Importance of Field Work in Natural Disaster Risk Assessments in High Mountains

Increase in glacier melting leads to the formation of new glacier lakes at the snout of glaciers in high elevations. It is common to find moraine-dammed lakes and glacier lake outburst floods (GLOFs) in different glacierized regions. GLOFs can affect fragile mountain ecosystems as well as economic activities due to the large magnitude of the flow. The complexity of this problem is increased by the remoteness of the areas and the lack of data, and the majority of risk assessments are based on remote observations such as satellite imaginary and aerial photography. The Dudh Koshi basin contains twelve of the twenty potentially dangerous glacial lakes of Nepal. In May 2012 ground penetrating radar surveys were completed at three glacier lakes in Nepal to measure the moraine thickness and detect the presence of ice in the moraines. The lakes were: Dudh Pokhari, considered to be high risk due to its volume; Tama Pokhari, considered to be low risk after a 1998 GLOF; and Imja Lake, considered to be high risk due to its large volume. Due to expansion in the direction of the Imja glacier instead of the moraine during the last decade and the extensive terminal moraine complex there has been a belief that Imja lake will not have a GLOF in the near future. This work highlights the idea that to have an accurate risk assessment, fieldwork needs to be one of the main sources of information, which could be mixed with remote sensing and numerical modeling. For these lakes we found: Dudh Pokhari does not show evident risk since it does not have evident triggers such as overhanging ice. Tama Pokhari was drained considerably in the 1998 GLOF; however, some risk remains since there are many hanging glaciers and the volume of the lake is not well known. Community members mentioned that twice some ice has fallen into the lake producing waves, which overtopped the moraine and generated floods downstream. At Imja Lake, extensive GPR surveys were run at the terminal moraine and indicated the presence of ice in the entire terminal moraine complex. The lake is expanding in both directions and exposed ice is visible at inner face of the south moraine. The terminal moraine is quite wide, but it is very unstable. We believe that the water is dammed by the ice instead of the moraine itself, and any change in the ice structure might lead to a collapse of the moraine, especially if an earthquake occurs, which is probably the worst scenario. Therefore, at all three lakes we found information that contradicts assessments based on remote sensing and prior site visits. For Dudh Pokhari, recent remote images indicated that it was dangerous; but after visiting, we believe it is not. At Tama Pokhari, the recent GLOF led to the assumption that it is not dangerous; but, based on our site visit, we believe it has some residual risk. For Imja Lake, people assumed it was not dangerous because it has such an extensive terminal moraine complex; but we believe in the near future it could be dangerous.