Erosion is the process of movement of loosened or weathered materials from one location to another (Islam, 2008). It happens as a result of natural or artificial processes, including wind, moving water, moving ice, pore water pressures, and moisture levels within the bank (Islam, 2008; Coleman and Wright, 1971; River Survey Project, 1996; Fujita et al., 2000). Every year the coastline island of the Bangladesh is eroded and sometimes accreted in a larger rate due to climate change, tidal fluctuation, sea level rise, and other man-made cause (Kratz, 1999; Mutton and Haque, 2004). The volume of water that flows after the upper catchment areas increase the intensity and vulnerability of floods as well as the related riverbank erosion (Hossain & Ferdousi, 2004). The three major rivers of Bang-ladesh, the Padma, Meghna, and Jamuna, have negatively impacted the lives and livelihood of the people by eroding thousands of the hectares of floodplain and a number of kilometers of roads and railways (Baki, 2014; Rahman, 2010; Haque, 1988; Haque et al., 1997). It dislocates thousands of people each year (Hossain & Ferdousi, 2004; Haque, 1988; Islam & Rashid, 2012). Rivers vary from one another in terms of their physical traits and behaviors (Yao et al., 2013; Surian, 1999; Alam and Hossain, 1998). The low-energy, multi-channel Meghna River fluvial system is made up of a network of connected channels and the inter-channel area (Alam, 1991). Alluvial rivers experience natural channel changes as part of their natural processes and the Meghna is no exception. Flooding, erosion, and drainage system congestion are major problems in the Meghna basin (Shahjahan and Reja, 2012; Nipa et al., 2022). 

The ideal technology for analyzing river bank erosion and accretion is GIS and RS since they can give a synoptic view of a vast area (Winterbottom and Gilvear, 2000). Remote sensing data integrated with GIS makes it an excellent tool for investigating the change in fluvial systems and mapping the likelihood of bank erosion (CEGIS, 2009; Milton et al., 1995; Mani et al., 2003; Sarkar et al., 2012; Green et al., 2000).  GIS and DEM techniques were used to describe the morphological change and quantify the temporal-spatial distribution of riverbed erosion and deposition (Sarma et al., 2007; Jian et al., 2009; Chu et al., 2006; Kummu et al., 2008; Ko-toky et al., 2005; Gurnell et al., 1994; Mahmoodzada et al., 2019; Bedini, 2007). Additionally, theodolite is widely used to track dynamic river systems (Chandler et al., 2001; Chu et al., 2006). In earlier research, by using satellite imageries the researcher found that the Meghna River in Chandpur had a higher rate of bar deposition than erosion (Nath et al., 2013). A study on the morphological changes of the Jamuna River, found that siltation covers 29.82 km2 and erosion covers 1235.25 km2 (Uddin et al., 2011). Another investigation found that the average width of the Jamuna River is increasing. They estimate that the average riverbank accretion is 166 km2 and the average riverbank erosion is 607 km2 (Khan et al., 2014). The Meghna River near Bhola has undergone morphological alteration, but very little research has been done to assess this change. So, it is critical to estimate the erosion and accretion rate of the Meghna River. Therefore, this study tried to identify the landform changes of the Meghna River by analyzing previous as well as present data. The studys conclusions may be useful to the government and other authorities in deciding how to manage riverbank erosion.

Study Area

This research emphasized mainly at Meghna river channel around Bhola district and tried to identify a trend of river course change. Meghna is one of the largest rivers where the Surma, Dhaleswari, Brahma-putra, Yamuna, and Ganges meet. The study region extents 46.5 km from the upper meghna to the lower Meghna, between latitudes 23 25 11 98N and 23 2 4 84N, and the longitudes 90 35 58E and 90 39E (Banglapedia). This study covered mainly the lower part of the Meghna River which flows across different islands of the Bhola district like Manpura, Tazumuddin, Sona Char, Char Bhuta and the Char Anandaprasad.

Fig. 1: Study Area Map.

Data Collection 

Landsat satellite images were acquired from the websites of the United States Geological Survey (USGS, 2016) to analyze 30 years of erosion and accretion patterns of the Meghna River. The details of these satellite imageries have been shown in the Table 1. To analyze the change of river bank, the total time was divided into four intervals 1992-2002; 2002-2012; 2012-2022; 1992-2022.  

Table 1: Information about the satellite images used in the study.

Data Processing and Analysis

The study used Landsat satellite images of 4-5 TM, 7 ETM+ and 8-9 OLI/TIRS sensors to detect the change of the Meghna River channel covering Bhola district. After collecting the images, pre-processing was done to remove errors and anomalies. Then, Band 1 to Band 7 was composited using the Arc GIS 10.3 program, and the mosaic function was used to merge two images. Then, an unsupervised classify-cation was applied to represent the river flow pattern of the designated area. The unsupervised classify-cation was performed through several steps which included shape file creation, polygon editing, mask extraction, and reclassification.  Far ahead, the raster file was transformed into a polygon and the shape of the river was extracted from the entire satellite image. The process was performed for all images of selected years of 1992, 2002, 2012, and 2022, and for the visualization of the change in the Meghna River course, cross-matching was performed. After that, the ‘intersect function was applied to identify the unchanged river area and the ‘erase function was applied to identify the eroded and accreted river area for the period of 1992-2092, 2002-2012, 2012-2022 and 1992-2022. At last, the area of each individual polygon representing of erosion, accre-tion, previous, current, changed, and unchanged areas was calculated through geometric calculation since the shape files were already projected to Bangladesh Transverse Mercator (BTM) projection with hector (ha) units in ArcGIS software. Finally, GIS maps were created and the whole process was completed carefully for all the datasets.

