Rates Of Land Subsidence Research Articles

The Influence of Sediment Isostatic Adjustment on Sea Level Change and Land Motion Along the U.S. Gulf Coast

AbstractSea level rise presents a hazard for coastal populations, and the Mississippi Delta (MD) is a region particularly at risk due to the high rates of land subsidence. We apply a gravitationally self‐consistent model of glacial and sediment isostatic adjustment (SIA) along with a realistic sediment load reconstruction in this region for the first time to determine isostatic contributions to relative sea level (RSL) and land motion. We determine optimal model parameters (Earth rheology and ice history) using a new high‐quality compaction‐free sea level indicator database. Using the optimal model parameters, we show that SIA can lower predicted RSL in the MD area by several meters over the Holocene and so should be taken into account when modeling these data. We compare modeled contemporary rates of vertical land motion with those inferred using GPS. This comparison indicates that isostatic processes can explain the majority of the observed vertical land motion north of latitude 30.7°N, where subsidence rates average about 1 mm/yr; however, subsidence south of this latitude shows large data‐model discrepancies of greater than 3 mm/yr, indicating the importance of nonisostatic processes. This discrepancy extends to contemporary RSL change, where we find that the SIA contribution in the Delta is on the order of 10−1 mm/yr. We provide estimates of the isostatic contributions to 20th and 21st century sea level rates at Gulf Coast Permanent Service for Mean Sea Level tide gauge locations as well as vertical and horizontal land motion at GPS station locations near the MD.

Minimizing the Residual Topography Effect on Interferograms to Improve DInSAR Results: Estimating Land Subsidence in Port-Said City, Egypt

The accurate detection of land subsidence rates in urban areas is important to identify damage-prone areas and provide decision-makers with useful information. Meanwhile, no precise measurements of land subsidence have been undertaken within the coastal Port-Said City in Egypt to evaluate its hazard in relationship to sea-level rise. In order to address this shortcoming, this work introduces and evaluates a methodology that substantially improves small subsidence rate estimations in an urban setting. Eight ALOS/PALSAR-1 scenes were used to estimate the land subsidence rates in Port-Said City, using the Small BAse line Subset (SBAS) DInSAR technique. A stereo pair of ALOS/PRISM was used to generate an accurate DEM to minimize the residual topography effect on the generated interferograms. A total of 347 well distributed ground control points (GCP) were collected in Port-Said City using the leveling instrument to calibrate the generated DEM. Moreover, the eight PALSAR scenes were co-registered using 50 well-distributed GCPs and used to generate 22 interferogram pairs. These PALSAR interferograms were subsequently filtered and used together with the coherence data to calculate the phase unwrapping. The phase-unwrapped interferogram-pairs were then evaluated to discard four interferograms that were affected by phase jumps and phase ramps. Results confirmed that using an accurate DEM (ALOS/PRISM) was essential for accurately detecting small deformations. The vertical displacement rate during the investigated period (2007–2010) was estimated to be −28 mm. The results further indicate that the northern area of Port-Said City has been subjected to higher land subsidence rates compared to the southern area. Such land subsidence rates might induce significant environmental changes with respect to sea-level rise.

Assessing the Impact of Building Volume on Land Subsidence in the Central Business District of Beijing with SAR Tomography

ABSTRACTHigh-rise buildings in the modern urban setting may have a considerable impact on land subsidence. The process of urbanization in Beijing has been found to be in direct relationship with the land subsidence. The permanent scatterers interferometric aperture radar (PSInSAR) technology is becoming a conventional tool for land subsidence monitoring. Synthetic aperture radar (SAR) tomography, as an extension of PSInSAR technology, allows a full 3D imaging, and is useful to reconstruct 3D urban building maps. In this study, a SAR tomography method “scale-down by L1 norm minimization, model selection, and estimation reconstruction” (SL1MMER) was applied to 33 TerraSAR-X images to retrieve building height. The object-oriented classification method based on the support vector machine was applied on the World View 2 image to extract geometry of building footprints and to aggregate SL1MMER derived building heights for each building. The building footprints and building heights were used to calculate the volume of buildings. The Stanford method for persistent scatterers, a PSInSAR methodology, was used to obtain land subsidence rates. The correlation coefficient between the building volumes and land subsidence showed no positive relationship for buildings that were below a certain volume. However, at local scale, with the increase of the building volumes, the correlation increased. The larger the volume of buildings, the stronger the relationship became. Once the volume of buildings became larger than 3.00 × 105 m3, the impact of building volume on land subsidence remained stable.

