Dorset WreckMap 2001

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Saturday, June 2, 2001

The WreckMap 2001 project evolved through consultation and collaboration between the NAS, the Dorset Coast Forum, the Dorset Wildlife Trust and the National Trust, with the aim of developing a methodology for an effective multi-disciplinary approach to seabed evaluation.

It is recognised that Studland Bay in Dorset is an area of high archaeological potential. Old Harry Rocks, at the southern extent of the Bay, are the remains of the land bridge that connected Studland Bay with the Needles on the Isle of Wight until the inundation caused by the end of the last ice age approximately 7000 years ago. Immediately north of the Bay is Poole Harbour, which has a long history of maritime use. In the 1960's a log boat was discovered off Brownsea Island in Poole Harbour and was radiocarbon dated to c300 BC, indicating maritime activity in the area since prehistoric times. In the 1st century AD Poole was used as an invasion port by the Romans, and by the 15th century the Harbour had been awarded Staple Port status as one of the few ports in the country where wool could legally be exported. The medieval period saw trading links with the Baltic, Spain and Italy and by the 17th and 18th centuries, transatlantic trade links were commonplace.

The close proximity of Studland Bay to Poole Harbour with its long and varied history of maritime trade and use would therefore suggest a high archaeological potential for the Bay, particularly with respect to maritime material. Indeed, the discovery in 1984 of the Studland Bay Wreck, one of only 52 wrecks designated nationally under the Protection of Wrecks Act 1973, would seem to confirm this. As a 16th century vessel of the Iberian-Atlantic tradition (Thomsen 2000: 69-85), she certainly attests to maritime activity in the area in the 16th Marker Buoycentury.

Image Right: Buoy marking the designated wreck site, Studland Bay, Dorset.

Studland Bay, Dorset
In January 2001 a ship's timber was washed up at the north end of Studland Beach inside the mole. The timber, approximately 2m across, had minimal marine growth and was well preserved with fastenings that included treenails and square shanked nails (only the holes remaining).
Timber
Above: Ship's timber recovered from Studland Beach in January 2001. See Appendix A of the printed report for the completed Timber Recording Sheet.

The National Trust have taken responsibility for the conservation and storage of this timber. They arranged for a dendrochronologist to examine it with a view to obtaining a date. Unfortunately, the timber was identified as fast-growing oak of an unsuitable nature for dendro-dating. Mike Markey of the Poole Bay Archaeological Group and licensee of the designated Studland Bay Wreck was consulted regarding the possibility that the timber had eroded from the protected site. He felt that the timber was too large to have been from that particular wreck.

Project Aims
The Dorset project aimed to test a methodology for an effective multi-disciplinary approach to seabed evaluation, while encouraging public involvement within a modest project budget.

Specifically the project aimed to:

  • Utilise skills and involve amateurs - Provide an opportunity for those involved in the NAS Training Programme to put their newly acquired skills into practice
  • Characterise the seabed (ground truthing)
  • Record sites and add information to SMR/NMR - Provide information for the Dorset Maritime Sites and Monuments Record, the National Monuments Record (Maritime) and the Dorset Coast Forum Marine Conservation Group
  • Disseminate information - Publish the project results among the participants, supporting bodies and more generally in the public domain

Image right: NAS member Martin Openshaw during one of the ground truthing dives.Groundtruthing

The project was divided into four broad phases:

  • Desk based assessment
  • Geophysical survey
  • Ground truthing of the geophysical targets and selected SMR records
  • Dissemination of the information acquired through the Sites and Monuments Record and publication.

Desk based assessment
A desk based assessment was carried out to assimilate existing archaeological and geophysical survey data for the area. This involved consultation of the Dorset Maritime Sites and Monuments' Record (DMSMR) which currently has 44 records for the Studland Bay area, comprising the designated wreck (see above), reported obstructions, anomalies from BP geophysical survey, Admiralty listings, multi-period shipwrecks and Valentine Tanks from the Second World War. This information, combined with that obtained from local dive literature, enabled the prioritisation of areas to conduct the geophysical survey.
GroupAbove: NAS members participating in Dorset WreckMap 2001.

Geophysical survey
The geophysical survey was undertaken between the 6th and 8th of June 2001. The aim of the survey was to map an area of seabed and identify targets of archaeological potential. Two survey systems were used, a Swath bathymetry system provided by Submetrix Ltd (SEA) and a Tritech 375Khz side scan sonar provided by Atlantas Marine of Langport, Somerset. The Swath system gathers depth data in a swathe across the seabed. The data can be viewed as a two-dimensional colour plot of bathymetry that clearly shows seabed features. The sideGeophys scan sonar system produces an image, based not on depth, but on the signal strength of the returned sound energy, providing a very clear indication of seabed relief, highlighting anomalies that could potentially be archaeologically important. The two systems were used in conjunction with one another as the Swath system produces quantitative high resolution data whereas the side scan produces a wider qualitative image of the seabed. The simultaneous use of these two systems optimises the information gathered on the seabed topography. The geophysical data were then processed over a 5 day period to produce data that could be assimilated into a Geographical Information System (GIS) and provide targets for ground truthing.

