Morphological changes of a developing sandbar along the shoreline of Sonadia Island, Bangladesh between 1972 and 2006 using remote sensing

ABSTRACT In a dynamic coastal environment, the morphology of a sandbar is anticipated to change regularly, particularly when there is a large annual sediment supply. The study explored the nature and magnitude of a developing sandbar’s morphological changes between 1972 and 2006 in Sonadia Island, Bangladesh, using five different satellite images. The sandbar’s changes across the shoreline were calculated using field survey data and manual-drawing outlines of overlaid maps. The study revealed continuous and rapid morphological changes of the sandbar along the Island’s shoreline. The net annual rate of shoreline displacement ranges between 3.94 m and 7.79 m during the period. These might directly result from the Island being exposed to huge wave and tidal action and excessive sediment supply around the Island. Transect 3 represents the sandbar’s head and appears to be the most active zone with the most significant annual shoreline displacement of 22.00 m. The sandbar’s far end around Transect 7 is more stable with a yearly shoreline displacement of 0.50 m. Changes were more drastic between 1999 and 2006 than between 1972 and 1999. These rapid changes after 1999 could indicate that the sandbar is becoming increasingly vulnerable to anthropogenic changes and the adverse effects of sea-level rise.


Introduction
Coastal regions are the sites of active and rapid geomorphological changes driven mainly by the slow however steady rise in sea level and the strong dynamic processes particularly wave action for erosion and sediment transport (Ritter et al., 1995;Bird, 2011;Kirwan & Megonigal, 2013;Oyedotun et al., 2018;Özpolat & Demir, 2019;Franco-Ochoa et al., 2020;S. Ghosh et al., 2020;Grases et al., 2020;Rizzo & Anfuso, 2020). Coastal geomorphological change with time is a worldwide phenomenon (Franco-Ochoa et al., 2020). About 70% of the world's coastline has shown net erosion over the past few decades, less than 10% has net degradation and the remaining 20% or so have remained relatively stable (Bird, 2011(Bird, , 1969M.K. Ghosh et al., 2015). Geomorphological changes in the coastal area of Bangladesh are taking place from time immemorial either as erosion or deposition, like in the case of any other depositional and erosional coasts in the world (Biswash, 1978;Curray & Moore, 1971). We have rather scanty records covering only a few decades about those morphological changes around the coast of Bangladesh (A.S. Islam et al., 2010;Crawford et al., 2021;Islam, 1991;Islam et al., 2016;M.K. Ghosh et al., 2015;M.M. Islam et al., 2020;Mullick et al., 2019;M. R. Islam et al., 2002;Salauddin et al., 2018;Sarwar, 2005;Sarwar & Woodroffe, 2013).
Bangladesh is a deltaic plain of three mighty rivers, namely the Ganges, the Brahmaputra and the Meghna commonly known together as GBM (Datta & Subramanian, 1997;Ahmad, 2003;Mukherjee et al., 2009;A.S. Islam et al., 2010). The contemporary major river systems in Bangladesh are originated from the Himalayas which are situated in the north to the Bengal delta (Mukherjee et al., 2009;M.H.R. Khan et al., 2019). Denudation of the Himalayas which is still active at a rate of about 70 cm per one thousand years resulted in the formation of the world's largest delta (Curray & Moore, 1971;Biswash, 1978;Allison & Kepple, 2001;Mikhailov & Dotsenko, 2007) as it allows to carry a total of about 2.4 billion tons of sediments per year into the Bay of Bengal by GBM (Holeman, 1968(Holeman, , 1969Viles & Spencer, 1995). These sediments interact with dynamic process within the Bay of Bengal leading to coastal morphological changes (Akter et al., 2016;Ali, 1989;Fein & Stephens, 1987;Ramage, 1971). The constant recycling of sediments and the annual additions from monsoon floods produce a complex pattern of erosion and accretion i.e., coastal morphological changes (Barua, 1991). A large amount of sediments is also thought to be carried by under-currents into the deeper Bay of Bengal and the Indian Ocean (Allison & Kepple, 2001;;Mikhailov & Dotsenko, 2007). The dynamic processes in the northern Bay of Bengal especially in the coast of Bangladesh (Khandker, 1997;Murty & Henry, 1983) are largely influenced by the bottom topography of the Bay which results in frequent morphological changes in the adjacent coast and Islands (Siddiqui, 1988;Barua, 1991;Viles & Spencer, 1995;Hoque et al., 2013).
Moheshkhali Island is located on the northwestern side of Cox's Bazar District of Bangladesh (Hoque, 2013). Sonadia Island is an extended part of Moheshkhali Island consisting of active tidal flats (M. S. H. Khan et al., 2005) primarily formed by sandy sediment deposition (Figure 1). Some khals (creeks) separate it from Moheshkhali Island (Islam et al., 2012;Salam & Hasan, 2019). Although Moheshkhali Island is little far away from the active delta formation region, it still receives a substantial amount of sediment (Hoque et al., 2013). It undergoes coastal processes that facilitate reshaping the Island's coastline, particularly the south and southeastern part including Sonadia Island (Hoque et al., 2013). Visible morphological changes are evident within the south-eastern shoreline of Sonadia Island, which therefore gets a sandbar formation (King & Williams, 1949;Hoque et al., 2013;Nelson et al., 2013). Study on formation and morphodynamical changes of sandbars in this geographical location seems important as it can help to understand dynamic coastal processes including wave and tide along with sediment supply (McNinch, 2004;Badrun, 2017). Knowledge of sandbars dynamics is also crucial for sustainable coastal zone management, lack of which may bring big failure to any development activities (Tătui & Constantin, 2020). Not too many but widely accepted research were accomplished on sandbar identification, morphology and dynamics and quantification of sandbars crest positions (Lippmann & Holman, 1990;Ruessink et al., 2002;Lafon et al., 2004;Almar et al., 2010;Aleman et al., 2015Aleman et al., , 2017Tătui et al., 2016;Tătui & Constantin, 2020;Walstra et al., 2016;Yuhi et al., 2016). Time series of satellite images can reveal the nature and magnitude of changes taking place within this sandbar across the shoreline of the Island. Thus, the attempt was undertaken based on the analysis of multi-temporal satellite images of the study area acquired between 1972 and 2006 supported by field observations. This study's main aim was to see the morphological changes of a developing sandbar along the south-eastern part of Sonadia Island and determine the direction and magnitude of morphological changes over time.

