Vertical Ground Displacements and Its Impact on Erosion along the Karachi Coastline, Pakistan

Abstract

This study employed remote sensing (optical and synthetic aperture radar) and data analysis techniques to quantify vertical ground displacements and assess their contribution to coastline erosion. To provide evidence from Pakistan, we selected the coast of Karachi—a mega-city located along the dynamic coastline of the Indus River Delta—which has been experiencing severe coastal erosion during the last few decades. Observations from the C-band Envisat/ASAR and Sentinel-1A sensors over the 2004–2010 and 2014–2016 periods, respectively, enabled us to study vertical ground displacements in the study area, providing a long-term assessment during 2004–2016. Results suggest that some areas along the Karachi coastline are subsiding at comparable rates to or even much higher than the relative sea-level rise (SLR, ~1.9 mm/yr), which may amplify the rates of relative SLR in coming years, along with accelerating coastal erosion. Various parts of the study area along the coast are unstable and undergoing displacement. Landsat images from 1989 to 2018 (10-year temporal resolution) were further used to examine the state of coastline erosion using three statistical approaches (i.e., End Point Rate (EPR), Linear Regression Rate (LRR), and Least Median of Squares (LMS)). While the erosion underlaid the majority of the eastern sections of the study area, the ground displacements were spatially heterogeneous across the study area and along the coastline. Erosion rates of ~2.4 m/yr spatially corresponded with ground displacement rates of up to ~−1.4 cm/yr, but not all the coastline segments with high annual mean erosion rates were associated with local mean subsidence. The causes of ground displacements and coastline erosion were analyzed, and results were interpreted by integrating spatial ancillary information. Results indicate that rapid urbanization, construction on reclaimed land, coastline erosion favoring seawater intrusion, failed drainage/sewerage networks, and soil liquefaction are contributing to the site-specific variations in the land displacement in Karachi.

Non-Linear Ground Deformation Detection and Monitoring using Time-Series InSAR along the Coastal Urban Areas of Pakistan

“…Flash ooding in the Malir Riverbed is another potential cause that could result in ground displacements. This could increase the soil's moisture content and weaken the bonds that hold the soil particles together, which would result in ground subsidence (Kanwal, Ding et al 2022).…”

“…11 and 7). According to previous study works (Amin, Shahzad et al 2022, Kanwal, Ding et al 2022, Tirmizi, Khan et al 2023, this is especially concerning given the substantial high-rise building and housing community construction on the reclaimed ground, and continued unchecked expansion could have disastrous outcomes. Soil liquefaction and construction load could be one of the main triggers for deformation in the study area.…”

Evaluation of predicted loss of different land use and land cover (LULC) due to coastal erosion in Bangladesh

“…In this study, the LRR rates were used to estimate shoreline change rates. This method is the most used method for coastline change analysis (Kanwal et al, 2022). The LRR rate is calculated by fitting a least-squares regression line to all shoreline points for along digital transects that are cast orthogonal from an offshore or onshore baseline to the mapped shorelines.…”

Land subsidence analysis using synthetic aperture radar data

“…The salinization of soils, ground, and surface water is caused by sea-level intrusion due to rising sea levels. The sea-level rise is also responsible for depleting the coastlines of Pakistan, with up to −2.43 m/year regression in the East and 8.34 m/year regression in the South [ 38 ]. Tide gauges are located at Gwadar, Ormara, Karachi, and Creek sites.…”

“…InSAR processes have been the most integrated and functional tool in measuring the displacement of the earth’s surface on a millimeter scale. It is used worldwide and has widespread utilization in measuring displacement related to natural hazards, for example, earthquake, volcanic activity, tsunami, and landslides [ 38 , 41 , 42 ]. The process involves a high density of measuring points over remote, unapproachable, and risky areas.…”