Coastal regions are dynamic environments that undergo continuous changes influenced by natural processes, anthropogenic activities, and climate variations. Understanding long-term shoreline dynamics is paramount for effective coastal management and sustainable development. This study embarks on a comprehensive analysis of the decadal evolution of the shoreline in south Kerala, a coastal area of significance, using advanced satellite imagery and Digital Shoreline Analysis Systems (DSAS).
The application of remote sensing technologies, particularly satellite imagery, provides a valuable tool for capturing the intricate details of coastal changes over extended periods. This study leverages the rich dataset from reputable sources, such as NASA and ESA, to thoroughly examine the coastline of south Kerala. The temporal range of this analysis spans multiple decades, enabling a nuanced understanding of the complex interactions shaping the coastal morphology.
The shoreline is a constantly changing system, where sediment gets eroded in one area and deposited elsewhere, striving to establish equilibrium with prevailing marine processes. Understanding the location and shifting boundary of the shoreline has become crucial for coastal engineers, scientists, and managers (Douglas and Crowell 2000). Regular examination of the shoreline helps identify natural processes causing changes and assesses the impact of human activities, enabling the development of effective management strategies. Shoreline changes can be classified as erosion or accretion, and this study utilizes remote sensing techniques and data to monitor coastal processes, including shoreline change, with reasonable accuracy, frequent observations, and cost-effectiveness.
Coastal erosion is primarily caused by sea and ocean forces acting on coastal rock particles. Hydraulic action of sea waves, abrasion, attrition, and corrosion are the main erosion processes. Rising sea levels, storm occurrences, global warming, direct wave action, littoral drift interference, sea level changes, river mouth alterations, and sand mining contribute to coastal flooding and erosional issues along the coastline. In the Indian context, erosion is mainly influenced by wave action, reduced sediment supply, tectonic movements, sea level rise, and human intervention.
The littoral processes operating along a coast involve the dynamic interactions between waves, currents, and sediment transport. These processes shape a coastline and influence the deposition and erosion of sediments along the coast. Accordingly, on the south Kerala coast, the coastal currents, wave action, and sediment transport have also played key roles in shaping its morphology over time.
Normally, human activities such as the construction of a port jetty or a groin may impact the coastal dynamics by altering sediment transport, leading to localized accretion and erosion. Consequently, there may be an alteration of the configuration of the coast.
Periodic monitoring of the coast to look for such changes, if any, will help to manage these changes, mitigate potential environmental consequences, and maintain the overall coastal equilibrium. The construction of a port can lead to accretion in certain areas due to changes in sediment transport. Dredging and modifications to coastal morphology can cause sediment deposition, resulting in the build-up of material in specific locations. This localized accretion may have both positive and negative effects, affecting ecosystems and navigation channels. Monitoring and environmental assessments are crucial to understanding and managing these changes. Localized erosion is also a common phenomenon in the vicinity of ports if coastal management measures are neither adopted nor adequate.
In the context of the south Kerala coast, it is pertinent to mention that the coastline is characterized by longshore sediment transport, which is the dominant cause of beach profile changes. Construction of the Vizhinjam Port may have altered the longshore sediment transport process, thus prompting us to take up a study. Thus, the long-term shoreline changes due to the construction of the Vizhinjam port facilities have been studied. In order to understand such an effect of the Port on the long-term shoreline change, a detailed analysis of the 40km stretch (20km on either side of the Port) and a comprehensive study of the above-mentioned factors was carried out and has been presented in this paper. The development of a port involves various activities like dredging, reclamation, and construction of coastal structures such as breakwaters, groins, and jetties. These activities often modify the coastal morphology. The study area, a 40km stretch along the southern Kerala coast, falls within the Thiruvananthapuram district of Kerala. It is bordered by Kollam District to the north, Kanyakumari District to the south, Tamil Nadu, and the Arabian Sea to the west, and is influenced by the Killi River and Neyyar River (Poovar estuary). The region experiences high humidity, reaching about 90% during the monsoon season. The average annual rainfall is 2035mm, with both southwest and northeast monsoons affecting the area. The turbidity falls within 5 NTU, and suspended sediments range from 0 to 10 mg/l. Geomorphologically, the south Kerala coast consists of coastal origin with younger coastal plains and denudational origin with a pediment pediplain complex. Rocky outcrops are found south of Kovalam. The study area falls under the Primary Sediment Cell of Kollam to Kanyakumari-PC10, with sub-cells classified as Kollam to Kovalam – 10a, Kovalam to Muttom – 10b, and Muttom to Kanyakumari – 10c, according to the Sediment Cell classification by NCSCM MoEFCC 2022.
