Maritime Archaeology Newsletter From Denmark 23, 2008

Maritime Archaeology

Newsletter No. 23

•

Spring 2008

Searching the left bank of the

from Denmark

Kongeå as a training exercise. Photo: Thijs Maarleveld.

N0. 23 • SPRING 2008

CONTENTS: Preparing for summer . . . . . . . . . . . . . . . . 2 An early “half-carvel” in the Northern Baltic . . . . . . . . . . . . . . . . 4 Knudedyb ahoy! . . . . . . . . . . . . . . . . . . . 10 From a grain of sand, a mountain appears. . . . . . . . . . . . . . . . . 15 Diving at Gredstedbro. . . . . . . . . . . . . . . 24

A fascinating endeavour:

ISBN 978-87-85180-41-4 Published by the Viking Ship Museum in Roskilde. DKK 99 / EUR 13.29 Also published in Danish: Velkommen om bord. Havhingsten fra Glendalough. Et genskabt langskib fra Vikingetiden. Electronic version (E-book) in preparation. 2

Preparing for summer It is the last days of spring, when this newsletter goes to the press. The term is nearing its end. The first students are fully engaged in writing their master theses. In the meantime preparations for fieldwork are in full swing. First, it will be England. There we are to contribute to the collaborative ‘Gresham wreck project’. We will continue the documentation of the hull of this late 16th century wreck that was ‘rescued’ from the Prince's Channel in the Thames, where it was in the way of shipping development. The construction is extremely revealing, which makes it exciting research. The fieldwork will facilitate the field school that the Nautical Archaeology Society (NAS) organizes in conjunction with the international IKUWA conference. The trip will be combined with a shared heritage conference in Wolverhampton and a UNESCO policy meeting at the British Academy. Then it will be Gotland in Sweden, where we will be engaged in an area survey that is financed as mitigation, compensation or interested goodwill by a projected pipe-line project. At the national scale, we have become used to projects that try and integrate archaeology and the reduction of impacts on the underwater cultural heritage resource. But for a large transnational project that crosses many nations and maritime zones to do so consistently is, as far as I am aware, something of a novum. Whether the survey and trial excavations will bring interesting results remains to be seen, of course. But it is certainly real world experience that will add to the students’ practical training and employability. Fieldwork in Greece will be limited this year, while larger operations are in a planning stage. But anyhow, it will be a very busy summer! When we come back, it will only be a few days before we wel-

come a new group of students. Apparently the group counts no less than nine different nationalities! Challenging times! And in the meantime, Havhingsten fra Glendalough will have finished its fascinating tour, after its return voyage from Dublin. The welcome on board publication is highly recommended. For me, it is something of a déjà-vu to start up a range of operations virtually without equipment. Any operation at sea needs so many little things and gadgets. The same is true for the laboratory. Starting anew, the whole collection needs building up. Diving gear and communication equipment are just part of it, although, not a part one can compromise on. We are choosing the best quality and start to have a good range of sets. Bauer compressors officially handed over a fine compressor during the geophysics course that we organized in February and that, with the help of Wessex Archaeology and other partners, we hope to develop into a recurring event. The Faculty of the Humanities put a minibus at our disposal. With ten years and over 200,000 kilometres, it is not exactly new, but it is marvellous and absolutely indispensable. A careful hand will do the rest. An inflatable was acquired and an outboard borrowed. The Viking Ship Museum helps with model building equipment. But another thing I am extremely glad of, is that for various reasons Hans Dal and Marine Arkæologisk Gruppe in Fredericia have decided to transfer their field equipment, including a pump and dredges, but also a very well assorted collection of stainless steel rods, buoys, sinkers, ropework, tackle and the lot to the Maritime Archaeology Programme. It is such a welcome and varied addition! That this avocational group disposes of its field equipment of many years standing, is a reason for some reflection. The group's interest in Prehistory seems to continue unabated. Is it a function of professionalisation then, that a private group radically parts with its past? Is it the inevitable outcome of development? Is it collateral damage that could

have been avoided? Is it damage at all? Or is it good riddance? Many perspectives are possible. Many perspectives are undoubtedly taken. It is not at all unique that the ways of vocational and recreational archaeologists split at some point. Each has their role, after all. In development led archaeology, where the negotiators for a mature discipline set realistic prices, there is no role for voluntary labour. Under other circumstances, this may be completely different. Joint efforts and operations may then be very effective, as they have often proved to be. The central role of the Marine Arkæologisk Gruppe in the early development of submerged Stone Age archaeology in Denmark is a well recognised fact. I take it as a sign of the group's collegial attitude to the profession, that they gladly offer their gear for the training of young professionals. It is a tribute to them, that we will make good use of it. Thijs J. Maarleveld

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An early “half-carvel” in the Northern Baltic The town of Sundsvall in northern Sweden was given town-privileges in 1621. It was part of the endeavour to establish a trade centre and a rifle factory in the northern parts of Sweden. The geography provided several fine natural harbours. Several of these are situated along Selångersån, the stream that runs through the central parts of Sundsvall. Land-elevation was so massive in this area that the harbours were already too shallow by the middle of the 17th century. The settlement had to be moved eastwards, towards the sea. Very little is known about the earliest settlement and the activities that took place at the spot that was later to become Sundsvall. Among the few traces of activities that predate the 17th century are the remains of a clinker built vessel. The wreck was found in two meters of water in 1994 when a water supply pipe was to be placed in the stream. The county museum surveyed the wreck and concluded that it must be quite old, perhaps medieval (Holmqvist 2000:9). Local divers, who regularly visited the site, have observed ongoing destruction of the site caused by currents and ice floes. Timbers from the wreck have been dispersed over quite a large area during the past few years. In a desperate attempt to stop the decay, the wreck has been wrapped in a net to prevent the current from making timbers drift away (Vikdahl & Högberg 1999:3f, Holmqvist 2001). Since then, ways of dealing with the wreck or some sort of “final” solution have been the subject of ongoing discussion. Shoes, bottles, and a well preserved hull

