SYNTHETIC HARBOURS

Korero (AEWS), Volume 3, Issue 6, 23 April 1945, Page 29

SYNTHETIC HARBOURS

The Best-kept Secret of the War

A KORERO Report

No secret of this war has been better preserved than was the intention

of the Allied High Command to construct our own harbours to speed our heavy equipment ashore after “ D ” day. There can be no doubt that the Nazis had perfected their anti-invasion strategy, and placed confidence in its success. ” It is not difficult to guess what that strategy may have been.

In any amphibious landingand particularly so when that landing takes place on a coast with few port facilities comes a time when the lightly equipped infantry of the invaders are precariously ashore, but are unsupported. The heavy equipmenttanks, tank destroyers, guns, ammunition and the trucks to carry it, are still striving to wallow through the surf to lend assistance.

his is the time when the defenders throw in their counter-attacks, with every chance of driving the attacking infantry into the sea. But when the Germans did launch their counter-attacks, they were first warded off and then soundly thumped by heavy equipment which the German High Command had every right to suppose could not be ashore at all. 1 he Allies had no major port at that time ; tiny French fishing harbours could not handle the traffic, and the tides and storms of the Channel precluded the landing of sufficient heavy equipment through the surf. How then was it done ? The answer represents, in the words of the late President Roosevelt, The most critical single project which the United Kingdom had undertaken in this war.”

This project was the construction, in England and Scotland, of virtually the entire artificial harbour equipment used on the beaches of France. This made possible the landing of enough supplies to ensure the Normandy breakthrough and the sweep through France and the Lowlands to the German border. All this with the possession only of Cherbourg,

never a great cargo port, and not open for heavy traffic until August months after “ D ” day.

Conception

At the Quebec Conference in 1943 the Combined Chiefs of Staff decided that the creation of artificial harbours would be absolutely essential. Only by such improvization would it be possible to unload over the beaches enough supplies to ensure a sucessful cross-channel invasion of Europe. Nothing like it had ever been attempted before, but since 1941 British engineers had been engrossed in experimental work, in which United States Naval Civil Engineers had joined. The final plans and specifications were completed in November, 1943, only seven months before “ D ” day.

Construction

Two complete port installations were required, one for use by the British Forces and the other for the Americans. The British port was to be at Arromanches. and the American at Laurent-sur-mer. Britain undertook the entire Construction programme for both forces, although American Seabees joined with the Royal Engineers and civilian labour in building and assembling the equipment. Some 50,000 men were kept hard at work on this giant task. Each of the two ports was to have a capacity equal to that of Dover, and was

designed to handle 12,000 tons of stores and 2,500 vehicles each day, for the first ninety days of the invasion. It was, therefore, plain that something more than merely extraordinary was required of our engineers. The three main elements in the design of the synthetic harbours were—(l) Reinforced-concrete caissons weighing up to 6,000 tons each, which could be towed across the Channel and sunk to form an inner breakwater : (2) Huge steel floats which could be moored in deep water to form an outer breakwater : (3) Floating steel piers hundreds of feet long that would reach from the beaches to ship-side and rise and fall with the tide. For the total project, something like 100,000 tons of steel and 600,000 tons of concrete were employed. The plan also involved the earmarking and preparing of sixty old warships and merchant vessels, twenty- of which were American. These were to be sailed to France, and there sunk in pre-arranged positions to form additional breakwater protection some 24,000 ft. long. One hundred and forty-six concrete caissons were built in specially excavated tidal basins. The caissons resembled roofless arks, and were built in six sizes, of which the largest had a displacement of 6,044 tons. In shape they resembled an inverted T, with the interior divided into compartments. The T construction was designed to give stability to the “ ark ” while afloat. Valves in the outer walls served as seacocks. A4O mm. Bofors gun was mounted on a steel platform on top of each caisson, and decked in

quarters provided for the gun crew during the tow across the Channel. Few members of the crews had any leisure to take a snooze during the trip. These guns, with those on the decks of the sunken blockships, were a vital part of the harbour defences. The steel floats which were to form the outer breakwater were built in sections weighing 15,000 tons each, and made ready for towage across the Channel. The floats of the portable pier equipment which was to be towed across in 480 ft. lengths were designed to cope with a 21 ft. tide, and were therefore able to sit down on rock or sand as the tide receded. Each pierhead consisted of a steel barge-like platform about 200 ft. long by 60ft. wide partially supported by hollowsteel columns. The columns sustained only enough of the load to anchor and steady the structure. The columns extended above the pierhead, and a most ingenious arrangement of sheaves, cables, and power-driven winches raised or lowered the whole structure with the tide.

