While the understanding and modelling of sea level rise (SLR) due to ocean density and mass changes have greatly improved over the past few decades, relative SLR contributions due to vertical land motions (VLMs) remain a major source of uncertainty. It is critical to downscale global and regional sea-level rise to local relative sea-level change as this is what causes coastal impacts and adaptation needs. In particular, land subsidence can strongly exacerbate coastal flood risk, saltwater intrusion, erosion and loss of wetlands and damage to infrastructure.

Here, we present the first analysis of pan-European coastal subsidence based on the European Ground Motion Service (EGMS) Ortho product. First, we perform a comparison between EGMS Ortho (Level 3) vertical velocity estimates and GNSS stations vertical velocity. This comparison reveals that the geodetic reference frame used to calibrate EGMS affects the vertical land velocity estimates and needs to be accounted for carefully, especially for the vertical land motions – including sub-millimetric/year velocities – that could affect local SLR estimates. After adjusting the EGMS calibrated product to the International Terrestrial Reference Frame (ITRF2014), we performed an assessment of VLM in European coastal flood plains. Our results show that half of the European area located in coastal flood plain is, on average, experiencing subsidence at a rate stronger than -1 mm/yr. More importantly, we find that urban area and population experience almost a -1 mm/yr subsidence on average (if we discard the uplifting regions due to Glacial Isostatic Adjustment) and for coastal airports and for harbours, the average land motion drops is even larger with -1.5 mm/yr subsidence rate. Finally, while our analysis allows identifying already well-known coastal subsidence hot-spots (e.g. Northern Italian coastal plain, Netherlands), we demonstrate with few examples that EGMS analysis also paves the way toward the identification of subsiding local scale coastal zones that have been ignored so far and for which flooding risk may become a major concern.

How to cite: Thiéblemont, R., Le Cozannet, G., Nicholls, R. J., Rohmer, J., Wöppelmann, G., Raucoules, D., de Michele, M., Toimil, A., and Lincke, D.: Current State of Coastal Subsidence in Europe Derived from the European Ground Motion Service, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15983, https://doi.org/10.5194/egusphere-egu24-15983, 2024.

The coast of Cameroon, which is approximately 590 km in length, is situated in the Gulf of Guinea and is characterized by its low elevation above sea level and sedimentary geology, making it particularly susceptible to erosion, subsidence, and sea level rise. The coast of Cameroon and its extensive mangrove forests are facing numerous economic pressures, including encroachment from urban expansion, agro-industrial development, port activities, oil and gas exploration and exploitation, and the increased pollution associated with these activities. Additionally, many rapidly growing cities located along the coast (Douala, Kribi, Tiko, Limbe) and neighbouring the mangroves (Duala estuary, Rio Del Rey estuary, and Ntem estuary) are currently experiencing alarming rates of coastal erosion, frequent flooding, complete loss of land, and evidence of subsidence from regional and continental research. Unfortunately, there have been no detailed investigations of the combined effects of land subsidence and sea-level rise, known as relative sea-level rise, and their present and future impacts on Cameroon's emerging coastal cities and mangroves under climate change. Therefore, this research aims to fill this knowledge gap by investigating, understanding, and projecting the causes, consequences, and coastal vulnerability related to land subsidence and sea-level rise to enable the development of information-based mitigation strategies and policies. We will use remote sensing data, InSAR analysis, hydrogeological investigations, and modelling tools to assess the real coastal elevation of Cameroon, determine the actual land subsidence rate, determine the actual local relative sea-level rise from tide gauge data, determine the factors influencing land subsidence, project future elevation evolution, and establish an integrated vulnerability assessment of the coastal areas of Cameroon. This research will contribute to a proper understanding of Cameroon's mangrove landscape dynamics, the vulnerability of coastal infrastructure, and its biodiversity to relative sea-level rise, subsidence, coastal retreat, and future flooding events. The outcome can be used to develop sustainable management strategies for Cameroon's coastal zone.