Above right: A geophysical anomaly highlighted on the side-scan sonar image.

The survey vessel Snapper was mobilised with the two acoustic systems. The data was collected and integrated with the Navigation system in the survey computer. The tow fish was trailed behind the survey vessel at a set distance to give an acoustic image of the seabed. Positioning for both systems was given by Differential GPS.

Ground truthing
The ground truthing of geophysical targets was carried out by NAS volunteer sport divers under the direction and guidance of NAS staff during the WreckMap 2001 field school 28th July - 3rd August 2001. Targets identified during the geophysics phase of the project were located using a Differential GPS (DGPS) receiver carried on a rigid inflatable. Divers were subsequently deployed to carry out a pre-disturbance survey of the anomaly using the Diving with a Purpose Dorset Dive Slate (see Appendix C), which encourages the collection of information concerning archaeology, the environment and ecology.

DisseminationReport

  • The information gained during the course of this project has been/will be disseminated in the following ways:
  • Report distributed to participants and sponsors
  • Report on the NAS website
  • Report in the NAS quarterly Newsletter
  • Completed Diving with a Purpose forms forwarded to Dorset Coast Forum and Dorset Wildlife Trust
  • New data added to the DMSMR where appropriate

Above right: NAS Project Report distributed in November 2001

Results:

Geophysics
Time restraints led to the decision to have two discrete survey areas, one to be surveyed on each day. Area A1 was to the North East of the Harbour entrance. This area was chosen as a zone of high sediment mobility. Area A1 has historically given up artefacts at different times, most recently ship's timbers have been revealed and noted by Mike Markey, in an area close to the known Antler wreck site. The second area, A2, was chosen as part of the Bay that had received minimal coverage during previous surveys and because logistically its location optimised the ground truthing phase of the project.

Survey lines in both areas were run with a north south orientation. Table 1 shows the survey scheme undertaken for the project. We wanted to achieve total ensonification of the seabed with the Swath bathymetry system. This system is dependent on water depth, and appropriate line spacings were chosen accordingly. The majority of Area 1 was surveyed with a line spacing of 40m because of its shallow nature. Area 2 was surveyed with a line spacing of 50m.

The data interpretation from the Swath bathymetry system has produced a total of forty targets. The majority of these targets are in Area 2. The targets range from small ephemeral impression on the seabed, that are most likely to be bed forms, to targets that are more likely to be of a cultural nature. Twelve of these targets were dived during the ground truthing phase of the project. From the side scan image produced by the Swath system we can identify a series of reef structures indicating emergence of the bedrock geology from the sediment. This was confirmed at the ground truthing stage: targets 20, 26, 36 and 40 proved to be pronounced eroding faces of rocky reefs. The faces of the reefs were perpendicular to the tidal flow, which would suggest that they are in a continual stage of development. From our findings during the ground truthing phase, these structures form a natural "trap" for anything flowing over the seabed, including cultural material.

It is apparent that the positions identified during our own geophysical survey differ from those already held on the SMR. This could of course be down to a number of factors including different positioning systems (e.g. GPS, DGPS, DECCA), active sedimentary regimes leading to covering/uncovering of targets, or simply many closely located targets.

It is intended that further interpretation of the data will take place. This will entail a detailed analysis of the bathymetry and an investigation into the sediment regime in these areas and its effects on the morphology of the bedforms.

Marine Identification Course
To encourage a multi-disciplinary approach to seabed evaluation and promote a holistic appreciation of the dive sites, a Marine Species Identification course was organised as part of the project. The course was tutored by Peter Tinsley of the Purbeck Marine Wildlife Reserve and attended by NAS volunteers and a visiting group of SeaSearch volunteers. This enabled project participants to identify environment and species as part of the seabed assessment during the ground truthing phase of the project. Feedback from project participants and professional archaeologists indicated that this was a worthwhile exercise and served to raise awareness of the natural environment.

During the WreckMap Dorset 2001 project, 12 geophysical targets were ground truthed. The results of these ground truthing dives are detailed in the dive log sheets, which are being held in the project archive in the NAS office. The information from these sheets has been assimilated into the ground truthing Sheets.

Diving with a Purpose Forms
The project aimed to familiarise participants with the NAS Diving with a Purpose form as a means of rapid pre-disturbance assessment of a potential site. The form used during the project was specifically designed for the Dorset region by the inclusion of five marine species of particular interest to the Dorset Marine Conservation group of the Dorset Coast Forum. During the ground truthing of targets, information concerning these species could therefore be collected. This information was of use to both the marine conservationists and the geophysicists (for interpretation purposes).

