The rain arrives this Thursday on La Palma. At another time, this would not be news precisely in the rainiest island of the Canary Islands. However, the Cumbre Vieja volcano has created an environment that forces us to be alert to the slightest change. The State Meteorological Agency (Aemet) foresees weak rains during the afternoon, which will reach a maximum of two millimeters per hour (mm / h), and also locally moderate, which will reach two to 15 mm / h. David Suárez, delegate of the Aemet in the Canary Islands, explains that this type of precipitation is expected in the north and east of the island. A priori, This would not generate great damage in the western part, the one affected by the eruption, although the roofs or roofs of the infrastructures that have thick layers of ash are endangered.
Lahars, a new threat that rain can bring on La Palma
The geologist and spokesperson for the Canary Islands Volcanological Institute (Involcan), David Calvo, explains that when the ash is soaked “it acquires a beastly weight” and can lead to the fall of weakened structures where there is an accumulation of material composed of fine rock and mineral particles. that spits out the volcano. They can also reach the sewers if the water runs through the streets, but this is something “secondary” in the scenario envisaged by Aemet. “There is not enough level for floods to occur,” adds Calvo, a situation that would be much more worrying.
What would happen if a second Filomena arrived?
The Atlantic storm soaked La Palma completely, with intense and intermittent rains; in some points it reached 41 liters per square meter in one hour last January. In the hypothetical case that another storm arrived, such as during the volcanic eruption, different episodes would be generated depending on the ground on which heavy rains fall. The forestry engineer and technical director of Emergencies of the Cabildo de Gran Canaria, Federico Grillo, explains that in La Palma there are three types: that of the flows, the forest areas and the land without vegetation.
Grillo, respected and applauded during the management in charge of the operation that fought the great forest fire of Gran Canaria in 2019, details that in the soil covered by lava flows there would be no problems because “it is quite robust and permeable”, that is, “drains very well and it is very difficult for dragging to occur ”. Vegetated areas “are like a sponge”, that is, part of the rain stays on the ground and the rest is released “little by little” in the form of ravines or runoff. On La Palma, the north and northeast of the island “supports 60 or 80 liters per square meter in an hour without problems.”
If ash has fallen, “it can be mobilized”, but at very different levels from those that would be generated in areas open to vegetation, especially in urban areas. “When there is no vegetation, which protects the soil from the impact of rain, the drops end up breaking the ground and generate a layer of dissolved soil that is incorporated into the water, and that gives it an aggressive character”, specifies Grillo. Rain in this type of terrain quickly concentrates and affects the channel through which it runs, creating greater depth and widening it. “It becomes harder, heavier water, which has abrasive and erosive capacity and a brutal destructive power,” adds Grillo. In appearance, it is like mud.
When it acquires these characteristics it is called lahar, one of the seven main volcanic hazards defined by the National Geological Institute (IGN). They are avalanches of unconsolidated volcanic material that, in addition to the effect of intense rains, can be formed by the melting of glaciers, overflowing or melting. This phenomenon can be triggered even after the eruption on La Palma ends if a torrential rain falls and due to the characteristics of the soil, which influences the destructive power of the lahar.
In the event that it is generated after the end of the eruptive process, the technical director of the Cabildo de Gran Canaria explains it through an analogy with the fire that devastated 10,000 hectares on the island. After the flames, the ground was “very sensitive, with tremendous slopes.” Before the first rains arrived, they carried out corrective measures to mitigate the possible effects, such as setting a kind of levee in strategic areas along the path of the floods to alleviate their force. But the rainfall “was very calm and the system had time to absorb; one part to the infiltration of the soil, another part retained and did not release much water. We did not have big problems ”.
Although much of the ash has been removed from the island’s buildings, as it falls constantly, it is relatively easy to generate problems in the sewers, because a very rapid concentration of water is produced that clogs and clogs the drainage system. The consequence, as it happens in periods of heavy rains in Las Palmas de Gran Canaria, is flooding.
For this reason, Grillo warns that it is very important to clean the ashes, especially from the roofs or downspouts. “If I have a layer of 3 or 5 centimeters on the roof and it rains, it moves the ash down the drain and plugs it. With 40 or 50 centimeters of water the ceiling can sink ”, he points out.