A NOVEL APPROACH TO STUDY THE IMPACT OF HUMAN INTERVENTION STRUCTURES ON THE SPATIO-TEMPORAL VARIATION OF SEDIMENT GRAIN SIZE IN THE EASTERN HIMALAYAN RIVER TISTA

Different methods have been used to evaluate grain size characteristics of the alluvial river deposits based on gauge data or field data to understand the impact of dams on the natural river processes. In case of Himalayan rivers, gauge data have limited access and historical field-based grain size data of pre- to post-dam period are hardly available for comparison. To overcome these limitations, 31 years’ of continuous satellite images (1987-2017) have been utilized for 75 km sandy braided stretch of River Tista to identify the alluvial landforms that formed prior to the dams and has survived, at least partially, till date. Grain size analysis of sediment samples collected from the deposits of surviving pre-dam landforms, has been compared with the grain size data of samples collected from the landforms that developed during the post-dam period, with an aim to assess the impact of dams/barrages. Landsat time series images were classified and overlay in GIS environment to prepare the composite maps. The composite maps help to group the landforms into three temporal classes: the TOP landforms formed prior to dams\barrage operation (till 1997); TMP landforms formed during the dam construction phases (till 2010) and TNP landforms developed during the full-swing operation of dams (between 2011-2017). We have collected 116 samples from 23 temporally grouped landforms. The D84 and D50 data have shown that TOP sediments are coarser than TMP and TNP landforms. TNP sediments are the finest. TOP and TNP samples show a natural downstream fining, in contrast to grain size parameters of TMP showing an unusual downstream coarsening pattern. Grain size fining in TMP and TNP bars are interpreted to represent the effect of trapping of the naturally supplied coarser sediments by the upstream reservoirs, whereas the downstream coarsening pattern of TMP is inferred to be related to the injection of a huge amount of sediment in the river during the construction of the upstream dams. This interpretation is also corroborated by a sudden jump in the suspended sediment concentration during the same period in the gauge data. The composite map shows its potential in identifying time of formation of the historical landforms and their utility as an inexpensive, easy-to-do technique to study the impact of climatic changes or human activities on the natural river processes.