High-resolution satellite imagery can help the country assess hazards from landslides and save lives
REMOTE SENSING: High-resolution satellite imagery, like this of Helambu on 8 May, can help the country assess hazards from landslides and save lives. Image: Digital Globe, made available by ICIMOD.
The 25 April earthquake and the swarm of high-intensity aftershocks in the past month have made the mountains of Central Nepal highly unstable, and the approaching monsoon rains could trigger landslides and block rivers.
After the earthquake, Nepal must now be prepared for secondary disasters like rockfalls, slope failures, glacial lake outburst floods and avalanches. Early warning could save lives, but how can we monitor these dangers?
An international volunteer team made up of the Kathmandu-based International Centre for Integrated Mountain Development (ICIMOD), the National Aerospace and Space Administration (NASA), the University of Arizona, Chinese Academy of Sciences, the US Geological Survey, British Geological Survey and Durham University have been using high-resolution satellite imagery to map and assess these hazards.
“The greatest risks right now are landslides,” says Arun Bhakta Shrestha of ICIMOD, which has located 3,000 landslides triggered by the quake, many of which could grow in size and destructive capacity.
Landslides mapped in the aftermath of the earthquake. Data until 17 May 2015. Source: NASA-USGS-Interagency Earthquake Response Team, ICIMOD, British Geological Survey/Durham University/Earthquake without Frontiers
Gorkha, Rasuwa, Dhading and Sindhupalchok have the highest number of landslides, and based on satellite imagery researchers now have an inventory of potentially deadly ones with their location and size pinpointed, and if they pose any danger to blocking rivers.
Before after images of Bhote Koshi area. All WorldView Image provided by Digital Globe, made available by ICIMOD.
Before after images of Laprak village of Gorkha District.
Before after images of Ward No. 5 of Barpak, Gorkha district.
One potentially dangerous landslide after the 25 April earthquake came down on the Marsyangdi River in Lower Pisang Village of Manang district. After analysing this river blockage, Shrestha says researchers created a model for which way the river would flow if the dam was to breach. At the moment, the threat of a landslide is still there if the dam is breached and a more accurate estimate of the volume of water is needed.
In other areas, concerned locals have directly reached out to scientists for information. For example, there was some panic that the Tso Rolpa glacial lake might burst following the 12 May aftershock because of its proximity to the epicentre.
Tsho Rolpa glacial lake as seen on 24 May. Image: Digital Globe, made available by ICIMOD
“Comparing before-after images of the area and monitoring the shake intensity, we came to the conclusion that there was no additional risk,” says Shrestha, and the government passed this information back to the Dolakha district administration.
But another team of researchers from the United Nations Development Programme (UNDP) made an on-site inspection of the moraine dam at Tso Rolpa and installed a community-based early warning system in coordination with the Department of Hydrology and Meteorology.
“The system is installed close to the lake. If the water rises to a dangerous level a warning will be received and disseminated through SMS to downstream communities,” says Vijaya Singh, assistant country director of UNDP.
Satellite imagery has also been used to assess the damage caused by avalanches, specifically in Langtang where an entire village of 500 people was buried by an avalanche that also destroyed ICIMOD’s Langtang observatory station.
Satellite imagery of Langtang as seen on 8 May. Image: Digital Globe, made available by ICIMOD
While GIS and remote sensing is a good starting point in the assessment of future hazards, it has its downsides. High-resolution imagery cannot be accessed on a regular basis and cloud cover can block views when they are urgently needed.
“We are planning on using radar imaging so that high-resolution satellite images can be taken despite the presence of clouds,” says Basanta Shrestha, Director of Strategic Cooperation at ICIMOD.
Researchers are now preparing for more detailed mapping of pictures so that they can try to predict which areas pose a danger to human habitation. They will be using both satellite imagery and site visits to cross validate. Alert mechanisms and early warning systems, similar to the one installed in Tso Rolpa, will then be placed.
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