Assessing channel changes of the Ganges-Padma River system in Bangladesh using Landsat and hydrological data

The Ganga/Ganges(1) is an important river system in South Asia which supports the life and livelihoods of millions of people both in India and Bangladesh. The system has a number of names throughout its length. Below its confluence with the Brahmaputra at Aricha it is known as the Padma, which in turn merges with the Upper Meghna at Chandpur below which the channel is known as the Lower Meghna.There is a growing concern about this large river system because its channels are subject to frequent migration, threatening engineering structures and resulting in various environmental and social consequences which may be compounded by climatic variability, land use change, and agricultural intensification as the basin experiences rapid population growth. Concerns have been expressed that the construction of a barrage just upstream of the Indo-Bangladesh border has adversely affected the Ganges reach in Bangladesh. Partly in order to investigate this, the planform changes of the Ganges and the Padma within Bangladesh was analysed over the period 1973 to 2011 using multitemporal Landsat images and long-term flow data in eight epochs with an average duration of 4.5 years. The Padma reach is less affected by the barrage and provides a useful control study. Areas of erosion and deposition were determined from sequential changes in the banldine positions. Mean channel width, sinuosity and braiding index were analysed using a Geographic Information System (GIS). Flood frequency, duration and magnitudes were studied using long-term discharge records. Generally, channel planform evaluation indicated that both the Ganges and the Padma experienced contraction, expansion and readjustment in configuration over the last 38 years. Erosion and deposition statistics of the Ganges indicate that 57 km(2) of land was lost along the right bank whereas around 59 km(2) has been gained along the left bank during the assessment period, suggesting that the erosion and accretion of both banks is roughly balanced with a general movement towards the right bank. The width of the Ganges varied from a maximum of 536 to a minimum of 3.23 km during the observation period. Changes to sandbar area are, in general, much more radical than changes to the overall width and area of the channel. Measurement of areas of erosion and accretion showed that both banks of the Padma experienced considerable loss of land. The total net loss for left bank and right bank was 155 and 28 km(2), respectively. The Padma is approximately twice the width of the Ganges and the changes to its channel area are not as temporally dynamic as the Ganges. The relationship between bank curvature and erosion/accretion of the river banks for both rivers was analysed and the results contradict established meander theory. Regression analysis between bank erosion rates, annual average discharge and mean flood flow data showed that bank erosion was significantly correlated with annual average discharge for the Padma (r(2) = 0.6283) and that the Ganges bank erosion rate is influenced by mean flood flow (r(2) = 0.6738). The flood frequency shows generally good stability across the first eight of the nine epochs for the Ganges but for the Padma the frequency showed even greater stability. We were unable to support the widely held belief that the upstream barrage has a deleterious effect on the Ganges but note that there is a slight effect due to the periodic release of sediment through scour sluices. (C) 2016 Elsevier B.V. All rights reserved.