Earth Surface Dynamics Research Group
By David Sear
Dunwich-'Britain's Atlantis' - has been revealing more of its secrets this week as a team led by Professor David Sear from Southampton University dived the site using a novel Sonar system that can see through muddy water.
As part of the '
Touching the Tide' Heritage Lottery project, run by Suffolk Council, the work at Dunwich has a number of aims, one of which is to dive on new sites revealed by a previous large scale mapping project back in 2011. The recent survey involved sending a diver down with a DIDSON sonar - a system that uses sound instead of light to 'illuminate' objects rather like a torch. Using coordinates from the earlier survey the team positioned the dive boat over the site and drop a weighted line with a buoy attached onto it. The diver clips onto this line and descends to the bottom where they 'shine' the DIDSON around them to identify what is protruding above the sand. The live acoustic images are projected in front of the divers mask so they can see what the sonar can see at the same time. The DIDSON also records all the data as a 'movie' which can be downloaded and analyzed frame by fram in the comfort of the diver boat or office. Back on the seabed, when a diver sees something of interest, they can alter the scale of the DIDSON to increase the resolution of the data - providing more and more detail - in some cases seeing objects of only 1cm across.
Diver holding the DIDSON acoustic imaging camera with head up display fixed on to the divers mask.
After sailing from the port of Lowestoft on Monday, the diver team set up at Dunwich, and began to sweep the site using a Sidescan sonar to check the conditions on the seabed after a winter of storm surges and high seas. This immediately revealed how dynamic the seabed is of this coast. Sand and silt had been moved over the site of the former town, burying all but the largest ruins. Further to the north of the town in the region of the old harbour, the seabed was not covered in sand, and we were able to locate two of the targets we were interested in. Diving Dunwich can really only be done during the change in the tide when the tidal currents are weaker on the turn. It is also safer to dive when waves are less than 1.5m. This means continuously checking the sea and weather forecasts to identify the best windows for diving around each turn of the tide. The result was diving into the night and early in the morning - which is no problem as it's black on the sea bed even during the day!
We dived the first site - located north of Dunwich and close inshore. The site proved to be a shipwreck. In the DIDSON, the diver could see the ribs of a wooden vessel, with piles of stone ballast lying in between each rib and in a pile on the landward side. Feeling around one of the ribs, the diver found it covered in a thin sheet of copper. Copper sheathing was put on the bottom of ships hulls after 1750AD so we knew we had a wreck from the last 250 years. The pile of ballast probably results from the ship leaning to landward as it was driven onto the sand bank or beach during a storn. At the moment we do not know the identity or type of wreck but someone somewhere will - so the next step is to work with local museums and archives to track this information down.
New unidentified wreck site. Left to Right - Site location relative to former town of Dunwich and current coastline, Sidescan image of the wreck from our 2012 survey - red dots are magnetometer hits indicating metal present, and DIDSON sonar images of bllast (upper) and wooden ribs (lower.)
Our next aim was to
revisit the ruins of St Peter's Church - these ruins are extensive and include larger blocks of masonry from the tower. We wanted to compare what we could see now to what we had been able to see in previous years to check the amount of burial by the sand.
We moored up over the church and weighted until early morning. There is something very special about lying in your bunk, knowing you are moored over the ruins of the old Town, listening to the dull roar of the waves breaking on the shore, while above, the same stars that those early Dunwich residents watched, still wheel above you.....but there were no bells to be heard this night!
Up at two-thirty in the morning and back on the dive. Check all equpiment is working, suit up the diver, check air lines and safety lines are secure, then over the side and into the dark waters. One signal to say all OK, and the diver disappears. The rest of us monitor the boats position, and watch the sea for signs of the diver resurfacing. After 20 minutes they appear, and we move carefully back towards them for recovery. The DIDSON images revealed that only the top 20 - 30 cm of the ruins now protrude above the seabed which means that around 1 metre of silt and sand has been deposited over this site since 2011.
Images from the four sites dived in the June 2014 survey. 1) is the unknown wreck, site; 2) shows blocks of peat from the former marshes; 3) shows the top of the ruins of St Peter's church, and 4) shows sand ripples over the site of All Saint's church.
We continued our search, diving on potential ruins around the harbour area, only to find that these were blocks of peat from the old marshes, revealed on the sea bed as the gravel barrier between Dunwich and Walberswick migrates slowly inland. Later in the week we tried to find the ruins of All Saints church, the last of the medieval churches to fall into the sea, and one seen in old photo's tottering on the cliff top. The ruins lie just off the beach, and were last seen in the 1980's in a small gully that forms between the beach and the first sandbank some 20m out to sea. With one of us directing from the cliff top where the last gravestones of All Saints churchyard can be seen, we set up a search area that would cut across the ruins of the church. The diver walked into the gully, and scanned the seabed with the DIDSON, but there was no sign of any ruins. All Saints, like almost all the ruins of Dunwich, lies beneath the sand.
So at the end of this phase of survey what have we learned? We have located a new wreck, which itself reminds us of the dangers of this coast, and the variety of the heritage that can be found in the coastal zone. We have also learned just how dynamic the sea bed is off this coast. Movements of sand and shingle occur on a daily basis but huge quantities are shifted around during storms, revealing and burying the archaeology. This emphasizes just how difficult it is to conduct marine archaeology in shallow coastal areas with shifting sand banks. We have been lucky in previous years to get the data we have from the Dunwich site. Now the site lies buried, and the ruins of lost churches and chapels are protected by a cloak of sand, to be revealed again sometime when the sea bed changes - which chould be next week or not for another decade. For Dunwich, we've added another archaeological site to the inventory of sites that make this among the most extensive and unique marine heritage areas in the world.
For more information visit
www.dunwich.org.uk.
Or read a paper published by David and the team in The International Journal of Nautical Archaeology (2011) on the Dunwich surveys,
here.
More information on the DIDSON sonar can be found,
here.
Original Article:
http://esdsoton.wordpress.com/2014/07/01/high-tech-sonar-reveals-new-wreck-through-the-muddy-waters-of-dunwich/