Water beneath a French glacier may have added to the 2024 La Bérarde flood

Possible water beneath the glacier

The researchers used a steady-state numerical model to calculate the pressure surface that guides water beneath Glacier de Bonne Pierre. Their results suggest that hydraulic barriers could, in theory, have impounded water volumes on the order of 10^5 cubic metres before the June 2024 flood. The largest modelled pocket was beneath a surface depression that had temporarily hosted a lake on the glacier’s surface.

When the researchers varied uncertainties in the glacier’s surface height, bedrock height and basal water pressure, the spatial pattern of basal water pressure had the largest effect on the estimated pocket volumes. The study does not establish that these pockets were actually present or that they contributed water to the flood.

Why hidden storage matters

The findings indicate that debris-covered glaciers with pronounced surface depressions may provide places for water to collect both on top of and beneath the ice. Accounting for this possible hidden storage could help researchers assess the potential water sources involved in glacial floods, although the study provides only a first-order estimate for this event.

Model estimates and limits

The study is based on a steady-state numerical calculation using the glacier’s surface and bedrock topography, combined with a stochastic analysis that propagates uncertainties in those measurements and in basal water pressure. There were no direct observations of subglacial water pockets. As a result, the model estimates where water might have been stored and how much could theoretically have accumulated, but it cannot confirm that the pockets existed or determine how much, if any, water entered the 2024 flood.