Displays

Unraveling the contribution of different natural and anthropogenic drivers to total subsidence can be a main challenge when studying changes of land elevation in a coastal-deltaic area. In fact, the contribution of a single driver often varies both in time and space and segmented land subsidence measurements only provide part of the solution. However, it is a crucial step required to facilitate the development of effective mitigation and adaptation strategies for sinking coastal-deltaic areas. This presentation highlights recent and future advances towards unravelling the contribution of different subsidence drivers for one of the largest deltas on the planet, the Mekong delta.

The multidisciplinary approach combined estimates of subsidence rates, both remotely-sensed (PS INSAR) and from field observations, with spatial data analysis and two complementary numerical modelling approaches, which bring together information and expertise from amongst others geology, hydrogeology and geomechanics. This multi-year effort provides insights in several significant natural (i.e. natural compaction) and anthropogenic subsidence (i.e. aquifer systems compaction due to groundwater extraction) processes that play a role in the Mekong delta system. Combining various advances enabled the creation of future elevation projections following groundwater-extraction scenarios, which provides valuable insights for Mekong delta’s policymakers but also shows the dire situation of the low-lying delta. Efforts towards further unraveling and quantification different subsidence drivers in the Mekong delta are ongoing and new Sentinel’s PS INSAR data provide exciting opportunities for detailed quantification of depth-depending sinking rates.

Present results make clear that the effectiveness of mitigation measures to reduce groundwater extraction-induced sinking rates will predominantly determine elevation evolution and thereby faith of the low-lying Mekong delta in the coming decades.

How to cite: Minderhoud, P. S. J., Erkens, G., Kooi, H., Zoccarato, C., Teatini, P., and Stouthamer, E.: Unravelling and quantifying natural and anthropogenic subsidence drivers in a mega delta, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9778, https://doi.org/10.5194/egusphere-egu2020-9778, 2020.

City level coastal subsidence can be caused by a number of factors, both natural (e.g. compaction) and anthropogenic (e.g. ground water extraction). Past observations in cities indicates that the rate of subsidence can be altered through policy intervention (e.g. Tokyo's ban on ground water pumping in 1970's). Given vertical land motion is a key component in local sea level projections where subsidence amplifies the onset of future damages, we test the extent to which intervention could reduce risk with a simple city level coastal damage model. We adjust water levels to embed different time dependent subsidence scenarios over the 21st century. We contend that local policy intervention to slow anthropogenic subsidence where possible will slow the onset of damaging sea level rise thus reducing potential coastal damages, and reduce the required increases in future flood protection heights. Performed in tandem with global mitigation efforts, cities currently under major threat may yet survive the climate crisis.

How to cite: Jackson, L.: Using subsidence scenarios to assess flood risk to delta cities under future sea level rise., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19378, https://doi.org/10.5194/egusphere-egu2020-19378, 2020.

Relative sea-level changes observed by tide gauges are commonly corrected for several components including crustal displacement, ocean dynamics, and vertical land motion. Vertical Land Motion (VLM) due to local land hydrology is a crucial component that observed as localized ground motion and varies with each tide gauges. Permanent GNSS stations are used to measure the VLM trend at tide gauges, however, only few tide gauges are equipped with collocated GNSS stations. Multi-temporal InSAR analysis provides ground displacements in both the spatial and temporal domains. Therefore, in our study, we applied the spaceborne Interferometric SAR technique to measure the local ground motion using Sentinel-1 SAR data. The Korean peninsula is surrounded by the East Sea/Sea of Japan, the Yellow Sea and the East China Sea have continuously monitoring tide gauges with a record length of more than 30 years. We acquire C-band Sentinel-1 SAR data (both ascending and descending mode) over the Korean Peninsula during 2014/11 and 2019/04. We estimate the high-resolution (~ 10 m) land motion at tide gauges (mm-level accuracy) over these 21 tide gauges and, compared with available collocated GNSS observations. 2D displacements (vertical and horizontal) are derived from ascending and descending mode InSAR displacements. The linear trend of VLM observed from our InSAR estimates is used to compensate for the relative velocity of sea-level changes observed from tide gauges.