In 2006, the Swedish Maritime Museums carried out a survey of the site. The aim was to examine the possibilities of long term preservation of the wreck at its present loca4

tion. A second aim was to gather as much information as possible about the origin of the vessel and its historical context. The fieldwork was carried out during one week in October 2006. The conditions in the stream Selångersån are far from ideal for archaeological fieldwork. Although the depth at the site is less than two metres, the visibility at the time of the fieldwork was less than 20cm. Most parts of the hull are embedded in sediments, and the survey consisted of digging a trench inside the hull. The trench was dug by hand, and sediments were removed using a water dredge. The parts of the hull that were visible as rising above the riverbed were recorded with a total station. The preserved length of the hull is 14.3 metres and the original length from stem to stern was probably 1617 metres. Quite a lot of the hull structure has been preserved embedded in sediments. The port side is the most complete. It has been preserved up to the bulwarks. This means that approximately 80% of the portside of the hull is available for study. On the other hand, only 30% of the starboard-side seems to have been preserved. The cultural layers in the hull proved to be disturbed. Find material consisted of bottles, shoes, and other items dating to the19th and 20th century. No cultural layers belonging to the shipwreck were observed. The only finds that might be the remains of an original cargo were some limestone pebbles and a fragment of an iron bar. Both types of material are possible cargoes for a vessel of this size. The find circumstances, however, are not conclusive. In the context of the earliest settlement of Sundsvall, it is as likely that they belong to the cultural debris of the town rather than belonging to the wreck.

Whereas the trial excavation did not produce much by the way of interesting associated material, the hull structure itself proved to be in very good condition indeed. Also, it showed remarkable and interesting features. The trench was opened amidships and addressed the deeper part of the vessel. The excavated section includes five strakes of the starboard-side, the keel, and three strakes of the port-side. The configuration of inner timbers is asymmetric. Whereas the starboard side is fitted with a thick stringer, the port side is fitted with a thin plank. One of the floors has been strengthened with a rider. The strakes have been fastened to each other with ironnails, which have been bent and hit back into the wood. The planking is fastened to the frames with tree-nails. Dating and provenience The hull provides good opportunities for dendrochronological analysis as the timbers are not worked more than necessary. As a result, bark is present on frames and

other strengthening timbers on the inside of the hull. Three samples were sent to the National museum of Denmark and the department for Environmental Archaeology. The analyses were carried out by Orla Hylleberg Eriksen, and the result was spot on. The timbers were felled in the summer of 1577. The provenience of the timbers could be estimated to some extent as the samples showed some matching to the chronologies of Jämtland (a northern province of Sweden) but also to Finland! The question of the vessel's origin must thus be left unsolved for now, although the analysis points to the northern Baltic Sea area. An early half-carvel Some of the loose timbers that were lying scattered around at the wreck-site were raised for further examination. Two of these turned out to be top-timbers that ended at the ceiling. Notably, these timbers lack the

This map was drawn in 1642 and shows the extension of Sundsvall (Lantmäteriet).

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characteristic clincher-hooks and revealed that the strakes of the uppermost planking are laid side by side in a carvel manner. At the carvel parts of the top-timbers, the tree-nails are oriented to the middle of the strakes. This is in contrast to the clincherparts where the tree-nails are oriented to the lower parts of the strakes as is common in constructions of bigger clinker-built vessels on the east coast of Sweden. Olof Hasslöf has described the tradition of building hulls in what he calls a “halfcarvel” manner (Sw “Halvkravell”) (see for instance Hasslöf 1970). The lower parts of the hull are built shell first in a clinker-manner. The strengthening inner timbers are put

in, and the side of the hull is then completed in a carvel-manner. Written sources, from the 1680s, testify that “half-carvel” hulls were considered as being clincher-vessels and more or less “peasants vessels” rendering them less privileges than carvel ships (see Eriksson 2008). There are several shipwrecks, dating from the 18th and 19th century, that give us example of the “half-carvel” technique, but none of these has undergone further examination. The Åkroken-wreck dated to the end of the 16th century is the oldest vessel yet known built in this technique. When carvel-built ships started to occur in the Baltic Sea in the early16th century, they represented the most

Sketch showing the excavated parts of the hull, viewed from the stem. Note the stringer on the starboard side. Illustration: Niklas Eriksson/SMM.

Right: The raised top-timber. Note the lack of clinker-hocks and the orientation of the treenails towards the middle of the planking in the uppermost parts of the hull. Illustration: Niklas Eriksson/SMM.

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technically advanced ship-constructions at that time. It is a bit surprising that what you could call “traces” of carvel-building and, most notably, skeleton construction occur in smaller clinker-built craft quite early after the big ships were built in this manner. What triggered the introduction of the “halfcarvel” technique is of course very difficult to tell from a single week of excavation in the muddy waters of Selångersån. Site formation On parts of the hull, traces of some sort of wood-decaying organism could be observed. The affected timbers were located at the starboard side. Organisms that break down wood and leave this kind of traces are not known in neither fresh nor brackish water. It is thus not likely that they have occurred at the present location of the wreck. It is possible that the hull had sunk in shallow water with parts of the starboard side above the surface during some period. If it was the currents, which sometimes can be quite massive at the site, that have moved the hull to deeper water, we do not know. The process of decay of the starboard side timbers stopped when the timbers were deposited under water. The present location of the hull is therefore not identical with the spot where the vessel became a wreck. The remains give the impression of a scuttled vessel that has been left on the shore of Selångersån.