As reported by Indu et al. (2021), over the past 127 years, Kerala has experienced five cyclones, with recent notable ones being Gaja in November 2018 and Ockhi from November to December 2017. Earlier cyclones occurred in November and December 1912 and November 1925. These cyclones primarily impacted southern Kerala and Tamil Nadu. The 1925 cyclone notably passed through north Kerala. Ockhi was the fourth cyclone in the Comorin Sea and the first to hit Kerala in 92 years. Gaja formed near Thailand, travelled across the Indian Peninsula, and reached Kerala's Arabian Sea coast in November 2018. It was the first cyclone since 1990, monitored by the India Meteorological Department, to travel via Kerala to the Bay of Bengal to the Arabian Sea.
Vizhinjam, also known as the 'Port of the Future,' is situated at 8° 2' 1" N and 77° 0' 0" E on the west coast of India (Fig. 1). It lies ten nautical miles off the international east-west shipping route and is well-connected to national and regional road and rail networks. The proposed site has a sea depth of 23 to 27 meters, making it suitable for berthing large container ships. Since India lacks ports with such depth, goods are currently transported on smaller vessels to the country. The Port of Vizhinjam is planned to be 150 meters wide, covering an area of about 250–275 hectares (600 to 700 acres) through reclamation of the sea. Two breakwaters measuring 1.5 km and 6 km will protect it, including a harbour basin and wharves. Also, the coast is protected with a series of coastal interventions such as seawalls and groynes. The 40 Km stretch of the study region has been protected with almost 24 groynes and a total seawall length of 13.155Km (out of which 8.75Km is damaged).
Detection of shoreline change, the rate of positional change, and future prediction play a crucial role in various aspects of coastal zone management, including hazard zoning, island development studies, marine transport, erosion/accretion assessments, sediment budgeting, and conceptual/predictive modelling of coastal morphodynamics (Zuzek et al. 2003; Maiti and Bhattacharya 2009). The integrated approach of remote sensing data and Geographic Information System (GIS) is instrumental in capturing, storing, managing, analyzing, and visualizing spatial or geographical data, making it valuable for shoreline extraction and mapping historical information for long-term analysis. However, this approach is subject to uncertainties about the old data collected from aerial photographs or topographic maps. Nonetheless, predicting coastline change using remote sensing along with GIS is considered one of the most promising methods as it continuously provides perspectives on coastal areas (Siriluk et al. 2012). The Digital Shoreline Analysis System (DSAS) is used in this study to compute shoreline statistics.
Shoreline changes are influenced by factors such as sediment supply and transport, sea-level changes, and interventions on the coastline. Additionally, hydrodynamics of the near-shore environment, river mouth processes, storm surges, and the nature of coastal landforms contribute to frequent changes in the shoreline (Scott 2005; Narayana and Priju 2006; Kumar and Jayappa 2009). Reliable predictions of coastal changes and thorough risk assessments over specified time periods are vital needs (Burgess et al. 2001). The sediment supply to the shoreline can result in three scenarios: a surplus, balance, or deficit in the sediment budget. A significant reduction or increase in sediment supply over a short or extended period can lead to a deficit or surplus in the sediment budget, subsequently causing shoreline change.
This research employs high-resolution satellite images and the Digital Shoreline Analysis System (DSAS) method. Digital Shoreline Analysis Systems (DSAS) form the methodological backbone of this study, offering a robust framework for quantifying shoreline changes. DSAS integrates geospatial techniques and statistical analyses to delineate and assess alterations in coastal land-water interfaces. By employing DSAS in conjunction with high-resolution satellite imagery, this research aims to uncover patterns, trends, and contributing factors influencing the long-term dynamics of the Vizhinjam shoreline.
Numerous studies highlight the pivotal role of satellite imagery in monitoring and analyzing coastal changes. High-resolution satellite data provide a valuable source for capturing spatial and temporal variations in shoreline morphology (Abdelhady et al. 2022).
DSAS has emerged as a robust tool for shoreline change analysis. Ahn (2015) demonstrated the effectiveness of DSAS in quantifying shoreline changes and highlighted its applicability in various coastal settings. The methodology integrates GIS techniques, allowing for precise measurement and analysis. Thieler and Danforth (1994) conducted a comprehensive temporal analysis of shoreline changes using DSAS and satellite imagery. Their study revealed notable patterns in erosion and accretion, emphasizing the significance of long-term observations for capturing cyclical trends. Their findings underscored the variability in coastal responses to environmental changes, necessitating site-specific analyses for effective coastal management.
The study also presents trend analyses of shoreline positions at hotspot locations. Validation of the shoreline analysis from satellite data is compared with beach profile data and field-collected shoreline data. The study utilizes satellite images of different resolutions to examine shoreline changes.