Santa Cruz de Tenerife (Spain), Jan 11 (EFE) .- The amount of ash emitted by the volcano on the Spanish island of La Palma, the rainfall in the area and the slopes of its ridge increase the risk of produce floods of mud, for which scientists and authorities have identified, through a pioneering computer model in Spain, about 30 points that could suffer a flood.
The end of the emission of pyroclasts and other volcanic products after the eruption that lasted 85 days on La Palma does not mean the cessation of the dangers associated with this natural phenomenon, and now is when three risks remain: the emission of gases, the temperature and the stability of runoffs and hyper-concentrated flows.
The research professor of the Department of Geological Risks and Climate Change of the Geological and Mining Institute of Spain (IGME-CSIC) Andrés Díez explains, in an interview with Efe, that these hyper-concentrated flows are called “lahars” in volcanic areas and are formed by highly ash-laden water and pyroclastic deposits from the volcano.
When it rains a lot, the ash is mobilized, forming mud flows that are channeled through the ravines and can reach nearby towns with destructive power, as happened with the Nevado de Ruiz volcano in Colombia in 1985.
It is a frequent risk in areas with high-altitude volcanoes, of more than 3,000 or 4,000 meters as in the Andes and the Kamchatka peninsula in Russia, and on whose summits snow or ice glaciers are deposited and after an eruption, with the Remaining heat melts and gives rise to these hyper-concentrated ash flows.
These flows could also occur on La Palma. Although the still unnamed volcano of Cumbre Vieja does not reach this height and there is no snow or ice at its top, there are three factors that can give rise to these lahars and, in fact, it is documented that the San Juan eruption in 1949 they formed two in the Las Manchas area, says Andrés Díez.
The first factor that comes together is the amount of ash that has accumulated in an “especially productive” eruption, since IGME researchers have measured thicknesses of more than two meters outside the cone in some areas.
It is a recently fallen ash and therefore it is loose, not compacted and is quite unstable, says the researcher.
The second factor that contributes to this risk is the enormous slopes of the area, since this volcano has emerged practically on the Cumbre Vieja ridge, with hills of about 1,800-1,900 meters of altitude and a few kilometers from the sea, that is, with a very steep slope in a short distance.
And the last, and most important, is the torrential nature of the rains on La Palma, with highly concentrated rainfall in time and space, continues Andrés Díez, who cites as an example that 80 percent of the rain that falls in the island occurs in the fourth part of the days that it rains.
In addition, 50 percent of the rain episodes are concentrated in a single day and in 45 percent of the rainiest days, more than 200 liters per square meter were registered.
The IGME has undertaken a comprehensive study on this risk that has begun with the analysis of the rain regime on the island, the measurement of the ash fallen in more than 700 points on La Palma and the use of the Iber computer program, developed by the University Polytechnic of Catalonia, in its version 3.1 -the first time it is used in Spain-, to develop a predictive model of what would happen if that ash reached the bottom of the ravines.
With this computer model, it is possible to “see” which areas could be flooded by the mud flow, how thick it would be and at what speed it could flow (and therefore its ability to destroy a bridge, farms or houses) through a multilayer analysis. in which the Complutense University of Madrid has also collaborated.
The Cabildo de La Palma has also provided an inventory of bridges, roads, towns and other infrastructures to determine their status and a list has been created with some 30 conflictive points that could be reached by a flow of mud.
In this way, if torrential rains were to occur on the island, it would be known in advance where it would be necessary to evacuate.