Afstudeerwerk door Tjitte Woudstra
As a child I grew up in the Noordoostpolder. At first sight a monotonous landscape reclaimed from the Zuiderzee without any historical highlights. However, as I later found out, the former bottom of the sea was inhabited by people before the Zuiderzee was formed. The main highlight that conserved the most traces of this habitation is Schokland. The first UNESCO world heritage site of the Netherlands. Action is needed to preserve the value of Schokland for future generations. In this location, ongoing soil subsidence poses a threat for Schokland.
Coastal- and delta landscapes over the world deal with soil subsidence. While the sea level is rising, low-lying landscapes are subsiding. In the Netherlands, the soil is subsiding for about 1000 years already. Often this is caused by human activities, such as dewatering and drainage for agricultural purposes. The figure above shows that polder landscapes with peat and clay soils are prone to subside.
The consequences of soil subsidence
Continuous heavy soil subsidence threatens the current use of the landscape. In locations with serious subsidence the future perspective of agriculture is declining. Where soil subsidence forces local water boards to lower the groundwater level for agricultural purposes, the problem will be even more intensified. Furthermore, a subsiding soil leads to the deterioration of infrastructure. Roads, foundations and underground infrastructure are vulnerable elements in a subsiding landscape. Finally, soil subsidence can also lead to the exposure of the archaeological collection and can threaten them to disappear. While this archaeological collection tells the story of the ‘genesis’ of our landscapes, By doing nothing ‘a part of our history can get lost’.
Soil subsidence and archaeology
An archaeological collection consists of traces that are marks of human interventions in the landscape of the past. Traces of these human interventions can be a ditch that leaves are dark imprint in the underground. Or an historical mound that is still somewhat visible in the landscape. Also, smaller objects such as tools and pottery tell us something about our former way of living and the use of the landscape. The combination of these traces tell the story of our past and is conserved in the underground.
Figure 1: Soil subsidence in the Netherlands (Source: Slappe-bodems.nl, 2020)
Figure 2: The location of Schokland
Location Schokland and the constant battle against the water
The landscape of Schokland is an example of a location dealing with soil subsidence for decades. At the same time, it contains valuable traces of habitation that date back to the prehistory (UNESCO, 1995). After the land reclamation of the Noordoostpolder in the 1940s, the former island of Schokland became part of a new developed agricultural landscape. The setting of the marine-clay layer after the land reclamation and the dewatering for agricultural purposes resulted in a subsiding soil. Now, the landscape of the former island of Schokland, which is based on a peat layer, subsides in a faster pace than its surroundings. The iconic elevation differences reminding of the former island are disappearing.
The research aimed to create an answer to the ongoing soil subsidence of Schokland. What hydrological measure can be taken to preserve the archaeological values around the former island. The second part of the research aimed to identify what historical layers defined the unique story of the constant battle against the water. The theory of the landscape biography divines two layer approaches: a division between anthropogenic, biotic and abiotic, and a division in historical layers. The rich layered history of Schokland is defined in the timelines below.
Figure 3: Biotic and abiotic landscape development of Schokland
To mitigate the soil subsidence around Schokland, three intervention zones were defined. These are the zones that contain the highest number of archeological values. The north-south shaped (former) island of Schokland connect these three intervention zones together.
Figure 4: Antropogenic adaptation to the changing landscape of Schokland
One of the intervention zones will be highlighted, this is the redesign of the Schokkerbos. Here there are remnants of human activity settling on the boulder clay formation, dating back to 9000 BC. The current Schokkerbos forest is located on this boulder clay formation, because this was during the land reclamation not suitable for agriculture. Around the forest, the agricultural values will be preserved by higher groundwater tables. The whole Schokkerbos will be turned into a park with open views into the polder landscape through patches of forest. The designs plays with elements from the late Pleistocene and the modernistic design of the Noordoostpolder, while preserving the archaeological values of Schokland. This shows the accumulation of Schoklands historical layers in the design for Schokland.
Figure 5: Threatened values of Schokland
Figure 6: Detailed Landscape design in the Schokkerbos area
Soil subsidence is a serious threat for the archaeology and history of the landscape in the Netherlands and Schokland. While our history is partly ‘stored’ in our underground, soil subsidence affects the longevity of these valuable sources. An important element of the thesis was to explore the possibilities to experience the history of the landscape, while implementing the needed intervention for the mitigation of soil subsidence. The combination of archaeology and hydrology made this thesis interdisciplinary.
This thesis researched the possibilities of landscape interventions in a landscape that sees its archaeological traces threatened. The case of Schokland was used, to test the needed intervention for the mitigation of soil subsidence in a landscape and use its site-specific archaeological and historical qualities. The end result of the thesis is a landscape design, which shows the possible end-results of this research. The design is based on the outcomes of the research directly. In the end, this project was an interesting design challenge. Several disciplines in our profession came together, such as archaeology, water technology and cultural history. The outcomes of this design challenge can be transferred to (Unesco World Heritage) sites dealing with soil subsidence. Looking at the historical layers and identifying archaeological elements that needs to be preserved is an important start to tackle this challenge.