of this is the phenomena of land-elevation. In the Sundsvall area, the land is rising with 7.3mm a year. The lower parts of the wreck are located at 34cm of water, and the deepest parts that are buried under sediments lie at approximately 2 metres. Within a not so very distant future, major parts of the hull will be lifted above the sea level and start to rot. Several different solutions have been discussed, but none has proved to be realistic in terms of preservation, impacts on local environment, and costs. For the moment, there are no plans of further excavations or measures for the preservation of the wreck. Niklas Eriksson Litterature Eriksson N. 2007. Förundersökning av fartygslämning från 1500-talet i Selångersån, Sundsvall, in print Hasslöf, O. 1970. Sømand, fisker, skib og vaerft: introduktion til maritim etnologi. Nordisk marinhistorisk arbejdsgruppe. Köpenhamn. Holmqvist, M. 2000. Skeppsvraket i Åkroken, Uppmätning och beskrivning av ett medeltida fartyg i Sundsvall. Rapport 2000:9, Länsmuseet Västernorrland. Holmqvist, M. 2001. Temporär övertäckning av det medeltida vraket i Selångersån, Sundsvall. Rapport 2001:18, Länsmuseet Västernorrland.

Future? The main aim of the survey was to examine the possibilities of a long term preservation of the wreck at its present site. Its present location means that the wreck is exposed to strong currents and ice-movements. On top Left: The data from the total station was analysed in a GIS-program in order to make a rough plan of the site. The red dotted line represents the estimated extension of the keel. Illustration: Niklas Eriksson/SMM. 9

Knudedyb ahoy!

In the autumn 2006 issue of this Newsletter (No. 21), Søren Byskov reported on parts of a wooden shipwreck found in the Wadden Sea. Since then, the remarkable keelson and related parts have been dated dendrochronologically by Aoife Daly, albeit on the basis of just one sample. Her results showed that our first impressions were correct. We are dealing with a ship from the High Middle Ages. The felling date could be specified as the summer season of 1264 AD. Where the tree was felled is less certain. The sample matched best to a range of northern German curves. The characteristics of the timbers combined with the dating gave an evident urge for surveying the site, both from a point of view of heritage management and from a point of view of scientific research. After all,

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wreck in the dynamic Wadden Sea area may well be worth protection. Preservation can be excellent. Sediment transport can warrant cheap ways of covering it. On the other hand, it may also be completely impossible to stabilize ongoing erosion. If the surfacing remains are important enough, there is every reason and a good opportunity to dismantle a site and sacrifice it to specific research questions. The beam trawler E4 “Ho Bugt” is used as a survey vessel. A trawler like this is not particularly well suited as a surveying platform, but the local knowledge of the crew compensated for all disadvantages. Photo: Thijs Maarleveld.

The Maritime and Fisheries Museum in Esbjerg, the Viking Ship Museum in Roskilde, the Strandingsmuseum St. George in Thorsminde, and the Maritime Archaeology Programme at the University of Southern Denmark all had an interest in knowing more, all from different perspectives. Such a range of interested parties may compromise decisive action, but it didn’t. Under the present regime of maritime archaeology in Denmark, with highly regionalized responsibilities, it is after all through cooperation that viable teams can be formed. Moreover, assistance and sponsoring by Esbjerg based underwater technology firm MacArtney A/S and local knowledge provided by the crew of the beam trawler E4 “Ho Bugt”, who had discovered the remains in the first place, proved vital for a successful operation. To begin with, the site was surveyed in mid-July 2007 using an Edgetech 4100 side scan sonar kindly provided by MacArtney. The survey was carried out by students and staff of the Maritime Archaeology Programme

in conjunction with the Viking Ship Museum in Roskilde and Jes Højlund of MacArtney. The trawler “Ho Bugt” was used as a survey vessel, and the two fishermen who originally discovered the ship timbers guided the archaeologists to the area where they had found the wreckage. The objective was to search for remaining parts of the wreck and to investigate additional “snags” reported in the area. During a five hour survey, 10 lines were run searching a total area of 1,124 km2. After post-processing, the most promising anomalies were chosen for a future diver inspection. Although a number of smaller anomalies were visible in the data, none of these could be immediately associated with a medieval shipwreck. Parallel to the activities in the field the salvaged timbers: the keelson, planks, and the presumed frame element or mast step Instruction

and

deliberation

on

the

job.

Photo: Thijs Maarleveld.

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The timbers as found in 2006 could originally have sat in the vessel in this way. Drawing: Mikkel H. Thomsen. buttress are undergoing conservation treatment at the conservation department in Ølgod. Their stay in the conservation tubs was temporarily interrupted for documentation in the Archaeological Workshop of the Viking Ship Museum in Roskilde. Here, they were documented in three dimensions by means of a FARO digitizer arm. Very little original surface was preserved on any of the timbers, but significant features survive. The keelson is of a typical medieval shape with scalloped edges between the notches for the floor timbers. The mast step has two treenail holes right at its aft corners. This may suggest that the mast step was cut after the keelson was fitted in the ship. The mast step has a single drain at its port side, and its bottom has some peculiar circular wear marks; perhaps originating from pebbles getting caught beneath the mast. On the front side of the mast step itself, there is a notch for a stanchion board supporting the mast and blocking it. It is a feature the Knudedyb wreck has in common with the

