Field tests of hydric tephra remobilisations at the Tajogaite 2021 volcano (La Palma, Canary Islands, Spain)

Abstract

The eruption of the Tajogaite volcano at the island of La Palma (Canary Islands, Spain) between September and December 2021 generated a huge amount of fallout deposits (tephra), of which there are still accumulations of more than 3 metres thick in sectors close to the cone. During and after the eruption, the tephra has been remobilised, both by anthropic actions of removal from buildings, roads and other infrastructures, and by natural processes due to volcanic, aeolian, gravitational and runoff dynamics. Several remobilisation tests were carried out on field to estimate the water remobilisation capacity and prevent possible hazards and risks associated with extreme lahar-type events due to intense rainfall. They consisted on artificial rainfall and concentrated runoff on experimental micro-plots and sectors of slopes and at the bottom of one of the ravines in the southwestern sector of the island (Tamanca ravine). Therefore, it has been possible to estimate which variables (slope, granulometry, type of surface cover) condition both the generation of runoff and the start of remobilisation, as well as some indicative threshold values. Among all the variables, the one that has the greatest influence on the onset of remobilisations is not the slope, as might be expected, but the concentration of runoff on impermeable surfaces (consolidated rocks, anthropised surfaces) or on the branches and stems of shrub and tree vegetation. These results are of practical use for preventive mapping of areas susceptible to suffer remobilisation. They may constitute the input for numerical models simulating the triggering and circulation of non-Newtonian flows on slopes and gullies, from which risk maps for land-use planning and emergency management will be derived.