Effects of anthropogenic land-subsidence on inundation dynamics: the case study of Ravenna, Italy

Abstract. Can differential land-subsidence significantly alter river flooding dynamics, and thus flood risk in flood prone areas? Many studies show how the lowering of the coastal areas is closely related to an increase in the flood-hazard due to more important tidal flooding and see level rise. The literature on the relationship between differential land-subsidence and possible alterations to riverine flood-hazard of inland areas is still sparse, although several geographical areas characterized by significant land-subsidence rates during the last 50 years experienced intensification in both inundation magnitude and frequency. We investigate the possible impact of a significant differential ground lowering on flood hazard over a 77 km2 area around the city of Ravenna, in Italy. The rate of land-subsidence in the study area, naturally in the order of a few mm year−1, dramatically increased up to 110 mm year−1 after World War II, primarily due to groundwater pumping and gas production platforms. The result was a cumulative drop that locally exceeds 1.5 m. Using a recent digital elevation model (res. 5 m) and literature data on land-subsidence, we constructed a ground elevation model over the study area in 1897 and we characterized either the current and the historical DEM with or without road embankments and land-reclamation channels in their current configuration. We then considered these four different topographic models and a two-dimensional hydrodynamic model to simulate and compare the inundation dynamics associated with a levee failure scenario along embankment system of the river Montone, which flows eastward in the southern portion of the study area. For each topographic model, we quantified the flood hazard in terms of maximum water depth (h) and we compared the actual effects on flood-hazard dynamics of differential land-subsidence relative to those associated with other man-made topographic alterations, which resulted to be much more significant.

Land subsidence in major cities of Central Mexico: Interpreting InSAR-derived land subsidence mapping with hydrogeological data

Significant structural damages to urban infrastructures caused by compaction of over-exploited aquifers are an important problem in Central Mexico. While the case of Mexico City has been well-documented, insight into land subsidence problems in other cities of Central Mexico is still limited. Among the cities concerned, we present and discuss the cases of five of them, located within the Trans-Mexican Volcanic Belt (TMVB): Toluca, Celaya, Aguascalientes, Morelia, and Queretaro. Applying the SBAS-InSAR method to C-Band RADARSAT-2 data, five high resolution ground motion time-series were produced to monitor the spatio-temporal variations of displacements and fracturing from 2012 to 2014. The study presents recent changes of land subsidence rates along with concordant geological and water data. It aims to provide suggestions to mitigate future damages to infrastructure and to assist in groundwater resources management.Aguascalientes, Celaya, Morelia and Queretaro (respectively in order of decreasing subsidence rates) are typical cases of fault-limited land subsidence of Central Mexico. It occurs as a result of groundwater over-exploitation in lacustrine and alluvial deposits covering highly variable bedrock topography, typical of horst-graben geological settings. Aguascalientes and Toluca show high rates of land subsidence (up to 10cm/yr), while Celaya and Morelia show lower rates (from 2 to 5cm/yr). Comparing these results with previous studies, it is inferred that the spatial patterns of land subsidence have changed in the city of Toluca. This change appears to be mainly controlled by the spatial heterogeneity of compressible sediments since no noticeable change occurred in groundwater extraction and related drawdown rates. While land subsidence of up to 8cm/yr has been reported in the Queretaro Valley before 2011, rates inferior to 1cm/yr are measured in 2013–2014. The subsidence has been almost entirely mitigated by major changes in the water management practices of the city, i.e., a 122km long pipeline bringing surface water from an adjacent state allowed to cease pumping in half of the wells.

