published in April 1947 showing the locations where Mulberry harbour sections were stored prior to use.


The open top design, although allowing for a quicker build (essential given the tight time scales) was a liability and many caissons collapsed when large waves over topped the caisson, filling the open internal compartments with water and causing them to burst outwards due to the weight of water inside.

After several units had failed, and with the expectation of further storms, the surviving caissons were 'winterised' by filling the internal spaces with sand then roofing over the open tops with metal shuttering. Later units, known as type Ax, not being subject to the need for great speed of build, were of an improved design that included a concrete roof with only small man holes allowing access to the insides.

Due to their size and lack of salvable material most Phoenix caissons remain where they were planted during WWII.  However, a small number were raised and reused to help rebuild the port of Le Havre while in the late 40s and early 50s others went as far as Sweden and Iceland.  The last movement of the units was in 1953 when several spare Phoenix stored in Portland Harbour were towed to Holland to help block the breaches in the dykes caused by the great storm of January 1953.  Four of these (all Ax types) now form the buildings of a museum dedicated to that event:

In Britain, several Phoenix caissons, wrecked while being stored in or towed to the storage sites, exist around the coast while two complete units (Ax types) are still in use as a wind brake helping ships berth at 'Q Pier' in Portland Harbour.

A drawing from 'The Story of the Mulberries' published by the War Office in April 1947 showing the general arrangement of a Phoenix caisson.  Note the walk way surrounding the caisson between the lower 'boat hull' and the upper caisson walls.  Also of note are the large cylindrical concrete 'clump anchors', resting on the walkway supplied to allow the caisson crew to anchor the caisson should the need arise.

A photo taken from the gun turret on an A1 caisson at high tide, probably showing Mulberry B's western breakwater.  Note the open tops and gun turrets, equipped with a 40mm anti aircraft gun, on each caisson. © IWM (BU610).

Mulberry B's eastern breakwater at mid tide.  The nearest caisson is almost certainly A1-15.  The photo was taken as the eastern breakwater was being 'planted' and further A1 Phoenix caissons will soon be located in front of A1-15, extending the breakwater westwards.  The ship visible in the mid distance is HMS Alynbank, the first blockship to be sunk.  Due to a miscalculation on the strength of the tide she swung 90 degrees out of position as she settled.  © IWM (A38428)

Phoenix caissons being built in a dry dock in Britain. © IWM (A25792)

A completed Phoenix caisson under tow, probably at Pagham or Dungeness.  While under tow the Phoenix caisson had a crew of about 12 to help handle the towing hawsers.  The crew lived in a small room built into the top of the caisson although most preferred to live outside given the mistrust in the ability of the caisson to float should it hit a mine or be torpedoed! © IWM (H39300)

A vertical aerial photograph of Mulberry B taken by 541 Squadron on 27 October 1944.  The line of Phoenix caissons at the upper left are those in the western break water and show the original open topped caissons, some 'winterised' caissons roofed over by the steel shuttering and some of the later, improved Ax type caissons (to the lower right of this line of caissons).  Note also the stores jetty made of spud pierheads and the floating bridges linking these to the shore (lower right).  To the top right of the image, the eastern breakwater is made of a mixture of Phoenix caissons (again some still with the open tops and others 'winterised') and corncob blockships (at the very right of the photo).  Within the sheltered water several ships are berthed or anchored as they disgorge their stores. The photo was used to produce revised charts of the harbour and the draughtsman marks can be seen in the image. UKHO Archives.

This image looks vertically down onto a set of caissons located in the western breakwater.  It shows multibeam data coloured by depth with blue being deepest and red/orange being the shallowest.  The light grey denotes data collected by the scanning laser over caissons that remain above the water surface.  Water depths are about 7m beside the caissons.

The lower line of caissons (shown in light grey) are the main line of caissons that are visible from the shore.  The remainder are submerged and only visible to divers or, now, via these images.  The grey caissons are of the improved Type Ax and were positioned during October/November 1944 late in the harbours life to reinforce it against the expected winter storms.  The other caissons are all Type A1s.  The upper walls have collapsed leaving only the boat hull from the walkway down and showing the internal construction of 22 compartments separated by concrete walls.  In some cases the boat hull has also collapsed leaving little more than a jumble of concrete on the sea floor. Rough weather caused many caissons to collapse while the harbour was still in use and spare units were brought in to fill the gaps.  Hence, the original line was the top most row of caissons with new units being added inshore of collapsed or collapsing units. 

As these replacement units then collapsed further units were positioned inshore again giving rise to the multi layer of caissons as seen.  In an attempt to prolong the life of the caissons, many were 'winterised' by decking over their upper surfaces with steel planks.  This stopped waves over topping the caissons getting into the internal compartments which was a major cause of the caissons collapse.  It is thought that the many long thin items on the seafloor outside the upper most line of caissons (towards the top of the image) are these decking planks. The later caissons (Ax type) were built with a concrete roof covering the internal compartments with only a small 'man hole' for entry into each one.  This roof is largely intact on most of the caissons although it has begun to collapse on the first Ax caisson in the row (centre of image).