How to cite: Palanisamy Vadivel, S. K., Kim, D., Jung, J., and Cho, Y.-K.: Multi-temporal Spaceborne InSAR technique to compensate Vertical Land Motion in Sea Level Change records: A case study of Tide gauges in Korean Peninsula, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12838, https://doi.org/10.5194/egusphere-egu2020-12838, 2020.

Vertical Land Motion (VLM) is a composite of several earth dynamics caused by changes of earth’s surface load or tectonics. In most of the Northern Hemisphere mainly two dynamics are causing large scale vertical land motion – Glacial Isostatic Adjustment (GIA), which is the rebound from the loading of the latest glacial cycle (10-30 kyr ago) and elastic rebound from contemporary land ice changes, that happens immediately when loading is removed from the surface.

With glacial mass balance data and observations of the Greenland Ice Sheet we have created an Northern Hemisphere ice history from 1996-2015 that is used to make a model for elastic VLM caused by ice mass loss that varies in time.

It shows that, in most cases, the elastic VLM model is able to close gaps between GIA induced VLM and GNSS-measured VLM, giving confidence that the combined GIA + elastic VLM-model is a better alternative to adjust relative sea level measurements from tide-gauges (where no (reliable) GNSS-data is available) to absolute sea level than 'just' a GIA-model. In particular for Arctic Sea Level, where elastic uplifts are prominent and large coastal regions have limited in-situ data available, the VLM-model is useful for correcting Tide Gauge measurements and thereby validate satellite altimetry observed sea levels, which is challenged by sea ice in the coastal Arctic.

Furthermore, our elastic VLM-model shows, that the uplift caused by the melt of the Greenland Ice Sheet (GIS) is far-reaching and even in the North Sea region or along the North American coast show uplift rates in the order of 0.4-0.7 mm/yr from 1996-2015. Interestingly, this is roughly equivalent to Greenland’s sea level contribution in the same period, thereby 'neutralizing' the melt of GIS. As GIS ice mass loss continues to accelerate, the elastic uplift will have increased importance for coastal regions and future relative sea level projections. Unfortunately, the opposite effect is true for the southern hemisphere or vice versa if Antarctic ice sheet mass loss would increase.

How to cite: Ludwigsen, C. A., Andersen, O. B., Khan, S. A., and Marzeion, B.: Importance of Northern Hemisphere Vertical Land Motion for Geodesy and Coastal Sea Levels, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19876, https://doi.org/10.5194/egusphere-egu2020-19876, 2020.

Lagoons and deltas are characterized by the presence of transitional environments, such as low-lying plains or islands, salt marshes, and tidal flats with fundamental value in terms of biodiversity, recreational activities, and protection of inland territories from storms. The fate of these morphological landforms is severely threatened by the ongoing rise of the mean sea level (SLR) and land subsidence (LS). The loss of elevation relative to mean sea level, i.e. SLR plus LS, must be counterbalanced by accretion of inorganic sediments and biodegradation of organic matter. A large contribution to LS of transitional landforms is due to auto-compaction of the Holocene sediments. In fact, the large porosity and compressibility of these recent deposits, especially when the organic fraction is high, are responsible for a significant thickness reduction because of consolidation when new deposition occurs on the surface. SAR interferometry on deep-founded and surface radar scatterers, ground-based monitoring equipment (deep levelling benchmarks, SET, accretion traps), and a novel in-situ loading test have been used in the Venice Lagoon to distinguish between deep and shallow LS contributions, i.e. LS occurring below and above the Pleistocene / Holocene bound. After a review of the available dataset, the present contribution describes the modelling activities that are ongoing to understand the collected measurements. In particular, an advance coupled mixed finite-element poromechanical model is used to reproduce the loading test carried out on the Lazzaretto Nuovo marshland on summer 2019. With the aim of reliably characterize the geomechanical properties of the Holocene sediments of the tidal-marsh, a number of plastic tanks were filled with seawater, reaching a cumulative load of 40 kN applied on a 2.5´1.8 m² surface. Specific instrumentations were deployed before positioning the tanks to measure soil vertical displacement and pore overpressure at various depths below the load and distances from the load center. The numerical model uses linear piecewise polynomials and the lowest order Raviart–Thomas mixed space to represent the three-dimensional porous medium motion and the groundwater flow rate, respectively. The model is applied to the various loading and unloading phases that superpose to the tidal fluctuation of the lagoon level recorded over the 4-day test duration. The geomechanical properties thus derived constitute a significant advancement to understand the LS drivers in transitional environments and predict their resilience to SLR.    

