Seismic survey of a devastating flood caused by the sudden draining of a glacial lake

On the 5th of July 2016, a wave of water about ten meters high swept down the steep slopes of Nepal’s Bhotekoshi / Sunkoshi Valley. This wave originated from a high mountain lake hitherto blocked by a pro-glacial moraine which, after sudden rupture, will release in one fell swoop more than 100,000 tons of water downstream, the equivalent of the weight from the Montparnasse tower. An international team of German (GFZ Potsdam), French (IGE Grenoble) and Nepali (Tribhuvan University, Kathmandu) scientists have studied this devastating event using a particularly innovative observation technique, based on soil vibration analysis. recorded by seismometers that had been deployed a year earlier following the Gorkha earthquake (April 2015).

In a study published in Science, the authors show that these flood events generated by the emptying of lakes, rare but particularly extreme, have a major impact on the long-term erosion of mountain valleys. Indeed, these unusual masses of water are the only ones to mobilize and transport the largest rocks present in the river, which usually protect the bed and limit its erosion. This discovery challenges the shared view that monsoon rainfall is the main force for erosion in mountain valleys. Thus, other forcings controlling the formation and evolution of glacial lakes, such as earthquakes and glacial dynamics resulting from climate change, could play a major role in fluvial erosion and thus in the evolution of the valleys of these rivers. regions.

Kristen Cook, a researcher at GFZ Potsdam, visited the valley before and after the flood, and describes the specific circumstances of this event. "The flood took place at the end of the day, when the inhabitants were still awake. It generated soil vibrations so strong that they could be felt by the population before the arrival of the wave. Two local residents who felt the flood approaching told him the facts : the youngest believed in an earthquake, but his eldest remembered the flood of 1981, and hastened to move away from the river, followed by other inhabitants. There were no casualties, but a lot of infrastructure was destroyed, mainly houses, bridges, hydroelectric dams and roads. For scientists who have deployed seismometers in the valley just before the flood, this event represents a rare opportunity for observation.

"With this seismic network, we could observe not a wave, but two ! "Says Florent Gimbert, researcher at IGE Grenoble. The first wave, consisting mainly of water, spread at a staggering speed of 30 km / h, 10 times faster than a normal flow in this type of river. The second, laden with many rocks and coarse sediments mixed with water, spread more slowly, but at 18 km / h all the same. "This second wave was the most energetic. Compared to the first, it was she who had to play a leading role in the destabilization of the river bed and banks. From these seismic observations, the researchers also show that the sediment transport in the river remained abnormally strong (up to 30 times stronger than normal) for several weeks after the flood. This recrudescence of transport materializes a non-equilibrium dynamic that shows that rivers keep in memory past extreme events. This component will have to be taken into account in erosion models. In view of the unique contributions of seismology to study these phenomena, a new project named SEISMORIV, led by Florent Gimbert, was funded by the National Research Agency. It aims to better understand the seismic signal generated by extreme flood events, in order to better assess their impacts in the context of global warming.

Source(s) :
Cook KL, Andermann C, Gimbert F, Adhikari BR, Hovius N (2018) Glacial lake outburst floods as drivers of fluvial erosion in the Himalaya, Science. doi : 10.1126/science.aat4981.

Contact(s) :
Florent Gimbert, IGE
florent.gimbert univ-grenoble-alpes.fr, 04 76 82 42 11

"Source : Actualités du CNRS-INSU"