Modeling Mitigation Strategies for Risk Reduction at Imja Lake, Nepal

A model was developed to assess the impact of a potential glacial lake outburst flood (GLOF) from Imja Lake in Nepal and its impact on downstream communities. Implications of proposed GLOF risk reduction alternatives, including one suggested by local community members, were assessed. Results provided three alternatives that offer significant risk reduction for the communities, including (1) no lowering of the lake and constructing a 60 m flood detention dam, resulting in a 43.2 percent reduction of risk, (2) lowering the lake 10 m with a 40 m dam, resulting in a 57.8 percent reduction of risk, and (3) lowering the lake 20 m with no dam, resulting in a risk reduction of 66.7 percent. An alternative to lower the lake by 3 m with no check-dam, currently under consideration by the Government of Nepal, would result in a 5.2 percent reduction of risk. This alternative does not appear to offer significant risk reduction benefits to downstream communities compared to lowering the lake by 20 m. Results suggest that either the lake must be lowered by significantly more than 3 m (20 m is recommended) or that a downstream flood detention dam be included in the project. One possible method of lowering Imja Lake is to use siphons to drain lake water by 3 m, excavate to the new water level, repeating the process until a total lowering of 20 m is achieved. This method would require the use of 13 pipes of 0.350 m diameter to lower the lake.