Hydraulic Reconstruction of the 1818 Giétro Glacial Lake Outburst Flood

Abstract

In the spring of 1818, ice avalanches from the Giétro Glacier created an ice dam, which in turn formed a glacial lake in the Drance Valley (Canton of Valais, Switzerland). Today, its maximum volume is estimated to have been 25×106 m3. Cantonal authorities commissioned an engineer named Ignaz Venetz to mitigate the risk of the ice dam's failure. He supervised the construction of a tunnel through which a large volume of water was drained as the lake rose (9×106 m3 according to his estimates and 11×106 m3 according to our model). After 2.5 days of slow drainage, the ice dam failed on 16 June 1818 and caused major flooding in the Drance Valley up to 40 km downstream, resulting in about 40 deaths. Venetz's lake monitoring notes, numerous testimonies gathered in the disaster's aftermath, and our field survey have made it possible to collect a wealth of information on this event, which is one of the world's major documented glacial lake outburst floods. Reconstructing major outburst floods remains challenging because not only do they involve enormous volumes of water spreading over long distances but they are also associated with additional physical processes such as massive erosion; intense transport of ice, sediment, and debris; and damage to vegetation and buildings. This paper attempts to reconstruct the 1818 Giétro flood by focusing on its water component. We develop a simple model to estimate the initial hydrograph during the slow drainage and failure phases. The flood's features are deduced by solving the shallow‐water equations numerically. The computational framework involves six free parameters, of which five are constrained by physical considerations. Using iterative manual parameter adjustments, we matched the numerical simulations to the historical data. We found that the peak discharge was close to 14,500 m3/s, the flood's front velocity was about 6 m/s, and flow depth varied considerably along the River Drance's bed (from 30 m just downstream of the ice dam to 2 m on the alluvial fan, 24 km west of the dam). To achieve a good agreement between computations and historical data, we had to select a high value for the Manning friction coefficient n (with n as large as 0.08 s/m1/3). As the Drance Valley is narrow, high flow resistance caused the flood's leading edge to behave like a plug, moving at a fairly constant velocity, with little dependence on what happened behind it. This result may explain why a simple flood routing model is able to reproduce the flood's features, because in an Alpine valley, a lateral spreading of the water volume is limited.

Reconstructing past flood events from geomorphological and historical data. The Giétro outburst flood in 1818

“…They also report that, downstream Martigny, the flood divided again into several branches and reached the Rhone River in different points on a distance of more than 4 km. The hydraulic modeling of the flood by Ancey et al (2019aAncey et al ( , 2019b shows that the water could have even reached the area of Charrat to the North-East. However, in absence of the exact former topography in the plain, the hydraulic model should be used cautiously here.…”

“…To reconstruct the spatial extent of the flood, we have adopted an interdisciplinary approach (Reynard et al, 2019) by compiling both historical sources and geomorphological observations (Table 1), and considering also the simulations of Ancey et al (2019aAncey et al ( , 2019b in the area of Martigny, where fewer data exist.…”

“…To prevent a new disaster, the latter mandated the cantonal engineer Ignace Venetz for mitigating the crisis. The solution imagined by Venetz was to empty the lake by digging a tunnel through the ice cone some meters above the lake level, so that the water would drain through the tunnel once reached its base, and, by thermal and mechanical erosion, would lower down the tunnel floor, which would gradually empty the lake (see Payot et al, 2018 andAncey et al, 2019a for additional details). The works were performed between the 11th of May and the 13th of June 1818.…”

“…After several precursor signals, the ice dam broke up the 16th of June at 4:30 pm, letting the 20 million cubic meters of remaining water emptying in 30 minutes. The peak discharge was estimated between 8000 et 15,000 m 3 /s (Ancey et al, 2019a(Ancey et al, , 2019bFunk et al, 2019). Charged with ice, sediment and wood, the flood devastated the valley and reached the city of Martigny one hour and half later, before getting to the Rhone River, at 459 m a.s.l.…”

“…Carrivick & Tweed, 2016;Haeberli, 1983). It is by far the most significant disaster of this type that occurred in Switzerland during the last centuries and is one of the world's most documented glacial lake outburst flood (Ancey et al, 2019a). It devastated all the valley down to the city of Martigny on a distance of about 35 km, killing 34 people and destroying hundreds of buildings as well as the road system and hydraulic infrastructures.…”