How to cite: Chounna Yemele, G., Teatini, P., and Minderhoud, P.: Vulnerability of coastal cities and mangroves to the combined effects of Land subsidence, relative sea-level rise, and groundwater extraction along the low-lying coastland of Cameroon, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8067, https://doi.org/10.5194/egusphere-egu24-8067, 2024.

Rosetta is a port city of the Nile Delta, 65 km east of Alexandria. The village is distinguished by its geographical location as the estuary of the Nile River and on the shore of the Mediterranean Sea. It is also historically distinguished as the original site for the discovery of one of the greatest heritage pieces in the world, the Rosetta stone at the village of Burj Rashid, which is currently in the British Museum. The construction of the High Dam reduces the natural mud deposits cause of the sinking and erosion of the crust there and the city experience variable high rates of erosion with accelerated coastal area lost on the last two decades.  This makes this city is very sensitive to any sea level rise.    Thus, Rashid is one of the most vulnerable coastal cities to the effects of climate change. 

The present study provides the contribution of the modernized geodetic and satellite techniques to take part into determination the effect of Mediterranean Sea level rise and land subsidence on the city of Rosetta. To reach the proposed objective the study utilizes   Global Navigation Satellite System (GNSS), tide gauges and satellite altimetry and gravity data. GNSS data has been used to determine the rates of the horizontal and vertical movements of the studied region   and linking the rates of vertical movement of the Delta to the temporal change of the sea level variation of the Mediterranean Sea by tying GPS measurements to tide gauge data. On the other hand, determination of temporal local and regional sea level variation of the southern part of the Mediterranean Sea using tide gauge and satellite altimetry data. Finally, total mass variation and the tectonic settings of the shore line features has been figured out using recent satellite gravity data. 

 Permanent GNSS network along the Nile Delta shows variable rates of land subsidence, with the subsidence rate of the studied area of about 6mm/y. Satellite altimeter data together with tide gauge data confirm the Sea level rise acceleration on this region with an acceleration of about 7mm/y.  On the other hand, the selected region shows complicated pattern of mass variation, land subsidence and Sea Level Rise. Therefore, impact of climate change may be the biggest challenge in this region. On this context, accurate monitoring on the land subsidence and the sea level rise is of great importance to the climate change mitigation and the protection of this city.

How to cite: Zahran, K.: Land Subsidence and Coastal Changes in Vicinity of the City of Rosetta, Nile Delta, Egypt Using Integrated Satellite and Ground-Based Techniques., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15568, https://doi.org/10.5194/egusphere-egu24-15568, 2024.

Subsidence research in the Netherlands primarily focusses on anthropogenic processes, specifically in the shallow and deep subsurface, resulting from land use, water management, and resource extraction. However, the 'intermediate' depth range, spanning hundreds of meters, has been relatively underexplored due to limited economic activities within this range. To accurately disentangle the overall human-induced subsidence, however, understanding the contribution from intermediate depth processes is essential.

We analyzed data from 20 extensometers monitoring subsurface movements between 1970 and 2021, at depth intervals ranging from ~10 to 400 meters. The extensometers were located in the hydrocarbon extraction areas operated by NAM (Nederlandse Aardolie Maatschappij) in Groningen, Friesland and Rotterdam.

Our findings highlight that secondary compaction, or creep, induced by overburden weight is the main driver of subsidence at intermediate depths. We found that subsidence rates of up to 0.6 mm/year in Groningen and 0.8 mm/year in Rotterdam account for over 10% of total measured subsidence in these regions. In Groningen, creep is attributed to Tertiary marine and Pleistocene eolian/fluvial deposits, whereas in Rotterdam, it is associated with Pleistocene shallow marine deposits. In all locations, the overburden weight of the Holocene deposits also affects the compaction at intermediate depth range.

Moreover, our analysis reveals cyclic responses in the extensometer data, including seasonal and tidal patterns. The strength of the seasonal signal corresponds to the inland salt-brackish groundwater boundary, while the tidal response is prominent near the coastline. Unexplained trend breaks, potentially linked to phreatic groundwater management, were observed in both regions.

