ICE TSUNAMI IN THE ALPS - Tambora: The Eruption that Changed the World - page 90

Figure 7.2. Portrait of Ignace Venetz (1826). The artist has ingeniously incorporated Venetz's most fam-
ous engineering achievement—the tunnel at the Giétro glacial dam—as the background Alpine vista. (©
Musées cantonaux, Sion; Photo: François Lambiel.)
DOOMSDAY IN THE VAL DE BAGNES
Two years after Shelleys' tour of Mont Blanc, in the spring of 1818, the residents of the Val
de Bagnes, forty kilometers to the east along the same mountain range, confronted a chilling
sight. The River Dranse, their major water source, which ought to have been swollen with
spring meltwater from the mountains, was reduced to a trickle. 14 Villagers sent to investigate
along the slopes of Mount Le Pleureur, where the Dranse flowed through a narrow gorge at
the head of the valley, returned with grim news. The decade's succession of cold summers,
more intense since 1815, had left a potentially disastrous legacy in the form of an ice dam cre-
ated by the Giétro glacier. Its lip advanced to the very brink of the narrow gorge, the Giétro
had begun to deposit huge blocks of ice into the river, forming a cone-shaped dam thirty
meters high. A huge lake had now formed behind the icy wall: three and a half kilometers
long, two hundred meters wide, and up to sixty meters deep.
Periodic buildup and release of large amounts of water are a natural characteristic of gla-
cial systems, and typically occur in the spring after a cold summer when meltwater coursing
from the peaks and slopes, then draining into a river system, meets with an unusually resili-
ent barricade of ice. 15 As the weather warms and the volume of water builds, the pressure on
the dam increases to a breaking point. In Iceland, the ensuing catastrophic floods are called
jökulhlaups , meaning “glacier-floods.” The French word débâcles is a more emotive term (at
least for English speakers) for the same phenomenon, conveying a sense of their devastating
impact on vulnerable Alpine communities.
Extreme glacier-flood conditions prevailed across the Alps in the spring of 1818. In the
rising midyear temperatures, and with its volume ever-increasing from seasonal runoff, the
dammed-up Dranse was poised to burst at any moment. The threatened flood would inund-
ate the pristine valley with over twenty million cubic meters of water, enough to submerge
the town of Martigny twenty kilometers away, and destroy all the farmland and villages in
its path. The worried peasants of the Val de Bagnes were fortunate in one regard, however.