Flash flood in Nepal causes loss of life and property

06 May 2012

On Saturday 5 May 2012, flash flooding in the Kaski district of northwestern Nepal resulted in the death of at least 31 people, left dozens more missing, and caused great loss of property including homes, businesses, crops, and livestock. The flood occurred in a tributary of the Seti River originating from the east of Mount Machhapuchhre. It is believed to be due to the outburst of a landslide-dammed lake. However, the exact location of the source of the flash flood has not yet been identified. The source of the flood water is considered to be located about 20 to 35 km from the city of Pokhara. Most of the damage and loss of life was near the confluence with the Sardi Khola. 

Following the news, ICIMOD was immediately called by Nepal’s Ministry of Home Affairs (MoHA), through the Joint Secretary, Shankar Koirala, to provide technical assistance based on its competence on the subject matter and potentially to provide support with satellite derived information for emergency response and management. The Emergency Operation Centre within the MoHA is the main authority coordinating efforts for emergency response and management in disaster situations. Other key national institutions involved include the Department of Hydrology and Meteorology, Water and Energy Commission, Armed Forces, Police Forces, and local government authorities. Present at the Ministry from ICIMOD were Basanta Shrestha, Pradeep Mool, and Birendra Bajracharya. ICIMOD experts were also present at a press conference organized by MoHA Saturday afternoon. 
ICIMOD has offered its technical assistance to the Government of Nepal to undertake the rapid field assessment of the site in cooperation with MoHA, the Department of Hydrology and Meteorology, the Water and Energy Commission Secretariat, and the Department of Water Induced Disaster Prevention. ICIMOD is also in touch with agencies such as the United States National Aeronautics and Space Administration (NASA) and the Japan Aerospace Exploration Agency (JAXA) for possible acquisition of pre- and post-disaster satellite images of the area for damage assessment. 
 
Interview requests have poured in to ICIMOD, and the Centre’s experts have contributed analyses of the situation to radio, television, and print media.

Likely cause of the event

Flash floods are flood events of great volume and short duration. They can reach their peak volume in just a few minutes and often carry large loads of mud, rock,and other debris. Flash floods can have different causes:
  • intensive rainfall or ‘cloudburst’ – the most common type of flash flood in the Himalayas;
  • glacial lake outburst – linked with retreating glaciers and therefore receiving wide attention;
  • landslide dam outburst – considered to have caused Saturday’s Seti flood.
Landslide dam outburst floods (LDOFs) can occur when large amounts of material from landslides or debris flows reach a river and temporarily block its flow, creating a reservoir in the upstream reach. As the reservoir level rises from river flow and overtops the dam crest, sudden erosion of the dam can cause an outburst. Overtopping can also be caused by secondary landslides falling into the reservoir. Internal instability of the dam might trigger an outbreak even without overtopping. 

Such floods scrape out beds and banks, causing heavy damage to riparian areas and huge sedimentation in downstream areas. A previous example of this type of flash flood occurred in Larcha, Nepal in July 1996, wiping out 22 houses and killing 54 people in a matter of a few minutes.

Landslide dam outburst events are generally random and cannot be predicted with any precision. About 25% of landslide dams burst out within one day of their formation. Therefore quick reaction is necessary to cope with LDOF risk. Hazard assessment and identification of risky areas, a good communication system, and preparedness at local level are some essential measures for minimizing the LDOF risk.

A lesson on the need for improved awareness

In the case of the Seti LDOF, some lack of awareness on the part of communities has been observed. When the landslide occurred and blocked the river, the downstream flow dropped drastically. This itself should have been a good early warning. However, the community did not have the awareness to react appropriately. This indicates a great need to increase awareness and preparedness at the local level. Further, there is a need to increase the capacity of concerned agencies, particularly at the district level, on measures to reduce flash flood risk. 

ICIMOD’s contribution

ICIMOD has developed resource manuals on non-structural and community-based flash flood risk management and training materials based on regional and international experiences. Similarly, ICIMOD has developed techniques to identify risk areas based on advanced geospatial data and tools. These materials can be used to build the capacity of concerned agencies in the future.

Other types of flash flood and associated risk management

Cloudburst – Short duration intensive rainfall is the most common cause of flash flood in the Himalayas. Real-time and near real-time precipitation (rainfall) information from direct observation stations and remote-sensing based rainfall estimation can be used in early warning systems to reduce the loss of lives and properties. However, the early warning system has to be accessible and understandable to the communities at risk.

Glacial lake outburst floods – Glacier thinning and retreat can result in the formation of new glacial lakes and the enlargement of existing lakes due to the accumulation of melt water behind loosely consolidated end moraine dams. Such lakes are inherently unstable and can be subject to catastrophic outburst. There are very few, small glacial lakes in the catchment east of Mount Machhapuchhre and they are not considered potentially dangerous. About 21 of the 1,466 glacial lakes mapped out in Nepal are considered potentially dangerous. The Government of Nepal carried out mitigation activities to reduce the GLOF risk in Tsho Rolpa glacial lake in 2000. ICIMOD, together with Nepalese institutions, has carried out detailed investigations of some glacial lakes and their downstream areas. ICIMOD is currently assisting Nepal’s Ministry of Environment and the United Nations Development Programme (UNDP) Nepal to develop a project proposal for reducing GLOF risk through mitigation and early warning systems in Imja and Tsho Rolpa glacial lakes.