Our analysis emphasizes the crucial role of intermediate depth contributions in obtaining a comprehensive understanding of the overall subsidence signal. Neglecting these contributions can lead to an incomplete interpretation subsidence, potentially overestimating the impact of mitigation measures. The intricate interplay of cyclic trends, salinity, water management measures, and secondary compaction emphasizes the necessity to expand monitoring efforts in the intermediate depth range.

 In response to the complexities identified in intermediate depth compaction analysis, we propose a monitoring setup with supplementary data sources, which maximizes the utility of the extensometer data. This setup is applicable to coastal plains and deltas worldwide, where a thorough understanding of subsurface processes is crucial for maintaining a sustainable living environment. Only with this comprehensive understanding can the total subsidence signal be accurately interpreted, ensuring that mitigation measures achieve their full effectiveness and avoid overestimation of other contributing sources.

How to cite: Verberne, M., Koster, K., de Bresser, H., and Fokker, P.: Unveiling the Hidden Depths: Insights in Intermediate Depth Compaction from 50 years of Extensometer Data in the Netherlands, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2120, https://doi.org/10.5194/egusphere-egu24-2120, 2024.

In light of the increasing frequency of severe droughts linked to climate change in central and southern Taiwan, the Choshui delta in central Taiwan has witnessed a notable surge in land subsidence. Consequently, the pace of land subsidence has peaked at 7.8 cm per year as of 2021 in the Choshui delta, Taiwan. The process of soil compaction in the Choshui delta primarily unfolds as a time-dependent geological phenomenon. A comprehensive understanding and characterization of this process are essential for the formulation of effective mitigation and adaptation strategies. This study presents a pioneering approach to characterize land subsidence in the Choshui delta, utilizing a random forest algorithm (RFA).

The random forest is an ensemble machine learning algorithm known for its versatility and effectiveness in various predictive modeling tasks. It is widely used due to its ability to handle complex relationships for tasks such as classification, regression, and feature selection in land subsidence. By leveraging this advanced modeling technique, we aim to identify and analyze the underlying causes of land subsidence, providing valuable insights into the dynamic interplay of environmental factors. This research contributes to a more comprehensive understanding of land subsidence patterns by incorporating a multi-factorial perspective in the face of changing climatic conditions. Using the RFA, we may identify and analyze the dominant factors affecting land subsidence. Subsequently, a land subsidence prediction model is established based on the RFA, considering the multi-factorial perspective, include cumulative compaction, groundwater levels, electricity consumption for pumping, and rainfall.

The RFA is utilized to identify the temporal patterns from historical time-series data spanning from 2008 to 2021, which is specifically associated with the land subsidence site in the study area. To validate the proposed model, we compare its predictions to the historical time-series data, utilizing metrics such as root mean square error, correlation coefficient, and coefficient of determination. Results demonstrate that, the optimal RMSE, R, and R² values during the prediction phase. The RFA in the context of land subsidence prediction performs exceptionally well, exhibiting high accuracy in predicting land subsidence patterns in the rapidly subsiding areas of the Choshui delta.

Keywords: land subsidence; climate change; random forest algorithm; groundwater; Choshui delta.

How to cite: Ku, C.-Y. and Liu, C.-Y.: Characterizing Land Subsidence Using Random Forest Algorithm in Taiwan, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1335, https://doi.org/10.5194/egusphere-egu24-1335, 2024.

Land subsidence in Kathmandu Valley has been recorded since 2003. The major cause of the Kathmandu valley subsidence is still unidentified and the subsiding depth or layer is not clear yet. Published research work has revealed the positive correlation between subsidence and sediment thickness of the valley. Kathmandu Valley, with a heterogeneous sediment thickness and different depths of bedrock, creates favorable conditions for differential settlement.