How to cite: Zoccarato, C. and Parrella, E.: Investigating land subsidence of transitional environments, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20314, https://doi.org/10.5194/egusphere-egu2020-20314, 2020.

 Transparent soils are developed as a physical modelling of macroscopic soil behaviors in geotechnical engineering aspect. Transparent surrogates with its index-matching fluid, called as transparent porous media or transparent soils, have been used for simulating geotechnical properties of natural soils. Visualization technique itself have been applied to microscopic level of soil deformation and soil flow problems such as X-ray, Computerized Tomography (CT), and Magnetic Resonance Imaging (MRI) cameras by very expensive apparatuses with highly operating skills. Geotechnical researches need rather understanding of macroscopic scale of larger test models with inexpensive experimental industrial substances. Transparent soils have been developed to achieve these needs with easy handling performance. 
 The authors demonstrated a pumping test in a glass tank of 30mm width by 80mm length by 70mm height filled with transparent hydrated superabsorbent polymer to represent aquitard (clay layer) over aquifer (saturated silica sand). The subsidence within the synthetic clay layer due to pumping of pore water from silica sand was constantly monitored by target racking method using four 8mm-diameter particles immersed in the synthetic clay layer. The test successfully visualized deformation due to vertical propagation of pore water pressure during subsidence event within the transparent synthetic clay layer. It was also found that this experiment result and the results from three-dimensional numerical simulation of poroelastic deformation were consistent with each other.

How to cite: Tabe, K. and Aichi, M.: Experimental technique for visualization of aquitard compaction over aquifer caused by excess pumping, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3265, https://doi.org/10.5194/egusphere-egu2020-3265, 2020.

Defining land subsidence causes is not an easy task, because ground lowering is a complex phenomenon due to the contribution of different physical processes related to natural contest and to anthropic actions. Indeed, such processes, which are characterized by a specific origin and may act in different spatial and temporal intervals, can overlap giving rise to a single surface land deformation, observable through conventional and innovative monitoring techniques (i.e. high-precision levelling, InSAR and GNSS). Of course, discriminating the individual causes is fundamental for reducing environmental and social harms, especially in deltas and coastal areas, where land sinking, coupled with climatic effects, can induce massive flooding. The present work concerns an application of a multi-component and multi-source approach, recently proposed by some of the authors for studying land subsidence in deltas. Such a methodology is aimed at understanding the processes causing both periodic and permanent components of the vertical land movement and at retrieving more accurate subsidence rates. It consists of three steps, respectively involving: a component recognition phase, based on statistical and spectral analyses of geodetic time series; a source (or physical process) selection phase, based on the comparison with data of different nature; a source validation step, where the selected sources are validated through physically-based models. The proposed procedure has been applied to the permanent component of subsidence in the Po Delta (northern Italy), an area historically affected by land subsidence and influenced by climatic changes, where continuous GNSS data and differential InSAR-derived time series were simultaneously acquired from 2012 to 2017. In particular, the exponential relation found between the mean SAR-derived LOS velocity and the thickness of the Late Holocene prograding deposits, pointed out the key role of the sedimentary compaction process with respect to the spatial distribution of the subsidence rates and confirmed the importance, already highlighted by other authors, of the consolidation of the shallower strata. In order to validate the consolidation process and to quantify also the deeper contributions of tectonics- and isostasy-depending mechanisms, 2D geological models have been constructed along two west-east sections across the central part of the Delta. Finally, the computed subsidence rates have been compared with the geodetic velocities estimated in Taglio di Po and Porto Tolle villages (Rovigo, northern Italy), clarifying the contribution of each geological mechanism to the observed delta subsidence.