Fruition

On “ D ” day, the synthetic harbours started movingin pieces—across the 100-odd miles of rough water to Normandy. This meant using eighty-five British and American tugs to tow 13 miles of piers, causeways, and breakwaters weighing in all far over 1,000,000 tons, besides the 500,000 tons of doomed vessels— be used as blockships gering along under their own power. Late in the afternoon of “ D ” day the first block ships arrived, and by nightfall twelve of them were sitting neatly on the bottom in line, some four hundred yards offshore. With the tide at its highest,

waves only just broke over their decks, leaving smooth water inshore suitable for landings from the smallest of craft. The sunken blockships, by the way, were the cause of the Nazis putting out jubilant claims of “ dozens of Allied ships sunk ” in the early stages of the landing. Sunk they cer-

tainly were, but the Nazis had no part in their sinking. Meanwhile, from the “ parking areas ” in Britain, the concrete caissons, accompanied by a cloud of tugs, had begun their journey. It is known that German reconnaissance planes had spotted the building of the caissons, but failed to appreciate their significance.

The first of the flock crossed the Channel unmolested, but by the second day the Nazis did attack from the air, only to find that the “ Noah’s Arks ” carried a sting in their ingenious shapes, in the form of their Bofors guns. This being too much like hard work, the Nazi planes concentrated on the tugs, and some men and equipment were lost. This had been foreseen, and even when the plan had been completed, there were still spare caissons in the British “ parking-places.” By “ D ” day plus three the caissons were arriving at both British and American ports at the rate of four to six a day, and the concrete sea-walls grew, enclosing an ever-larger area in which great ships could be unloaded in safety. The steel floats were moored about a mile to seaward of the concrete harbour walls. A great wave lost half its force in breaking

over the floats, and arrived at the concrete wall as a smaller wave no longer dangerous. By D day plus twelve both the British and the American ports were more than half-completed, and many supplies were being landed. The German observers on the Normandy coast seem to have been curiously obtuse as to the

purpose of the curious happenings under their very noses. By the time they attempted air assault on the synthetic ports it was much too late. Every caisson and every blockship flew its own barrage balloon, and most intense antiaircraft fire was put up by the batteries installed on both caissons and ships. To make assurance doubly sure, fighter-cover was maintained overall, and German planes stood no chance. On “ D ” day plus twelve, June 18, the worst gale for forty years blew in from the north-east at 70 m.p.h. The American port, less fortunate than the British one, which was partially protected by a reef, was completely wrecked. The gale lasted for three days, during which time mountainous seas tore loose the 15,000 ton floating breakwaters, smashed the concrete caissons, and ripped up pierheads and bridging.

The British harbour, on which construction was less advanced, suffered much less. It was decided to abandon the American harbour, and such material as could be salvaged was towed to the harbour on the Arromanches.

The value of this joint harbour can be -estimated in the light of the statement by Lieutenant-General Brehon B. Somervell, Chief of U.S. Army Service Forces, who said that in 109 days since “ D ” day, the Allies had landed nearly 2,500,000 troops, 500,000 vehicles, and 17,000,000 ship-tons of supplies. The fall of Cherbourg went some way to replace the loss of the American synthetic port, but the lion’s share of this vast total tonnage was handled by the surviving prefabricated harbour at Arromanches.

There was no intention of using these ports indefinitely. It was thought that if they lasted until Cherboug fell, they would have justified their cost. If they lasted until the autumn so much the better. The conception of the use of

prefabricated harbours has been brilliantly vindicated by results. The primary purpose— of landing heavy equipment in vast quantity very quickly was achieved by a stupendous BritishAmerican effort.

A British naval officer, at a recent showing of the port models in Washington, said : “ This gigantic project, which cost 100,000,000 dollars, was worth every cent of it. Without the prefabricated ports to insure a flow of supplies over the beaches the invasion of Europe would never have been attempted.”

Nor did Admiral Alan Kirk, commander of amphibious operations in the Atlantic, use exaggeration when he described the synthetic ports as “ The Wonder of the Age.” There can be little doubt that history will assess the Allied conception and use of prefabricated harbours for the successful invasion of Europe as being the outstanding engineering achievement of the war.