Outburst flood from Glacier de Tete Rousse: A past and future threat

To protect the 3.000 inhabitants of the France village of Saint-Gervais–Le Fayet from a possible glacier outburst, the authorities have decided to drill into and install pumps on the Glacier de Tête-Rousse (Mont Blanc Massif), where a larger volume (65.000 cubic meters) of stagnant water is presumed. A supraglacial lake containing estimated 25.000 cubic meters water was discovered during this March, the authorities now fear that the water could be released in a sudden outburst when the surrounding icewalls collapse or the water excavates an outlet. In the course of this week the base camp at 3.200m a.s.l will be prepared, the drilling and installing of the pumps will presumably be concluded until October, before the onset of the winter.

Meanwhile the residents of the villages were warned of the possible danger and a evacuation plan is in elaboration.
The precautions are not entirely unfounded; in the night between July 11. and 12., 1892 the village of Saint-Gervais was severely damaged and 175 peoples killed by a 200.000 cubic meters outburst coming from the Tête-Rousse Glacier.
In the Alps, outburst floods from intraglacial cavities are not rare but generally lead to only small discharges and debris flows causing little or no damage. The outburst flood from Tête-Rousse was, however, one of the deadliest disasters ever caused by a glacier.

Before 1878, in a period with increased rate of ablation, a supraglacial lake formed in the centre of the glacier, this lake subsequently became covered by ice and snow.
The collapse of the glacier tongue in 1892 finally released the accumulated water, a large cavity 40m in diameter and 20m high containing estimated 20.000 cubic meters water at the glacier terminus remained as testimony. From this lower cavity, an 85m long intraglacial conduit led to the upper cavity (the former lake) with an additional volume of 80.000 cubic meters.

Fig. 1. The lower cavity at the terminus of the glacier, note epeople for scale. A part of the snout has been torn from the glacier. Photograph by H. PELLOUX, September 1892, figure from VINCENT et al. 2010.

Fig. 2. The upper cavity (former supraglacial lake) at the centre of the glacier. Photograph by M. KUSS, 13 August 1893, figure from VINCENT et al. 2010.

Fig. 3. Longitudinal section of the tongue, sketch from VALLOT and others (1892), figure from VINCENT et al. 2010.

After the catastrophe a monitoring program was initialized and in 1898-1899 a horizontal tunnel drilled to prevent water accumulation inside the glacier. In 1901, a 50m long and 40m deep crevasse became filled with water, so until 1904 a new tunnel was constructed, and 22.000 cubic meter water drained. This tunnel still exists and is supposed to prevent water accumulation close to the bedrock of the glacier.

Fig. 4. Map of surface and bedrock topography in 2007. The locations of the upper cavity and lower cavity (green dashed curve) and the excavated tunnels 81899 and 1904) are shown.

References:

VINCENT, C.; GARAMBOIS, S.; THIBERT, E; LEFEBVRE, E.; LeMEUR, E. & SIX, D. (2010): Origin of the outburst flood from Glacier de Tète Rousse in 1892 (Mont Blanc area, France). Journal of Glaciology, Vol. 56(198): 688 - 698