The Sindhupalchowk floods serve as a powerful reminder that there remains a high risk of slope failure and landslides this monsoon
Hardly 15 days since the monsoon started in Nepal, and it is already leaving a trail of destruction. Retaining walls have collapsed burying school children, and floods and landslides in various parts of the country have killed more than four dozen people and rendered hundreds homeless.
The 2015 Gorkha earthquake and its repeated aftershocks not only triggered thousands of landslides, but also weakened the soil and destabilized Nepal’s steep slopes. The 2015 monsoon was anticipated to further weaken the slopes and trigger landslides but the weak monsoon prevented many landslides and secondary hazards from occurring. Both the intensity and the amount of rainfall are crucial factors that could influence the occurrence of these landslides.
The monsoon is different this year. Experts have predicted ‘above-normal’ rainfall over much of South Asia. Consensus on this was reached at the eighth 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. The strong El Niño/Southern Oscillation (ENSO) conditions prevailing over the Pacific Ocean since July 2015 have now weakened to moderate level.
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 Department of Hydrology and Meteorology of Nepal (DHM), the rainfall this monsoon is expected to be about normal in the country, although with a great deal of spatial variability—with above-normal rainfall over far-western region, below-normal rainfall over eastern region, and normal over most of central region. For example, although the average rainfall of Nepal is around 1,800 mm, there is large spatial variation with Lumle receiving around 5500 mm per year and upper Mustang only 140 mm.
The Sindhupalchowk district is amongst the worst affected by the 2015 Gorkha earthquake and its subsequent aftershocks. The recent floods in the Bhotekoshi River, a trans-boundary river originating on the southern slopes of the Himalayas in the Tibet Autonomous Region and flowing through Sindhupalchowk, swept away more than 50 houses with 200 more at high risk. It created widespread destruction, an anticipated hazard waiting to happen after the Gorkha earthquake. Fortunately, timely evacuation of human settlements along the river bank ensured that no human lives were lost. However, the floods eroded stretches of road from Kodari to Jure landslide area and affected hydro projects such as the Upper Bhotekoshi and Sun Koshi causing disruption in the movement of people, goods, and services, and supply of energy.
Though the exact cause of the flood is yet to be established, the huge volume of water and debris brought down suddenly by the river points to the likelihood that a landslide dam upstream may have breached. Satellite images have indicated heavy rainfall in the upstream areas, which could have triggered a landslide in an area rendered geologically weak following the earthquakes. Information passed on by the Institute of Mountain Hazards and Environment, Chinese Academy of Sciences, suggests that a heavy rainfall of about 85 mm in 24 hours caused a landslide in the area near bridge No. 707 in the Zhangzhangbo valley blocking the river and accumulating water behind it. The subsequent outburst of the landslide dam caused the flood. Human settlements in Tatopani, Barhabise, Lamosanghu, and Khadichaur remain at high risk of inundation from such floods. This event serves as a powerful reminder that there remains a high risk of slope failure and landslides during this monsoon not only on the country’s steep mountainous terrain, but also in its adjoining regions which may block rivers and cause outburst floods.
Efforts on increasing preparedness, and putting mitigation measures into place, will help minimize the adverse impacts of possible landslides and floods. Real-time monitoring of river valleys and their tributaries is required to check the condition in the rivers. The Department of Hydrology and Meteorology (DHM) has, for the second year, put a 24/7 flood watch with a telephone hotline for providing timely warning of imminent flood situations.
Actions such as preventing people from living in flood plains, hazard zoning land use planning, and improving preparedness of communities, could help reduce the losses. Early warning systems play an important role in alerting the communities. Warnings that are understood and accepted by communities and prompt communities to take informed actions lead to saving lives and properties. Further, an inventory of landslides, their categorization according to the risk and landslide susceptibility zoning, is essential for the relocation of inhabitants and resettlement and construction. Assessment of risk and vulnerability reduces the exposure of people to these multi hazards and minimizes the adverse impacts. But the implementation of such measures is limited because of poor technical and managerial knowledge about such measures combined with a lack of financial resources, institutional arrangements, and legal provisions.
The Bhotekoshi flood has also reemphasized the need for trans-boundary cooperation. The floods in the Bhotekoshi River originated in Tibet, China, but caused destruction downstream in Nepal. An early warning system in the upstream can provide adequate lead time and save lives and infrastructure from flashfloods such as glacial lake outburst floods and landslide dam outburst floods. A real-time data sharing mechanism and an early warning system are essential.
In this regard the International Centre for Integrated Mountain Development in collaboration with the World Meteorological Organization and hydromet agencies from Bangladesh, Bhutan, Nepal and Pakistan has established a regional flood information system for sharing real-time data and information and promoting trans-boundary cooperation. The results of the regional flood outlook piloted in 2014 indicate that it is a promising tool that may support effective flood forecasting in the Himalayan region.
Mandira Singh (firstname.lastname@example.org) is Programme Coordinator, HYCOS Initiative/Senior Water Resources Specialist; and Arun Bhakta (email@example.com) is Regional Program Manager, River Basins Management /Senior Climate Change Specialist at ICIMOD