How to cite: Vitagliano, E., Di Maio, R., D'Ambrogi, C., Calcaterra, D., Fiaschi, S., and Floris, M.: Integration of geodetic observations and geological models for investigating the permanent component of land subsidence in the Po Delta (northern Italy) , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3279, https://doi.org/10.5194/egusphere-egu2020-3279, 2020.

Estimation of coastal land subsidence rates due to GIA and tectonic factors on millennial scale has become an urgent task for the hazard assessment of future rising sea level. Whilst geophysical simulation is a promising approach, the modeling uncertainty is still difficult to constrain due to the lack of accurate sea-level data. Another practical approach is based on the present elevations of paleo indicative landforms of known ages, such as coastal terraces and MIS5e marine sediments. However, this method also suffers from uncertainties associated with the measurements of landform relicts. In order to obtain a robust estimation of long-term coastal subsidence rates along the southern China coast, an active economic zone vulnerable to future sea-level rise, this study applies a statistical method to determining the high-probability land subsidence histories of six coastal sectors (the Yangtze Delta, Fujian & Taiwan Strait, Han River Delta, East Guangdong, Pearl River Delta, and West Guangdong & Hainan Island) over the past six millennia. The land subsidence histories of the six sites are produced by comparing their RSL histories reconstructed from qualified sea-level index points (SLIPs) with those of the Malay Peninsula, based on the assumption that the Malay Peninsula has been tectonically stable. Therefore, the RSL history at each site is considered as a function of eustatic sea-level change, global GIA (e.g. ocean siphoning), local GIA (e.g. coastal levering) and tectonic movement. Therefore, a subtraction of RSL histories between the China sites and the Malay Peninsula will result in land vertical movement trends consisting of both the local GIA and tectonic components. The result shows that the coast of southern China has been undergoing linear land subsidence over the past 6000 years. The subsidence rates of the six sites average at about 1.2±0.1 mm/yr, with the highest rate of 2.1±0.1 mm/yr in the Han River Delta and the lowest rate of 0.5±0.1 mm/yr in West Guangdong & Hainan Island. In order to separate the tectonic subsidence rate from the local GIA rate for each site, outputs of GIA models (a 3D Earth model HetM-LHL140 coupled with ICE-6G_C) for China and the Malay Peninsula were obtained. The result suggests that the local GIA component (mainly coastal levering) might have accounted for half of the land subsidence along the China coast over the past 6000 years. This estimation of long-term land subsidence rates should form an integral part of the hazard assessment for the coastal communities in China.

How to cite: Xiong, H. and Zong, Y.: Millennial scale land subsidence history along the southern China coast, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4509, https://doi.org/10.5194/egusphere-egu2020-4509, 2020.

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.

How to cite: Dörr, N., Schenk, A., de Wit, K., Lexmond, B. R., Minderhoud, P. S. J., Neussner, O., Nguyen, D. K., and Nguyen, T. L.: Recent Subsidence Rates in the Mekong Delta derived from Sentinel-1 SAR-Interferometry, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18912, https://doi.org/10.5194/egusphere-egu2020-18912, 2020.

Land subsidence caused by seasonal fluctuation of groundwater level caused by agricultural groundwater use was numerically simulated in this study. In the study area, Kawajima, Saitama prefecture, Japan, the hydraulic head has been gradually increasing over time with seasonal fluctuations and the subsurface formations have repeated expansion and compaction. However, the land subsidence progressed because the compaction included the plastic deformation. In this study, vertically one-dimensional model to numerically simulate coupled groundwater flow and soil deformation in Kawajima was developed with modified cam-clay model. Because of the lack of subsurface information, it was difficult to set the physical properties such that the simulated subsidence and the observed subsidence are satisfactorily close to each other. This study applied a genetic algorithm in order to search the set of underground physical properties. The improved set of underground physical properties succeeded to reproduce the observed land subsidence in Kawajima.

How to cite: Akitaya, K. and Aichi, M.: One dimensional numerical modeling of land subsidence caused by seasonal groundwater level fluctuations in Kawajima, Japan, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6592, https://doi.org/10.5194/egusphere-egu2020-6592, 2020.

