Learning more about land subsidence in Kanis, Kamerik and Kockengen

Land subsidence inflicts extensive damage every year in the villages of Kanis, Kamerik and Kockengen, which are built on peat. Peat oxidation caused by exposure to air, and peat compaction as a result of loading, are two important causes. However, until now, it was not known how much land subsidence could be attributed to oxidation and how much to compaction. An interdisciplinary team of researchers from Utrecht University and Deltares conducted a study of the three villages going back 1000 years. They published their findings in Science of the Total Environment. The results can be used as a basis for effective action to slow down subsidence and to structure the peat-rich area on more sustainable lines.

The land in Kanis, Kamerik and Kockengen has subsided by up to four metres in the past 1000 years. In recent years, the rate of subsidence has been 14 centimetres a year in some locations as a result of drainage and the loading of the peaty subsurface to make the area suitable for agriculture and habitation. These villages are therefore forced to spend large amounts on raising, and repairing damage to, roads, mains networks and buildings. There is also more damage as a result of flooding.

Effective measures to combat or reduce land subsidence depend on a proper understanding of the processes involved. Biologists and physical geographers from Utrecht University and land subsidence experts from Deltares therefore decided to quantify the relative contributions of the compaction and oxidisation of peat as factors in the overall process of land subsidence in the three villages. A better understanding of the subsidence process will also help to decide who is responsible for mitigating the effects.

Compaction in built-up areas, oxidation in rural locations

The study showed that subsidence occurs every time an additional load is placed on the peat soil or when the groundwater level is lowered. ‘The relative contributions of the compaction or oxidation of peat vary widely in space and time, with compaction playing the largest role in built-up areas and oxidation dominating in rural locations,’ says Sanneke van Asselen, a researcher at Utrecht University and Deltares. ‘We also found out that the there is still potential for ongoing subsidence in the built-up locations in our study area.’ The loading duration and the groundwater level are also factors, as are the composition and thickness of the raised layers. Sanneke Van Asselen: ‘Locations with a thick peat layer that is close to the surface and where there has not yet been any major loading are highly susceptible to subsidence. In places where the peat layer has been subjected to a load for a long time and that are below the groundwater level, subsidence is much slower. It should be pointed out that subsidence associated with creep does continue to play a role here.’

Case for a global problem

Hundreds of millions of people around the world live in coastal areas, and it is expected that the population density in these areas will only increase. At the same time, many coastal areas are exposed to land subsidence in combination with rising sea levels. The researchers expect that the results from this research also apply to other coastal areas around the world with peat subsurfaces. Specific information about the subsurface is essential for spatial planning and strategies so that the most suitable construction sites can be selected and so that effective steps can be taken to reduce land subsidence, for example by building structures that can cope with subsidence. The study is part of the Future Deltas research programme at Utrecht University, which focuses on the development and integration of knowledge for worldwide sustainable delta management, with land subsidence as one of the leading priorities.

Publication

Download the publication via ScienceDirect The relative contribution of peat compaction and oxidation to subsidence in built-up areas in the Rhine-Meuse delta, The Netherlands, Sanneke van Asselen, Gilles Erkens, Esther Stouthamer, Hessel A.G. Woolderink, Rebecca E.E. Geeraert, Mariet M. Hefting, Science of the Total Environment 2018, https://doi.org/10.1016/j.scitotenv.2018.04.141