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Lahar advisory for Mayon volcano under Tropical Storm “Choi-wan” (Dante), Philippines

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Prolonged and heavy rainfall produced by Tropical Storm "Choi-wan" — known as Dante in the Philippines — may generate post-eruption lahars on major channels draining Mayon Volcano by incorporating loose material from remnant pyroclastic density current (PDC) deposits from the January – March 2018 eruption, PHIVOLCS said on June 1, 2021.

Based on the Tropical Cyclone Bulletin # 11 issued at 03:00 UTC on June 1, 2021 (11:00 LT) by the Philippine Atmospheric Geophysical and Astronomical Services Administration (PAGASA), the province of Albay has been placed under Tropical Cyclone Warning Signal No. 2 due to Tropical Storm "Choi-wan."

Choi-wan is expected to be over the coastal waters of Eastern Albay Province by the morning or early afternoon (LT) of June 2, 2021.

Due to its trajectory, and potentially heavy to at times intense rainfall, Choi-wan can be expected to generate volcanic sediment flows or lahars, muddy streamflows or muddy run-off in rivers and drainage areas of Mayon Volcano.

On the forecast track, the center of the tropical storm is forecast to make its initial landfall over Eastern Samar between tonight and tomorrow early morning, June 2 (LT). This tropical cyclone is more likely to remain as a tropical storm category prior to landfall and weaken into a tropical depression on Thursday, June 3 and to Low Pressure Area on Saturday, June 5.

Tropical Storm "Choi-wan" at 07:40 UTC on June 1, 2021. Credit: JMA/Himawari-8, RAMMB/CIRA, TW

DOST-PHIVOLCS thus strongly recommends increased vigilance and readiness of communities in pre-determined zones of lahar and related hazards.

"Prolonged and heavy rainfall may generate post-eruption lahars on major channels draining Mayon Volcano by incorporating loose material from remnant pyroclastic density current (PDC) deposits from the January-March 2018 eruption," PHIVOLCS said.

The bulk of erodible PDC deposits occupies the watershed areas of the Miisi, Mabinit, Buyuan and Basud Channels. In addition, older and erodible eruption deposits occupy the watershed areas on the eastern and western slopes of the edifice and can be remobilized as non-eruption lahars by erosion of banks and channel beds.

Potential lahars and sediment-laden streamflows may occur along the Miisi, Binaan, Anoling, Quirangay, Maninila, Masarawag, Muladbucad, Nasisi, Mabinit, Matan-ag, and Basud, Bantayan Channels in Albay Province.

Mayon lahars can threaten communities along the middle and lower slopes and downstream of these channels with inundation, burial and wash away.

DOST-PHIVOLCS strongly advises the communities and local government units of the above identified areas of risk to continually monitor the typhoon conditions and take pre-emptive response measures for their safety.

Flooding rains brought by Choi-wan have already claimed the life of one person and left another missing along the Banga River nir Norala on May 30.

Featured image: Tropical Storm "Choi-wan" at 07:40 UTC on June 1, 2021. Credit: JMA/Himawari-8, RAMMB/CIRA, TW

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