Predicting the future land subsidence caused by groundwater abstraction is necessary for the planning and decision-making of groundwater usage in coastal area. Although numerical modeling is expected to quantitatively predict land subsidence, a single calibrated model cannot provide a reliable prediction because of the uncertainty on properties and conditions in the subsurface. In addition, applying ensemble Kalman filter or ensemble smoother to land subsidence modeling is not straightforward because of the highly nonlinear and hysteric characteristics in clay compaction process.

This study developed a smoother algorithm with a multiple calibration-constrained null-space Monte Carlo method for a numerical simulator of groundwater mass balance with modified Cam-clay model. The developed algorithm calibrates a model ensemble using a newly obtained observed value in each observation step. Based on the calibration-constrained null-space Monte Carlo method, a new model ensemble in the null-space is produced in each observation step. In this step, both the current and past state as well as parameters in the model are updated like ensemble smoother in order to follow the hysteretic behavior in the soil compaction. The produced ensemble can be used not only for prediction uncertainty analysis at that step but also as initial estimates of a multiple calibration-constrained null-space Monte Carlo method in the next observation step.

The proposed method was applied to the land subsidence modeling in the Tokyo lowland area, Japan. The proposed method could make model ensemble with satisfactory good reproducibility and show the range of uncertainty of future prediction for several scenarios of future groundwater level change.

How to cite: Aichi, M.: Land subsidence prediction with uncertainty analysis by a smoother algorithm with a multiple calibration-constrained null-space Monte Carlo method, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16906, https://doi.org/10.5194/egusphere-egu2020-16906, 2020.

Munro Thuruth (Island) is an island group comprising of 8 medium size and a few tiny islands located in the backwaters of the famous Ashtamudi lake in Kerala, South India. The Munro Island with an area of 13.4 sq.km is situated at the confluence of the Ashtamudi Lake and the Kallada river (9^oN Latitude and 76^o 37’ E Longitude). It is an artificial island built during the 18^th Century by reclamation of the Kallada river delta on the downstream side, where it debouches into the Arabian Sea. The individual islands of the Munro group, with an elevation of 3.3m (approx.) above MSL, remained more or less stable till 1965. However, during the last two decades media reports on sinking/subsidence of the individual islands have drawn attention of scientists, politicians, administrators as well as the government.  As per the reports, the subsidence or rise in surrounding water level has been rather alarming and this is quite evident from the perennial inundation observed at certain critical low-lying areas in the island. Speculations on the causative factors responsible for the permanent/alarming rates of inundation witnessed in the island are linked to both local and global changes in the environmental conditions. According to one school of thought it is land subsidence due to tectonic activity combined with sea level rise due to global warming that has contributed to the sinking of the Munroe Island.  But there is another group that advocates that the flooding/inundation reached the critical level after the 26 December, 2004 Tsunami which struck the Kerala coast.  In this study, the various causative factors and their respective roles in the rise in water level/ subsidence reported at various locations is being critically reviewed and the salient conclusions that emanated from the analysis are presented. The analysis reveals that the flooding in the Munro Island can be attributed to a multitude of factors like rise in sea level linked to climate change, wave setup and wind setup which act individually or in combination under favourable conditions;  changes in morphology (post Tsunami) of the Ashtamudi estuary and changes in the inlet geometry over a period of time; land subsidence due to primary and secondary (creep) consolidation as well as the impact of ground vibration due to the movement of high speed trains ; groundwater drawdown due to excessive water extraction and reduction in fresh water flow from the Kallada river; sand mining from the river and reduced sediment inflow to the system because of the Kallada dam construction. The importance of carrying out numerical model studies to understand the tidal dynamics as well as the combined effect of tides, wind and waves on the water surrounding the islands which are located at varying distances of 8-10km from the Ashtamudi tidal inlet is also emphasized.

How to cite: Nair L, S., Krishna P. S., S., Ravindran, P., and I Varghese, T.: A critical examination of the flooding and its impact on the Munro Island in Southwest India, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21525, https://doi.org/10.5194/egusphere-egu2020-21525, 2020.