# Hazard Assessment for a Glacier Lake Outburst Flood in the Mo Chu River Basin, Bhutan
###### Abstract  
The frequency of glacier lake outbursts floods (GLOFs) is likely to increase with the ongoing glacier retreat, which produces new glacial lakes and enlarges existing ones. Here, we simulate the outburst of a potentially dangerous glacial lake in Bhutan by applying hydrodynamic modelling. Although the lake volume is known, several parameters connected to the dam breach and the routing of the flood are rough estimates or assumptions, which introduce uncertainties in the results. For this reason, we create an ensemble of nine outburst scenarios. The simulation of magnitude and timing of possible inundation depths is an important asset to prepare emergency action plans. For our case study in the Mo Chu River Basin, the results show that, even under the worst case scenario, little damage to residential buildings can be expected. However, such an outburst flood would probably destroy infrastructure and farmland and might even affect the operation of a hydroelectric powerplant more than 120 km downstream the lake. Our simulation efforts revealed that, by using a 30-m elevation model instead of a 5-m raster, flood magnitude and inundation areas are overestimated significantly, which highly suggests the use of high-resolution terrain data. These results may be a valuable input for risk mitigation efforts. glacier lake outburst flood; GLOF; jokulhlaup; hydrodynamic modelling; HEC-RAS; risk assessment; Bhutan 2021 25 September 2021 2021 7 October 2021 12 October 2021 lowers it again, leading to progressive erosion. GLOFs can also be triggered externally by mass movements such as ice avalanches, calving, rockfalls or bigger landslides [2; 8; 9], which have their own causes such as earthquakes or heavy precipitation. These events cause waves and subsequent overtopping of the dam, also followed by incision. A third possibility is the collapse of the dam due to hydrostatic pressure or internal structural failure, often connected with ice cores or seepage along pipes. A first glacial lake inventory for Bhutan [10] was carried out by the International Centre for Integrated Mountain Development (ICIMOD) and contained 2674 entries, 24 of which were considered \"potentially dangerous glacial lakes\" (PGDLs). The criteria for hazardous lakes were: recent expansion, lake area (\(>\)0.1 km\({}^{2}\)), distance to glacier (\(<\)0.5 km), water depths, and moraine dam conditions (shape and material). A more recent inventory, based on high-resolution Advanced Land Observing Satellite (ALOS) imagery from 2006 to 2011, was compiled by Japanese researchers [11; 12]. They have delineated 733 lakes at altitudes between 4000 and 6000 m a.s.l. ; the significantly lower number compared to the first inventory is most probably due to a different methodology and not to a disappearance of lakes. The National Center for Hydrology and Meteorology (NCHM) of the Royal Government of Bhutan has conducted a re-assessment of potentially dangerous lakes by field observations and high-resolution satellite imagery from 2016. Based on bathymetry data, the surrounding topography and findings of the first inventory [10], 12 lakes were identified as potentially dangerous [13]. Most of these lakes (nine), among them are the most dangerous ones, are located in the Pho Chu River Basin. For this reason, most of the research [6; 14] and mitigation measures are focused on the headwaters of Pho Chu. In this study, we want to concentrate on the Mo Chu catchment. Although only two potentially dangerous glacial lakes (PGDLs) are located here, they also may threaten rural communities or Punakha town and therefore should not be disregarded. In recent years, attempts to model GLOFs became more and more successful. Simple geometric models estimate the maximum travel distance by the average slope from the starting point to the farthermost area of damage [9]. In contrast to 1-D flood models, two-dimensional hydrodynamic models are capable to simulate turbulent flow, eddy fluxes, lateral spreading behavior, and other important features of a GLOF.