Assessing downstream flood impacts due to a potential GLOF from Imja Tsho in Nepal

The Imja glacial lake, located in the Khumbu region, has the potential to generate a GLOF, which can increase risk due to flooding of downstream communities and cause substantial impacts on regional social, environmental, and economic systems. The Imja lake, formed on top of the Imja glacier, is bounded by the Lhotse Shar and Imja glaciers to the east, lateral moraines to the north and south, and a 700 m wide by 700 m long ice-cored terminal moraine complex to the west. The lake has experienced rapid growth in area and volume, with a volume of 35.8 ± 0.7 million m3 by 2002 and 61.7 ± 3.7 million m3 by 2012. The western, down-valley expansion has stabilized in recent years, while the eastern expansion continues unabated, mostly through calving from the glacier terminus. The characterization of the risk of Imja Tsho is somewhat controversial, with some researchers declaring it to be relatively dangerous and others concluding that it may be stable. ICIMOD identified Imja Tsho as one of six high-priority glacial lakes in Nepal that require detailed investigation. The United Nations Development Program (UNDP) is implementing the Community Based Flood and Glacial Lake Outburst Risk Reduction Project to reduce the possible risk to downstream communities posed by the lake. According to the UNDP project strategy, the GLOF risks arising from Imja Tsho will be significantly reduced by reducing the lake volume through an artificial controlled drainage system combined with a community-based early warning system. The village of Dingboche, located about 8 km downstream from the lake, is the most risk-prone area from a potential Imja GLOF. The village has extensive agricultural lands and buildings within 10-20 m elevation from the stream that will be flooded in the event of a GLOF unless flood prevention measures are taken. Two villages in the path of a potential GLOF from Imja Tsho have been the focus of the work reported here: Dingboche and Phakding. Dingboche is a village of about 200 residents with an economy that depends on local agriculture and trekking and climbing tourism, while Phakding is a village of about 1500 residents with an economy that depends on local agriculture and trekking tourism. A new, two-dimensional debris-flow model has been developed for predicting the potential GLOF hazard from Imja Tsho in terms of inundation depth in downstream communities. The model has been applied to four scenarios: current lake conditions, and three risk mitigation scenarios with the lake water level lowered 3, 10, or 20 m below the current level. The results indicate that only minor flood impact reduction is achieved in the downstream community of Dingboche with modest ( ∼3 m) lake lowering. Lowering the lake by 10 m shows a significant reduction in inundated area, while lowering the lake by 20 m almost eliminates all flood impact at Dingboche. Further downstream at Phakding, the impact of the GLOF is significant, and similar reductions in inundation are likely as a result of lake lowering.