Because of the lack of any adequate deepwater ports along the open stretch of the Normandy coast selected for the assault it was necessary to provide shelter and large-scale unloading facilities for the build-up, until such time as Cherbourg and Le Havre could be captured and put into operation. Two quite separate projects were undertaken to fulfil these needs – Corncob and Mulberry.
‘Corncob’ provided artificial breakwaters by scuttling blockships – four old warships and 54 merchant ships – off each of the assault beaches. The five shelters, which were code-named Gooseberries and were laid between 7 and 10 June, formed a lee for the smaller landing craft and also served as bases for maintenance and repair parties; the French battleship COURBET continued to fly her ensign and man her AA armament.
The two ‘Mulberry’ harbours were far more ambitious in their concept and execution. Each was to provide a sheltered anchorage equivalent in area to Dover harbour, with unloading facilities which could handle 6,000 tons of stores and 1,250 unwaterproofed vehicles daily by the fourteenth day after the initial assault; due to a lack of tugs, these targets were extended, four days before the operation began, to the twenty-first day. The life of the harbours was to be 90 days.
Each Mulberry comprised three main components:
- 200-ft floating steel cruciform structures moored end to end offshore to reduce wave energy and provide shelter for a deepwater anchorage; in practice, they were found to reduce wave height by up to 40 per cent.
- Concrete caissons, uniformly 200 feet long but varying in displacement between 2,000 and 6,000 tons, sunk on the 10 metre (5% fathom) line to form breakwaters for the inner harbour; the Gooseberries off ‘Juno’ and ‘Omaha’ beaches were incorporated into these breakwaters. The Phoenixes also provided accommodation and AA gun positions for the defence of the harbour.
- the floating pierheads, piers and roadways within the port; prefabricated in steel and concrete sections, they were assembled on arrival. Like the Phoenixes, the Whales were a War Office design and production responsibility: towing characteristics were not prominent among the design criteria and caused problems – after the loss of four Whale tows through bad weather, sailing in wind strengths above Force 3 was not permitted.
The moorings for the Bombardons were laid off ‘Omaha’ (Mulberry A) and Arromanches (Mulberry B) on D+ 1 and the first units were moored on the next day. Phoenixes were laid down from D+3 as surveys of the intended lines were completed and on the same day the first Whale pier was begun in the Arromanches harbour. The bad weather interfered with the programmed work but by D+ 10 piers were operational in both harbours and Mulberry B was handling a dozen coasters and 1,500 tons of stores daily; Mulberry A was not quite so far advanced.
Between noon on 19th and midnight 20th/21st June, the Channel and Seine Bay were affected by a Force 7 gale which produced waves of an average height of eight feet in the assault area. The more exposed Mulberry off ‘Omaha’ beach was wrecked, two thirds of the Phoenix units collapsing and the main pier destroyed by up to 30 LCTs and other craft being driven against it. Mulberry B was more fortunate and remained virtually intact, although over 800 craft of all types from LCTs downwards were stranded; When stock was taken of the damage and loss (which included 22 Whale tows – 2½ miles of roadway – which sank offshore), it was decided that Mulberry A would be abandoned and all resources would be devoted to clearing and expanding the Arromanches harbour. The ‘life’ of the harbour was to be extended to permit it to continue operating into the winter.
The surviving Mulberry was not completed until 20 July. It was, however, already operating beyond its planned capacity and an average of 6,750 tons per day was cleared between 20 June and 1 September. Compared with the tonnage delivered over the open beaches from LSTs, LCTs and lighters – a daily average of 15,000 tons over just the two US beaches, this ‘dryshod cargo’ figure may not seem impressive, but among its other virtues the Mulberry was able to handle certain loads which could not be simply driven ashore. Cherbourg fell to the US Army on 27 June but the very thorough German demolition of the facilities prevented its reopening until September; thereafter, a daily tonnage of 12,000 was soon reached.
The Arromanches Mulberry remained open well into the autumn for although Le Havre and Antwerp were captured during the first half of September, neither could be reopened until November, the former because demolition, by the RAF as well as the retreating Germans, had been so comprehensive and the latter because the heavily-mined approaches to the undamaged port were dominated by enemy-held territory, necessitating a further major amphibious operation (the invasion of Walcheren), followed by a major mine clearance operation before the first cargo could be delivered. Antwerp was opened to large ships on 28 November 1944 and, with a daily capacity of 40,000 tons, thereafter became the principal Allied supply port for the advance into Germany.