Application of Satellite Image Time-series in Evaluating the Impact of the Dams on Sediment Transport Pattern of the Himalayan River Tista

Previous studies on the impact of dams in influencing sedimentation patterns in a river have used pre-/post-dam gauge data or historical grain size data. However, historical bedload sediment data are hardly available in the river basins of the Himalayas. In order to overcome the limitation, the present study uses 31 years (1987-2017) of satellite image time-series (SITS). The study considers a ~75 km stretch of the sandy braided Tista River in the eastern Himalayas that extends from the Gazoldoba barrage, located 25 km downstream of the mountain front, to the India-Bangladesh border. A total of 13 dams have come up upstream of the barrage, and the study is designed to explore the effect of these upstream engineering structures on the sediment character of the downstream Tista River. We used NDVI and NDWI indices to extract the landforms (channels, bars, vegetated islands/banks). A composite map was then produced by superposing these extracted features. The composite map allowed us to identify the year of the initial development of river landforms and their subsequent modifications. The developmental history of 23 such landforms could be successfully traced, whereas many were completely eroded during subsequent years. Six landforms were identified to have developed during the pre-construction period, five developed during the construction period, and 12 landforms were identified to have developed post-construction of all the dams and the barrage. The sediment character of the landforms developed during the pre-, syn- and post-construction periods preserves the sedimentation pattern during these periods, and thus sampling of these landforms made up for the lack of historical bedload data. We collected 116 sediment samples from these landforms, and their analysis suggests an overall decrease in the sediment grain size during the post-dam period compared to the pre-dam period. In addition, the coefficient of downstream fining was remarkably reduced during the post-dam period compared to the diminution coefficient during the pre-dam period. These changes reflect the effect of sequestration of coarser bedload in the upstream engineering structures and reduction of flow velocity and discharge during the post-dam period.