Evaluating Land Subsidence Rates and Their Implications for Land Loss in the Lower Mississippi River Basin

High subsidence rates, along with eustatic sea-level change, sediment accumulation and shoreline erosion have led to widespread land loss and the deterioration of ecosystem health around the Lower Mississippi River Basin (LMRB). A proper evaluation of the spatial pattern of subsidence rates in the LMRB is the key to understanding the mechanisms of the submergence, estimating its potential impacts on land loss and the long-term sustainability of the region. Based on the subsidence rate data derived from benchmark surveys from 1922 to 1995, this paper constructed a subsidence rate surface for the region through the empirical Bayesian kriging (EBK) interpolation method. The results show that the subsidence rates in the region ranged from 1.7 to 29 mm/year, with an average rate of 9.4 mm/year. Subsidence rates increased from north to south as the outcome of both regional geophysical conditions and anthropogenic activities. Four areas of high subsidence rates were found, and they are located in Orleans, Jefferson, Terrebonne and Plaquemines parishes. A projection of future landscape loss using the interpolated subsidence rates reveals that areas below zero elevation in the LMRB will increase from 3.86% in 2004 to 19.79% in 2030 and 30.88% in 2050. This translates to a growing increase of areas that are vulnerable to land loss from 44.3 km2/year to 240.7 km2/year from 2011 to 2050. Under the same scenario, Lafourche, Plaquemines and Terrebonne parishes will experience serious loss of wetlands, whereas Orleans and Jefferson parishes will lose significant developed land, and Lafourche parish will endure severe loss of agriculture land.

Modelling the impact of land subsidence on urban pluvial flooding: A case study of downtown Shanghai, China

This paper presents a numerical analysis of pluvial flooding to evaluate the impact of land subsidence on flood risks in urban contexts using a hydraulic model (FloodMap-HydroInundation2D). The pluvial flood event of August 2011 in Shanghai, China is used for model calibration and simulation. Evolving patterns of inundation (area and depth) are assessed over four time periods (1991, 1996, 2001 and 2011) for the downtown area, given local changes in topography and rates of land subsidence of up to 27mm/yr. The results show that land subsidence can lead to non-linear response of flood characteristics. However, the impact on flood depths is generally minor (<5cm) and limited to areas with lowest-lying topographies because of relatively uniform patterns of subsidence and micro-topographic variations at the local scale. Nonetheless, the modelling approach tested here may be applied to other cities where there are more marked rates of subsidence and/or greater heterogeneity in the depressed urban surface. In these cases, any identified hot-spots of subsidence and focusing of pluvial flooding may be targeted for adaptation interventions.

Assessing the potential of the multi-aquifer subsurface of the Mekong Delta (Vietnam) for land subsidence due to groundwater extraction

Abstract. Land subsidence rates of ~ 1–4 cm yr−1 are measured in the low-lying Vietnamese Mekong Delta. These relatively high subsidence rates are attributed to groundwater extraction, which has increased drastically over the past decades due to growing domestic, agricultural and industrial demands. As a result, hydraulic heads in aquifers are dropping, on average 0.3–0.7 m yr−1. There is an urgent need to go from measurements to predictions in order to test possible future groundwater management scenarios and to reduce the increase of flood risk, salt water intrusion and, on the longer term, prevent the delta from drowning. In this study, we aim to assess the subsidence potential of the multi-aquifer subsurface of the Mekong delta due to groundwater extraction. The first step is to gain a thorough understanding of the complex sedimentary architecture of the heterogeneous subsurface. Combined with the related geotechnical properties, the subsurface build-up determines the subsidence potential. Here, we present our approach to develop a 3-D geo-hydrological model based on lithological borehole data, geophysical sedimentary properties, palaeogeography and conceptual models of delta evolution.

