Seismic network can serve as early warning of floods

Seismic networks could be used to establish a flash flood early warning system for high mountain regions, according to a new study published in Science.

Researchers from the German Center for Research in Geosciences (GFZ), together with their colleagues from the National Geophysical Research Institute of India (NGRI), have studied minute by minute with data from a network of seismometers the catastrophic rockfall of the past 7th of February in the Ganges Valley of Dhauli, India, with the subsequent flood that killed a hundred people and destroyed two hydroelectric plants. (early warning of floods)

early warning of floods

The seismometers registered the signal at 10:21 and 14 seconds, local time. 54 seconds later, the mass hit the bottom of the valley at 3,730 meters high, generating an impact equivalent to a magnitude 3.8 earthquake.

In the valley, the mixture of rock and ice mobilized additional debris and ice, which - mixed with water - rolled down the valleys of the Ronti Gad and Rishi Ganga rivers like a gigantic flow of debris and flooding.

First author Kristen Cook of the GFZ estimates that the mass initially shot downhill at nearly 100 kilometers per hour and after about ten minutes the movement slowed to just under 40 kilometers per hour.

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At 10:58:33 AM, the flood reached a major road bridge near Joshimath. In a matter of seconds, the water rose 16 meters. Thirty kilometers further down the valley, the Chinka gauging station recorded a 3.6-meter drop in the water level, and another sixty kilometers further down, the level still rose one meter.

Based on the ground shaking signals recorded by seismic stations, the collective research of partners from the GFZ Geomorphology, Seismic Hazard and Risk Dynamics, and Earthquake and Volcano Physics sections, together with colleagues from the NGRI, identified three distinct phases of the catastrophic flood.

catastrophic flood

• Phase 1 was the rockslide and its massive impact on the valley floor.

• Phase 2 followed, with the mobilization of huge amounts of material - ice, debris, mud - creating a devastating wall of material that rushed through a narrow and winding valley, where a large amount of material and energy remained. it decreased rapidly over time. This lasted about thirteen minutes.

• Phase 3 (fifty minutes) was more like a flood, with huge amounts of water that flowed downstream, dragging large rocks up to 20 m in diameter.

• They emphasize that the most important finding is that "the data from seismic instruments are adequate as the basis for an early warning system that warns of the arrival of these catastrophic debris flows, " says Niels Hovius, last author of the study and scientific director at functions of the German Center for Research in Geosciences GFZ.

• Another key point is the availability of a dense seismic network, operated by Indian colleagues from the National Geophysical Research Institute of India (NGRI).

Hovius's colleague, Kristen Cook, the first author of the paper, adds that "the alert time available for places located in valleys depends on the distance and speed of the downstream flow front."

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For example, Joshimath, where the river level rose 16 meters during the flood, was 34.6 km downstream of the landslide. Kristen Cook explains that "this means that the inhabitants of Joshimath and its surroundings could have received a warning half an hour before the flood arrived ." For the regions higher up, where the wave arrived only a few minutes after the detachment, it could have been enough to shut down the power plants.

regional seismic network

The solution the GFZ researchers are working on with their colleagues in India and Nepal is a compromise: install stations in strategic locations that will form the backbone of a high mountain flood early warning system.

According to Marco Pilz, “this compromise, in a way, is an optimization problem that future studies will have to tackle and in which we have already made systematic progress, for example in the German region of the Lower Rhine Bay ”. A more in-depth analysis of flash floods and debris flows will help better understand how seismic signals can aid early warning. "

The first ideas to establish such an early warning system based on a seismological approach emerged long before the catastrophe as a result of a joint workshop of Helmholtz researchers and Indian colleagues in Bangalore in the spring of 2019.

The current study project was initiated by NGRI's Virendra Tiwari and Niels Hovius. It took advantage of flood placement and a regional seismic network already established by the National Institute of Geophysical Research of India.

Hovius says that “ early warning is increasingly urgent as mountain rivers are increasingly used for hydroelectric power generation, seen as an engine for economic development in some of the poorest mountain regions in the world. Given that catastrophic floods are also likely to be more frequent with the warming of the climate - he adds -, which will cause the rapid retreat of glaciers and the precarious stagnation of meltwater in high places, future risks will increase even more”.

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