Redefining dangerous glacial lakes in Bhutan by integrating hydrodynamic flood mapping and downstream exposure data

Dangerous glacial lakes in Bhutan have primarily been identified considering the likelihood of producing a GLOF, which in turn has been assessed only based on upstream lake area/volume and their surrounding topographic conditions. However, this approach is incomplete as it ignores the at-risk downstream exposure and vulnerability thus the actual impacts. Here we redefined dangerous glacial lakes by considering the impact of the simulated most likely scenario GLOF on downstream exposed elements at risk. Our study shows that a total of approximately 22399 people, 2613 buildings, 270 km of road, 402 bridges and 20 km2 of farmland are exposed to potential GLOF inundation in Bhutan. We classified lake130 (Thorthormi Tsho) as a very high danger glacial lake in Bhutan, five lakes as high danger and 21 other lakes as moderate danger. Among these high danger glacial lakes, three of them: lake93 (Phudung Tsho), lake251, and lake278 (Wonney Tsho) were not recognized as dangerous in previous studies. Our assessment further revealed five downstream local government administrative units (LGUs) are associated with very high GLOF danger while nine others are associated with high GLOF danger. Six of these LGUs had not been previously documented as being at risk from GLOF including: Chhoekhor and Bumthang town in Bumthang, Paro town and Lamgong in Paro, Nubi in Trongsa and Khoma in Lhuentse districts. Our study underscores the significance of integrating potential inundation mapping and downstream e data to define dangerous glacial lakes. We recommend strengthening and expanding the existing GLOF disaster preparedness and risk mitigation efforts in Bhutan to reduce future damage and loss in high GLOF danger LGUs identified in this study.