Drought as a result of climate change, with all its consequences, is increasingly in the news. Drought also threatens our archaeological heritage. This has been going on for decades, but more dry periods give it an extra boost.
We all notice it, our summers have been extremely dry and warm in recent years. There are more heat waves and it is not raining enough everywhere in the country to sufficiently replenish the groundwater, our underground water buffer. This causes the soil to dry out. This is not only detrimental to nature, but also to archaeology. Most archaeological monuments in the Netherlands are hidden underground, where they are conserved by the groundwater.
Water conserves organic materials, such as wood and skeletons. As soon as oxygen is added to this, they perish. “Archaeological sites in moist soils, for example with remains of prehistoric settlements, contain a lot of vulnerable material. When they are less than a meter below ground level, a too low water level threatens their survival. And with that you lose an important source of information,” explains Iepie Roorda, archeology advisor at the National Office for Cultural Heritage.
Drought damage to archaeological monuments is therefore a water problem, not a temperature problem. In archaeological monuments that can be seen above ground, such as the dolmens in Drenthe, the wood and bone remains in the ground have long since decayed. They are therefore not affected by this type of damage.
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Modern water management
Drought due to climate change is a recent problem, but the excessively dry soil has been threatening our archaeological heritage for decades. This also applies to the archaeological monument Aartswoud, in West Friesland. “In this agricultural area, just below ground level, there are many archaeological remains from the New Stone Age and the Bronze Age. Due to the fall in the groundwater level, the organic material of this national monument has almost completely perished”, says Roorda.
Archaeological investigations from the 1970s have found traces of habitation in Aartswoud, such as wooden posts for houses. Archaeologists have regularly described the condition of the wood, from a well-preserved wooden post to very thin remains of wood. At the beginning of this century, test samples were taken at the same locations, and many of these wooden remains were found to have completely disappeared.
Our modern water management is causing this decline. In the Netherlands, the water boards regulate the level of surface water that influences the groundwater level. They keep our feet dry, but also the farmland. With the advent of modern agriculture after World War II, with its scaling up and increasingly heavier farm equipment, farmers needed a lower water level. Wet land is bad for the harvest and heavy machinery sinks into the mud.
Own groundwater level
Hans Huisman, professor of geoarchaeology at the University of Groningen, has also seen the disastrous consequences of drought for archaeology for years. He has done a lot of research on the former island of Schokland in the reclaimed Noordoostpolder, now a UNESCO World Heritage Site. He noticed how the receding water affected this archaeological monument.
“Schokland contains many traces of habitation, sometimes more than ten thousand years old. There are, for example, graves from the Stone Age, but also artificial mounds from the Middle Ages. The falling water threatened all of this, as did the island itself, which is made up of peat and settles in when the groundwater level drops.” Because Schokland is a World Heritage Site, it is considered irreplaceable, unique and property of the whole world. It is therefore of great importance to preserve the island.
The solution was as expensive as it was intensive:the farmers on the eastern part of the island were bought out and since then the water board has kept the groundwater level there higher than in the rest of the area, partly by means of weirs. They have been measuring the water level several times a day for twenty years to check whether the archaeological remains below ground level are not endangered.
Huisman:"The amount of data we have is enormous and we have seen changes occur since the extremely dry summers. There was always a fluctuation in the water level. In the summer the water was lower than in the wetter winter. But now the groundwater in the In the summer they are lower and lower for longer and in the winter it only recovers later, so we have to stay extra alert to see whether this continues to go well.”
Limesweg gone?
The Limes, the northern border of the Roman Empire that ran along the Rhine River in the Netherlands, has also been nominated as a World Heritage Site by UNESCO. Do the Roman remains still suffer from drought damage? Roorda:“In South Holland, wooden remains of the Roman road along the Limes (near Valkenburg – ed.) and wooden revetments of the Corbulo Canal (Voorschoten and Leidschendam – ed.) and a harbor have been found at Forum Hadriani. This is located at the current Voorburg. The wood in this area does not suffer much from the drought problem, since it is deep enough below ground level in this area.”
“In recent years, a lot of archaeological research has been done into the course of the Limesweg through the Netherlands. At locations where we thought the Limesweg was running, we sometimes found nothing at all. It is possible that the remains there had already rotted away completely due to a too low groundwater level, but it does not have to be. In some places the Limesweg consists only of gravel. And if archaeological traces are just below the surface, they may have disappeared due to all kinds of earthworks,” explains Roorda.
Pack shipwrecks
What we do know for sure is that the shipwrecks in Flevoland are slowly decaying. Until the reclamation of the last century, the turbulent Zuiderzee was located here. For centuries ships sank to the bottom and after the Prohibition hundreds of wrecks emerged. Most were immediately cleared, but about 70 shipwrecks have been preserved just below the surface.
Huisman:“A small drop in the groundwater level already causes a lot of damage and the wooden wrecks are now disappearing. For protection, we were able to wrap about twenty of them with plastic, so that they stay moist, and then covered with soil. However, this is not possible everywhere, for example at wrecks that lie on the land of agricultural companies that are actively in use.”
All those shipwrecks together contain a lot of information, which is now partly lost. Not only the wood disappears and with it large parts of the ship structures, but also the organic cargo such as textiles. “You won't find that anywhere else in the world, in this context. Over time, you will only be left with glass, ceramics and iron”, says Huisman.
Drains
Water shortage is the greatest threat to underground archaeological remains. But due to the current climate change, in which summer rain showers are less frequent but much harder, another problem arises:erosion. At sites on slopes in agricultural areas, layers of soil are washed away during a heavy downpour. And with it the archaeological remains that lie just below the surface and the deeper remains rot away when they come into contact with oxygen. This problem already existed, but it seems to be getting worse, according to Huisman.
“This mainly takes place on the mounds in Friesland, wierden in Groningen and on the loess shellings in Limburg. Near Meerssen, for example, are the remains of a Roman villa. Research shows that more and more archaeological material disappears here every year. To combat this, plowing is now less frequent and less intensive, so that the agricultural land is bare for as short a time as possible. Because when there are crops, much less soil washes away.”
Visible traces
Fortunately, the drought does not only mean misery for archaeology. With the aid of satellite images and aerial recordings, greater color differences can be seen in a drier soil than in a moist soil. Take clay soils for example. Anyone who has clayd as a child knows that wet clay is dark and becomes light when it dries.
“Archaeological remains have a different composition than the soil and are darker. When the soil retains less moisture, they stand out more strongly against the natural soil. This makes the archaeological remains a lot more visible, especially from the air," says Roorda.
What we can also see clearly from the air are maps of disappeared buildings. In places where walls used to be, the grass withers faster and you can see the color differences from the air. And so we discover, or rediscover, new archaeological sites.