Dynamics of the glacial lake at Flatbreen in Fjærland, Western Norway, and its role in the flood regime of the Tverrdøla catchment

Abstract

Glaciers are severely affected by climate change as they retreat due to rising temperatures. This increase in meltwater and unveiling of depressions has simultaneously caused a rise in the size and number of glacial lakes globally, further resulting in an increased risk of glacial lake outburst floods (GLOFs; Shugar et al., 2020). Along with warming temperatures, GLOFs can be influenced by extreme precipitation (Harrison et al., 2018). The patterns of such precipitation have already intensified and are expected to become more extreme as global warming increases (Seneviratne et al., 2021). Given the global growth of glacial lakes, it is imperative to comprehend both the causes of their flooding and their vulnerability to climate change.

This study focuses on the glacial lake at Flatbreen glacier in Fjærland, Western Norway. Although relatively small, the Flatbreen glacial lake has a history of flooding the Tverrdøla river. The most recent event occurred in November 2022 when a period of extreme precipitation contributed to a debris flow, destroying a road and isolating locals. Suspected of being a flash flood from extreme precipitation and/or GLOF activity, the exact cause of the flooding was unknown. This study uses a hydrological model (HBV-light) to predict discharge values and a hydraulic model (HEC-RAS) to determine if corresponding water levels match geomorphological evidence of erosion along the channel. Both models help simulate the event and better understand the dynamics of the glacial lake and its role in flooding the Tverrdøla catchment.

The results suggest that significant subglacial drainage channels became blocked and rerouted water to the glacial lake in November 2022. The drainage network likely started to close in the fall, and the heavy rainfall overwhelmed the system. In turn, water deviated from its normal course to the icefall and instead flowed out of the lake through the moraine-breach outlet, leading to the debris flow in the Tverrdøla catchment. The simulation provides a peak discharge of 26.4 m3/s at the bridge crossing. However, this does not correspond with evidence of erosion along the channel and is likely underestimated due to some misrepresentation of snow/glacier melt and glacial outflow in the model. Based on expected increases in temperature and heavy precipitation due to climate change, peak discharge in the study area may increase up to 64% by 2060 and 125% by 2100 during a similarly shaped precipitation event. The findings underscore the urgent need to enhance our understanding of glacial lake dynamics and their flood risks, as climate change continues to amplify extreme weather and enlarge glacial lakes.