World War II caisson, West Knock sandbank, Shoebury Ness


Heritage Category:
Scheduled Monument
List Entry Number:
Date first listed:


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The building or site itself may lie within the boundary of more than one authority.

Southend-on-Sea (Unitary Authority)
Non Civil Parish
National Park:
National Grid Reference:
TQ 92027 82433

Reasons for Designation

D-Day, June 6th 1944, is one of the most significant dates in modern history, defining the start of the final phase of World War II in Europe. After two to three years of preparations, the assault phase of `Operation Overlord' - codenamed Neptune - lasted for little over three weeks and by 30 June had landed over 850,000 men on the invasion beachheads, together with nearly 150,000 vehicles and 570,000 tons of supplies. Much of the success of Overlord was due to the presence of an artificial harbour (known as a `Mulberry' harbour) at Arromanches, built in sections (and different components generally at separate sites) and towed across the channel for the disembarkation of troops and landing of supplies. Although one other harbour failed, the successful one at Arromanches was significant in providing the tactical advantage of surprise, and the logistical advantage of not having to land on a defended shore and at the mercy of the weather. As Prime Minister Churchill said, these harbours were `a principal part of the great plan', and were decisive in the first days of the invasion. Many sites were involved in the harbours' construction, stretching at least from Southampton, via south coast ports and London, to the North East. These sites were chosen and, in some cases, designed for the manufacture of either: the `phoenix' caissons (fully submerged breakwaters made of cement; the largest was 60,447 tons) and bombardons (partially submerged steel breakwaters; up to 1000 tons) which made up the outer harbour; or the pierheads (`spuds'), floating piers (`whales', with their steel-spanned roadways), and the pontoons (`beetles', some of steel, some of concrete) that supported them. These construction sites were located either in largely unmodified dry docks or slipways, or in excavated basins or prepared beaches. Beach construction sites are the most visible and may retain construction platforms, slipways and winch house foundations. A small number of excavated basins are also known. Some of the components also survive, mostly at sea, having sunk while on tow, but occasionally on land where they were left as surplus to requirements. Although in military archaeology fixed defences will often survive better than materiel representing the mobile offensive, sufficient of the preparations for D-Day in England remains to give an impression of the scale of the Operation, and the variety of the specific tasks involved. All sites where surviving remains provide an impression of the scale and nature of construction work, including construction sites and surviving components, will be considered of national importance.

The Shoebury Ness caisson is a Type C1, of which 17 were constructed out of a total of 212 caissons of all types built. These huge concrete structures formed part of a system of temporary `Mulberry' harbours built to provide sheltered mooring close to the shallow shelving beaches of Normandy, facilitating the rapid re-supply of the invading D-Day forces in June 1944. The historic significance of this monument can best be appreciated by understanding a little of how it was, or would have been, used as part of a temporary harbour works following the D-Day landings in 1944. Part of the invasion strategy was to create beachheads where the enemy least expected attack and offered less resistance. The lessons learnt after the disastrous failure at Dieppe in August 1942 ruled out a direct frontal assault on any French north coast port, such as Le Havre and Cherbourg. These were heavily fortified against such an eventuality and would have resulted in heavy Allied casualties. A period of complex deception, called Operation `FORTITUDE', was intended to fool the Germans into thinking the invasion would be focused where the sea crossing was at its narrowest at the Pas de Calais.

Operation `OVERLORD', the plan to eventually liberate occupied Europe and subdue Germany, began with the invasion of the eastern flank of the Cotentin peninsula in Normandy, at five shallow sandy beaches codenamed `Utah', `Omaha', `Gold', `Juno' and `Sword'. The key to this, involving the largest invasion force in history, was for the Allies to take their own temporary harbours with them - floated over in sections and assembled just off shore. However, these harbours would become largely redundant following the eventual capture of more established port facilities elsewhere.