Land subsidence and ground fissures in Xi'an, China 2005–2012 revealed by multi-band InSAR time-series analysis

Xi'an, China has been undergoing significant land subsidence along with ground fissure development. These geohazards have brought about severe damages to buildings, bridges and other facilities. In order to warn of and mitigate disasters, it is urgently necessary to obtain the latest rate, extent, and temporal evolution of land subsidence in Xi'an. With multiple SAR datasets as well as leveling, GPS and ground water level of aquifers, we study and map the spatial and temporal evolution of land subsidence and ground fissures over Xi'an. First, 62 synthetic aperture radar (SAR) images acquired by Envisat, ALOS, and TerrSAR-X during 2005–2012 are used to form three independent interferometric stacks to unveil the spatial and temporal variations of land subsidence and ground fissures by using the time-series interferometric synthetic aperture radar (InSAR) technique. GPS and leveling measurements are applied to calibrate the InSAR results. Precision of our InSAR annual subsidence results is less than 9mm. We derive the east–west and vertical components of the observed land deformation in 2009 using descending and ascending InSAR observations, finding out that the horizontal component of land deformation cannot be ignored if the deformation is large or ground fissures are active. Second, four main land subsidence zones are detected in Xi'an, with an average subsidence rate of 50mm/a during 2005–2012. Time-series InSAR results indicate that land subsidence rates in Xi'an increased by 200% from 2005–2007 to 2008–2010, extending existing ground fissures. Third, InSAR-derived land subsidence correlates with the change in ground water level, and seasonal variations in subsidence correlate with changes in ground water pumping. Last, the consistency on the spatial–temporal distribution variation between ground fissures and land subsidence could be drawn from the time series results and profile analysis. Shapes of subsiding zones follow the general trends of mapped ground fissures and main faults in an ENE direction. Changes in the subsidence gradient are also observed over most of the ground fissures and faults. Subsidence-triggered fissures can cause localized surface displacements, aggravate localized subsidence, discontinue the integrity of ground water flow, and limit the horizontal spread of subsidence funnels. With continuing mass construction projects in Xi'an, monitoring ground deformation with satellite InSAR therefore can provide efficient means to image land subsidence, movements of fissures and faults, and the associated geohazards.

Bangladesh’s dynamic coastal regions and sea-level rise

The physical geography of Bangladesh’s coastal area is more diverse and dynamic than is generally recognised. Failure to recognise this has led to serious misconceptions about the potential impacts of a rising sea-level on Bangladesh with global warming. This situation has been aggravated by accounts giving incorrect information on current rates of coastal erosion and land subsidence. This paper describes physical conditions within individual physiographic regions in Bangladesh’s coastal area based on ground-surveyed information, and it reviews possible area-specific mitigation measures to counter predicted rates of sea-level rise in the 21st century. Two important conclusions are drawn: the adoption of appropriate measures based on knowledge of the physical geography of potentially-affected areas could significantly reduce the currently-predicted displacement of many millions of people; and the impacts of a slowly-rising sea-level are currently much less than those generated by rapidly increasing population pressure on Bangladesh’s available land and water resources and by exposure to existing environmental hazards, and the latter problems need priority attention.

Is land subsidence increasing the exposure to sea level rise in Alexandria, Egypt?

AbstractDelta margins are subject to relatively high rates of land subsidence and have the potential to significantly exacerbate future changes in sea levels predicted by global warming models used in impact studies. Through a combined analysis of GPS and persistent scatterer interferometry data, we determine that most of the coastline of Alexandria has been subject to moderate land subsidence over the past decade (0.4 mm/yr on average and up to 2 mm/yr locally). This contrasts to previous studies that suggested subsidence in excess of 3 mm/yr. Based on our findings, we infer that on multi‐century to millennia timescales, land subsidence in the area of Alexandria is dominated by tectonic setting and earthquakes or gravitational collapse episodes of a growth fault, whereas on shorter interseismic decadal to century timescales, subsidence rates are likely steady and moderate, in agreement with natural compaction and dewatering of the observed Holocene sediment layer.

