PhD thesis defence: Peat soil in the countryside is subsiding faster than in the city

PhD candidate Kay Koster has mapped out peat soil in the Netherlands and concluded that peat soil in rural areas and small towns will subside faster than in major cities. Koster defended his doctoral thesis on 3 November 2017 in the University Hall of Utrecht University.

Subsidence is as Dutch as windmills and clogs. Subsidence is nevertheless dangerous, because it causes a comparative sea-level rise, an increased risk of flooding and damage to buildings. Peat, in particular, which was once in abundance here in the Netherlands, has diminished significantly in density. This is due to centuries of dewatering, which was necessary to make the delta suitable for agriculture and habitation.

Peat consists of decaying plant matter, water and a little clay. Centuries of dewatering partly exposed the peat to the air, which in turn rotted away the plant matter. The peat layers that remained below the groundwater became thinner as a result of compression. As current groundwater levels are continuing to drop, the land continues to subside.

The proportion of decaying plant matter, water and clay in a peat layer determines the extent to which it reduces in thickness when groundwater levels drop. The varying proportions for each peat layer have now been mapped out in 3D for the Dutch delta and coastal plain. Koster then used the 3D maps in scenarios for future drops in polder groundwater levels.

The major cities located in areas of peat soil, such as Amsterdam and Rotterdam, are least prone to subsidence, whereas rural areas and smaller towns such as Gouda will subside the fastest. The difference is caused by the build-up of thick layers in the major cities, which has resulted in significant levels of water being squeezed out of the peat and consequently in protecting the residual plant matter against rotting processes.