The Mekong delta, like many deltas around the world, is subsiding at a relatively high rate, predominately due to natural compaction and groundwater overexploitation. Land subsidence influences many urbanized areas in the delta. Loading, differences in infrastructural foundation depths, land-use history, and subsurface heterogeneity cause a high spatial variability in subsidence rates. While overall subsidence of a city increases its exposure to flooding and reduces the ability to drain excess surface water, differential subsidence results in damage to buildings and above-ground and underground infrastructure. However, the exact contribution of different processes driving differential subsidence within cities in the Mekong delta has not been quantified yet. In this study we aim to identify and quantify drivers of processes causing differential subsidence within three major cities in the Vietnamese Mekong delta: Can Tho, Ca Mau and Long Xuyen. Satellite-based PS-InSAR (Persistent Scatterer Interferometric Synthetic Aperture Radar) vertical velocity datasets were used to identify structures that moved at vertical velocities different from their surroundings. The selected buildings were surveyed in the field to measure vertical offsets between their foundation and the surface level of their surroundings. Additionally, building specific information, such as construction year and piling depth, were collected to investigate the effect of piling depth and time since construction on differential vertical subsidence. Analysis of the PS-InSAR-based velocities from the individual buildings revealed that most buildings in this survey showed less vertical movement compared to their surroundings. Most of these buildings have a piled foundation, which seems to give them more stability. The difference in subsidence rate can be up to 30 mm/year, revealing the contribution of shallow compaction processes above the piled foundation level (up to 20 m depth). This way, piling depths can be used to quantify depth-dependent subsidence. Other local factors such as previous land use, loading of structures without a piled foundation and variation in piling depth, i.e., which subsurface layer the structures are founded on, are proposed as important factors determining urban differential subsidence. PS-InSAR data, in combination with field observations and site-specific information (e.g., piling depths, land use, loading), provides an excellent opportunity to study urban differential subsidence and quantify depth-dependent subsidence rates. Knowing the magnitude of differential subsidence in urban areas helps to differentiate between local and delta wide subsidence patterns in InSAR-based velocity data and to further improve estimates of future subsidence.

Many of the world’s major river systems seemingly have one or a few disproportionally large meanders, with tight bends, in the fluvial-tidal transition (e.g., the Thames in the UK, and the Salmon River in Canada). However, quantitative studies on meanders have so far primarily focused on rivers without tidal influence or on small tidal meanders without river inflow, providing relations between channel geometry and meander characteristics (length, amplitude, and sinuosity). Physics-based predictions of meander size and shape for the fluvial-tidal transition zone remain untested for a lack of data. Therefore, it remains unclear whether the dimensions of meanders in the fluvial-tidal transition zone are indeed disproportionally large, and whether meander characteristics can be used as an indicator for tidal influence. Here, data from 823 meanders in 68 fluvial-tidal transition zones worldwide are presented that reveal broad-brush relations between channel geometry and meander dimensions. Our results show that fluvial-tidal meanders indeed become larger in the seaward direction, but the dimensions are proportional to local channel width, as in rivers. Sinuosity maxima are an exception, rather than the rule, in the fluvial-tidal transition zone. Surprisingly, the width of the upstream river correlates with estuarine channel width and tidal meander size even though river discharge constitutes only a fraction of the tidal prism. The new scaling relations can be used to constrain dimensions of rivers and estuaries and their meanders.

Soil shrinkage characteristic curves are used to describe the shrinkage behavior and hydraulic properties of unsaturated soils. To construct soil shrinkage characteristic curves, a high‐data‐density measurement method is needed that relates water content to soil volume changes. We present a fully automated soil shrinkage measurement setup, based on the simplified evaporation method, to characterize the shrinkage behavior of undisturbed natural expansive clay soils. The high data density creates the opportunity to produce soil shrinkage characteristic curves without the need for a mathematical model. The technique allows for resaturation of the samples after drying, enabling differentiation between reversible and irreversible shrinkage. The setup consists of the commercialized HYPROP2 apparatus combined with optical distance sensors to measure the horizontal and vertical dimensions of the samples, yielding data on the sample volume, weight, and soil water suction. The measurement frequency is once per 10 min, and the measurement period is up to 4 weeks, providing a detailed time series of the drying and shrinkage characteristics. The setup can capture the different shrinkage phases and offers the opportunity to relate the soil shrinkage characteristic curve to soil water suction. The measurement data acquisition rate and accuracy enable detailed interpretation of soil water retention curves for nonrigid soils and are shown to be essential for understanding and quantifying the shrinkage potential of several types of deposits.