Inside the caisson wall (to the right of the image) are a few small wrecks, the identities of which are unknown and it is not known whether these date from WWII or later.

Another view of the caissons in the western breakwater, this time looking north west.

A view looking north east along the western breakwater.  The caissons that remain clear of water and captured by the laser scanner are now shown in yellow/red for clarity.  These are all of the improved Ax type with the concrete roof.  All the visible A1 type caissons have collapsed to at least the walkway if not further.  In the distance (top right) is the start of the eastern breakwater with the main harbour entrance between them.

A view looking east along the eastern breakwater.  The nearest caisson appears to have broken its back with the two halves lying at different angles.  Beside this caisson are some of the long, narrow features thought to be the 'winterisation' decks added to help prolong the life of the caissons as winter storms became more common.  Note also the two small features lying to the left of the second caisson.  These are potentially two of the concrete clump anchors as fitted to each caisson.  In the background can be seen one of the blockship wrecks and the start of the Rocher du Calvados, a shallow rocky area located to the north east of Mulberry B.



To provide sheltered water for deep draft vessels that would find it difficult to enter the harbour, a floating breakwater was anchored to seaward of the main 'Phoenix' breakwater.

The floating breakwater was made of 200 ft long cruciform sectioned steel floats called bombardons.  A row of 24 bombardons, moored end to end with 50ft gaps created a breakwater one mile long.

Unfortunately, the design specification for the bombardons required them to withstand waves generated by winds of up to and including force six.  This they did admirably.  However, between 19 and 22 June (D-Day plus 13 – 16) a massive storm hit the Normandy area with winds sustaining force seven for prolonged periods and force eight in gusts.  This created waves far in excess of those considered in the design of the bombardons and, despite holding up effectively unscathed for 30 hours, by the end of the storm all bombardons had been wrecked, either sinking at their moorings or being ripped free and wrecked on the shore.

A drawing from the War Office book 'The Story of the Mulberries' published in April 1947 showing the general layout of the bombardons.

Bombardons under construction in Southampton's King George V dry Dock. © IWM (A25813)

Bombardons in action off Mulberry B.  © IWM (A24370)

A view of the bombardon wrecks lying north west of the main harbour.  It appears that these bombardons broke in two at approximately their mid point since the combined lengths of the two parts equate to the expected length of a complete bombardon.  The water depth here is approximately 12 metres with the bombardon wreckage extending a metre or so above the surrounding sea floor.  The identity of the small objects is not know, these may be the anchors used to moor the bombardons or could be more modern crab pots or other debris.



To ensure that sheltered water was available on the Normandy coast as soon after the initial invasion as possible some 60 old ships were prepared as blockships.  As with all elements of the Mulberries codenames were used with the individual blockships being referred to as Corncobs.  Preparation as a Corncob involved removing useful items (e.g. winches), cutting through all water tight bulkheads, ballasting to a standard draft of 18 ft and fitting scuttling charges to the hull about 3 ft below the water line.  The corncobs then gathered at Oban and Methil in Scotland before sailing for Weymouth bay on the south coast.  From Weymouth bay they sailed on the afternoon of D-Day to arrive off Normandy on June 7.  All five landing beaches received a breakwater made of these corncobs. The breakwaters were codenamed Gooseberries with Gooseberry 1 being located at Utah beach, Gooseberry 2 at Omaha, Gooseberry 3 at Gold, Gooseberry 4 at Juno and Gooseberry 5 at Sword.  Each Gooseberry was made of approximately 12 Corncobs.

Gooseberries 2 and 3 were enlarged with Phoenix caissons etc to form the two Mulberry harbours.

After WWII, many of the blockships were raised and towed away for scrapping.  However, several had deteriorated to a point where raising was impossible and these were scrapped in situ, effectively being cut off as low as the salvager could get, leaving the bottom plates where the ship was scuttled.

Blockships, known as Corncobs, mass off Oban prior to their final voyage to Normandy.  The ships sailed for Normandy in three convoys called Cob1, Cob2 and Cob3.  Cob1 contained the ships for the US Gooseberries (at Utah and Omaha beaches), Cob 2 contained the ships destined for the British and Canadian beaches (Gold, Juno and Sword beaches).  Cob 3 contained the slowest and most decrepit ships whose speed of advance was deemed to be so slow that they would hold up even the snail like progress of Cobs 1 and 2! © IWM (A 27070)

HMS Durban and NMNLS Sumatra the final two Corncobs planted in Gooseberry 5 at Ouistreham photographed at high tide.  The ships were all scuttled so their upper decks remained dry at all states of the tide and hence their superstructures could be used as accommodation for the crews of the many small landing craft which used these improvised breakwaters for shelter. © IWM (A 24054)

The image shows the remains of four blockships from Gooseberry 3 located within the eastern breakwater of Mulberry B.  The large wreck to the top left is the remains of 'Empire Bittern' with 'Saltersgate' lying alongside it (immediately below it in the image).  To the right is the 'Georgios' while to the far right is the 'Njejos'.  The Empire Bittern was not part of the original set of blockships but came later to reinforce the gooseberry after the great summer storm of 19-21 June had badly damaged many of the corncobs.

