**1. Introduction**

The Sundarban area of the South Asian Region is located between 21o N to 26o 30'N and 88 o E to 92o 30′E in the Bay of Bengal. It consists of a cluster of low-lying islands covering an area of ~10,000 km2 that were formed during the last 11,000 years. It is the second-largest river delta built by the Ganga-Bramhaputra river system driven by southwest monsoon rains. The rivers carry large amounts of sediment loads, ~10 9 t/ year, from the Himalayas and upper parts of the Bengal delta. The mangrove forest of the Sundarbans is shared between India (38%) and Bangladesh (62%). The Royal Bengal Tiger, Ganga River Dolphin, and certain endangered species, such the River Terrapin, may all be found in the Subarnarekha Mangrove woods, which are rich in biodiversity. In both nations, the vital ecosystem is protected and listed as a UNESCO World Heritage Site.

The mangrove ecosystem holds immense ecological and economic importance. It is home to numerous organisms that possess substantial ecological and economic values. These ecosystems play a crucial role in supporting both terrestrial and aquatic food chains, thereby sustaining a diverse range of plant and animal species. One of the notable contributions of mangrove ecosystems is their ability to serve as natural barriers, protecting shoreline and island areas from various natural hazards such as cyclones, hurricanes, and tsunamis. By absorbing and dissipating the force of waves, they effectively mitigate coastal erosion. Moreover, mangroves act as biological filters, helping to maintain water quality by trapping sediment and nutrients, thus purifying polluted coastal waters.

Furthermore, mangroves play a vital role in maintaining the carbon balance in coastal areas. They sequester and store large amounts of carbon dioxide, contributing to climate change mitigation efforts. Additionally, these ecosystems hold significance for tourism and recreation purposes, attracting visitors who appreciate their unique beauty and ecological value [1]. Overall, the preservation and conservation of mangrove ecosystems have far-reaching ecological, economic, and societal benefits.

Mangroves demonstrate remarkable ecological stability in terms of persistence and resilience. However, they are highly sensitive to changes in hydrology. Therefore, it is essential to prioritize the protection and restoration of mangrove ecosystems. Since gaining independence, a significant number of homeless individuals have migrated and settled in the reclaimed Indian Sundarban region. Despite various efforts to safeguard mangrove resources, they face substantial anthropogenic pressure resulting from unsustainable exploitation for multiple purposes, such as wood harvesting, fodder collection, fuel extraction, and charcoal production [2]. Additionally, the conversion of forested areas into aquaculture and agricultural lands, as well as the construction of jetties and harbors to meet the demands of the growing population, further exacerbate the challenges faced by mangroves [2–4].

On an average, 5–6 cyclones annually hit the Sundarban area. Of these cyclones, two are of severe category. Among these, Amphan in 2020 had the highest impact with an estimated loss of 128 lives and > USD 13 million in damages. The diversity and extent of the Sudarban are constantly declining due to anthropogenic and natural causes. Regular monitoring of extent and quality is necessary for this area to control or even regain the loss if given constant effort for a longer period of time. Land dynamics of delta coastline are controlled by three major factors.


Studies suggest the subsidence rate of the Bengal delta area is in the range of 15–50 mm annually. The exploration of oil and gas from delta, trapping of sediments due construction of reservoirs upstream, and other anthropogenic activities are considered the main cause of the subsidence of Bengal Delta. Also, the estimated rise of sea level of Bay of Bengal is at >10 mm/year which is among the world's highest (Based on global sea level data and modeling, Eriscson et al. [5] are noticeable this coastline.

The objective of this study is to observe the change of shoreline in Bay of Bengal through time and its impact on the extent of mangrove forest cover area. The rate of shoreline erosion or accretion and movement of coastline are calculated using the series of Landsat images available from 1975 to 2022. Conducting

### *A Study of Morphological Changes in the Coastal Areas and Offshore Islands of Sudarban… DOI: http://dx.doi.org/10.5772/intechopen.112243*

field surveys in the swampy mangrove forest can be extremely challenging due to their inaccessibility (Nandy et al. [6]). In such circumstances, remote sensing techniques have emerged as increasingly valuable tools for mapping and monitoring mangroves in a timely manner [7]. While remote sensing data cannot replace field surveys, it offers several advantages. These include synoptic coverage, the availability of free or low-cost satellite data, and repetitive coverage [8, 9].

Remote sensing allows for the collection of data over large areas, providing a broad overview of mangrove ecosystems that would be difficult to achieve solely through field surveys. Additionally, satellite data can be obtained at regular intervals, enabling the monitoring of changes and trends in mangrove extent and condition over time. The availability of free or affordable satellite data further enhances the accessibility and affordability of remote sensing technology for mangrove monitoring purposes. While remote sensing data is a valuable tool, it is important to note that it should be complemented with field surveys to validate and ground truth the remote sensing results. By integrating remote sensing with field-based data collection, a comprehensive and accurate understanding of mangrove ecosystems can be achieved.

Allison [10] mentioned the Bengal delta is in a net erosional state at a rate of ~1.9 km2 per year and the coastline retreat of ~3–4 km in some areas of the western edge since 1792 (~21 m year−1). Some studies mention the accretion rate as ~7 km<sup>2</sup> year−1 along the river mouth regions [11].