Fast water infiltration: a mechanism for fracture formation during land subsidence

A mechanism for fracture generation and for triggering land subsidence is presented. Infiltration through a pre-existing fracture zone into a two-layered system, as well as the deformation of unconsolidated sediments on the land surface, was numerically investigated. The numerical simulation of infiltration is based on a two-phase flow-model concept for porous media, and for the deformation, it is based on a Mohr-Coulomb model concept. Different studies with variations of the fracture parameter and infiltration conditions have been carried out. The infiltration results show that fast infiltration in a partially saturated aquifer leads to land subsidence, extension of pre-existing fractured zones and the generation of new cracks. If the water column is only on the fracture, the clay layer acts like a barrier and inhibits the infiltration through the fracture. If the water column covers the entire surface, the barrier effect is overcome; the infiltration intensity depends on the height of the water column, the fracture permeability and the fracture width. The deformation results show that a strong rainfall event of 2 h leads to deformations that are about 30 % of the vertical and 70 % of the horizontal annual land-subsidence rates.

Enhanced algorithm based on persistent scatterer interferometry for the estimation of high-rate land subsidence

Persistent scatterer interferometry (PSI) techniques using amplitude analysis and considering a temporal deformation model for PS pixel selection are unable to identify PS pixels in rural areas lacking human-made structures. In contrast, high rates of land subsidence lead to significant phase-unwrapping errors in a recently developed PSI algorithm (StaMPS) that applies phase stability and amplitude analysis to select the PS pixels in rural areas. The objective of this paper is to present an enhanced algorithm based on PSI to estimate the deformation rate in rural areas undergoing high and nearly constant rates of deformation. The proposed approach integrates the strengths of all of the existing PSI algorithms in PS pixel selection and phase unwrapping. PS pixels are first selected based on the amplitude information and phase-stability estimation as performed in StaMPS. The phase-unwrapping step, including the deformation rate and phase-ambiguity estimation, is then performed using least-squares ambiguity decorrelation adjustment (LAMBDA). The atmospheric phase screen (APS) and nonlinear deformation contribution to the phase are estimated by applying a high-pass temporal filter to the residuals derived from the LAMBDA method. The final deformation rate and the ambiguity parameter are re-estimated after subtracting the APS and the nonlinear deformation from that of the initial phase. The proposed method is applied to 22 ENVISAT ASAR images of southwestern Tehran basin captured between 2003 and 2008. A quantitative comparison with the results obtained with leveling and GPS measurements demonstrates the significant improvement of the PSI technique.

On causes and impacts of land subsidence in Bandung Basin, Indonesia

The Bandung Basin is a large intra-montane basin surrounded by volcanic highlands, in western Java, Indonesia, inhabited by more than seven million people. The basin, an area of about 2,300 km2, is a highland plateau at approximately 650–700 m above sea level and is surrounded by up to 2,400 m high Late Tertiary and Quaternary volcanic terrain. Based on the results of nine GPS surveys conducted since 2000 up to 2011, it was shown that several locations in the Bandung Basin have experienced land subsidence, with an average rate of about −8 cm/year and can go up to about −23 cm/year in certain locations. A hypothesis has been proposed by several studies that land subsidence observed in several locations in the Bandung Basin has been caused mainly by excessive groundwater extraction. It is found that there is a strong correlation between the rates of groundwater level lowering with the GPS-derived rates of land subsidence in several locations in Bandung Basin. The GPS results in this study detected significant subsidence in the textile industry area, where very large volumes of groundwater are usually extracted. The impact of land subsidence in Bandung can be seen in several forms, mainly in the cracking and damage of houses, buildings and infrastructure. Land subsidence also aggravates the flooding in Bandung Basin, which has brought huge economic losses and deteriorated the quality of life and environment in the affected areas.