As time and availability of units allowed the Gooseberry was also reinforced with Phoenix caissons to provide better protection and several of these are visible in this image.  The white area is unsurveyed due to the existence of large numbers of fishing pots and their attendant floats.


Whale was the code name for the pier-heads and floating roadways connecting these to the land.  Most of the Whale items were removed once the harbour was decommissioned with items either being returned to Britain for scrap or being reused in France to repair bridges destroyed during the fighting.

What does survive are some of the concrete floats.  Nicknamed 'beetles' on account of their shape (beetle was however never an official code name) they were made of either steel or concrete.  Steel was the preferred material but a chronic shortage of this in Britain during 1943/44 meant that where the beetle would remain floating at all states of the tide concrete was used.  Only concrete beetles remain within the harbour, the steel ones having been removed and, presumably, scrapped.

After the harbour was decommissioned the concrete beetles were often towed away and used in coastal protection schemes.  Several were used to help block the holes in the dykes surrounding Walcheren Island in the Netherlands while many more can be seen in Dibden bay, Southampton where they are used as coastal protection.  A number of concrete beetles remain on the beach at Arromanches-les-Bains.  These had been damaged while in service and were moved to the beach so as not to be in the way of further operations.

A view along the stores pier at Mulberry B.  The main pier is made of 'spud pier heads', 200ft long steel pontoon fitted with legs (spuds) at each corner.  These spuds allowed the pontoon to be raised and lowered in synch with the tide while also anchoring it firmly to the seafloor.  The pier heads were connected to the shore by floating bridges made of steel bridge spans supported on concrete or steel floats (known as beetles).  One of these bridges can be seen to the left foreground of the image while another is in the middle distance. © IWM (BU987)

The image shows a concrete beetle resting upright on the seafloor just north of the harbour.  This beetle is a so called PP7 type.  PP stands for Pier Pontoon and the 7 for the 7th type.  PP1 – PP4 were all prototype units that were only used in Britain – mainly in the testing area around Cairnryan in Scotland.  PP5 and PP6 were smaller types designed to support the light duty roadways rated at 25 tons.  The PP7 was the largest beetle and was intended to support the heavy duty roadway rated at 40 tons as needed for transporting tanks and other heavy armoured vehicles.  Several man holes allowed access to the interior to check for leaks etc and two of these are open.

Not the Mulberry harbour but Port en Bessin, the small fishing port about five miles along the coast and the berth for the survey boat.  The image shows the water front of the port as surveyed by the scanning laser during a test run around the basin.  The laser returns are coloured by intensity which allows different colours to be determined on the buildings – note the shop name visible above the shop in the centre.  The scanning laser provides a great density of data with returns being obtained from very small features such as the rigging of the yacht and the ladders on the quay walls. Each return is geo-referenced allowing precise measurements to be taken if required.  The laser is a low powered infra red laser which is completely safe, allowing it to be used when people are present in the area.

Another view from the trial laser run, this time a 'team photo' of the Mulberry Harbour survey team!  Note the shadows behind the people and the mirror image below the water where the laser has reflected off the smooth water surface.

Xplorer moored in Port en Bessin.  The multibeam echo sounder is mounted on the pole lying along the working deck.  The multibeam is at the rear of this pole protected by the orange fender.  The forward end of the pole is fastened to the boat via a pivot which allows the pole to be rotated to hang vertically made rigid using the bracket visible at the forward end of the pole.  On the rear of the upper deck (just above and behind the '' legend is the scanning laser.

Mulberry B Survey Data DVD Offer

If you want to explore the remains of Mulberry B further you may wish to receive a DVD containing the survey data and other images. The contents of the DVD are described in the attached Readme file.

UKHO is making this DVD available for free but require a donation to be made to the UKHO's charity for 2014.
To obtain the DVD please post a stamp addressed envelope (make sure it is large enough to hold a DVD) and a cheque for at least £5 made payable to 'UKHO Charity' to:

Michelle Blagdon
Admiralty Way

UKHO's charity for 2014 is the Taunton Sea Cadet Corps, and we hope to be able to buy a replacement for their aging minibus so that the young cadets can attend training and stewarding events across the south west.