This study is focused on understanding and analyzing the early evidence of land subsidence in different parts of Kathmandu Valley, with direct field observations. Developing empirical relationships to better understand the underlying causes of land subsidence, primarily, multiple deep tubewells (DTW) sites across the valley were observed with the aim of analyzing changes in tubewell head and the impact of subsurface geology and tubewell lithology being studied. 

Tubewell constructed over bedrock shows the upheaval of the tubewell head in UN Office, Pulchowk, Jagdal Gulma, Chhauni, Prime Hotel, Thamel, where depth to bedrock is 126, 100, and 210 meters respectively. Tubewell upheaval has been recorded since 2017 in Pulchowk and Chhauni and since 2020 in Thamel with the rate of tubewell upheaval is similar to land subsidence recorded from InSAR and DGPS surveys. A similar pattern of tube well upheaval is visualized in and around the surrounding area, where tubewells are constructed over bedrock. In the same area, tubewells constructed on soil subsurface with relatively shallow depth remain the same. A 550 meter long horizontal surface crack around the Manbhawan area indicates the differential settlement of ground surface due to land subsidence. However, no strong evidence is recorded around the central part of the valley floor, where the depth of bedrock is deep.

The evidence recorded by tubewell upheaval at Pulchowk, Chhauni and Thamel validates the regional compaction of sediment layers deposited over the bedrock and the surface crack around the Manbhawan area validates the differential settlement due to change of subsidence rate over the same area. No change of tubewell head in shallow tubewell in the same area validates the deep aquifer compaction due to groundwater extraction and sediment load.

How to cite: Khatiwada, B., Rana, S., Khanal, A., Khatiwada, B., Tiwari, N., and Mishra, B.: Early evidence of land subsidence in Kathmandu Valley, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6841, https://doi.org/10.5194/egusphere-egu24-6841, 2024.

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.

How to cite: Lexmond, B., Janssen, C., Erkens, G., Griffioen, J., and Stouthamer, E.: Applicability of measured soil shrinkage characteristic curves to in-situ land movement monitoring data for expansive soils, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3252, https://doi.org/10.5194/egusphere-egu24-3252, 2024.

We introduce a novel groundwater flow model designed for the rapid estimation of seawater intrusion (SWI) in coastal aquifers. Drawing on Girinskii's potential theory (1947) for 2D aquifer flow, the model extends Strack's (1976) adaptation to coastal aquifers under transient state conditions. Key features include applicability to heterogeneous unconfined aquifer systems under time-dependent pumping and spatially distributed groundwater recharge. Assumptions include a sharp seawater-freshwater interface, prevailing horizontal flow, and neglect of flow rates within the saltwater wedge.

Built upon a finite-element 2D saturated groundwater simulator, the model derives a solution for the potential using a non-linear formulation of the classic flow equation and can accommodate prescribed-head or prescribed-flux boundary conditions at inland boundaries, as well as time-varying head conditions at the shore boundaries, which is crucial for addressing long-term sea-level rise (SLR). Noteworthy modifications enable the model to estimate land subsidence through a 1D vertical consolidation model directly in terms of potential, providing a holistic view of aquifer dynamics. The model is successfully tested against analytically based solutions and can facilitate swift calculations of aquifer vulnerability indicators, such as SWI toe location, increase in dissolved salt mass, and apparent land subsidence due to compounded effects of SLR, groundwater pumping, and long-term variable recharge conditions. Our presentation will showcase the model's features, preliminary results focusing on the effects of aquifer heterogeneities on the intensity of SWI and land subsidence patterns, model strengths and limitations. Emphasis will be placed on the model computational efficiency, enabling rapid estimation of crucial SWI indicators. Furthermore, the discussion will outline future steps, highlighting the need to balance the simplicity of working hypotheses with the computational demands of more intricate variable density models in assessing complex coastal aquifer dynamics.

How to cite: Yu, W., Baù, D., Christelis, V., and Geranmehr, M.: A Model for Fast Estimation of Seawater Intrusion in Coastal Aquifers: Simulating Flow and Land Subsidence, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-21064, https://doi.org/10.5194/egusphere-egu24-21064, 2024.

