Satellite-Based Assessment of Flood Inundation, Vegetation Damage, Channel Change and Environmental Impacts of the August 2026 Rasuwa Flood, Nepal Himalaya

Abstract

    The devastating flood event in northern Nepal that occurred in August 2026 brought significant modifications to the river channels, valley floors, vegetation, and sediments of the Bhotekoshi/Lhende Khola and Trishuli river corridors. The present study integrates a remote sensing, geographic information system (GIS), terrain, and hydrometeorological analysis to understand the event in the Rasuwa region of the Central Nepal Himalaya. An optical and radar satellite remote sensing approaches were used to assess impact on the study area’s surface-water extent and flood-affected regions respectively. In addition, the study made use of Sentinel-2’s multispectral imagery to describe the environmental disturbance, vegetation condition, exposed sediments, debris, and river-channel modifications. The normalized difference vegetation index (NDVI) and normalized difference water index (NDWI)/modified normalized difference water index (MNDWI) were applied to assess impacts on vegetation and surface-water conditions. A digital elevation model (DEM) was evaluated to determine the influence of elevation, slope, drainage, and valley morphological control, on flood and debris-flow paths. In addition, pre- and post-event imagery comparisons were made to identify riverbank erosion, channel widening, sedimentation, and debris accumulation. The Department of Hydrology and Meteorology hydrometeorological records were used to assess rainfall-temporal relationships, river level, and satellite flood extent correlations. Finally, flood extend, vegetation loss, erosion, sedimentation, topography, drainage, agricultural land, and settlements, and infrastructure were merged using a weighted multi-criteria GIS method to develop an environmental flood impact index. The results suggest that the event’s largest environmental impacts were recorded along the Bhotekoshi/Lhende Khola and Trishuli river corridors as the flood inundation, vegetation disturbance, channel modification, erosion, sedimentation, steep terrains, and human exposure overlapped. Overall, the study demonstrates the importance of remote sensing and terrain information in combination with hydrometeorological records to evaluate flood and cascading mountain hazards in the Himalayas.