Glacial lake flood hazard assessment and modelling : a GIS perspective

Abstract: High mountain regions have experienced decreased glacier stability due to varying climate conditions. As glaciers melt, they are being replaced by glacial lakes of different sizes, some of which are prone to outburst flooding, and have caused catastrophic damage to downstream settlements and infrastructure. The study proposes a framework for a first stage hazard assessment of glacial lakes using GIS techniques and a digital elevation model. It introduces a new dynamic flow runoff model in a glacial lake hazard context. Based on a triangular form based multiple flow algorithm, the model is used to estimate flood magnitude and hazard degree. The assessment is applied to the Pho Chu sub-basin in the Himalayan region, Bhutan. The results show 6 lakes in the basin of area greater than 0.2 km2 and 6 of them classified as potentially hazardous by at least one hazard indicator. It is found that there could be different ways to determine moraine dam steepness and several spatial methods are attempted. The possibility of measuring moraine dimensions is limited by the digital elevation model’s resolution. The assessment method can be further improved by including a hazard indicator for rock avalanches. Flood routing from Raphstreng lake is modelled over a pilot area in the sub-basin to demonstrate the application, assuming partial lake drainage. The maximum flood depth reached during model run time of 300 minutes, mostly falls between 1 to 15 m. Spatially concentrated in the main river channel, the flood extent enters the first settlement area at about 140 minutes from the start time. Rating hazard degree, the results show that most of the inundated extent fell under the extremely hazardous category. Where flood data are available from post-flood field surveys, it is recommended that the model be validated. A useful aspect of implementing the dynamic model in the future is that analysis of flood arrival time with respect to different settlements and infrastructure can be carried out, as water depths in the study area are saved for each time step.