Potential glacier contributions to the 2024 La Bérarde flood

Abstract. On 20–21 June 2024, an unprecedented flood of the Etançons river caused important damage to the village of La Bérarde (Écrins, France). An analysis of the event showed that the flood was caused by the combination of an intense rain-on-snow event at high altitude and the subglacial drainage of a supraglacial lake from Glacier de Bonne Pierre. In this study, we quantify the water volume that could have also been trapped beneath the glacier in local minima of the hydraulic head, i.e., in locations that could host so-called glacier water pockets impounded by hydraulic barriers. In the absence of direct observations of water pockets, we use a numerical, steady-state approach that computes the subglacial hydraulic head from surface and bedrock topography of Glacier de Bonne Pierre. As of June 2024, hydraulic barriers at Glacier de Bonne Pierre could, in theory, have impounded water volumes on the order of 105 m3, with the largest modeled water pocket beneath a surface depression that temporarily hosted a supraglacial lake. These results provide a first-order estimate of the potential subglacial water storage capacity prior to the June 2024 flood. We propagate uncertainties in surface elevation, bedrock elevation, and flotation fraction (the ratio of basal water pressure to ice overburden pressure) through a stochastic framework and show that spatial variability in the flotation fraction dominates the uncertainty in the resultant water-pocket volume. This highlights the strong sensitivity of subglacial water-routing results to poorly constrained basal water pressure conditions. While acknowledging that the actual presence and contribution of such water pockets cannot be confirmed from available observations, our study highlights the glacial flood potential of debris-covered glaciers with pronounced surface topographic depressions, which can promote both supraglacial and subglacial water storage.