Subsidence occurs in many alluvial depositional environments, causing substantial socio-economic losses. Thick aquifers with interbedding or lens-structured clay can contribute significantly to compaction. The poromechanism of aquifer compaction is usually complicated, and its spatial variation is poorly understood, inhibiting our ability to fully explain the region-wide nature of groundwater-exploitation-induced land subsidence. In this study, we explore the poromechanism of aquifer deformation in the Choushui River alluvial fan, Taiwan. Groundwater level and multi-layer compaction data were collected for analyses. Elastic, plastic, and viscous characteristics all appear in the deformation data to various extents. A visco-elasto-plastic (VEP) model composed of a visco-elastic set and a visco-plastic set in series, which are associated with different viscous dampers, is proposed to model the deformation. Compared with existing models, the proposed model is superior in terms of simulating compaction as well as its applicability and versatility. The proposed VEP model is applied to observation wells in the proximal, middle, and distal fans of the Choushui River alluvial fan to explore the spatial variation in the poromechanical properties. The results show that the Young's modulus of the elastic spring increases with depth and with the distance from the distal fan to the proximal fan. The Young's modulus of the plastic element decreases with depth. The system response factor of the visco-plastic set decreases with depth and with the distance from the distal fan to the proximal fan. Aquifer recovery incapability increases with depth. Both the trend and dynamic fluctuations in compaction for the Choushui River alluvial fan are well captured by the proposed VEP model, which is essential for assessing long-term land and water resource management and evaluating potential short-term threats to infrastructure caused by seasonal subsidence.
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