Materials and methods
Nine (09) selected transects roughly normal (perpendicular) to the shoreline ( Figure 1) covering the whole sandbar were selected using GPS survey for ground- truthing. The transects were chosen randomly at an interval between 1 and 5 km except Transect 1 and 2. During field observation and visit, sediment samples, beach profiling across the transects and geographic positions (fixes) were collected and analyzed. The sediment sorting results along with beach profile were used to study the relative wave-energy condition that are presented and discussed earlier in Hoque et al. (2013). Transect 1 and 2 are not situated in the sandbar or Sonadia Island but used for ground truthing and sediment studies earlier. They are situated in the Moheshkhali Island, close but separated from Sonadia Island by a tidal creek. Hence, the remaining seven transects (3-9) representing the sandbar were studied to detect the shoreline change.

Satellite images with source
Three Landsat satellite images, one ASTER image and a browse image of QuickBird Satellite over a period between 1972 and 2006 were used in this study to analyze changes in the Sonadia's shoreline. The acquisition time and year of each satellite image are shown in Table 1.
Landsat satellite images have been acquired from the image archive of the Global Land Cover Facility of the University of Maryland, USA. The ASTER image was ordered from the EROS (Earth Resources Observation System) Data Center's Distributed Active Archive Center (EDC-DAAC), a joint NASA-USGS data archive; and the QuickBird browse image was captured from the Goggle Earth application interface.

