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The natural surface area of many coastal and delta plains has been increased by land reclamation in response to growing populations. These reclaimed lands are often experiencing subsidence. The reclaimed South Flevopolder in the coastal plain of the Netherlands has experienced severe subsidence after its reclamation in 1968. The subsidence is caused by phreatic groundwater level lowering and the associated aeration of the former subaqueous shallow subsurface and increased effective stresses. In this study, surface elevation measurements that quantify the subsidence, conducted annually between 1968 and 1993, and in 2009 and 2012, have been used to constrain and estimate the parameters in models that describe subsidence. For the estimation an Ensemble Smoother with Multiple Data Assimilation was employed. For the forward models, we employed correlations for compression (primary consolidation and creep), oxidation, and shrinkage of coastal deposits. Shrinkage of the aerated clay and organic clay layers was found to be the main contributor to subsidence, and the measurements could be represented well. The quantification of the model parameters allows for better subsidence forecasts. The stochastic method that was employed further facilitates to define a quality measure for forecasts in terms of a covariance matrix or a confidence range. Annual subsidence measurements were used for compression parameter estimationAn Ensemble Smoother with Multiple Data Assimilation was employedShrinkage of clay and organic clay was found the main contributor to compression

This research targets disentangling shallow causes of anthropogenically induced subsidence in a reclaimed and urbanized coastal plain. The study area is the city of Almere, in the South Flevoland polder, the Netherlands, which is among the countries' fastest subsiding areas. The procedure consists of integrating Interferometric Synthetic Aperture Radar (InSAR) data with high‐resolution phreatic groundwater and lithoclass models, and a database containing construction details. The InSAR data were derived from Sentinel‐1, one ascending and one descending track, over a period from March 2015 until June 2020. The two main parts of the workflow are isolation of the InSAR points of structures without a pile foundation and a data assimilation procedure. The shrinkage of surficial clay beds by phreatic groundwater level lowering is identified as the main cause of subsidence in the area, with an average contribution of 6 mm per year. The history‐matched physics‐based model predicts that 1 m drop in phreatic groundwater level now translates into 10 mm of subsidence in the next 5 years. Additionally, a groundwater deficiency due to severe dry periods should be considered as an accelerator of subsidence. To ensure a robust network to estimate subsidence, we recommend a consistent monitoring strategy of the phreatic groundwater level. The city of Almere, in the Netherlands, is part of a polder that was reclaimed in 1968. Land reclamation is accompanied by the lowering of groundwater levels, which can cause land subsidence. Almere is situated on top of about 9 m of soft soil layers. These layers were deposited after the last ice age and consist predominantly of clay and peat. It is important to understand and quantify the subsidence processes in these Holocene layers, to be able to mitigate subsidence. By lowering the groundwater level, the soft soil layers are dried. Clay shrinks when it dries out and organic material (within peat) oxidizes. Lowering the groundwater level also causes the load of the layers below to increase, which can result in the compaction of the layers (reduction in size by pressing together). This study targets the behavior of these processes. Results of our study indicate that the shrinkage of clay is the dominant driver of subsidence in Almere. One meter lowering in groundwater level now results in approximately 1 cm subsidence in 5 years. To improve our understanding of the non‐trivial link between groundwater fluctuations and subsidence, higher spatial‐temporal resolution groundwater monitoring is required. Interferometric Synthetic Aperture Rader data on objects constructed on soft soil without a foundation are used for subsidence measurementsShrinkage of clay by aeration as a result of artificially lowered phreatic groundwater levels is identified as the main source of subsidenceOne meter drop in phreatic groundwater level now translates into 1 cm of subsidence in 5 years Interferometric Synthetic Aperture Rader data on objects constructed on soft soil without a foundation are used for subsidence measurements Shrinkage of clay by aeration as a result of artificially lowered phreatic groundwater levels is identified as the main source of subsidence One meter drop in phreatic groundwater level now translates into 1 cm of subsidence in 5 years