Century-old wreck uncovered while performing routine Port Company survey

The first sighting of the wreck: Data collected with a Teledyne RESON SeaBat 7101 Multibeam Echosounder. Data processing in Teledyne PDS.
The first sighting of the wreck: Data collected with a Teledyne RESON SeaBat 7101 Multibeam Echosounder, Data processing in Teledyne PDS
Teledyne PDS is used for processing the data and the initial stage of charting before exporting to AutoCAD
Teledyne PDS is used for processing the data and the initial stage of charting before exporting to AutoCAD
The Wreck seen in maximum resolution:  Data collected with a Teledyne RESON SeaBat 7101 Multibeam Echosounder. Data processing in Teledyne PDS
The Wreck seen in maximum resolution: Data collected with a Teledyne RESON SeaBat 7101 Multibeam Echosounder. Data processing in Teledyne PDS

Emily Hand, Hydrographic Surveyor at the Bristol Port Company tells the interesting tale of uncovering the wreck of the ‘Brunswick’ a metal-hulled cargo vessel lost off the UK’s Port of Bristol in 1900 and rediscovered in 2017.

The entire buoyed channel of the Bristol Port Company’s statutory harbour area is surveyed in full over a three-year period, and the area for investigation in 2017 was the Bristol Deep, off the coast of Clevedon. The 28th June was day two of what turned out to be a nine day survey spread over the spring tides from June to August. The survey started in the deep shipping channel and progressed north towards the drying sand bank of the Middle Grounds. The Middle Grounds has a spit of sand known as the Welsh Hook.

This arrowhead shaped spit of sand this year was found pointing to a wreck at its tip. The data was a little noisy originally due to the very high turbulence around the wreck but with an additional infill line, full coverage was possible and there was no doubt as to the existence of the wreck.

A WRECK APPEARS FROM THE DATA
At first I didn’t realise this was a new wreck as there is one relatively close by called the Johann Carl and there was no reason to expect anything new in this area as it had been surveyed many times before. On the transit back to port though and then returning to the office it began to dawn on me that this was something previously unknown. As the area was partially outside of our RTK radio transmitting zone I had to wait a while to gather the POSpac data in order to be able to process but as soon as possible I began processing the data. I showed the image to the Deputy Harbour Master Conservancy who was very surprised. He looked back over all the surveys of the area and in over 40 years the wreck had not been detected. It would seem that during all previous surveys, the Middle Grounds sand bank had been around 200m further south showing the location of the wreck as 2m below chart datum. The wreck lies below this at 5.5m below chart datum.

Going back to the site for second survey a couple of weeks later, on the 11th July, we took the opportunity to gather more data over the wreck to produce a clearer image. Our Teledyne Reson 7101 multibeam is usually operating at just 10pps creating a 0.5m grid but for this second survey of the wreck we ran at maximum capacity of 50pps creating a 0.25m grid. After processing, the image was clear enough to begin to attempt to identify the vessel. A raised forecastle and aft deck were noticeable with the bridge amidships and a cargo hold forward of this. This along with the size of 65m long by 9m wide led to estimates of it being a late 19th or early 20th century cargo ship.

THE FATE OF BRUNSWICK REVEALED
Much research followed the discovery to attempt to determine the specific vessel and the cargo vessel. Brunswick was deemed the most likely candidate by both ourselves and then Historic England. Of the 29 wrecks in the area the Brunswick is the only large, metal-hulled cargo vessel that matches the dimensions. It was also described as having grounded on the Welsh Hook on a foggy Christmas Eve in 1900.

Finding the wreck was a very interesting day ‘at the office’; however potentially more interesting to us here in the survey team is what caused the sand bank to shift so much further than ever surveyed before? Where in the estuary system has this sand gone and of course what else may lie beneath the sand of the Severn Estuary?

Estuary surveys are undertaken based on our survey programme, with the most frequent surveys being the Severn Bar and Denny Shoal sand banks surveyed every 2 months, the final approach to the ports, known as King Road, surveyed annually and the rest of the buoyed channel within the harbour area surveyed in full over a 3-year cycle.

NAVIGABLE FLUID MUD
With the third highest tidal range in the world at 13.2m MHWS, our sandbanks are ever changing, however, a further challenge is the addition of mud within the environment. The tides cause the mud particles to become fluid and highly mobile. The result of this is unreliable multibeam data detecting soft pockets of navigable fluid mud.

It is for this reason that our almost weekly entrance surveys are conducted using a Teledyne Odom Hydrographic CV200 and Stema Silas density software to detect the navigable horizon. The surveys further out in the estuary can be affected by the fluid mud on neap tides as the multibeam signal struggles to penetrate the “fluff” or it may detect a layer of fluid mud in the deeper areas of the estuary. For this reason, surveys are conducted on spring tides where the predicted tide is greater than 12.6m. Couple this with needing a good weather window and it can become difficult to fit in all the surveys. The bigger areas can take the whole summer to complete during which time the sand can shift quite dramatically; this area is so dynamic that sand waves do not always match up even after just 20 minutes between survey lines. These factors take a little work initially, however with experience coming from being solely based in this area we know most of the quirks of working in the Severn Estuary.

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