Disentangling Shallow Subsidence Sources by Data Assimilation in a Reclaimed Urbanized Coastal Plain, South Flevoland Polder, the Netherlands

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.