As Tropical Depression "Quinta" is poised to make a landfall over the Bicol Region, its heavy rains could generate post-eruption lahar along the slopes of Mayon Volcano, threatening nearby communities.
In an advisory issued Saturday, the Philippine Institute of Volcanology and Seismology (Phivolcs) warned residents in "pre-determined zones" to be vigilant and ready for such a possibility.
‘Quinta’ is expected to make a landfall over the Bicol Region between Sunday evening and Monday morning where it will bring light to moderate, with at times heavy rainfall, the Philippine Atmospheric Geophysical and Astronomical Services Administration (PAGASA) said in its latest forecast.
"Prolonged and heavy rainfall may generate post-eruption lahars on major channels draining the Mayon Volcano edifice by incorporating loose material from thick pyroclastic density current (PDC) deposits and remnant ashfall from the January to March 2018 eruption," Phivolcs explained.
According to Phivolcs, the bulk of erodible PDC deposits occupy the watershed areas of the Miisi, Mabinit, Buyuan, and Basud Channels, while older and erodible eruption deposits occupy the watershed areas on the eastern and western slopes of the mountain.
These deposits, Phivolcs added, can be remobilized as "non-eruption lahars" by the erosion of banks and channel beds and can threaten communities along the middle and lower slopes and downstream of these channels with inundation, burial, and wash away.
Phivolcs said lahar flows were already observed in barangays Maninila and Tandarora in Guinobatan, Albay last October 15 and 20 due to torrential heavy rains brought by Typhoons "Ofel" and "Pepito," respectively.
With this, the bureau warned of potential lahar flows and sediment-laden streamflows on all river channels draining the slopes of Mayon Volcano, particularly the Miisi, Binaan, Anoling, Quirangay, Maninila, Masarawag, Muladbucad, Nasisi, Mabinit, Matan-ag, and Basud Channels, that may be generated by heavy rainfall brought about by the tropical depression.
Communities and local government units beside these drainages are also advised to be very vigilant and to move residents to high ground should heavy rains occur.
Alert Level 1 remains in place over Mayon Volcano which means that it is at an "abnormal condition."
Although no magmatic eruption is imminent, Phivolcs strongly advised the public to refrain from entering the six-kilometer radius permanent danger zone due to the perennial life-threatening dangers of rockfall, landslides or avalanches at the middle to the upper slope, sudden ash puffs, and steam-driven or phreatic eruptions from the summit.