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Kolding Cog and other shipfinds from the high or late Middle Ages. The Copper Wreck in Gdansk, the recently surveyed Avaldsnes ship in Norway, and the Medemblik ship in the Netherlands come to mind. A few carpenter’s marks have also been preserved marking the position where the notches for the floor timbers should be cut. The second timber is a slightly curved internal timber of roughly square cross-section. It has no preserved edges for clinker planks, and the thicker end is preserved to its butt end. If the piece is indeed a frame element, it looks like a half frame, which would be highly unusual for the period in question (mid 13th century). Alternatively, the piece should be interpreted as a mast step buttress; one of a set supporting the keelson either side of the mast step. Two plank fragments were also salvaged. They are both of rectangular cross-section with treenails and penetrating iron nails but no clinker nails or sintels. They are interpreted as ceiling planks. If these interpretations are Starboard view of the keelson, 1:25. Recording: Morten Ravn & Mikkel H. Thomsen.

correct, the finds assembly begins to make sense as the timbers – all from the inner bottom part of the vessel – could very well have been connected or at least closely spaced and thus caught in the trawl in one single pass in 2006. In October and November 2007, it was finally time for a diver inspection of the site. The strong tides in combination with limited daylight hours provided for only one hour of diving per day. The inspection, which covered 590m2 in total, focused on two circular areas: one centered on an anomaly indicating a depression, the other on the find spot as reported. While the latter spot produced no finds aside from the presence of stones, which are most likely dumped there as a result of human activity, the site of the anomaly did indeed produce ship timbers. The visible timbers were another presumed ceiling plank, which might well derive from the medieval wreck. It lay completely loose on the seabed. Also, there was an extremely worm-eaten internal timber which disappeared in the current before it could be documented. Most surprisingly, however, the inspection produced … another keelson…!

This new keelson is also detached from any other wreckage, but its massive size (10.55m long, more than 50cm wide, and 50cm high at the mast step) means that, unlike the plank, it is not so easily moved. Tidal currents, storm surges, and extreme seastates can exert enormous forces on this strectch of coast, especially if they are right in line with the tidal inlet. So what is the conclusion?

First of all, it has become clear that there is more than one wreck in the area; something also demonstrated by the many different timbers caught in Knudedyb by fishermen over the years. The new keelson is undated. It might, however, refer to the 18th century or later. Possibly, it is from the heyday of shipping on the neighbouring island of Fanø, which fell between c. 1740 and 1900. And what of the medieval wreck? There may very well still be a more or less coherent wreck structure in the area. It is clear from the barThe survey area including net snags and sonar anomalies. The central cluster was the focus for the diving inspection.

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A cold November morning on the westcoast is not enough to deter maritime archaeologists in a small dinghy or students from Ireland and Greece. Photo. Ntina Vafiadou. nacles on the first four timbers that, unlike the plank found during the diver inspection, they did not lie loose on the seabed when the trawl caught them. Most likely the wreck lies covered – once again – by sand waves travelling across the area. But the remains themselves are worthwhile and relevant to the researchers of the Viking Ship museum and the Maritime Archaeological Programme. Finding the wreck again will require repeated surveys. At some point, the wreck will reappear and in one such window of opportunity, it will be possible to find the wreck using high frequency side scan sonar. In cooperation with the Strandingsmuseum in Thorsminde, which now has the official responsibility for the recording of archaeological finds on the west coast, students of the Maritime Archaeology Programme will 14

continue to document finds and sites that are observed by local fishermen. When the weather allows, new ‘snags’ will be dived. As a matter of practical training, they will also be exposed to new geophysical surveying ventures. Jens Auer, Mikkel Haugstrup Thomsen, and Thijs Maarleveld References

Byskov, S. 2005: Parts of a wooden shipwreck found in the Wadden Sea. Maritime Archaeology Newsletter from Denmark, 21: 16-17 Daly, aly, A., 2006: Dendrokronologisk undersøgelse af trae fra skibsvrag, Knudedyb, Ribe Havn. (Dendro.dk rapport 1:2006). Ejstrud, B; Maarleveld, T. 2006: Et middelalderligt fragtskib – Marinarkæologi i Knudedybet. Sjæk’len 2006: 132-141

From a grain of sand, a mountain appears Sediment erosion is one of the major threats to submerged archaeological sites. Depending on the type of seabed and the strength of the currents around a site, sediment may be eroded and transported leading to exposure of underlying archaeology, which would then also be susceptible to attack by wood boring organisms. Jesper Frederiksen and Claus Skriver illustrated this scenario perfectly in their article on the Gåsehage wreck in the previous issue of the newsletter. Sandbags are often used as a means of stabilising archaeological sites underwater. However, their deployment is labour intensive, especially when working in areas with strong currents. Recently, maritime archaeologists and conservators have been trying to stabilise sites in situ using sediment transport to their advantage by entrapping sediment particles carried in the water column and creating an artificial seabed, or burial mound, over the threatened site. Notable examples of this are the use of artificial sea grasses on the wrecks of the William Salthouse (Hosty, 1988; Harvey, 1996) and the James Matthews (Godfrey et al. 2005) and various types of netting (shade cloth, debris netting, wind netting), which was pioneered by researchers at the Netherlands Institute for Ship Archaeology (NISA) and has been used on several wrecks in the Netherlands (Manders, 2003), Sri Lanka (Manders, 2006) and also trialled on the James Matthews (ibid.). The artificial sea grass and the nets effectively function in the same way. The plastic fronds of the artificial sea grass trap sediment particles in the water column as water passes through them. Due to friction, the water is slowed causing the sediment particles to fall out of the water column resulting in an artificial seabed/mound. The net func-

tions by fixing it loosely over the structure to be protected, so that it billows in the water column. As with the artificial sea grass, suspended sediment in the water column passes through the net, but as it does, it is slowed by friction: the sand falls out of suspension and creates a mound under the net. For any of these methods to be effective, there has to be sediment transport over the site. But even so, it is hard to tell what would be most effective. For this reason, a comparative study was devised. The aim was to trial both an artificial sea grass and four different grades of netting (strength and mesh size) in an area where sediment transport is present. Sedimentary processes were also studied in order to better our understanding. Central, however, was to compare and contrast how the different materials worked. The long term aim being to establish general “rules of thumb” for their use. The site of the wreck of a small 13th century oak clinker built vessel, known as the Hårbøllebro wreck, was chosen due to its dynamic environment. Although the wreck has been stabilised with sandbags, the environment is ideal for these trials. The Hårbøllebro Wreck

