Abstract
According to the Intergovernmental Panel on Climate Change (IPCC), global mean sea levels may rise from 0.43 m to 0.84 m by the end of the 21st century. This poses a significant threat to coastal cities around the world. The shoreline of Karachi (a coastal mega city located in Southern Pakistan) is vulnerable mainly due to anthropogenic activities near the coast. Therefore, the present study investigates rates and susceptibility to shoreline change using a 76-year multi-temporal dataset (1942 to 2018) through the Digital Shoreline Analysis System (DSAS). Historical shoreline positions were extracted from the topographic sheets (1:250,000) of 1942 and 1966, the medium spatial resolution (30 m) multi-sensor Landsat images of 1976, 1990, 2002, 2011, and a high spatial resolution (3 m) Planet Scope image from 2018, along the 100 km coast of Karachi. The shoreline was divided into two zones, namely eastern (25 km) and western (29 km) zones, to track changes in development, movement, and dynamics of the shoreline position. The analysis revealed that 95% of transects drawn for the eastern zone underwent accretion (i.e., land reclamation) with a mean rate of 14 m/year indicating that the eastern zone faced rapid shoreline progression, with the highest rates due to the development of coastal areas for urban settlement. Similarly, 74% of transects drawn for the western zone experienced erosion (i.e., land loss) with a mean rate of −1.15 m/year indicating the weathering and erosion of rocky and sandy beaches by marine erosion. Among the 25 km length of the eastern zone, 94% (23.5 km) of the shoreline was found to be highly vulnerable, while the western zone showed much more stable conditions due to anthropogenic inactivity. Seasonal hydrodynamic analysis revealed approximately a 3% increase in the average wave height during the summer monsoon season and a 1% increase for the winter monsoon season during the post-land reclamation era. Coastal protection and management along the Sindh coastal zone should be adopted to defend against natural wave erosion and the government must take measures to stop illegal sea encroachments.
Highlights
A shoreline is defined as the boundary between land and water
The mean accretional End Point Rate (EPR) for the eastern zone was observed to be 14 m/year, which indicated that 95% of the total transects of the eastern zone encountered accretion
The pre- and post-land reclamation hydrodynamics revealed that an average wave height increased by 2.95% during the south-west summer monsoon season and 0.97% during the NW winter monsoon season, indicating potential future threats to the shoreline position
Summary
A shoreline is defined as the boundary between land and water. The position of the shoreline is dynamic both spatially and temporally, due to hydrological, geological, climatic, and economic developments in coastal areas [1,2]. Short-term geomorphological changes in the shoreline are caused by extreme geological, climatic, and oceanic events (i.e., earthquakes, tsunami, seasonal variation in waves, tides, and storms conditions), such changes are less predictable, while long-term changes to the shoreline are caused by relative changes in astronomical, meteorological, and regional climatic variations (i.e., tides, waves, sea-level rise, and storm surges), and are somewhat predictable [6,19]. Both types of shoreline change are important for understanding trends in coastal sustainability for different times and spaces [3]
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