To enable the forces already on the Normandy coast on D-Day to be reinforced and kept supplied with utmost speed it was necessary that two invasion ports, each roughly the size of Dover Harbour, should be constructed in England, then be transported across 100 miles of sea and placed in position off the enemy occupied coast. The story of this staggering achievement was told in the “War Illustrated” in November 1944.
Long before the invasion was launched it was obvious that vast quantities of stores and equipment would have to be landed on open beaches. It was estimated that about 12,000 tons, plus 2,500 vehicles of all kinds, would have to be handled in this manner each day for 90 days at least. There was only one possible plan; this was to have ready prefabricated sections which could be assembled at the spot where two vast invasion ports would be most useful. A harbour for British use was to be located at Arromanches, another for American use, at Port-en-Bessin. And each approximately the size of Dover Harbour, which took seven years to complete! These two were to be set up in days, to give full shelter and all usual port facilities to vessels whilst unloading. This entailed the construction of 150 caissons, enormous hollow blocks for the harbour walls. The largest of these caissons had a displacement of over 6,000 tons. Each contained crews quarters for use during the passage, the crew being partly naval, and partly from the Royal Engineers (or American Seabees) for carrying out the operation of sinking. At a later stage, Bofors guns, 20 tons of ammunition, and rough shelters for a guns crew, were placed on the top of most caissons as additional A.A. protection for the harbour. Caissons were towed across the Channel, each by a tug of about 1,500 hp
Mr. Jack W. Gibson, 57 year-old Yorkshire man and Director of Civil Engineering at the Ministry of Supply, one of the chief figures on the construction side of the prefabricated ports.
On arrival they were manoeuvred into position with the help of small tugs, then special valves were opened in each, allowing water to fill it and sink it where it was to remain. It took roughly 22 minutes to sink the largest. These had all been built during the winter months, a few in graving docks, the rest in emergency basins constructed behind river-banks, the banks were then dredged a way so that the partly completed caissons could float out and be towed to wet docks for completion of the concrete work. Towing of these to the Normandy coast commenced on D-Day plus one. On D-Day blockships sailed to where this work was to be completed, to provide breakwaters for the immediate shelter of hosts of small craft. Sixty ships of various types and sizes were earmarked for this purpose, including old warships. These made the crossing close behind the assault forces and all arrived safely. They were sunk by explosive charges, and their crews were then brought back to England. To complete each port, internal equipment such as piers was essential. It was no easy matter to construct a pier hundreds of feet long on a flat beach, with a rise and fall of tide of over 20 feet, and which sometimes may be floating and at other times be resting on sand or, worse still, rock. This problem had been under examination for many months, and in 1942 the Prime Minister took a personal interest in the matter. In typical Churchillian style he wrote, “Piers for use on beaches. They must float up and down with the tide. The anchor problem must be mastered. Let me have the best solution worked out. Don’t argue the matter. The difficulties will argue for themselves.”
Pier-Heads Built as ships
The result was eminently satisfactory. After severe sea tests the equipment went into production amounting to 7 miles of pier and all necessary appurtenances. Pier heads were built, as ships, in various ports on the coast, from Leith round to Glasgow, four by the R.E’s at military ports in Scotland. Most of the remaining equipment was prefabricated all over the country, and then assembled at the Army depots at Southampton and Richborough. About 240 contractors were employed on this particular task, and 50,000 tons of steel were used. The work of assembly-on-the-spot went on whilst the sinking of caissons was being completed. By D-Day plus 12 more than half of these were in position, and the harbours were already an impressive sight. Up to this time the operation was going according to plan. Very few accidents had occurred, and the air superiority was such that enemy interference had caused little trouble. Floating breakwaters, consisting of steel floats, had previously been moored end-to-end in a long line, the object of these being to have a damping effect on the sea in strong winds; 15,000 tons of steel were used in their construction. The Army Fire Service helped in sinking the pier heads to the correct level, as well as manning and floating land based fire stations.
And then, on D-Day plus 13, there occurred the biggest June gale for 40 years. It blew from the worst possible direction, the harbours were exposed to its full force, and these were only at the half-way stage. The American harbour suffered very severely and the breakwaters were largely broken up, so much so that the work on this harbour was discontinued. All the pier equipment which was on the voyage across when the gale started was sunk, but only one caisson failed to weather the journey.
Gigantic prefabricated pier for one of the home made invasion harbours being towed across the Channel. Average speed for this operation was 4 knots, the towing distance for each unit averaging a hundred miles. The total tug fleet available for the assembly of the harbour parts off the Normandy coast was 85, varying from vessels of 1,500 hp to those of 600 hp, the latter not generally used in the open sea. Each round trip took about three days and the transport of well over 1,000,000 tons was involved.
After the gale subsided the work of construction continued on the British harbour at Arromanches, though a long spell of rough weather prevented pier equipment from being towed over, so that the remaining harbour was not unloading to maximum capacity until well into July. But even on the worst day 800 tons of petrol and ammunition, as well as many troops, were landed over the piers. Eventually it was completed, and a port bigger than many with famous names had been built in a few weeks against a lonely French beach. Day after day, in all weathers, scores of ships of all sizes had moored within its shelter or berthed in unbroken lines along its quays. Never, even at the height of a peacetime trade boom, had so much shipping used such limited accommodation at one time. As a result of the craftsmanship of vast numbers of British workmen hundreds of thousands of tons of vital supplies, scores of thousands of men and many thousands of vehicles had been put ashore in the most rapid military build-up ever undertaken. The prefabricated port made possible the liberation of Western Europe. Furthermore, when the concrete sections were assembled for trial, in shoal water off Dungeness shortly before D-Day, they were seen by German air reconnaissance and led the German High Command to the erroneous conclusion that the Pas de Calais, with its flying-bomb launching sites, was the objective. This inaccuracy caused the retention of large enemy forces in the wrong parts of Northern France, enabling the landings to go ahead before German troops could be switched to the real zone of operations.