There were four major components to Mulberry harbours. The first component was formed of 60 cement-filled `blockships', sailed across the Channel under their own steam then, when in the correct location, lined up bow to stern and scuttled, creating secondary or `Gooseberry' breakwaters for the harbour site. The second component involved 150 massive `Phoenix' hollow concrete caissons of various sizes, each towed by a tug. When in their correct location within the ring of Gooseberries, flood valves were opened and the caisson settled on the bottom leaving approximately two-thirds of each structure above the water to form an inner breakwater. The third component comprised several floating wharves or `Spuds' anchored within the harbour perimeter thus formed, and able to rise up and down according to the state of the tide, each between four huge steel pylon legs with splayed feet secured to the sea bed. The fourth component consisted of floating road bridges connecting the Spud wharves with the land, composed of steel and concrete pontoons or `Beetles' that supporting sections of roadway. An additional element was positioned farther out to sea in the form of a floating breakwater constructed of hollow tubes or `Bombardons' designed to reduce the height of incoming waves. This whole infrastructure was on a massive scale, designed to cope with large numbers of troop transport and supply ships. Each harbour was to extend along the shore about 3.5km and reach out into the water by 1.75km.

Two Mulberry harbours were started, one in the British sector on `Gold' beach at Arromanches; the other farther west in the American Sector on `Omaha' beach. After towing the sections across the Channel, assembly started on the 9th June and by the 18th two great arcs of caissons were in place. The following day, however, a very heavy storm, lasting three days started, entirely wrecking the incomplete harbour on Omaha and severely damaging the one on Gold. What could be salvaged from Omaha was transferred to the British, so the capture of Cherbourg with its port became an urgent task for the Americans.

Mulberry harbours were originally designed to endure for 100 days; nearly sixty years later some thirty great Phoenix caissons remain in position in the Arromanches `temporary' harbour wall. Apart from the Shoebury Ness example, at least three other caissons remain in British waters, having never made it over the Channel. Recorded examples lie within English waters at two locations: two units at Castletown Pier, Portland Harbour, Dorset; and one unit in Langstone Harbour, near Portsmouth.

The Shoebury Ness caisson is therefore a rare surviving example, which chanced to be stranded in British waters rather than reach its intended destination. It reflects an extraordinary engineering project designed to facilitate perhaps the most significant event of World War II - the D-Day counter- invasion and liberation of Continental Europe - and it remains a high visible symbol of this critical point in the history of the war.


The monument includes a Phoenix caisson, its back broken, lying on West Knock sandbank in the Thames Estuary some 1.8km SSW off the point at Shoebury Ness. The caisson lies in two sections, end to end, orientated north west-south east. The larger is approximately 35m long; the smaller 26m. The width of both sections is about 9.75m and height up to 6m. The exact sizes are difficult to confirm as both sections are listing longitudinally, partially sunken into the sandbank, and surrounded by a `moat' of deep water caused by tidal scouring. It is constructed of a steel framework and concrete base and walls about 0.3m thick. The sections are hollow and open to the sky, sea water periodically inundating part of each section according to the state of the tide. Internally, it is divided by cross-members into square cells, 18 in all and two-abreast for the full length of the structure.

According to an eye witness, Royal Navy signalman Ian Gordon, who was on board the caisson when it was being towed from Immingham on the Humber en route to Southsea, it sprang a leak so was brought into the Thames Estuary and allowed to sink.

The following items are excluded from the scheduling: all fixtures and fittings relating to the modern shipping navigation light and tower, the platform on which it stands, access walkways and ladders, and any electrical cabling and additional mooring bollards. The caisson structure beneath them, or to which they are attached is, however, included.

MAP EXTRACT The site of the monument is shown on the attached map extract.


The contents of this record have been generated from a legacy data system.

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This monument is scheduled under the Ancient Monuments and Archaeological Areas Act 1979 as amended as it appears to the Secretary of State to be of national importance. This entry is a copy, the original is held by the Department for Digital, Culture, Media and Sport.

End of official listing

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