Reconstruction of active surface deformation in the Rishi Ganga basin, Central Himalaya using PSInSAR: A feedback towards understanding the 7th February 2021 Flash Flood

Active surface deformation, displacement pattern, and erosional variability is estimated using the geomorphologically sensitive morphometry along with the Persistent Scatterer Interferometric Synthetic Aperture Radar (PSInSAR) technique using the Sentinel-1 Adata (119 images) acquired between 07- 02-2017 and 10-02-2021. The average velocities for this dataset are estimated to be between +/- 11 mm/y. The Raunthi River catchment from where the flood was triggered is undergoing similar to 8 mm/y subsidence and similar to 10 mm/y uplift. Compared to this the basin wide deformation (Rishi Ganga basin) is estimated to be around +/- 10 mm/y with commulative ground displacement of around +/- 45 mm. The times series analysis suggests an increase in the ground displacement by around 5 mm/y and seems to be responsible for the expansion of pre-existing cracks in the vicinity of the Vaikrita Thrust (VT) and subsequent failure of the northern face of Nandi Peak on 7th February 2021. The Global Positioning System (GPS) derived strain distribution pattern indicate a relatively higher accumulation of strain (>0.35 mu strain/y). The normalized steepness index (k(sn)) variation along the longitudinal section of Rishi Ganga and Raunthi River sub-basin in Central Himalayan region shows anomalous increase at the glacio-fluvial transitional processes. Moreover, the chi profiles as well as planform plots shows anomalously lower values within the Raunthi River sub-basin when compared with the Rishi Ganga basin. Based on the lower values of chi it is observed that Raunthi River sub-basin is undergoing high erosion which can be caused by the presence of sheared lithology and incision of the relict glacial and paraglacial sediments. We negate the suggestion that abrupt rise in the temperature was the major triggering mechanism for the recent disaster, instead it is the sheared lithology and preexisting fissure developed because of differential uplift and subsidence in Raunthi River that led to the wedge failure and subsequent flash flood. Had the climate was the major driver of the recent tragedy ?, it should have impacted multiple hanging glaciers in the Rishi Ganga valley. Therefore, the study calls for detailed geomorphological, structural and glaciological investigation in regions dominated by glacial and paraglacial processes in the strategic regions of the Himalaya. Towards this, the state of art PSInSAR technique seems to provide fast and reliable detection of terrain instability/stability along with identification of potential areas of slope failures in near future in the glacial and preglacial zones. (C) 2021 COSPAR. Published by Elsevier B.V. All rights reserved.

Automated summary

This study estimates the active surface deformation, displacement pattern, and erosional variability using the geomorphologically sensitive morphometry and the PSInSAR technique. The results show that the Raunthi River catchment is undergoing subsidence and uplift, leading to high erosion in the Raunthi River sub-basin. The study refutes the notion that temperature rise was the major trigger for the recent disaster, but suggests that sheared lithology and preexisting fissures played a key role. It calls for detailed investigation in glacial and paraglacial regions of the Himalayas.