Image analysis
Landsat and ASTER images came with a gross globally accurate georeferencing, however not adequate for a local application like the present study. Therefore, they have been freshly georeferenced using ENVI Remote Sensing Analysis software, taking the Landsat ETM image as the base (reference) image. QuickBird browse image was also georeferenced in the same process. For the image to image georeferencing, distinct landscape features like road junctions & bends, stable ground features, small manmade ponds and ditches etc have been used to collect Ground Control Points (GCPs) in the reference image, and they were matched on the other images for corresponding GCPs. Based on a carefully taken set of 15 GCPs the images were rectified using Polynomial Image transformations. The Root Mean Square Errors (RMSE) of the image corrections process for each image is shown in Table 2.
After successful georeferencing, the desired part of the image was cropped (subsetted) for subsequent analysis (Figure 2).

GIS analysis of morphological change
Five cropped images (Figure 2A-Figure 2E) of the study area between 1972 and 2006 were overlapped, and the outline of the Island was drawn in a single page from each image ( Figure 2F). Morphological changes were determined from those outlines. A line near to perpendicular to the most of shorelines was drawn first, and distance of each shoreline at each Transect from that perpendicular line were measured manually. The differences between two shorelines of two consecutive images was calculated by subtracting the values of the previous image from its successive next image. This difference signifies as morphological changes. The values could be found as positive or negative.

Results
This study revealed that the outer shoreline changes ( Figure 2F) its position from time to time which indicates regular morphological changes were taking place between 1972 and 2006. Recurrent morphological changes in the size and shape of the sandbar were observed throughout the period. Here morphological changes include erosion, accretion along with inward and outward movement of the developing sandbar of the south-western shoreline of Sonadia Island. The study conjointly found that if erosion is observed along the sandbar of the Island once, immediately afterwards deposition of sediment accumulates along the sandbar. This phenomenon was common throughout the study area, hence in the entire  shoreline of the island. Besides, the sandbar also had a net upward shifting toward the north over time. From keen observation, it was also clear that the sandbar has almost a stable shore in the south-western and south-western side of the sandbar, while the eastern part of the sandbar is protruding towards north-east direction rapidly.
The changes in the study area in the form of horizontal shifting of the shoreline positions across the transects in various years interval are given in Table 3 and Figure 3. The average annual rate of displacement of shoreline was 3.96 m, 3.94 m, 7.79 m and 7.07 m during the 1972-1989, 1989-1999, 1999-2004 and 2004-2006 periods, respectively. Highest annual displacement of shoreline was 22.00 m, which was found at Transect 3 during 2004-2006. Lowest annual displacement of shoreline was 0.50 m, which was found at Transects 7 between 1999 and 2004. The changes were   1972-1989 1989-1999 1999-2004 2004-2006 between 1999 and 2004 and between 2004 and 2006 than between 1972 and 1989 or between 1989 and 1999. The magnitudes of changes were high at Transects 3, 4 and 5 while the changes were subtle at Transects 6, 7, 8, and 9, i.e.,, in the sandbar's crest region. These spatially variable rates of changes indicate that morphological changes are high in southeastern part, i.e., the active sandbar region through shifting of sandbar toward the north and net accretion throughout the periods except net erosion around the south-western shoreline of the island between 1989 and 1999. Morphological changes including erosion, accretion and inward and outward movement of the study area in different periods are also shown in Figure 3.