Status of Landform Change of Meghna River

Depending on the physical characteristics Meghna Rivers platform constantly changes and is extremely difficult to predict due to the dynamic nature of these waterways and islands.

Table 2: The area that has been eroded and accreted during the past 30 years.

The Table 2 displays the area that has been added to and lost throughout the course of various time periods. In case of erosion, a declining tendency was seen from the years 1992-2002 to the years 2002-2012, going from 11466.41ha to 9943.77ha and then again decreasing to 8846.89 ha in the years of 2012-2022. Between the years of 1992-2002 and 2012-2022, the erosion rate decreased from 11466.41ha to 8846.89 ha. Likewise, this study, studies conducted on on Meghna river in Chandpur district, found that 3517 square meter area was eroded during 1990 to 2002 (Nath et al., 2013). In case of Amu River during period of 2000-2005, 2005-2010 and 2010- 2014, eroded area was calculated 1,984 ha, 1,410 ha, and 1,680 ha respectively (Mahmoodzada et al., 2019). On the other hand, in the case of accretion, the study found the rate of accretion was smaller between 1992-2002 and increased steadily between 2002-2012 from 11086.74 ha to 11868.41 ha. A sharp growth is visible from the period of 2002-2012 to the period 2012-2022. Additionally, the findings of this study are consistent with research on Padma river between 1998 and 2017, found that Padmas average rates of erosion and accretion were 1472.056 and 1610.152 ha/year, respectively (Islam, 2009; Ophra et al., 2018). In the case of the Jamuna River the research found net erosion of 607km2 while the net accretion was 166km2 (Khan et al., 2014; Khan and Islam, 2003). Investigations carried out by the Krantz found that during 1990-1995 the erosion on the Meghna River along Bhola island was 4.3 km2 and accretion 20.9 km2 (Krantz,1999).

Landform Change of the Meghna River between 1992 to 2022 

The Meghna River channel along the Bhola District is constantly moving due to the intense water flow during the monsoon. From the analyses of satellite images of 1992 to 2022 it can be said that the river bank has been eroded and the channel moves in different directions at different times within several locations. Fig. 2 depicts Meghan Rivers erosion and accretion from 1992 to 2022. The study found average erosion in the Meghna River was about 537.93 ha/yr. In a prior study, the author discovered that the Bhola districts total area of erosion and accretion was 399 and 583 km2, respectively (Hassan et al., 2017). Massive water flow, conse-quent bank slumping, and its dynamics character-istics are responsible for the erosion of the Meghna River similar results also found in different studies (Islam and Rashid, 2011). The average accretion rate OF Meghna was about 1244.94 ha/yr and the probable reason for accretion was siltation (Hassan et al., 2017). The study also reveals that the amount of siltation varies according to the average water discharge, flow rate, watercourse power, unit size and gradation, and temperature (Islam and Rashid, 2011; Islam, 2008; Khan et al., 2014). 

Fig. 2: Erosion and Accretion area of Meghna River from 1992 to 2022.

Status of Total Changed and Unchanged Area

According to the analysis, there were significant and unstable alterations to the river channel. Fig. 3 indicates the overlapping of rivers in 1992 and 2022. The study found that in 1992 the total area of the Meghna River was about 98350.19 ha but in 2022 the area was about 77139.81ha. In previous work, the author found that the total landmass in a region of the islands northern section was 240.9 km2 in 1990 and 257.5 km2 in 1995 (Krantz, 1999). The study found the overall changed area is about 21210.38 ha and the unchanged area is about 56454.18 ha.

Fig. 3: Overlapping of Meghna River between 1992 to 2022.

In this study, the dynamic characteristics of the Meghna River bank are shown through the analysis of a collection of Landsat images from 1992 to 2022. The analysis found that the erosion rate significantly decreased between 1992 and 2022. The analysis also revealed that in 2022 accretion was greater than erosion. The main factors contributing to erosion at the Meghna are believed to be tides, the combined Padma and Meghna flows, sediment load deposition, floods, and bank shifting. The rivers irregular changing behavior has an adverse effect on the rural floodplain population of the Bhola. Future channel shifting patterns and erosion rates should be deter-mined by additional research, and effective steps should be made to lessen the severity of erosion and its detrimental effects.

Authors heartiest thanks go to others who were involved and helped directly and indirectly through-out the research work.

Regarding the research, writing, and publication of this paper, the authors indicated they had no poten-tial conflicts of interest.