Differential subsidence in the urbanised coastal-deltaic plain of the Netherlands

Abstract

The urbanised peat-rich coastal-deltaic plain of the Netherlands is severely subsiding due to human-induced phreatic groundwater level lowering, as this causes peat layers to compress and oxidise. To determine the potential susceptibility of this area to future subsidence by peat compression and oxidation, the effects of lowering present-day phreatic groundwater levels were quantitatively evaluated using a subsidence model. Input were a 3D geological subsurface voxel-model, modelled phreatic groundwater levels, and functions for peat compression and oxidation. Phreatic groundwater levels were lowered by 0.25 and 0.5 m, and the resulting peat compression and oxidation over periods of 15 and 30 years were determined. The model area comprised the major cities Amsterdam and Rotterdam, and their surrounding agricultural lands.The results revealed that for these scenarios agricultural areas may subside between 0.3 and 0.8 m; potential subsidence in Amsterdam and Rotterdam is considerably lower, less than 0.4 m. This is due to the presence of several metres thick anthropogenic brought-up soils overlying the peat below the urban areas, which has already compressed the peat to a depth below groundwater level, and thus minimises further compression and oxidation. In agricultural areas peat is often situated near the surface, and is therefore highly compressible and prone to oxidation. The averaged subsidence rates for the scenarios range between 7 and 13 mm a −1 , which is corresponds to present-day rates of subsidence in the peat areas of the Netherlands. These results contrast with the trend of coastal-deltaic subsidence in other deltas, with cities subsiding faster than agricultural areas. This difference is explained by the driver of subsidence: in other deltas, subsidence of urban areas is mainly due to deep aquifer extraction, whereas in the Netherlands subsidence is due to phreatic groundwater level lowering.

Disentangling and Parameterizing Shallow Sources of Subsidence: Application to a Reclaimed Coastal Area, Flevoland, the Netherlands

“…Studies conducted in the Netherlands that focused on peat oxidation generally use a fixed annual oxidation rate with respect to the actual thickness of a layer (Koster, Stafleu, & Stouthamer, 2018;Van der Meulen et al, 2007;Van Hardeveld et al, 2017). Because only the organic matter oxidizes, a residual thickness representing admixed clastic sediments should ideally be taken into account.…”

“…Subsidence models are generally based on field and laboratory measurements (e.g., Schothorst, 1982;Van Asselen et al, 2018), sometimes incorporating a physical basis to describe the underlying subsidence processes (e.g., Gambolati et al, 2006;Koster, Stafleu, & Stouthamer, 2018;Van der Meulen et al, 2007). However, model outcomes are critically dependent on model parameters, soil parameters, and the drivers that cause the subsidence.…”

“…We identified the major contributors to subsidence and obtained model parameters for three processes: compression, oxidation, and shrinkage caused by phreatic groundwater level lowering. Of these three, shrinkage had been added as an additional process to the traditionally used other two in the Netherlands (e.g., Koster, Stafleu, & Stouthamer, 2018;Muntendam-Bos et al, 2009;Nieuwenhuis & Schokking, 1997;Van der Meulen et al, 2007), because the study area consists of reclaimed subaqueous deposits in which first-time groundwater lowering might induce a compaction mechanism in fine grained soil that would not otherwise be accurately represented (e.g., Doornkamp, 1993).…”

Can we elevate the subsiding coastal plain of the Netherlands with controlled sedimentation?

“…Van der Meulen et al (2007) estimated that 9.4 to 28 million m 3 yr −1 of additional accommodation space is created by oxidation and compression of these shallow peat layers. Although rates of future peat oxidation and compression are not easy to estimate, as they depend on unknown choices in future groundwater management (Koster et al, 2018), we use these maximum oxidation and compression rates and added these to the provisioned accommodation space by the scenarios of 21st century MHW.…”

Towards regionally forecasting shallow subsidence in the Netherlands

“…Recent subsidence studies deploying GeoTOP focused on past peat compaction (Koster et al, 2018a), peat oxidation potential (Koster et al, 2018b), and scenarios of near future subsidence in urbanized and agricultural peat areas (Koster et al, 2018c). However, parameterizing peat and clay voxels on a regional scale with properties essential to forecast compaction and oxidation under changing groundwater levels remains challenging.…”