Subsidence and carbon loss in drained tropical peatlands

Abstract. Conversion of tropical peatlands to agriculture leads to a release of carbon from previously stable, long-term storage, resulting in land subsidence that can be a surrogate measure of CO2 emissions to the atmosphere. We present an analysis of recent large-scale subsidence monitoring studies in Acacia and oil palm plantations on peatland in SE Asia, and compare the findings with previous studies. Subsidence in the first 5 yr after drainage was found to be 142 cm, of which 75 cm occurred in the first year. After 5 yr, the subsidence rate in both plantation types, at average water table depths of 0.7 m, remained constant at around 5 cm yr−1. The results confirm that primary consolidation contributed substantially to total subsidence only in the first year after drainage, that secondary consolidation was negligible, and that the amount of compaction was also much reduced within 5 yr. Over 5 yr after drainage, 75 % of cumulative subsidence was caused by peat oxidation, and after 18 yr this was 92 %. The average rate of carbon loss over the first 5 yr was 178 t CO2eq ha−1 yr−1, which reduced to 73 t CO2eq ha−1 yr−1 over subsequent years, potentially resulting in an average loss of 100 t CO2eq ha−1 yr−1 over 25 yr. Part of the observed range in subsidence and carbon loss values is explained by differences in water table depth, but vegetation cover and other factors such as addition of fertilizers also influence peat oxidation. A relationship with groundwater table depth shows that subsidence and carbon loss are still considerable even at the highest water levels theoretically possible in plantations. This implies that improved plantation water management will reduce these impacts by 20 % at most, relative to current conditions, and that high rates of carbon loss and land subsidence are inevitable consequences of conversion of forested tropical peatlands to other land uses.

Use of local tidal records to identify relative sea level change: accuracy and error for decision makers

The Intergovernmental Panel on Climate Change (IPCC) considers eustatic sea level rise to be a major impact driven by climate change. Relative sea level change, whether positive or negative, will affect industries, communities and ecology along the world’s coastlines and estuaries. Estimates of global eustatic sea level rise between 1961 and 2003 are 1.8 ± 0.5 mm a−1, reflecting results from validated global tide gauge records. Over the last two decades, several studies have used automatic tide gauge records with at least 80 years of data to generate global prediction models. The IPCC recognises that global change is not uniform, therefore local policy for flood management and coastal protection should rely on local change models that incorporate glacio-isostatic adjustment (GIA) and apply accurate data correction techniques. Some of the longest tidal records are held within the Northern Hemisphere, e.g. Cascais, Amsterdam, Aberdeen, Sheerness and Newlyn. The UK provides several important case studies highlighting changes in relative sea level between the north and the south, primarily due to variations in GIA rates of land uplift and subsidence. Tide gauge records are held by a variety of governmental, non-governmental and private organisations. However, each source may typically compile data in different ways, relying on diverse equipment and recording techniques, often with variations in frequency, length, quality and corrections applied. Even within a single organisation there may be differences in dataset quality. This paper examines some of the key sources of error when working with historical tidal datasets in local geographic areas and aims to identify the limitations of locally derived data thereby assisting in the determination of relative sea level trends that are of widespread value to infrastructure and policy makers.

Using Holocene relative sea-level data to inform future sea-level predictions: An example from southwest England

Holocene relative sea-level data contain information on vertical land movements along coasts and, hence, can provide vital input for predictions of future sea-level change. At Thurlestone, in southwest England, late Holocene coastal sediments were cored and sampled in coastal back-barrier marshes. The presence of a basal sedimentary unit containing salt-marsh microfossils made it possible to obtain precise estimates of late Holocene relative sea-level change from the sediments. This is important because previous studies have suggested that the southwest of England is experiencing the fastest rates of land subsidence in the British Isles. Ten new late Holocene basal sea-level index points fill an important gap in the palaeosea-level data set for southwest England. Another 15 early and middle Holocene sea-level index points are available from previous work. The data show that relative sea level rose by about 10 m between 9000 and 7000 cal. yr BP and a further 8 m in the last 7000 yr. In the last 2000 yr, relative sea level rose on average by 0.9 mm/yr. The coast is currently subsiding by 1.1 mm/yr due to ongoing glacial isostatic adjustment (GIA). The Bradley et al. (2009) GIA model, which is used in the United Kingdom to determine land-motion rates for input into future sea-level predictions, underestimates the rate of coastal subsidence by about 0.16 mm/yr, but performs better than other models. Our data validate the land-motion rates currently used in regional sea-level projections.