Discussion
Remarkable morphological changes of the sandbar along the shoreline of Sonadia island was observed throughout the period between 1972 and 2006. Morphological changes include both erosion and accretion of the Sandbar and of the shoreline of Sonadia Island; along with inward and outward movement of the sandbar. Net accretion on the Island and across the coastline during the period is evident. This phenomenon is common in some other coastal region enriched with heavy sedimentation (Bird, 1969;Lafon et al., 2004;Pape et al., 2010). Once the island undergoes erosion at times followed by accretion in subsequent periods, which shows much more similarities with the report of SPARRSO (1987SPARRSO ( , 1993 and findings of Pramanik (1983,1988); and Pramanik et al. (1981) in some other coastal islands of Bangladesh. Some recent studies suggest such inward and outward movement may be because of seasonal to annual dynamics of the sandbar morphology (Tătui et al., 2016;Vidal-Ruiz & De Alegría-arzaburu, 2019;Tătui & Constantin, 2020).
Sonadia especially the south-eastern part of the island undergoes active morphological changes where there is a still developing sandbar (Hoque et al., 2013). The sandbar is prograding in the southeastern direction of the island. Besides, this active sandbar undergoes northward shifting. These sorts of morphological changes take place only in the southeastern portion of the island may be due to less exposure to open the Bay of Bengal and wave action than the western portion encountering continuous wave action (Tătui & Constantin, 2020;Tătui et al., 2016). Shore features like sandbars are very dynamic mainly governed by waves, currents and regional sediment characteristics along with geological setting that may control the nearshore morphology by influencing the stability of sandbar morphology and position as well as surface sediment characteristics (McNinch, 2004). Similar findings were also supported by moderately sorted grain size distribution around Transect 3 and 4 compare to well sorted to highly well-sorted sediment distribution across the rest of the shoreline of Sonadia Island (Hoque et al., 2013). These changes also may be attributed to longshore current around the island, because the longshore current has an active role as waves generally do not approach shoreline parallel to the shore and the component parallel to the shore can  . between 1972 and 1989 B. between 1989 and 1999 C. between 1999 and 2004 D. between 2004 and 2006). move sediment. The second concept was also suggested by Ali (1989) and Ali, 1995. He described discharge current, tide, monsoon current and storm surges are the main causes of coastal erosion and accretion, i.e., morphological changes in Bangladesh. Beach drift may also have a contribution in the changes of the study area as beach drift plays a role as waves approaching at an angle to the beach, but retreating perpendicular to the shoreline, results in the swash of the incoming wave, moving the sand up the beach in a direction perpendicular to the incoming wave crests and the backwash, moving the sand down the beach perpendicular to the shoreline (Fox et al., 1966). Thus, with successive waves, the sand will move following a zigzag path along the beach (Ahnert, 1998;Ritter et al., 1995;Stephen, 1999). Over time, the effects could be large. Sediment is moved and redeposited to increase the size of the shore of the island. The effects on the land surface were seen by examining the shore profile. Beaches occur where sand is deposited along the shoreline (Hayes, 1975). These physical forcing can give the extreme output as Moheshkhali Channel brings enormous sediment from the hilly region of Myanmar and Bangladesh (Islam & Hoque,). Sediment also transported here from the great GBM river system (Holeman 1968;Ali, 1989Ali, , 1991.

Conclusion
Morphological changes especially erosion and deposition in the coastal area of Bangladesh are taking place from the time immemorial. We have records of these changes covering about two hundred years, but they are mostly scanty, and not continuous. Records of study on morphological changes in Sonadia Island is not available. This study is the first one of its sorts and very important particularly in the context of the island being developing as a new deep seaport. Therefore, a greater understanding of the hydro-geomorphology of the Island system is critical for a megaproject like this. Further studies based on shoreline detection, near shore satellite-derived bathymetry or the very recent automatic procedure to extract and analyze sandbar morphology and changes from satellite images should be followed.

Acknowledgments
Special thanks go to the Global Land Cover Facility of the University of Maryland; the USA as Landsat satellite images have been acquired from its' image archive. Thanks to the EROS (Earth Resources Observation System) Data Center's Distributed Active Archive Center (EDC-DAAC), a joint NASA-USGS data archive for the ASTER image and the Goggle Earth application interface for the QuickBird browse image.

Disclosure statement
No potential conflict of interest was reported by the author(s).