The Hårbøllebro wreck is a small 13th century oak and clinker built vessel that sank in Grønsund off the south eastern coast of Denmark. It is appr. 8m long and 3m wide. The wreck lies in 11 metres of water and is one of the rare examples where the keel is uppermost, so that remnants of rigging and other contents of the vessel may still be preserved in the hull. The wreck was reported by sports divers early in 2002. Apparently, it had been exposed when a storm removed overlying sediment. A series of wooden (pine) test blocks were 15

placed around the site in the autumn of 2002 and assessed after a year’s exposure. These blocks showed attack by the wood borer Teredo navalis. The presence of the shipworm and potential threats of physical deterioration from the strong water currents around the site were reason enough for stabilisation and protection. A total of 80 sandbags were placed over the site in the course of 50 diving hours in 2002 and 2003. Weather conditions and sediment transport in Grønsund Although the wreck had been exposed due to a storm, the site is also regularly affected by strong local currents, which potentially remove sediment. These currents are caused by water being “pressed” into the small Grønsund strait from the Baltic Sea when winds are north easterly to south easterly. Conversely when the winds are north westerly, water is pressed from Storstrømmen into Grønsund and out into the Baltic Sea. Hypothetically, the site will be more affected by winds from the Baltic Sea both due to the longer fetch (50km) between northern Germany and Grønsund and the sea horizon angle of the coast (>120°) as The Hårbøllebro wreck as discovered in 2002. Photo: Thomas Christoffersen.

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opposed to the shorter fetch and narrow sea horizon angle between Storstrømmen and Grønsund. The likelihood of local sediment erosion and transport can be estimated from the particle size of surface sediment and water current speeds. Even though currents may be present, they may not be strong enough to lift sediment from the seabed into the water column. Sediment samples from around the site were taken for particle size analysis and current speed data was collected using an Aanderaa RCM9 current meter, which was deployed from 7 September to 9 October 2006. The instrument measured the current speed and direction, water depth, and suspended sediment (turbidity) 50cm above the seabed. It was positioned in the locality where the trials of the various materials for sediment entrapment were to take place. In addition, meteorological data were accessed for Møn Fyr (DMI Station 31251, WMO 06179) from the Danish Meteorological Institute (http://www.dmi.dk/dmi/vejrarkiv) for the complete duration of deployment of the current meter and of the subsequent experiments. Water current speeds for the period measured was on average 0.25ms-1. However, there were three distinct periods where the speed increased to 0.50ms-1 (data not shown). The particle size analyses showed

that the sediments immediately around the wreck were sandy with a high clay (approx. 14%) and silt fraction (approx. 4%). A cautious interpretation of this data indicates that under the measurement period there was limited local sediment transport as the sediments are almost cohesive and require at least 0.3ms-1 to overcome the forces that bind the particles and keep them on the seabed (Soulsby, 1997). That the results should be treated with caution – due to the limited time when data was collected – was attested when further sediment samples were taken from under the artificial sea grass and under the nets after eight months. These results showed the sediment particle sizes and composition to be almost identical indicat-

ing that there was some local transport. Nevertheless, the turbidity results (data not shown) showed three distinct periods where there was an increased concentration of suspended particulate matter. However, they did not show a direct correlation with the increased current speeds measured. This would indicate that some sediment transported over the site is not necessarily eroded locally but is coming from the Baltic Sea to the east or Storstrømmen to the west. The source of this non local sediment from an easterly direction may be explained on the basis of observations made during visits to the site and discussion with local fishermen. North easterly winds generate currents, which erode the chalk cliffs of Møns Klint to the north east of the site and act as a source of sediment. Two or three days after such weather, the eroded sediment finds its way into Grønsund turning the water milky. Where the sediment from a westerly direction comes from, is presently a little less certain and is currently under investigation. However, meteorological data (not shown) from the period when the current meter was logging showed that at the times when turbidity was high on the site, there had been high winds (>10m/s) recorded two or three days prior to the increased turbidity. The highest turbidity measurements corresponded with a period when there had been either strong easterly or north westerly winds. Artificial Sea Grass For the experiments, an artificial sea grass mat (5m x 3m) produced by Seabed Scour Controls (http://www.scourcontrol.co.uk) was used. The mat as supplied is wound onto

Top left: Location of the wreck site in respect to Storstrømmen, Grønsund, the Baltic Sea and Møns Klint. North is orientated uppermost. Bottom left: Aanderaa RCM9 current meter in situ. Photo: David Gregory. 17

Artificial sea grass mat anchored to the seabed and prior to releasing the plastic fronds. Photo: David Gregory.

