# Hazard Assessment for a Glacier Lake Outburst Flood in the Mo Chu River Basin, Bhutan
## 5 Discussion and Conclusions  
We applied the HEC-RAS hydrodynamic model to simulate GLOF scenarios with a reasonable effort in data preparation and computational time. The duration of one model run ranged between 37.5 h for the best case scenario and almost 42 h for the worst case. This variation is caused by the computational method of HEC-RAS, which is more complex for wet cells. The most challenging steps were the creation of outburst hydrographs and the determination of the channel roughness. The first depends on the outburst volume and on dam parameters, which are generally unknown. The second is difficult to estimate by means of remote sensing as well as by field techniques. For these reasons, several outburst volumes and \(n\)-values were selected to create an ensemble of possible combinations. Future research should aim at the quantification of uncertainties, for example by probabilistic dam breach modelling using a Monte Carlo approach. This could help to keep input parameters within realistic ranges. Another limitation of our results is the fact that sediment transport could not be considered. Here, more case studies in regions with better data availability could deliver valuable information on reasonable model input. Our modelling results in the Mo Chu basin in Bhutan revealed that channel roughness is not essential for the extent of the inundation, but plays an important role for the velocity and arrival time of the flood. These findings are in agreement with the results from other studies [23]. The hazard potential from a jokulthaup triggered by the outburst of Sintaphu Tsho is comparably small. Even under a worst scenario, which assumes that 90% of the lake is drained and the channel roughness is rather low (\(n=0.04\)), only few buildings will be affected. However, along its route through sections with narrow valley, such a flood would surely damage bridges and roads, causing logistic problems in this mountain region which is already difficult to access. Attempts to reduce the risk potential could aim at a lowering of the lake level by artificial outlets, pumping or tunneling [31]. Since the remoteness of the lake might hamper extensive construction works, a feasible way can also be to widen or deepen the river channel along the GLOF pathway. A different strategy is to reduce the vulnerability of the valley to flood hazards. This can be achieved by technical measures, such as the strengthening of bridges or the construction of dams. Another important adaption method is the prevention of new buildings in threatened areas through appropriate legislation. Here, our results can help to identify prohibitive zones. The vulnerability can also be reduced by early warning components, enabling residents to save valuable goods or at least their lives. Compared to rapid mass movements, the response time for GLOF events is longer and allows warning systems or the design of an evacuation plan. In the Mo Chu Valley, a cost-friendly version could be the nomination of a local observer in Gasa Hot Springs and to equip him, if necessary, with means of communication in order to alert authorities in the case of a flood event. More security can be provided by an automatic warning system, especially in the case of events which happen during nighttime. In the Punakha-Wangdue Valley, where larger glacial lakes exist, an automatic early warning system is in operation since 2011. There, automatic weather and water level stations are connected to 17 sirens which warn the valley population. The NCHM is planning to extend such systems to other valleys (www.hydromet.gov.bt, accessed on 15 August 2021). Hydrodynamic modelling could be a helpful tool in designing such a system or in preparing flood hazard maps. Integrated disaster management should also undertake awareness and capacity building programs to train the population in identifying hazards and knowing safe evacuation zones. Due to the catastrophic outburst of Lugge Tsho in 1994 and smaller events afterwards, the awareness of GLOF hazards is comparably high in Bhutan.