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Mulberry B Harbor Multibeam Survey
High-Resolution Coverage of World War II Artifacts in French Port

Chris Howlett

Duncan Mallace

Point-cloud image of the degraded wreckage of a bombardon. Much of the bombardon has corroded away, leaving only (it is thought) the water-tight floatation cells. This is probably a single bombardon, broken in two, since the combined length of the two pieces equates to that of a full bombardon.
Between September 26 and October 4, 2011, the United Kingdom Hydrographic Office (UKHO) and MMT-NetSurvey Ltd. (Banbury, England) undertook a detailed bathymetric and topographic survey of the Mulberry B World War II harbor off Arromanches'les'Bains, France. Intended primarily as a training event for UKHO's Seabed Data Centre staff and MMT-NetSurvey staff, the survey was also designed to return detailed images of the harbor remains as a baseline from which future studies could determine the rate of decay of these important artifacts.

MMT-NetSurvey supplied the survey equipment, which consisted of a RESON (now Teledyne RESON of Slangerup, Denmark) Seabat 7125SV2, Optech (Vaughan, Canada) ILRIS-3D laser scanner, Applanix (Richmond Hill, Canada) POS MV inertial sensor and a Geometrics (San Jose, California) G-882 magnetometer.

Building of Mulberry Harbors
The Mulberry harbors were the solution to the seemingly impossible problem of keeping the Allied army in France supplied with the vast quantities of food, ammunition and reinforcements needed to defeat the defending German army.

Logically, only a fully functioning port could be expected to handle, in all weathers, the huge tonnage of supplies needed by a large mechanized army. The Germans knew this and had heavily fortified all major ports, expecting them (with some good reasons) to hold out until any invading army was thwarted by dwindling supplies. The Allies also knew this and, after the disastrous experiment at capturing the port of Dieppe in France in 1942, knew that capturing a port by frontal assault in the early part of an invasion was effectively impossible. Hence, the Allies needed a port if the invasion was to succeed, yet the Germans had fortified all suitable ports to the point where they could not be captured. To overcome this stand off required some nimble thinking, and this is where Mulberry came in.

Although exactly who thought of the idea has slipped into the mists of history, the idea was simple in its scale: if the Allies needed a port but could not capture one, they would have to build one and take it with them. Preposterous as this idea seemed, it is exactly what happened and resulted in possibly the largest and quickest civil engineering project ever conceived, with two full harbors being designed, built in sections and towed 100 miles across open seas to France all within nine months: Mulberry A located at Saint-Laurent, France, for use by U.S. forces and Mulberry B located off Arromanches-les-Bains for use by the British and Canadian forces.

The Mulberry harbors were created from four main elements. First to arrive were a number of old ships that were scuttled in line to form a breakwater known as a Gooseberry. This immediate breakwater was then extended with giant concrete caissons known as Phoenix. Within the sheltered water formed by the Gooseberry and Phoenix breakwaters, jack-up pierheads were connected to the shore by floating roadways. The floating roadways used an ingenious type of lightweight anchor called a kite anchor to secure each of the supporting pontoons, or beetles as they were called. Collectively, the pierheads, roadways and beetles were known as Whale. Finally, moored to seaward of the Phoenix breakwaters was a further, deepwater breakwater made of large steel floating units known as Bombardons.

Having begun construction on June 7, 1944 (one day after the British and American forces assaulted the D-day beaches in Normandy, France), both harbors were essentially complete by June 17. However, on June 19 the worst storm to hit the English Channel in 40 years seriously damaged the prefabricated harbor units. The Bombardons were all wrecked and never replaced. The Phoenix and Gooseberry were damaged, although the Whale units in Mulberry B, protected by the outer breakwater, survived intact. The Whale units in Mulberry A were destroyed, not by the storm directly but by drifting ships that had run out of fuel or broken down and were driven against them by the winds.

After the storm subsided, the Americans at Mulberry A dispensed with the Whale units and landed stores over the beaches sheltered by a reinforced Phoenix breakwater. At Mulberry B, the Whale was enhanced, partly using items salvaged from Mulberry A, and both the Gooseberry and Phoenix breakwaters were reinforced, allowing the Mulberry harbors to remain in operation until November, by which time the great port of Antwerp had been captured and took over the role of supplying the Allied army ashore.

After the Mulberry harbors were shut down, useful items were salvaged, while the wind and sea gradually reduced the other items to rubble. However, substantial elements still remain, many permanently submerged, and form a lasting monument to the audacity of the planners and engineers who built the harbors during the mid-1940s. It is also reported that many of the kite anchors remain since they grip the seafloor so well that, when being recovered, cables have snapped before the anchors were dragged out, leaving them embedded and buried in the seafloor.

Mulberry B Multibeam Survey
Although the U.S. had surveyed in 2001 much of the remains of their Mulberry A with multibeam sonar (RESON SeaBat 8125), the British Mulberry B had never been systematically explored with multibeam equipment, although the French Hydrographic Office (SHOM) had conducted a survey in 1993 using single beam and side scan as a precursor to the 50th anniversary celebrations planned for June 1994. With the SHOM survey being complete, it was not expected that anything new would be discovered, although the new survey, using modern multibeam sonar, would provide much better images of the wreckage and, hence, provide a baseline for any future studies regarding the rate of deterioration.

The plan was to berth the survey boat in Port-en-Bessin in France about 5 miles west of Mulberry B. Port-en-Bessin is a tidally constrained port with the lock gates only opening for four hours around each high water, so the survey boat, Xplorer, hired from Falmouth Divers (now Marine & Towage Services of Brixham, England), was working roughly 14-hour days, leaving on the early tide then returning on the later one. Xplorer is a 12-meter twin-hulled workboat with a 1.2-meter draft and capable of 25 knots but also suitably maneuverable at slow speeds. To continue this article please click here.

Chris Howlett leads UKHO's Foreign Government ENC quality assurance team. Prior to this, he led UKHO's Seabed Data Centre, being responsible for ensuring bathymetric surveys arriving at UKHO were fit for purpose. He also chaired the International Hydrographic Organization's working group that developed a new edition to the S-44 Hydrographic Survey Standards.

Duncan Mallace is the managing director of MMT-NetSurvey. MMT is a marine survey company dedicated to providing the highest-quality hydrographic, geophysical and geotechnical services worldwide. His survey career has taken him around the globe, and he has been lucky enough to have had a wide range of projects and equipment. He is an expert in multibeam sonar technologies and their uses.

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