Project participants found the DWAP form useful and easy to use in the initial stages of the project. Experienced recorders, however, found it restrictive and eventually reverted to their own recording methods. NAS recognises the limitations of the DWAP form but sees it as a useful introductory tool to promote constructive recording techniques.

Retrieval of glass bottle
During a ground truthing dive on 31/07/01 an intact glass bottle was found between rocks in a small reef. After consultation with the National Trust as landowners it was decided to raise the bottle for the following reasons:

  • The intact bottle, though apparently not very old, was unusual in appearance and was felt to be at risk from the elements if left in-situ.
  • As an educational exercise in the administrative, logistical and practical implications of carrying out such an activity.

The National Trust were consulted with respect to the proposed retrieval of the bottle. They approved and agreed to undertake responsibility for the conservation of the bottle once it had been recovered. The necessary materials for safe retrieval were obtained and two project participants raised the bottle on the 2nd of August 2001, the final diving day of theGlass Bottle project. Having been successfully recovered, the bottle was photographed and put in passive conservation.

The bottle is now in the keeping of the National Trust archaeologists based in Corfe, Dorset. The bottle was found to have the letters DAWS on it. The find was reported to the Receiver of Wreck on Droit No: 209/01.

Above right: Photo of the bottle recovered during the project.

Project Outcomes

  • Successful ground truthing of geophysical targets
  • Better understanding of the dynamic nature (unquantified) of the seabed in Studland Bay
  • Provided information on marine species for Marine Conservation Group
  • Public involvement and development of stewardship
  • Promoted use of the Diving with a Purpose form
  • Increased public awareness (evening talks by Gordon Le Pard and Ed Cumming)
  • Educational value
  • Public involvement in archaeological ground truthing process
  • Understanding of process of artefact retrieval

It is felt that amateurs have a clear role in the ground truthing of the underwater cultural heritage of the UK. The project clearly showed that volunteers, with some guidance and direction from archaeologists, can make a valuable contribution to the archaeological record. While this is not a new concept, NAS is trying to provide a national framework (Diving with a Purpose) within which numerous groups can participate. Such groups will have access to an organisation that can respond to requests for support and provide guidance. We would anticipate that information retrieved from these groups would, in the first instance, go to the local SMR and other appropriate bodies. Sport divers have a valuable role to play in the long-term monitoring of sites as well as ground truthing and discovery. They can therefore play a positive role in the subsequent management of the coastal resource.

The Future : Geophysics
The two days of geophysical survey at Studland cost the NAS approximately £500. This figure is very significantly subsidised by time and equipment provided free of charge by the project sponsors. This approach to geophysics survey can only be sustained for a small number of projects, as it depends upon the generosity of sponsors, and our own limited finances.

This study shows, however, that coupling the geophysical survey with a volunteer diver ground truthing exercise greatly improves our coverage and understanding of underwater archaeological resources. Extending this ability in future will rely on the ability to raise sponsorship for more extensive surveys of this type.

Note: this section on the future for geophysics investigation has been heavily copy edited.

The Future : Ground truthing
Use existing SMR as a basis for future ground truthing, or use targets generated by dedicated geophysics.

Who does the ground truthing?

  • NAS continue to ground truth targets using teams of volunteers, or
  • NAS publish positions of geophysical targets (e.g. via NAS website) and invite sport divers to ground truth targets independently.

The first option offers more control and ensures that any information finds its way back into the archaeological record. The second option provides an opportunity for a greater number of participants, but does rely on the integrity of the contributing sport divers. Independent groups may not provide information in a useable form and with this approach there is also the possibility that an important site could be found by a group and not reported.

The Future : Monitoring
Establish monitoring stations on selected sites, e.g. Antler wreck, Valentine tank, reef, or other appropriate sites.

Such activity would provide quantifiable information concerning the dynamic nature of the seabed and environment in Dorset, an essential factor for decisions in future coastal management.

Who does the monitoring?
NAS would recommend that NAS supported volunteer teams carry out this work under the Adopt a Wreck/Site scheme, thus promoting public participation and developing a sense of local ownership of the underwater cultural heritage.

Dorset Coast Forum might wish to consider other 'at risk' areas where this methodology could be applied.

Conclusion
NAS is committed to continuing its involvement in Dorset and would like to enter into discussion with all interested parties concerning the issues raised above. It is important that all views are taken into consideration in the formulation of a future plan.
Writing upAbove: Photo of the group members writing up the day's activities.

The NAS would like to thank:
Captain Ron Howes and his boat Snapper, who diligently steered survey lines for many hours beyond the call of duty. Steve Pierce from Poole Harbour Commissioners supplied tidal data and valuable information on the survey area. Charlie Foll from Atlantas Marine, for the supply of the side scan sonar, and Paul Byham from Submetrix Ltd (SEA). NAS would also like to thank Simon Corfield (DERA) and David Kemp and staff at the National Trust Studland Beach & Nature Reserve visitor centre.

International: 
No
Area: 
England