an iron pole with lifting rings to aid deployment. Even though the mat weighed 100 kg, it was relatively easy to manoeuvre from a small diving vessel (6 metres long). Once lowered to the seabed, the mat was unrolled by divers and fixed to the seabed by a series of “anchors”, which were hammered into the seabed to a depth of approx. 50cm. Two divers could easily manoeuvre the mat on the seabed, and even though an air hammer was not available to affix the anchors (as the

manufacturers recommend), it was a relatively simple task to hammer the six anchors into the sea bed with a sledge hammer. For diver safety, the plastic fronds are retained in a thin net, which is removed when the mat has been securely fixed, thus limiting the risk of the diver being entangled. Once the net was removed, the fronds naturally begin to float but were also helped along by a diver ruffling them up. The design of the sea grass mat is extremely well thought through, and there is minimal risk of divers being caught up in the plastic fronds. It was relatively easy to deploy, and from start to finish the operation took approx. 2 hours. One problem from an archaeological perspective, is that the large anchors need to penetrate 50cm into the seabed, which could easily penetrate any underlying archaeology. Several methods were tried to assess how much sediment the artificial sea grass had trapped, but the simplest was to measure water depth with a dive computer (± 5cm) at positions every 50cm over the top of mat/the trapped sediment. The depth of the surrounding seabed was used as a baseline. Measurements were corrected for any tidal fluctuations using a local tide gauge, which Artificial sea grass after 12 months exposure. Photo: David Gregory.

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also enabled subsequent measurements to be compared. Subtracting one from the other gave a measure of how much sediment had been caught. The mat was assessed after two weeks, four weeks, eight months, and twelve months. Within the first month, the sea grass had trapped 5-10 centimetres of sediment (measured with a ruler as it was too close to the resolution of the dive computer). This had increased to 20-35 centimetres after eight months, although, it was apparent that most sediment had been caught at either ends of the mat – those which were most exposed to either easterly or westerly going currents. After 12 months, there had been a loss of sediment with more of the artificial sea grass being exposed. Thus even though the sea grass is effective at trapping sediment, it did not, in this instance, create a stable seabed. In order to give a better overall picture of the extent of sediment caught in the artificial sea grass, 3D models of the sediment depths were made (Surfer™). Data after 12 months exposure are shown in the 3D model on A completed net waiting to catch sediment. Photo: David Gregory.

page 22. Using the Surfer software it was also possible to calculate the volumes of sediment trapped over time. After eight months, a total volume of 3.64m3 sediment had been trapped. However, after 12 months, only 1.16m3 remained – a loss of approx. 2.5m3. Debris Netting

Four different grades of netting supplied by Petonic Trailer Nets (http://www.petonic.dk) were trialled: • • • •

Shade cloth 95gm-2, mesh size 10x5mm Debris netting 130gm-2, mesh size 3x3mm Debris netting 180gm-2, mesh size 3x3mm Windbreak net 230gm-2, mesh size 5x2mm

Separate 6 x 3 metres sections of each net were used. Bronze eyelets were fixed at regular intervals along their edges, so the nets could be fixed to the seabed with metal hooks. A series of small fishing net buoys were also attached to the nets to give them extra buoyancy. The nets were relatively quick and simple to prepare and deploy. They could either be taken down by the diver or simply lowered to the seabed and

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carried to where it was to be laid out. It must be noted that for diver safety, one edge of the net should be weighted so as to avoid the diver becoming entangled. During one deployment where the net was not weighted, the net floated vertically in the water, was caught by the current, and dragged over the top of the divers enveloping them. Subsequently, one end of each net was affixed to a 2.5cm diameter iron pole. Once on the seabed, the leading edge of each net was attached to the seabed with 50cm long metal hooks, and the nets unrolled 5 metres – the extra metres allowing the net to billow upwards. The other end of the net was then similarly fixed with metal hooks and the sides fastened with 25cm long metal hooks. Extra fishing net buoys were added to increase buoyancy if necessary. The photos on pages 20 and 21 show the setting out of a net, and the photo on page 19 shows a completed net. After some practice, it was possible to deploy one of the nets in approx. 30 minutes. As with the artificial sea grass, the vari-

ous nets were assessed after two weeks, four weeks, eight months, and twelve months using a dive computer. After two weeks, the nets had started to catch sediment but this was difficult to quantify as in all cases, it was on average less than 10cm (too close to the resolution of the dive computer). After four weeks, it was thought that the nets were not functioning as there had been no great change in the volumes of sediment caught. The nets had sunk to the seabed, and it appeared that the mesh in the nets, regardless of their size, had become blocked with algae and other detritus. Further buoys were added and the nets shaken so as to remove sediment that had been deposited on top of the nets and prevented them from billowing in the current. This appeared to help, as after 8 months, all nets had caught sediment, 90% of the volume, which was recorded after 12 months and is shown in the 3D models on page 23. The calculated volumes of sediment trapped under each net type after 12 month deployment were: 130gm-2 (4.41m3) >

A prepared net being carried to the site. Photo: Jørgen Dencker.

Fixing the leading edge of the net with metal hooks. Photo: Jørgen Dencker.

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230gm-2 (4.35m3) > 95gm-2 (3.46m3) > 180gm-2 (1.96m3) respectively. As with the artificial sea grass, the current flow over the nets was predominantly bi-directional as more sediment was caught at the easterly and westerly edges of the nets. However, it appears that the sediment trapped under the nets was still mobile as the form of the mounds changed. Importantly, the calculated volumes of sediment after each assessment showed sediment was retained as the calculated volumes never reduced. It is currently difficult to explain the differences between the effectiveness of nets. As no measurable loss of sediment but only stasis or accumulation were recorded, the mesh size of the nets surprisingly has little effect. Suspended particulate matter was fine enough to penetrate all the nets, and interestingly, even the large mesh size of the 95gm-2 (10 x 5mm) retained sediment. It is thought more likely that it is the actual positioning of the nets on the seabed and the local conditions around them which has the greatest effect. It transpired that the

130gm-2, 230gm-2, and 95gm-2 nets were all placed within the full flow of the current, whereas the 180gm-2 was slightly out of the current and may have been slightly shielded by an underwater outcrop, which was 10-15 metres eastward. This may have meant that there has not been as much sediment available for entrapment. It is now planned to set out further nets where all will be where the current flow is.

