Hydrodynamic reconstruction of the paleoflood from the Early Holocene ice-dammed lake Nedre Glomsjø, Norway

Study region: Norway Study focus: Numerous geological traces provide evidence for the existence, size and catastrophic drainage of the Early Holocene glacial lake Nedre Glomsjø in southern Norway. We present, for the first time, a hydraulic reconstruction that links the three domains of the glacial lake outburst flood process chain: lake hypsometry, tunneling under the remnant ice sheet and subsequent deluge downstream. This is done by first reconstructing the hypsometry of the lake using paleo shorelines observed from modern high-resolution elevation data. The development of an ice tunnel under the remnant ice sheet is then simulated using an ensemble approach to determine a suite of hydrographs for the range of possible parameter combinations. These hydrographs are applied as upstream input to a 2D hydrodynamic model of the flood propagation. The flood marker dataset includes abundant landforms and flood traces at multiple scales, such as erosive paleostage indicators, sediment records and large-scale flood bars. The results from each domain are constrained by the flood marker dataset, which adds to the robustness of our interpretation. New hydrological insights for the region: The reconstruction shows that glacial lake Nedre Glomsjø covered roughly 1250 km2, and that ∼100 km3 of water was available for drainage. The peak discharge of the glacier lake outburst flood was at least 1.5 ×106 m3/s and probably closer to 2.0 ×106 m3/s. Our results demonstrate the benefit of assembling the entire process chain from glacial lake, tunnel expansion and flood propagation in a single modeling framework to give robust results that satisfy multiple geological constraints for each component of the glacial lake outburst flood (GLOF) system. The methodology described in this paper can be applied to contemporary and future glacial lakes to better predict GLOFs in a changing climate.