FORMATION AND EVOLUTION OF SANDBARS IN THE PADMA RIVER, BANGLADESH

Abstract

Sandbars are natural formations in river channels, typically composed of sand and other sedimentary particles, created by river flow and erosion-deposition The shapes, sizes, and types of these sandbars exhibit diversity depending on factors such as river flow, sediment composition, hydrological conditions, and erosion-deposition processes. These sandbars are an integral part of the river ecosystem and influence the water flow patterns, navigation routes, aquatic habitats and even provide shelter to humans and habitats. In the context of Bangladesh, there is a considerable size of population that dwell in these sandbars. The process of sandbar formation and recession is quite dynamic, and its complexity is further accentuated within the confluence zones. The Ganges, the Brahmaputra, and the Meghna River systems together form the largest delta (GBM Delta) in the world which is characterized by numerous interlaced rivers, canals, and streams. These river systems contain numerous sandbars which contribute to its complex river morphology. These sandbars are home to many people, animals, and wildlife. However, the dynamic nature of sandbars, high monsoon discharge, flooding and erratic erosion-deposition patterns make the sandbar formation and recession process quite unpredictable and puts the lives of people living on it in danger. This paper aims to study the morphological changes in sandbars, the erosion-deposition patterns within these river systems causing the sandbar formations, compare the sandbar formations in the two confluence zones of the Padma River, determine the changes in water-sandbar surface area ratio, visualize the evolution of sandbars, and quantify the movements of sandbars. The study revealed that over the past two decades (1990-2010), the main channel of the river was dominated by sandbar area. In contrast, in the past 7 years (2016-2022), the average surface area of water within the main channel of the Padma River was approximately 73,387 ha while the sandbars covered an area of around 51,274 ha, highlighting the recent dominance of water in the main channel. Conversely, in a similar time frame (2016-2022), at the two confluence zones of the Padma River, the average surface area of sandbar was greater than that of water within a 30km radial distance from each confluence node. At the upstream confluence (Padma-Jamuna-Ganges), the average surface area of sandbars was approximately 40150 ha while water covered an area of around 28531 ha. Similarly, at the downstream confluence (Padma-Meghna), sandbars covered an area of 43696 ha on an average while water covered an area of around 40889 ha. This showed that sandbars have been rapidly accumulating at the two confluence zones. It is also observed that the sandbars are very dynamic as both lateral and longitudinal migrations are observed. It is however evident that the littoral sandbars are more stable compared to the medial sandbars and therefore more suitable for habitation. From visual inspection it is evident that the bankline of the medial sandbar (Amirabad) shifts more rapidly compared to the littoral sandbar (Harirampur within the period from 2016 to 2022. Additionally, the coefficient of variation of area for the littoral sandbar and the medial sandbar are approximately 2.11% and 5.37% respectively. This indicates that the area of the littoral sandbar is more stable over the years compared to the medial sandbar as it has a lower coefficient of variation and therefore is less active. Over the years, the confluence nodes on either side of the Padma River have constantly shifted due to rapid sandbar accumulation. Between 1990 and 2022, the maximum shifting of the upstream and downstream confluences reached approximately 7500 meters and 18000 meters respectively, both in the southeast direction.