Bankline Shifting and Associated Geomorphological Changes with Related Human Induced Drivers of the Lower Turag River, Savar, Dhaka, Bangladesh

Abstract

Almost all the rivers that flow through Bangladesh are alluvial in nature. All alluvial rivers generally change its position and shape  continually as a consequence of hydraulic force acting on its bed and banks and these changed to the geometric characteristics of the  river. These changes may be slow or rapid and may result from natural environmental changes or from changes caused by man's  activities. Turag is one of the important Rivers in terms of siltation, discharge and channel shifting. The distributaries to move over the  whole of floodplains over a long period of time. Accretion of sediments and changes in channels still seem to be in progress. But due  to the expansion of the Dhaka city and several activities of the local inhabitants of the study area, there is a possibility of shifting  tendency of river course and geomorphological changes on surrounding landscape is emerged. The Remote Sensing (RS) and Geographic Information System (GIS) methods and other statistical data techniques are used for the assessment of River bank  erosion-accretion and identification of bank line shifting pattern. Normalized Difference Water Index (NDWI) is applied for  differentiating the land-water interface based on Landsat Thematic Mapper (TM) images for the year of 1995 and 2005. Landsat Operational Land Imager (OLI) image is used for the year of 2020. 10 crosssections are taken for analyzing the channel shifting  pattern. Considering all cross-sections, erosion is dominant than accretion along the bankline. Within this 30 years, the percentage of  variation between erosion (-) and accretion (+) is -75.94%. 60 random points are considered along the demarcated banklines and across the cross sections from both of the left and right bank of the river which are tested as positional accuracy based on Google  earth pro. The overall accuracy of detecting banklines is 93.3%. Bank line shifting is occurred in the right bank of the river which is  dominant at the upper portion of the selected stretch of the River. The surrounding area of the Turag River has also been changed due  to the actions of several human induced drivers; such asindiscriminate settlement expansion, market place expansion, and sand  transportation are substantial.