Phoenix Caisson (outer) off Pagham Harbour


Heritage Category:
Scheduled Monument
List Entry Number:
Date first listed:
Location Description:
3.56km south-east of Pagham Harbour, West Sussex


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

Location Description:
3.56km south-east of Pagham Harbour, West Sussex
National Grid Reference:


The asset comprises a rare type of steel reinforced concrete caisson unit, code-named Phoenix, built as part of the artificial Mulberry Harbours that were assembled as part of the follow-up to the Normandy Landings on 6 June 1944.

Reasons for Designation

The Phoenix caisson (also known as Outer or Far Mulberry) located off Pagham Harbour, West Sussex, is scheduled for the following principal reasons:

* Period: as a material record and an eloquent witness to the engineering achievements and logistical preparations around England’s coast for the largest amphibious invasion ever undertaken, on 6th June 1944; * Potential: forming a primary source of evidence relating to the ingenuity of the wartime Allies in creating, and building, artificial harbours for use in Normandy, and; * Rarity: as the only confirmed surviving A-1 type of Phoenix caisson Unit anywhere in the UK.


In June 1944 the Allies opened a Second Front in Europe with Operation Neptune, comprising a largely amphibious invasion. In order to supply the armies once they had landed, harbours were necessary to allow supply ships to unload. In the expectation that captured harbours would have been rendered unusable by the enemy, the Allies intended to build two artificial harbours; one in the US sector and one in the British sector. These harbours were code-named Mulberries A and B respectively and Operation Neptune is still considered one of the greatest and most challenging logistical operations of all time.

The Mulberries consisted of four major component structures, code-named Gooseberry (a network of artificial breakwaters), Bombardon (floating breakwaters), Whale (floating piers) and Phoenix (sunken breakwaters).

Phoenix Units comprised large concrete caissons which varied in size from about 2000 to 6000 tons, with a maximum draft of 20ft (~6m). They were constructed by civil engineering contractors around the coast of Britain and were designed to be towed across the Channel and sunk off Normandy to form breakwaters to protect unloading ships. During the days before the D-Day landings several of these large concrete caissons were sunk in the shallow waters off Pagham ready to be re-floated and towed to Normandy. In fact, the marine area off Pagham, commonly known by mariners as the ‘Park’, contains an exceptional number of submerged (i.e. parked) Second World War sites that are associated with Mulberry harbour operations. Phoenix Units were also assembled off Dungeness and Allhallows, Kent, prior to embarkation.

It appears that in June 1944 the Unit was ‘parked’ off Pagham awaiting passage to Normandy. Its ballast tanks were pumped out to re-float it but as its Tug wasn’t ready, the Unit was re-sunk. In the process, it swung in the tide and settled across its own previous scour in the seabed, where it has remained since.

The Unit was first charted in 1944 when it was noted to be ‘visible at all states of the tides’ though three years later the UK Hydrographic Office recorded that the caisson was not visible anymore. Anecdotal evidence suggests that at some point in 1945, the Unit was used for target practice by Hawker Typhoons based at RAF Selsey, Church Norton, Selsey.


The structure comprises a hollow box divided into 22 compartments that could be flooded and emptied with pumps to control the buoyancy of the Unit and the ingress of water was regulated with a wire-guided system of sluice valves. The internal watertight concrete bulkheads were supported by a central fore and aft buttress that had openings similar to limber (drain) holes to allow the water run freely within the structure and this cellular structure added strength and stability to the caisson. The upper part of one of these deep rectangular slots was used during navigation as quarters for the crew. The lower structure was scow-shaped (flat bottomed) with sheered ends to allow a certain degree of navigability and stability when the unit was towed and the flat base meant that the unit could sit firmly on the seabed. The upper structure was open on top and had high walls to provide shelter and mooring points. The top of the Unit could be accessed through ladders that were put at various points along the walls and a gangway allowed personnel to walk its length.

Constructed of steel reinforced concrete, the steel reinforcement is subject to saltwater corrosion although the rate of this corrosion is not known. However, the structure does not seem to have been particularly affected by decay in the last 73 years and appears to be in an apparently stable condition. The Unit lies in an upright position, roughly aligned north/south, with a coherent structure although it is damaged at the southern end (most likely as a result of strafing in 1945). The northern end is the most coherent and in good condition. Overall, the structure still retains its original shape and recognisable features such as internal compartments and bollards on the top of the wall. The extreme southern end is broken with features hard to distinguish amongst the rubble of concrete and steel. Overall the structure is large measuring approximately 60m by 17m, standing up to 8m high.

Eighty of the 212 Phoenix Units manufactured were of the A-1 type but very few of the caissons are known to survive archaeologically; only five others are known in UK waters. Two of these are located in Portland Harbour, Dorset (listed Grade II 1993; NHLE entry 1203075), one lies off Shoeburyness, Essex (scheduled 2004; NHLE entry 1021090), another in Langstone Harbour, near Portsmouth (National Record of the Historic Environment or NRHE reference 1398041) and one lies off Littlestone-on-Sea, Kent (scheduled 2013; NHLE entry1415588). None of the designated caissons are of the A-1 type while the Langstone Unit is recorded as either an A-1 or C-type. As such, the Pagham Phoenix caisson comprises the only confirmed A-1 type, the largest of all of the original Phoenix Units built, anywhere in the UK.

Records held by Historic England indicate the presence of two unconfirmed Phoenix Units lying off Cornwall: one lies in Mounts Bay at a depth of 51m (NRHE ref 918818) with another lying on the seabed some 2 nautical miles north-west of Land's End at a depth of 55m (NRHE ref 830065). Others are reported to lie south of Portland, south of the Isle of Wight and off Watchet, Somerset. In addition to their current lack of identification and unrecorded condition, it is not yet known why Phoenix Units would be present in the far south-west of England.


Mayor, A., 2015, Outer Mulberry, in SCUBA magazine Wrecked! edition, BSAC
Mayor, A., 2015, Report on investigations of the remains of the artificial harbours codenamed “Mulberry” which assembled along the south coast of England prior to Operation Neptune and D-Day
McDonald, K., 1995: Dive Sussex: a diver guide, Underwater World Publications
Wessex Archaeology, 2017, Mulberry Units Pagham Harbour, West Sussex, unpublished report for Historic England


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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