Dorset WreckMap 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.
Right: Map of Poole Harbour and the Studland Bay area.
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 century.
Above 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).
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
Above right: NAS member Martin Openshaw during one of the
ground truthing dives
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.
Above: 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 side 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.
Dissemination
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 the 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.
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.
Above: 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.
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