When the levee breaks: Reducing GLOF risks through dam breach modelling

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The 2016 Bhote Koshi floods put the lives of more than 200 households in Sindhupalchowk, Nepal, at risk. The flooding was initially thought to be a result of heavy rains, but an analysis conducted by Department of Hydrology and Meteorology (DHM) officials a year after the floods identified a glacier lake outburst flood (GLOF) in Tibet as the cause. GLOFs occur when water breaches the surrounding wall of a lake body – the destructive nature of these floods can be attributed to the suddenness of the breach and the large quantity of water that is released downstream. In the case of the Bhote Koshi flood, the GLOF breach originated from a glacial lake that was spread over 9,000 m2 – equivalent to almost seven Olympic-size swimming pools.

Given the Koshi basin’s susceptibility to disasters, many communities near the Koshi River are constantly at risk. In a bid to address these risks, a collaborative effort between the United Nations Development Programme (UNDP) in Nepal, DHM, and International Centre for Integrated Mountain Development (ICIMOD) has focused on training DHM personnel on the principles of dam breach modelling (DBM). This modelling helps gauge the impacts of a potential flood event. Focusing on primarily two aspects of the breach, DBM analyzes the occurrence of the breach as well as the flow of water from the resulting breach “… to determine dam failure consequences.” DBM prior to such events can help local authorities and communities prepare and put in place evacuation and other disaster response plans. Juan Fernandez, a dam breach modeler at UNDP Nepal who served as a resource person during the training, explained that a starting point was the mapping and identification of potentially dangerous glacial lakes on the basis of ICIMOD’s remote-sensing data. To further expand the skillsets of DHM personnel, the training was also linked to Nepal’s application to the Global Climate Fund (GCF) in adaptation and resilience building. The DHM will be one of the primary bodies involved in the application process.

The Tsho Rolpa Lake, one of the biggest lakes in Nepal, has grown considerably over the last 50 years as a result of glacial melting in the Himalaya (Photo: Nabin Baral/ICIMOD)

According to a case study conducted by ICIMOD on flash flood risk management, around 5,800 people face the risk of another major GLOF event in the Bhote Koshi Valley. Moreover, more than 600,000 people could be indirectly affected from its impacts on trade, hotels, transport services, and tourism. Deepak KC, Senior Programme Officer for Integrated Climate Risk Management at UNDP Nepal and resource person for the training, shared that the training participants could become advocates in their own right in convincing authorities on the need for timely assessment and monitoring of GLOFs. The runtime of the simulation when increased GLOF risk has been assessed follows a timeframe that could take longer than the time required to mobilize efforts on ground. Along with this, real-time changing parameters of the actual on-going disaster could result in miscommunication. If the DHM and other local institutes have in-house capacity in such modelling, they can visit the disaster site and change parameters accordingly. This also means that a mitigation strategy can be devised, which would strengthen risk resilience. KC also pointed out that the current focus is on rescue and relief operations, but a well-formulated preparedness strategy is necessary. 

Riverbank settlements like those in Khadichaur are particularly at risk from sudden GLOF events and ensuing floods (Photo: Jitendra Bajracharya/ICIMOD)