**5. Results and discussion**

Accretion and erosion were not the same throughout the different segments of time considered for the study. For example, erosion was dominating in the 1975–1988 interval with a land loss of ~12 km2 /year, while erosion declined in the following periods to a rate of ~8 km<sup>2</sup> per year. If we look at results as a whole erosion wins over accretion and the coast registers a net land loss of ~280 km<sup>2</sup> in this study period. Accretions in the 1994–2005 and 2005–2015 periods were ~ 10 km2 and ~ 5 km2 per year respectively. In this segment, a total gain of ~210 km<sup>2</sup> of land can be observed.

According to the present study, the erosion and accretion rates in the Subarnarekha Delta have been highly dynamic over the past 47 years. It is observed that the delta has experienced both erosion and accretion at different times during this period. The total land change estimated over the last 47 years in the Subarnarekha Delta is a loss of 250 km2 of land. This suggests that the delta experienced a net loss of land during this time period. The study also indicates that accretion has been dominant in the east direction, while erosion has been more pronounced in the south-towest direction. This spatial variation in erosion and accretion patterns suggests that

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

different parts of the delta have been subjected to varying degrees of land loss or gain. These findings highlight the dynamic nature of the Subarnarekha Delta, with ongoing processes of erosion and accretion shaping its coastline over the years. Understanding the spatial patterns and rates of erosion and accretion is crucial for managing and mitigating the impacts of coastal changes in the delta region.

A notable pattern observed in the land dynamics of the Sundarbans coastline is the declining rate of accretion in successive periods. This trend could be attributed to the overall sediment deprivation of the delta caused by human activities such as dam construction and other anthropogenic disturbances upstream (**Table 2**) [17].

Erosion is observed in all directions except for the landward directions (N, NE, NW), suggesting the potential influence of sea level rise (SLR) impacts in the Bay of Bengal. The variation in erosion rates in different azimuthal directions may be attributed to a combination of surface wave and tidal actions. Surface waves primarily originate from the southwest direction in the Bay of Bengal, while tidal actions predominantly occur from the south [18]. Additionally, the East India Coastal Current (EICC) flows northward during the rainy seasons (March–September) and reverses its direction during the dry seasons (October–January). As a result, the combined forces of waves and tides are stronger in the south azimuthal direction compared to other directions.

These factors contribute to the complex interplay of erosion and accretion along the Sundarbans coastline, with variations in sediment dynamics influenced by SLR, wave action, tidal forces, and seasonal currents (**Figures 4** and **5**).

Since there is no dike or other construction to safeguard the shoreline, the coastline retreat in the Sundarban area was expected due to sea level rise. By storing sediments upland and reducing their availability at the shore, sediments from dam construction have also had a substantial impact. While tidal action occurs from the south, surface waves in Sundarbans coastline region are primarily from the southwest. The East India Coastal Current (EICC) flows southward during the dry seasons of October to January and northward during the rainy seasons of March to September. The southern region is more affected by the combined impact of waves and tides than other regions. During the final 47 years of the research period, the Subarnarekha coast lost 270 km2 in total, or 5.7 km2 per year. This number is higher than what preceding researchers had predicted. There has been a loss of land, but not equally. Despite the fact that some new islands have appeared along the coast, the overall picture shows a loss of land mass there.


#### **Table 2.**

*Accretion and erosion rates in the Sundarbans coastline estimated.*

#### **Figure 4.**

*Erosion and accretion over the past 47 years in suburban coastal area.*

**Figure 5.** *Accretion and erosion of the coastline between 1975 and 2022.*
