How Do Changes in Glacier and Basin Geometry Affect the Evolution of Ice-Dammed Glacier Outburst Floods?

We combine a glacier outburst flood model with a glacier flow model to investigate long-period variations in outburst flood characteristics. In this study, we consider outburst floods originating from a marginal lake basin dammed by a retreating glacier. Marginal basins can form due to the retreat and detachment of tributary glaciers, a process which often results in remnant ice being left in the basin. The remnant ice can act as an ice shelf or break apart into a pack of icebergs which limits the basin storage capacity but also exerts pressure on the underlying water and promotes drainage. Despite large changes in glacier and remnant ice volumes that are expected to impact flood hydrographs, we find that during glacier retreat there is a strong, nearly linear relationship between total flood water volume and peak discharge for individual basins. Consequently, peak discharge increases over time as long as there is remnant ice remaining in a basin and peak discharge begins to decrease once a basin becomes ice free. Similar size outburst floods can occur for very different glacier volumes as a result. We also find the mechanism of flood initiation impacts the temporal variability in outburst flood magnitude. Basins that connect to the subglacial hydrological system only after reaching flotation yield greater long-term variability in outburst floods than basins that are continuously connected to the subglacial hydrological system (and therefore release floods that initiate before reaching flotation). Our results highlight the importance of improving our understanding of changes in glacier catchment geometry and outburst flood initiation mechanisms in order to better assess outburst flood hazards and impacts on landscape and ecosystem evolution.