Globally, land subsidence caused by groundwater pumping is a common phenomenon. Numerous subsurface processes, both natural and anthropogenic, contribute to its occurrence. In coastal regions, severe land subsidence exacerbated by groundwater pumping is particularly detrimental. On top of that, coastal areas affected by natural compaction, river delta consolidation processes, and additional exposure to drainage-induced aquifer-system compaction are especially susceptible to flooding and salinization due to the steadily rising sea level triggered by climate change.The Mekong delta, one of the world's largest deltas, is densely populated and crucial for agricultural production. The delta is low-lying and has a high rate of natural compaction, whereas human activities accelerate land subsidence. A numerical model of groundwater-extraction-induced aquifer-system compaction was developed in 2017 to demonstrate the effects of 25 years of groundwater extraction on land subsidence in the delta. The model encompassed the time range from 1991 to 2016 using geological, hydrogeological, geomechanical, and remote sensing data. In 2020, the model was updated to include a surface water network. Six scenarios were developed to simulate potential future pathways of hydraulic head evolution and aquifer-system compaction in the Mekong delta from 2019 to 2100.Our research aims to enhance the reliability of the existing numerical model of groundwater extraction-induced aquifer-system compaction in the Mekong delta, given the significance of such scenarios in the development of policies to mitigate the negative effects of groundwater pumping. Our research focuses on four steps.First, a novel subsurface model representation.The 3D subsurface model of the Mekong delta was developed using ten hydrogeological cross-sections derived from 96 geological borehole logs interpolated linearly. This resulted in a subsurface model consisting of 15 layers, including seven aquifers, seven aquitards, and a phreatic top layer. The goal of the current study is to develop a new schematisation of the aquifer system within the Mekong delta based on 522 borehole logs and to investigate the spatial variability of the aquifer system using advanced geostatistical tools.Second, a hydrogeological schematization enhancement.In the current schematisation, aquitards are discretized as a single layer, resulting in the inability to simulate delayed groundwater pressure propagation within the aquitard. Several additional models with refined aquitard discretization are constructed and compared to evaluate the effect.Third, the quantification of the influence of deterministic modelling on compaction.The hydrogeological model is deterministically parameterized and calibrated using hydraulic head time series. Utilizing stochastic modelling of hydrogeological parameters, the impact of this deterministic modelling approach on simulated compaction is determined.Fourth, a hydrogeological and geomechanical parameters consistency improvement.The previous hydrogeological and geomechanical model parameterizations are inconsistent since the groundwater model and the geomechanical module were initially parameterized and calibrated independently. To address this issue, an iterative procedure is used to calibrate storage and compression indexes consistently for each individual model layer. This is accomplished by utilising groundwater head datasets recorded by 358 piezometers and land subsidence datasets retrieved by InSAR from 2006 to 2010 and 2016 to 2019.

Coastal subsidence increases the vulnerability to flooding risk, salinization of water resources and permanent inundation. For the Mekong Delta, whose mean elevation is less than 2 m above sea level, subsidence rates of up to several centimeters per year have been reported recently. This leads to a growing risk for the resident population, infrastructure and economy, increased by the accelerating sea level rise. Land subsidence in Mekong Delta has different causes, most prominently natural compaction of young deltaic sediments, but also overexploitation of groundwater aquifers with accompanying head decline. Precise monitoring of the subsidence rate is necessary for analyses of cause and hazard as well as planning and assessment of countermeasures. Here, we present and discuss recent land subsidence rates in the Mekong Delta derived from satellite-based SAR-Interferometry.We use Sentinel-1 scenes acquired between 2015 and 2019 to analyze recent land subsidence in the lower Mekong Delta. The Persistent Scatterer Interferometry technique (PS-InSAR) is applied, which allows for the estimation of displacement rates of coherent backscatter targets with mm-accuracy. Separate analyses of time series from ascending and descending observations and comparison with other studies based on data of the same sensor give insight into the accuracy of the parameter estimation and the error budget.The observed subsidence rates of up to 6 cm/yr feature mainly three different spatial characteristics: (i) interconnected areas of little to no subsidence, (ii) isolated urban hot-spots with high subsidence rates and (iii) larger regions with increased subsidence rates covering urban as well as rural areas. Points on deeply founded infrastructure frequently exhibit lower subsidence rates than adjacent ground surface points. We study this phenomenon at different buildings since subsidence rates with respect to different foundation depths can be used as a proxy to constrain the effective depths of sediment compaction. Further, the correlation of observed subsidence rates and spatial distribution of lithostratigraphic units from quaternary sedimentary depositions is investigated. Finally, we show changes and commons in the spatial distribution of the subsidence rates compared to a previously published study on subsidence in the Mekong Delta covering data from 2006 to 2010.

Normal seasonal variations in wet and dry conditions cause vertical land movement, due to shrinkage and swelling, particularly in deposits rich in expansive clay minerals or organic matter. This land movement can be up to decimeters depending on the characteristics of the deposit, causing damage to infrastructure and buildings. Furthermore, the increase in drought duration and intensity results in a lowering of the groundwater table to unprecedented levels in shallow deltaic and coastal subsurface deposits. Exposure to resulting low water contents in the unsaturated zone induces irreversible shrinkage and densification of the soil structure, contributing to land subsidence.Understanding the relation between water content and bulk volume over time is crucial for predicting the land movement and assessing the potential for land subsidence and associated structural damage. The shrinkage behaviour is commonly characterized using soil shrinkage characteristic curves, which relate the water content to bulk volume.In this study, we characterized the soil shrinkage characteristic curves (SSCCs) of undisturbed natural samples extracted from the capillary fringe zone of shallow, marine clay-rich deposits in the Netherlands. The sample site was selected based on the presence of an extensometer, constantly monitoring the water content and thickness of the soil layer. The SSCCs were created by measuring the water content and sample volume during air drying of the samples. The sample volume was measured with optical distance sensors. The water content decreased from 0.97 to 0.08 during the experiment and the void ratio from 0.97 to 0.34. The SSCCs of the samples show a linear relation between water content and void ratio, for a water content between 0.97 and 0.34.By applying the established linear relation to the monitored in-situ soil moisture content at the sample site, we predicted the change in thickness of the soil layer. The prediction overestimates the measured changes in land movement, indicating the importance of in-situ conditions. The measured land movement seems to be more responsive to the groundwater level than the water content. The and the coming results underscore the importance of measuring land movement in-situ.