Visualization methods using transparent synthetic soils (TSS) have been developed as a physical model of macroscopic soil behavior from a geotechnical engineering perspective. Transparent surrogates containing transparent porous media and pore fluids have been used to simulate the geotechnical properties of natural soils. Studies to experimentally verify local deformation in subsidence phenomena require the use of inexpensive laboratory industrial materials to understand the macroscopic scale of larger test models. TSS made of polymeric polymers used in this experiment are one of those employed to accomplish this need and are easy to handle.

In the present study, a larger-scale pumping test than previously reported was conducted using a 300 mm wide x 250 mm long x 249 mm high acrylic tank filled with transparent hydrated polymer to represent an aquitard (clay layer) over an aquifer (saturated silica sand). Settlement within the synthetic clay layer due to pumping of pore water from the silica sand was constantly monitored by the target racking method using 100 particles of 3 mm diameter immersed in the synthetic clay layer. This experiment successfully visualized the deformation inside TSS due to vertical propagation of pore water pressure in the TSS during pumping and after pumping, in more detail than in the previous experiment. The experimental results showed good agreement with the numerical results of the three-dimensional pore-plastic deformation theory.

How to cite: Tabe, K. and Aichi, M.: Visualization technique in a laboratory experiment for the deformation distributions of subsidence of aquitard over aquifer due to excess pumping, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6862, https://doi.org/10.5194/egusphere-egu24-6862, 2024.

Peat soils typically have a high void ratio and compressibility. As a result, land subsidence is a common threat to peatlands. Geotechnical models are used to predict subsidence and the spatial variation therein to assess the potential for land subsidence and the effect of mitigation measures. In these models compaction is often calculated using parameters describing the consolidation and creep behaviour of the soil. Consolidation, a result of excess pore pressure dissipation, is calculated using the compression (CR) and recompression ratios (RR) together with the effective stress and preconsolidation stress. Creep, viscous compression unrelated to dissipation of excess pore pressure, is calculated using an independent secondary compression parameter (C_α) and the preconsolidation stress which are derived from laboratory compression tests. Pristine peat soils have a much higher high organic content (OC), a higher compressibility and are more susceptible to creep than clay soils. This study explores the relation between compressibility and OC by looking at the OC end members, peat and clay, and specifically looking for patterns in mixtures of the two.

A large collection of compression test data is used to evaluate the relation between the compressibility of peat and its organic content. Holocene peat and fluvial and marine clay samples from the Netherlands are analysed. The composition of the samples is described by parameters such as wet unit weight, water content and organic content. A relation between water content and loss-on-ignition was determined by fitting a trend to a different extensive collection of Holocene and lake-infill peat samples from the Netherlands. This relation was used to acquire OC for all compression test samples. Analysis results show that for low OC (<30%) samples values of CR, RR as well as C_α increase with increasing OC following a significant trend. The high OC (>30%) samples show relatively high values for all three parameters compared low OC, about twice as high on average, but due to high variability in these values no trend can be discerned.

These differences between the low and high OC results point out that compressibility is related to the OC for clay rich samples, whereas the compressibility of organic rich samples is not dictated solely by the OC. Using this outcome a distinction was made between clay-dominated and peat-dominated compaction behaviour. This analysis enables an estimation of the three compaction model parameters for clay-rich samples based on the OC. Hence, for example, an OC-dependent C_α can improve subsidence modelling compared to a static C_α which does not incorporate changes in soil composition over time: loss of OC of peat soils and organic clays via oxidation or anoxic decomposition will influence the compressibility of the soil rather than or besides the volume loss directly linked to the loss of organic content. This will alter the way we calculate not only compaction but might also impact the calculation of greenhouse gas emission as a result of decomposition, which is a current topic within land subsidence.

How to cite: van Elderen, P., Erkens, G., Zwanenburg, C., Kooi, H., and Stouthamer, E.: The relationship between organic content and compressibility of peat and clay soils, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8379, https://doi.org/10.5194/egusphere-egu24-8379, 2024.