A China-Nepal joint expedition team released its initial reconnaissance findings on Oct 13, including the first full-basin remote-sensing map of the affected area with a resolution of 0.5 meters, following an ice-rock avalanche in August on the Nepali side of the Himalayas that triggered cross-border impacts, China Central Television reported.

The map overlays pre- and post-disaster imagery on a single interpretation sheet, covering the entire path from the avalanche source to downstream areas. It provides an important reference for reconstructing the disaster chain's movement path, triggering mechanism and basin-wide impacts.

The disaster began on Aug 26, when a mass of ice and rock broke away from a high-altitude slope on the Nepali side of the Himalayas, triggering a massive avalanche that rapidly transformed into a destructive debris flow. The flow swept downstream, striking Gyirong Port in the Xizang autonomous region and causing extensive damage to buildings and infrastructure before triggering flooding that spread into Nepal. The disaster caused heavy casualties on both sides of the border.

To clarify the disaster mechanism and support post-disaster recovery, experts from China and Nepal set out from Chengdu, Sichuan province, on Oct 4 for a roughly 20-day joint emergency expedition in Nepal to investigate major cryosphere-lithosphere hazards. The team comprises more than 50 researchers from over 10 universities and research institutes, including China University of Geosciences (Beijing), the Nepal Academy of Science and Technology and Chengdu University of Technology.

A major challenge facing the team was the persistent cloud cover along the southern Himalayan flank, which makes it extremely difficult to obtain cloud-free optical satellite imagery. To address this, researchers combined multiple optical datasets with radar imagery and repeatedly checked key sections. They produced the first cloud-free pre-disaster base map of the entire basin, which they overlaid with post-disaster imagery to create the interpretation map.

Guo Zhaocheng, a senior engineer with the expedition team's remote sensing group, told CCTV that the map enables researchers to identify and analyze the scale of ice-rock avalanches, the characteristics of slip surfaces, the distribution of loose moraines within channels, and areas of erosion and deposition along the disaster path.

Guo said that, unlike previous investigations that focused primarily on disaster-affected areas within China, this expedition also examined source areas outside the country, including areas upstream of Gyirong Port and downstream regions in Nepal. This broader approach enables researchers to trace the disaster chain's evolution from its source to downstream areas and systematically assess its impacts across the entire basin.

During the first phase of reconnaissance, the team conducted about 120 kilometers of ground surveys in the affected Trishuli River basin in Nepal and used a helicopter to inspect about 40 km of road blockages below Gyirong Port. Researchers also conducted additional surveys on the Chinese side, bringing the total length of ground and aerial verification across the basin to about 160 km.

Wang Chengshan, the Chinese chief scientist of the joint team and a member of the Chinese Academy of Sciences, told CCTV that the first phase aims to trace the entire disaster chain, from its source and the port area to downstream areas, to develop a comprehensive, firsthand understanding of the event.

In the next phase, researchers plan to conduct detailed field investigations guided by specific scientific questions. They will systematically sample gullies, channel sections and deposits associated with the disaster. Through segmented drone surveys and on-site sampling, the team aims to develop high-precision three-dimensional models to reconstruct the disaster chain's movement path, deposition patterns and underlying mechanisms.

The expedition is expected to provide scientific evidence for disaster prevention and mitigation along the China-Nepal border and improve safety in Himalayan border corridors. It will also generate firsthand data to help researchers understand large-scale disasters in the context of global warming, with a focus on interactions between the alpine cryosphere and lithosphere.

limenghan@chinadaily.com.cn