Rain water which in another time would be good news for farmers in western La Palma, but today, when lava flows have destroyed more than 251 hectares of crops – 30% of the total area affected -, Precious showers become one more complication. The culprits of this radical change are not lava flows, but the ashes expelled by the volcano. Huge mountains of pyroclasts less than two millimeters in diameter get heavier in contact with rain, becoming materials that can unravel new hazards. In this way, the ashes can end up clogging the sewers, collapsing roofs or even, and in more exceptional cases, generate lahars.
The Federation of Entrepreneurs urges residents to clean the ashes from sidewalks and streets on the weekend
At the moment, with the population of the exclusion zone evacuated and under the influence of very weak rains, the main risk facing the population of the island is the clogging of its sewers. This was stated the Federation of Entrepreneurs of La Palma that, in his Twitter account, he urged citizens to sweep the ashes off “sidewalks and streets”, given the possibility of weak and occasional rains. And it is that the ash, in contact with water, in addition to gaining weight, turns into a “paste” that can cause problems of various kinds in the sewers. According to the forecasts of the State Meteorological Agency (Aemet), these showers will fall with greater probability on the north and east slopes of the Isla Bonita – that is, on the opposite side to the place where the volcano is located. «We are ending the episode starring the Saharan air mass and starting tomorrow [hoy para el lector] La Palma will be under the influence of a maritime air mass, “he said. David Suárez, provincial delegate of the Aemet in the Canary Islands. This will cause the probability of rain to increase, although as highlighted, if it ends up falling, it will be very weak given that the cloudiness comes from the north slope. This weather forecast will last for the weekend. Another possibility is that, due to the weight acquired by the ashes mixed with the water, the roofs of the houses may collapse. Hence, the continuous removal of these small pyroclasts from the roofs of the houses is requested.
For a lahar to occur, it must rain heavily for a long time in an area with a slope
But mixing ash with water has more associated risks. The most extreme, although also the least likely on the island, is that of the lahars. These structures are formed as a consequence of the dragging of volcanic materials when very intense rains occur, forming slides of materials down the slope. There are several records that report events of this type throughout history on the island. A few years after the San Juan volcano erupted in 1949, an ash slide on the ground generated new losses beyond those caused by the volcano itself. As Carmen Romero, a geographer at the University of La Laguna (ULL), recounted, a few years after the eruption “road traffic recovered in the area”, but in these laborious works the secondary effects of the ashes were not taken into account. Hence, with the first rains, small lahars formed “that caused the loss of livestock and at least eight victims among the workers who were in the area.” But you don’t have to look so far back to find the occurrence of a phenomenon like this on La Palma. In 2009, after a large fire, the occurrence of a phenomenon with similar characteristics to lahar was also reported. “It was actually a derby flow that went down to the sea and destroyed several banana trees”, emphasizes the geographer of the Chair of Disaster Risk Reduction and Resilient Cities of the ULL, Pedro Dorta.
In 2009, after a large fire, an ash slide similar to lahar occurred on La Palma, known as ‘derby flow’
For it to happen, however, “it has to rain heavily for quite some time,” as Dorta writes. In addition, the accumulation of ash would have to be large and the slope very steep. The climatic conditions are only met in Los Llanos, El Paso and Tazacorte – the area of exclusion of the volcano – when a southwest storm enters. “These types of phenomena generate very intense rainfall, but they are also very rare,” recalls Dorta. Even so, if it does occur, the “effects may be very local.” That is, it is likely that it only happens in those areas that meet all the requirements.
The storm will enter from the north and will leave weak showers and some cloudiness in the east of the island
In any case, it must be a risk that is taken into account in the emergency action plans, since in addition, it could occur long after the eruption ended, as it happened in 1949. «The risk of it happening is very low, but it exists, and that is why we have to draw up plans and monitor their occurrence “, reviews Dorta. These works must be added to the revaluation of the associated risks of the terrain once the eruption is over, since “the orography has totally changed”, so the risk that future runoff will produce on the new terrain must be re-measured.