The allied invasion on the European mainland brought several problems with it. The most important was the problem of supply. Next to supplying the troops that had already landed new divisions had to be transported in order to expand the force on shore. These troops had to be supplied as well.
Possible solutions to the problem of supplying the landing in Normandy were a frontal assault at a large harbour or to land supplies on the beach with the aid of landing ships. Already before the dismal raid on Dieppe, conquering a large port was deemed to be almost impossible, for the Atlantikwall was at its strongest around and in these harbours. Next to this,German troops were expected to mine and destroy the installations in a harbour, rendering it out of use for months on a row before being fully operational again for supplying the landings.
The possibility of landing supplies, troops and vehicles directly on the beaches was considered to be a possible short term solution but would not be adequate for the structural arrival of supplies. Freighters would have to unload their cargo in landing boats or on pontoons that would ferry to the coast after which the cargo would be loaded unto trucks. The landing boats would suffer a lot of wear and tear on their hulls and thus require more maintenance. Besides, using these boats could only be done once every tide because the boats would have to wait for the high tide in order to get from the beach. Supply would in this way be very slow and would require a large number of vessels. Various small fishing ports at the coast could help out, but their capacity was not big enough.
Transportation 5 (Tn.5)
Headed by Major Allan H. Beckett in the beginning of 1941,a new organisation within the Ministry of Defence was founded with the name Transportation 5 (Tn.5). Tn.5 got the task to supervise construction of harbour facilities with related equipment as dredgers, floating cranes, repair vessels as well as the construction of several rail roads in England. The experience gained by this organisation and the training of qualified personnel which this involved, made Tn.5 the ideal organisation to find a solution for the supply problem.
The solution to the supply problem was ,according to Tn.5, floating harbours. Various parts that were needed for the floating harbour were already invented by Tn.5 for other applications.Some parts were already in a prototype stage and were subjected to tests in e.g. stormy weather.
The plan of Tn.5 prevised a floating harbour which would be built in parts in Great Britain and which would be towed by tugs to the area of the invasion. After this,the parts were to be assembled in place. This plan was received in a rather reserved way at first because of the breathtaking complexity in design, planning and secrecy.However, on 23 Augustus, 1943,during the Largs-conference where a large number of people was present who were involved in the planning of Overlord, agreement was reached on the plan through the personal action of Lord Louis Mountbatten. During the Quebec conference called Quadrant, the COSSAC plan, the overall plan for operation Overlord, was accepted by,among others , Franklin Roosevelt and Winston Churchill. One of the parts of the plan was the floating harbour.After this a central managing board was created that would take care of the design and construction of special piers, artificial harbours and breakwaters .They would organize and manage the development of the conquered harbours. For the command of the two artificial harbours at a later hour Vice-Admiral W. G. Tennant was appointed. Tennant was chosen because he had shown considerable skills during the evacuation from Dunkirk.On a certain stage during the evacuation of the 338,000 troops from the beaches of Dunkirk it was decided to drive the trucks from the British Army into the sea, thus creating an artificial pier and in the same time denying the Germans the use of these trucks.These would suffer considerable from the salt water.
The codename Mulberry was chosen for the project after the whole project of the floating harbour proceeded a long way.. During the Quebec conference in 1943 it was decided that two Mulberries had to be made. Mulberry A for the Americans at Colleville and Mulberry B for the British at Arromanches. It was decided that the various parts for the artificial harbours should be ready in May, 1944. Furthermore it was decided that it should be feasible to construct the harbours in two weeks on location and each harbour was supposed to have a capacity comparable to Dover, which is 6,000 tonnes and 1,250 vehicles a day.
Plan of supply
The total supply of the troops in Normandy was provided for only partially by the Mulberries. The port of Cherbourg was to be conquered and made operational as soon as possible. According to estimates this would take certainly two months after the conquest.The small fishing ports that lay within the landing area like Carentan, Grandcamp, Isigny, Barfleur and St. Vaast all contributed their share from within several weeks after the landing, although they could not be used during the low tide.
The invasion beaches that did not get an artificial harbour received breakwaters. In this way a sheltered strip of beach was created so that landing ships could unload. These breakwaters consisted of scuttled blockade ships and caissons.