HMS Colossus - stabilisation and recording

Between 2003 and 2005 stabilisation trials were underaken on the Colossus stern site which aimed to determine suitable methods for stabilising the timbers exposed on the seabed. These exposed timbers have deteriorated considerably in the years since survey began in 2001. Various strategies exist for protecting sites. The specific aim of the stabilisation trials was to establish the efficacy and economic viability of different protection strategies in the conditions prevailing on this site. For further information on the stabilisation trials see the Colossus - overview page and the Download Centre.

It became clear from the stabilisation trials which of the trialled methods would be the most effective way of stabilising the exposed timber of this wreck. However, as this trial was only over a two year period we do not understand the longer term effects of stabilisation on the site. As such it was decided to protect a relatively small area (3.8 x 5.5m) at the stern of the site using a Terram 4000 mat held in place with sand bags.

The two areas which were recorded are shown outlined and hatched. The control area is shown in green and the Terram area in blue


The reason for choosing this small area at the stern of the vessel is twofold. Firstly, excavation at the stern of the vessel carried out by the ADU in 2001 demonstrated the high quality of the remains on this part of the wreck. The possibility of further carved material lying buried in this area is considerable. Secondly, this is a visually unappealing part of the wreck, so covering it with Terram will not detract from the appeal of the wreck to visiting divers. Currently three local dive charter boats in Scilly take visiting divers to Colossus, around which a Dive Trail was installed in 2009.


Planning Frame Survey

So that the long term effects of the stabilisation can be determined, the small area of timber to be protected, along with an associated control area, were recorded at a scale of 1:5. This was achieved using 1m square planning frames positioned along tape baselines. The principle advantage of drawing at a scale of 1:5 is that it is possible at this scale to record more detail. For example, at the more usual scale of 1:10, an average treenail (25mm diameter) is only 2.5mm on the drawing, making it very difficult to record details such as wedges.

CISMAS members performing the planning frame survey. Planning frame in position on its tape baseline.


The recorded area and location of planning frames


Comparing the planning frame survey made this year with that previously drawn in 2003 gives a graphic illustration of how the timbers have deteriorated in the intervening five years. Figs 7 and 8 below show detail from the 2003 and 2008 surveys. Those timbers shown in pink on the 2003 survey no longer survive. Comparison of the two plans also shows many areas where timbers have decayed to a smaller size in the five years which passed between the two surveys.

Detail from the 2003 and 2008 plans showing the area around the rearmost upper gun deck port. Note how those timbers coloured pink are no longer present in 2008



The installation of the small area of Terram 4000 was undertaken in June 2008. On installation it became clear that several upstanding iron concretions were preventing the mat from sitting properly on the seabed. Consequently, it was decided to remove a small part of the mat on the northern edge to avoid these concretions and allow the mat to sit flat on the seabed. The most important elements of the stern wreckage were still protected by this modified mat shape.

The Terram mat and seabed sign in place on the seabed


The Terram mat was secured using sandbags placed in a continuous line around the edge of the mat. Additional lines of standard sandbags were also placed in the centre of the mat and around the upstanding lead pipe. Work on the stabilisation trials suggests that this mat will quickly become colonised with weed and will accumulate a layer of sediment within a few months. The ultimate longevity of the Terram mat will depend to some extent on future sediment movements on the site. Indications are that the Terram itself will have a long lifespan. Experience has shown, however, that the sandbags will start to deteriorate within five to ten years. By then, there should be sufficient sediment accumulated on the mat to hold it securely in position. Careful inspection during the routine sediment monitoring of the site will document the condition of the Terram mat.


The HMS Colossus Stabilisation and Recording project was funded by English Heritage.

English Heritage



HMS Colossus

A memorable dive
Debris field survey
Site stabilisation
Dive trail
Site plan