Rolling out the net.

Fixing extra fishing buoys for extra buoyan-

Photo: Jørgen Dencker.

cy. Photo: Jørgen Dencker.

Preliminary conclusions and ongoing work The water current speed and particle size analysis indicated that there would be limited local sediment transport as the currents were not strong enough to bring the sediments into suspension. However, turbidity measurements taken at the same time showed that there was sediment in the water column, which was more than likely from a non local source from the Baltic Sea or Storstrømmen. In terms of assessing the suitability of a site for using these methods, the presence or absence of sediment in the water column is paramount and measuring

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3D model of the artificial sea grass after 12 months deployment. turbidity is a simple way to confirm sediment transport. The artificial sea grass was successful in trapping sediment at its maximum 3.64m3 but in this instance, it did not create a stable seabed, as approx. 2.5m3 of sediment was subsequently lost. The four different types of debris net all worked better than the artificial sea grass trapping upwards of 4m3 of sediment. Mesh size of the net did not seem to play a large factor in their effectiveness – all were large enough to allow the suspended sediment to pass through and small enough to retain deposited sediment. However, it remains important to understand the interrelationship between mesh size and the suspended particle size. After all, one needs to ensure that particles can actually get through the net. Presently, a sediment trap has been deployed on site to assess the size of particles in the water column in order to compare these results with what is actually being caught by the nets. The different weights of the nets did not seem to be a factor in the success rate either, although it was important that enough fishing buoys were attached to prevent the 22

nets from totally sinking to the seabed. Furthermore, the heavier grade the more robust the nets were to work with and more resistant to abrasion when on the seabed. One of the nets (180gm-2) caught less sediment than the others, but this is thought to be due to a lack of available suspended sediment in the area where it was deployed. Economically, there is between a 30 and 500 fold difference between the price (per square metre) of the various nets and the artificial sea grass and a four fold difference in the deployment time. Although the results presented are preliminary, it would appear that in areas with sediment transport, debris nets are a very cheap, simple, and effective way of catching suspended sediments, retaining them, and creating an artificial burial mound. In this way, they can facilitate the protection of our submerged cultural heritage from the threats of sediment erosion and wood boring organisms. David Gregory, Rasmus Ringgard, and Jørgen Dencker

3D models of the four types of debris netting used after 12 months deployment. Vertical scaling and orientation of the nets is as for the artificial seagrass. Numbers in brackets are volumes of sediment. References Godfrey, I.M., Reed, E., Richards, V.L., West, N.F. & Winton, T. 2005. ‘The James Matthews Shipwreck – Conservation Survey and In-situ Stabilisation’, in Proceedings of the 9th ICOM Group on Wet Organic Archaeological Materials Conference, Copenhagen, 2004, eds P. Hoffmann, K. Strætkvern, J.A. Spriggs & D. Gregory, The International Council of Museums, Committee for Conservation Working Group on Wet Organic Archaeological Materials, Bremerhaven, pp.40-76. Harvey, P. 1996. A review of stabilisation works on the wreck of the William Salthouse in Port Phillip Bay. Bulletin of the Australasian Institute for Maritime Archaeology, 20(2):1-8. Hosty, K. 1988. Bagging the William Salthouse: site stabilization work

on the William Salthouse. Bulletin of the Australasian Institute for Maritime Archaeology, 12(2):13-16. Manders, M., Safeguarding: The physical protection of underwater sites. In C.O Cederlund (ed.) Moss Newsletter, 4, 1821. Manders, M.R., 2006, ‘The in situ protection of a Dutch colonial vessel in Sri Lankan Waters’, in: Robert Grenier, David Nutley & Ian Cochran (eds); Heritage at Risk – Special Edition – Underwater Cultural Heritage at Risk, 58-60. Acknowledgements The instigation of the project was made possible with a grant from the Danish Directorate for Cultural Heritage (Grant number 2003222-0121). The authors would also like to thank their respective institutions; The National Museum of Denmark, Institute for Geography, Copenhagen’s University and The Viking Ship Museum, Roskilde for supporting and financing the monitoring phase of the project.

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Diving at Gredstedbro Maritime Archaeology deals with an uneven distribution of archaeological wreck finds. While some periods boast many finds and some even have historical evidence supporting the archaeological data, other periods are only very scarcely represented. This makes it difficult to evaluate the development of ships on the basis of comparative and diachronic study. One period that is highly underrepresented is the Early Middle Ages in Northern Europe. As clearly illustrated by Klavs Randsborg in 1991, the period from 400 to 800 AD only has very few known wrecks. This situation has not fundamentally improved since 1991, making it necessary to use the few known finds to their maximum scientific potential. One such find is the Gredstedbro ship (Crumlin-Pedersen 1967; 1968). Discovered in 1945 during a normalization project on The frequency of wreck finds, status 1991. After Randsborg 1991.

the stream Kongeå in South Western Jutland, it was originally interpreted as the remains of a bridge. Parts of the timbers were broken off, and three of these were handed in to the museum in Ribe. Only some 20 years after their discovery the remains were recognized as being timbers from a ship dating to the 7th century AD. Ships from this period are very scarce, and although the actual archaeological evidence from Gredstedbro only consists of three broken off pieces of timber (parts of a keel, a floortimber and a stem- or sternpost), the find has received widespread attention in the archaeological literature. The site is scientifically significant, and merits further investigation. Unfortunately the site was not precisely recorded in 1945 and today the exact location of those parts of the ship which still remain in the stream bank is unknown. Several attempts have been made to relocate the Gredstedbro ship. Just after the ship timbers were recognized as such by Ole Crumlin-Pedersen in the mid-1960s, the The location of the Gredstedbro find.