Drought-driven transient aquifer compaction imaged using multitemporal satellite radar interferometry

In unconsolidated, heterogeneous aquifer systems, low rates of pore-pressure diffusion of applied effective stresses due to the drainage of thick, low-permeability, clay-rich layers with time constants of decades to centuries cause delayed, residual permanent compaction and land subsidence. Current application of satellite differential radar interferometry (DInSAR—differential interferometric synthetic aperture radar) to estimate aquifer hydraulic properties (compressibility and/or storage) in these systems is limited by the temporal availability of synthetic aperture radar data (1992–present). In this paper we study the degree of aquifer compaction due to water extraction using DInSAR through an example in southeast Spain. Ground deformation data indicate large-scale deformation and in particular the discovery of the highest rates of groundwater-related land subsidence recorded in Europe (>10 cm/yr), affecting the Guadalentin River basin (>200 km 2 ), the largest tributary of the Segura River. Modeling of the ground surface time series of the Guadalentin Basin indicates that deformation is mainly driven by nonlinear time-delayed flow processes in the underlying aquifer. After a drought period (1990–1995), the aquifer responded with an exponential decay of the land subsidence (lasting ∼8 yr), suggesting transient groundwater pore-pressure flow. We show that multitemporal satellite radar interferometry analysis and its modeling can be a stimulating way to study nonlinear soil mechanics and groundwater flows at aquifers. A deeper understanding of such processes could help the management of water resources and land subsidence of unconsolidated coastal and Quaternary alluvial aquifers in a highly evolving climate region (the Mediterranean Sea and elsewhere).

Alexandria-Nile Delta coast, Egypt: update and future projection of relative sea-level rise

Previous studies have indicated that the Nile River deltaic plain is vulnerable to a number of aspects, including beach erosion, inundation, and relatively high rates of land subsidence. This issue motivates an update and analysis of new tide-gauge records, from which relative sea-level changes can be obtained. Estimated rates from five tide gauges are variable in terms of magnitude and temporal trend of rising sea level. Analysis of historical records obtained from tide gauges at Alexandria, Rosetta, Burullus, Damietta, and Port Said show a continuous rise in mean sea level fluctuating between 1.8 and 4.9 mm/year; the smaller rate occurs at the Alexandria harbor, while the higher one at the Rosetta promontory. These uneven spatial and temporal trends of the estimated relative sea-level rise (RSLR) are interpreted with reference to local geological factors. In particular, Holocene sediment thickness, subsidence rate and tectonism are correlated with the estimated rates of relative sea-level change. From the relatively weak correlation between them, we presume that tectonic setting and earthquakes, both recent and historical ones, contribute more to accelerated RSLR than that of dewatering and compression/dewatering of Holocene mud underlying the Nile Delta plain. As a result, large areas of the coastal plain have been subsided, but some sectors have been uplifted in response to tectonic activities of thick underlying older strata. Projection of averaged sea-level rise trend reveals that not all the coastal plain of the Nile Delta and Alexandria is vulnerable to accelerated sea-level rise at the same level due to wide variability of the land topography, that includes low-lying areas, high-elevated coastal ridges and sand dunes, accretionary beaches, and artificially protective structures. Interaction of all aspects (tectonic regime, topography, geomorphology, erosion rate, and RSLR rate) permitted to define risk areas much vulnerable to impacts of sea incursion due to accelerated sea-level rise.

Detection of land subsidence in Semarang, Indonesia, using stable points network (SPN) technique

Land subsidence in Semarang is a severe hazard threatening people and urban infrastructure. The evolution of land subsidence has been monitored between 1996 and 2000 with few leveling stations. Hence, maps show an overall distribution of land subsidence, but are inaccurate in detail. Stable points network (SPN) technique has been applied to improve land-subsidence mapping. SPN measures ground motion using Synthetic Aperture Radar satellite images. For the SPN processing, 35 radar satellite images acquired between 2002 and 2006 have been selected. Derived land-subsidence rates vary from fractions of 1 mm/year to values of 10 cm/year and even beyond. Classification and visualization of the ground motion data illustrate the boundary between mainly stable ground in the south of Semarang and increasingly land-subsidence-affected land toward the coast. Final result is a map featuring the land subsidence with much higher accuracy as had been possible before. This article presents a brief description of the land subsidence in Semarang and introduces briefly the concept of SPN technique. Results of SPN-based ground motion detection in Semarang are presented and discussed from a user’s point of view.