Diligence required to prevent landslides in quake aftermath
About a month after the April 25 earthquake hit Nepal, a landslide occurred about 10 kilometers north of Beni, the headquarters of Myagdi district.
The landslide, which consisted of loose, fine sediments, blocked the Kaligandaki River, causing the river’s water to accumulate in a reservoir behind the landslide dam. The water overtopped and breached the natural dam, sending a flood of more than 2 million cubic meters of water downstream. Fortunately, swift government action to evacuate villages at immediate risk prevented human casualties, and the landslide dam was breached in less than 16 hours. Had the lake that formed upstream become larger, with the river water retained for a few more days, the resulting flood would have caused catastrophic damage downstream.
It is important to note that this landslide occurred as a result of natural processes that were further aggravated by the earthquake. And learning from other earthquakes in the Hindu Kush Himalayan region, this is not a surprising event after a magnitude-7.8 earthquake.
After Wenchuan and Kashmir
In the aftermath of the magnitude-8.0 Wenchuan earthquake in Sichuan, China, in 2008, scientists recorded more than 12,000 landslides along the region’s river valleys. The earthquake and numerous strong aftershocks triggered many large landslides that blocked rivers, resulting in the formation of more than 30 lakes. One high-risk event was the formation of a landslidedammed lake near Mount Tangjia, Sichuan, which blocked the Jian River. More than 200,000 people living downstream and at immediate risk were evacuated, and the government constructed a sluice to safely drain the lake and prevent large-scale devastation.
Another example is the magnitude-7.8 Kashmir earthquake in 2005 that affected India and Pakistan. The tremor triggered several thousand landslides — mainly rock falls and rock slides — around the epicenter in Pakistan. Several of these landslides dammed rivers, forming lakes upstream — including the Hattian landslide approximately 32 kilometers southeast of the metropolis of Muzaffarabad — and posing a risk to thousands of people. Observations on the ground, combined with a comparison of satellite imagery before and after the earthquake, supported the hypothesis that landslide hazards increased in the Neelum and Jhelum valleys after the 2005 quake.
Immediately after the earthquake in Nepal, a NASA-led interagency team of international experts, of which Icimod was a core member, collaborated to map landslides in the earthquake-affected area. The team has identified more than 3,000 landslides — a majority of which are concentrated along river valleys.
Weak Monsoon, No Comfort
The earthquake and repeated aftershocks have not only triggered thousands of landslides, they have also weakened the soil and destabilized slopes in Nepal’s steep mountain valleys. It is anticipated that monsoon rainfall could further weaken the slopes, which could result in more landslides. The intensity and amount of monsoon rainfall are crucial factors that could influence the occurrence of these landslides.
Experts have predicted below normal rainfall during the 2015 monsoon. Consensus on this was developed at the sixth South Asia Climate Outlook Forum, based on an expert assessment of prevailing global climate conditions and forecasts from different climate models from around the world. Currently weak El Nino/Southern Oscillation (ENSO) conditions are prevailing over the Pacific Ocean. The ENSO is a global climate phenomenon that has a significant influence on the year-to-year variability of the monsoon over South Asia. According to the Indian Meteorological Department, the rainfall this monsoon over the whole of India is expected to be about 12 percent less than the long period average from June through September. Experts have also indicated that across Nepal the probability of below normal rainfall this monsoon is around 45 percent, with the likelihood of above normal rainfall around just 20 percent.
So how could a weak monsoon affect the potential for landslides in Nepal in the aftermath of the earthquake? Let’s look back at the Wenchuan earthquake for more information. Before the Wenchuan earthquake, the average intensity of rainfall needed to trigger debris flows was around 80 millimeters in 24 hours.
Researchers have found that after the earthquake, as little as 30 millimeters of rainfall in a 24-hour period resulted in debris flows, with the magnitude of debris flows much larger than those before the earthquake. In Nepal, this could indicate that even if the amount of rainfall this monsoon is less than normal, as has been predicted, there still remains a high risk of slope failure and landslides because of the effect the earthquake has had on the country’s steep mountainous terrain.
Thus, it is important that efforts be made to increase preparedness, and that mitigation measures are put into place to help minimize the adverse impacts of possible landslides. Appropriate monitoring of river valleys and their tributaries is also required, including in rivers that have been affected by landslides like the Marshyangdi near Lower Pisang and in the Budi Gandaki, which have been deemed safe for now, but may change under monsoon conditions. For this, the Department of Hydrology and Meteorology has dispatched field teams to affected areas to ensure that existing real-time water level and rainfall stations are in operation, which will be vital for providing early warning during the monsoon period.
Enhancing disaster preparedness, improving the immediate response, and ‘building back better’ during recovery, rehabilitation and reconstruction is the fourth priority for action of the Sendai 2015 Framework for Disaster Risk Reduction endorsed by 187 countries around the globe, including Nepal and other countries in the Hindu Kush Himalayan region. The framework sees disaster as a critical opportunity to build back better — including through the integration of disaster risk reduction in development measures — in order to make nations and communities more resilient to disasters.
The current priority of the government is reconstruction and rehabilitation, for which the identification of safe settlement areas and hazard-prone areas is essential. The inventory of landslides, their categorization according to the risk, and landslide susceptibility zoning is essential for the relocation of inhabitants and resettlement and construction. For long-term risk identification, hazard zoning, proper land use planning, and “building back better” could help avert future disasters and support more sustainable development. Shrestha is the program coordinator of the HYCOS Initiative under River Basins Programme at the International Centre for Integrated Mountain Development