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Museum in Ribe augered along the bank, while local sports divers searched the stream bed. But none of these searches were successful. In 1981 a georadar survey was carried through. An area of c. 90 × 14 meters along the stream bank was surveyed. The main result of this was that the old stream bed was identified, despite the changes that had occurred in 1945 and since. The location of the ship was not revealed by the georadar, nor by subsequent coring in the area. New investigations on Gredstedbro

As part of the training for the professional diving qualification which is offered to Maritime Archaeology students at the University of Southern Denmark, an underwater survey was carried out on site in January 2008. The purpose of this work was two-fold: apart from the obvious ambition of finding the ship, an equally important goal Sketch of the area (redrawn from orthophoto at www.dkconline.dk) with penetration depths (in ns) from the 1981 georadar survey superimposed. Data from Christoffersen & Pedersen 1981. Scale 1:2500

was to get to know the environment and to train the students under the relative adverse conditions of river diving. The students worked in two dive teams, each with a diver, a tender and a supervisor. A third diver was standing by as safety diver. Working in the substantial currents of a Danish stream in January, the conditions were adverse indeed. Nevertheless, the southern bank of the river was systematically surveyed. In practice this meant visual inspection, as the visibility was as good as up to 30-50 cm. More important perhaps were the hands, systematically feeling their way along the river bank. The divers worked downstream from four stations, spaced 2030 m apart, and in all about 90 m of the river bank was surveyed. The ship was not found. But a single piece of worked oak was picked up from the bottom sediment about 30 m from the present bridge (cf. the plan of the georadar survey). The piece is irregularly shaped with dimensions of c. 29×16×6 cm. The surface is partly deteriorated, but there are visible tool marks on the piece. Both ends are broken off.

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The piece is too small and uncharacteristic to identify. It is interesting that it is found exactly off the position in the bank, where the ship is most likely to be located, according to evidence that is derived from an interview with one of the workers of the 1945 normalization project and from the 1981 georadar survey. Indeed we know that the workers in 1945 broke off sufficient parts of the wood to be able to continue their work, and this could be one of the remains. But numerous man-made objects would end up in a river, and the fragment may equally well be the remains of a discarded fence post, flushed down by the current. The ship is still unfound, but pending further work, the newfound wood fragment has been send to the conservation laboratory in Ølgod for further processing. Further work

A unique archaeological ship is located in 26

The Kongeå is not a deep river, but its strong current and low temperature in January is certainly a challenge for the aspirant commercial divers. Photo: Thijs Maarleveld. the bank of Kongeåen in South Western Denmark. We know it is there, but still do not know exactly where. The site is situated a mere 15 minute drive from the university campus in Esbjerg where we teach Maritime Archaeology. As a training site for our diving course, the site is both conveniently placed and professionally relevant. Monitoring changes in the river bank, through the seasons and gradually improving surveying skills is to be a recurrent exercise. In that respect work on Gredstedbro continues within the University’s Maritime Archaeology Programme. But the site has potential for more than merely training students in river diving. The ship was well preserved in 1945, and although partly destroyed by the dig-

Maritime Archaeology

ging, there is a good chance that substantial remains still rest in the bank. Even if these have lost their original integrity, they are still of utmost importance. If we are able to find the ship or even parts thereof, and if we can find sufficient funds and relevant partners to excavate and process them, it will in all likelihood make an important contribution to the archaeology of ships in early medieval Europe. So far, we need to content ourselves with background studies, a reconstruction of the environment and the development of additional strategies for field survey.

Newsletter No. 23 • Spring 2008

from Denmark

ISSN 1902-0708 EDITORS: Thijs J. Maarleveld & Helle Kildebæk Raun Lay-out: Jens Lorentzen & Ewa Britt Nielsen DTP: Helle Kildebæk Raun

Bo Ejstrud

References

Christoffersen, H. & J. Pedersen 1981: En georadar undersøgelse ved Gredstedbro med henblik på at lokalisere et skib i åbrinken til Kongeåen. Unpubl. report in ASR 155, Geocon A/S. København. Crumlin-Pedersen, O. 1967: Gredstedbroskibet. Mark og Montre. Crumlin-Pedersen, O. 1968: The Gredstedbro Ship. The Remains of a Late Iron Age Vessel Found in 1945 in South Jutland. Acta Archaeologica 39, pp. 262-267. Randsborg, K. 1991: Seafaring and Society in South Scandinavian and European perspective. In O. Crumlin Pedersen (ed.), Aspects of Maritime Scandinavia AD 200 -1200, Roskilde, pp. 11- 40.

PRINT: PE offset A/S, Varde © Centre for Maritime and Regional Studies and authors 2008

Maritime Archaeology Newsletter from Denmark is a continuation of Maritime Archaeology Newsletter from Roskilde, Denmark and is published twice a year by: The Maritime Archaeology Programme, University of Southern Denmark at the Centre for Maritime and Regional Studies Niels Bohrs Vej 9 • DK-6700 Esbjerg Tel. +45 6550 4177 • Fax +45 6550 1091 e-mail: [email protected] The Newsletter is supported by: University of Southern Denmark Centre for Maritime and Regional Studies Fiskeri- og Søfartsmuseet The National Museum of Denmark The Danish Institute in Athens Langelands Museum The Viking Ship Museum Holstebro Museum Strandingsmuseum St. George Bangsbo Museum Moesgård Museum Haderslev Museum

Practical training in field methods. Photo: Jens Auer.