Lake outburst with seismometers listening to the flood

An international team of scientists including french researchers from the Institut des Géosciences de l’Environnement (IGE/OSUG, Université Grenoble Alpes/IRD/CNRS/Grenoble INP) have recently studied a particularly devastating flood caused by the sudden drainage of a pro-glacial lake in the Bhotekoshi / Sunkoshi Valley, Nepal. Using a particularly innovative seismic technique that consists in evaluating the ground shaking caused by the flood, the scientists are able to quantify flood processes that were impossible to observe using traditional techniques.

On July 5, 2016, a devastating wall of water gushed down the Bhotekoshi/Sunkoshi River in Nepal. It came from a lake that had been dammed by a glacial moraine but the dam broke and discharged more than 100,000 tons of water all at once. An international team of GFZ and Nepali scientists was able to record the sudden outburst with seismometers deployed the year before in the wake of the catastrophic Ghorka earthquake in April 2015.

In a study published in SCIENCE, the authors argue that such major flood events have a greater impact on erosion rates than the annual monsoon rainfalls. The reason is that the water masses mobilize large boulders and coarse sediments that usually protect the riverbed, even under monsoon-type floods during which those large boulders remain stable. "Erosion rates may be strongly influenced by non-climatological drivers such as earthquakes and the climatic factors that affect the size and distribution of glacial lakes", the authors write in their paper.

Kristen Cook, member of Niels Hovius’ team at the GFZ and first author, visited the valley before and after the flood. Several lucky circumstances surrounded the event: The flood occurred in the evening when people were around their homes home but not asleep, and it was so powerful that it caused to ground to shake, says Kristen. She talked with two local residents who felt and heard the flood approaching. The younger one told her that he had thought of an earthquake but his elder companion remembered another flood from 1981 and urged to climb away from the river. Other people reacted similarly and fled in time so no people were hurt. However, there was substantial damage to infrastructure, bridges were destroyed as well as hydro-power stations and roads. From the scientists’ point of view, the researchers were lucky to have deployed seismometers in the valley well before the flood occurred.

"With this seismic network, we observed not one wave, but two!", notes Florent Gimbert, researcher at IGE Grenoble. The first wave was essentially made out of water, and propagated downstream at a staggering speed of 30 km / h, which is about 10 times faster than regular flow in such a river setting. The second was instead essentially made out of rocks and coarse sediments mixed with water, and propagated downstream at a slower pace, although still at the particularly high speed of 18 km / h. "This second wave was the most energetic and probably responsible for 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, so as to better assess their impacts in the context of global warming.