Tropical storm ‘‘Basyang’’ has dumped extremely rare volumes of rain that exceeded Northern Mindanao’s 100-year return period and contributed to flooding in Iligan City due to overwhelmed rivers and drainage systems.
The Philippine Atmospheric, Geophysical and Astronomical Services Administration (Pagasa), in a Facebook post, said the rainfall amounts exceeded the 100-year return period in the Northern Mindanao area.
‘‘This means that in any given year, there is a one percent chance that this kind of extreme rainfall event—or worse—could occur,’’ Pagasa said.
Basyang poured more than 350 millimeters of rain in several upstream areas, causing severe flooding that devastated communities, displaced thousands of residents, and claimed lives in Northern Mindanao.
Based on Pagasa’s 24-hour rainfall accumulation data from ground-based stations recorded on February 6, the Hindang automatic rain gauge (ARG) recorded 354.5 mm (heavy); Juan Tabla ARG, 361.0 mm (heavy); Doroteo D. Lloren ARG, 383.0 mm (heavy); Dulag ARG, 253.0 mm (heavy); Banggolo Bridge ARG, 280.5 mm (heavy); Iligan ARG, 144.5 mm (moderate); Agus 6 HEPP ARG, 130.5 mm (moderate); and Rogongon ARG, 145.5 mm (moderate).
‘‘Satellite images show that tropical storm ‘‘Basyang’’ produced a significant amount of rainfall over Mindanao and the Visayas. Rainfall warnings and weather and flood advisories were issued over these areas due to the storm,’’ the weather bureau said.
The Global Facility for Disaster Reduction and Recovery (GFDRR) pointed out that a 100-year flood refers to a one-in-100 probability that a similar or more severe event could occur in any given year.
Such an event could likely happen two years or more in a row, but ‘‘this remains extremely rare,’’ it said.
‘‘A longer return period (for example, 100 years versus 20 years) suggests a lower probability that an extreme hazard will occur in any single year,’’ it added.
‘‘The most common misconception is that a 100-year flood will occur only once per century, but that is not true. There is a small probability that such an intense event could occur every year,’’ it further stated.
Meanwhile, Pagasa attributed the rainy weather on Sunday to the combined effects of the shear line and the northeast monsoon, or amihan.
In an 11 a.m. advisory, Pagasa forecast 50 to 100 millimeters of rain over Quezon, Oriental Mindoro, Camarines Norte, Camarines Sur, Catanduanes, Albay, and Sorsogon due to the shear line until Monday noon, February 9.
The same weather conditions are expected to affect nearly the same provinces, including Northern and Eastern Samar, over the next few days.
The weather bureau said no new low-pressure area is being monitored after former tropical storm ‘‘Basyang,’’ internationally named Penha, dissipated at 8 a.m. Saturday.
The National Disaster Risk Reduction and Management Council said Basyang affected 64,360 families in Western Visayas, Negros Island, Central Visayas, and Caraga.
Office of Civil Defense spokesperson Junie Castillo, citing 6 a.m. data, said 12 fatalities have so far been recorded, all subject to validation. Four deaths were caused by a landslide in Cagayan de Oro City, while the others were due to drowning.
Castillo said floodwaters have since subsided and clearing operations are under way in affected areas.
The OCD reported 445 damaged houses, including 38 totally destroyed and 407 partially damaged, noting that the cost of damage to infrastructure and agriculture has yet to be consolidated.
In a separate radio interview, Department of Public Works and Highways Secretary Vince Dizon said Basyang damaged one bridge and one unfinished flood control project.
‘‘We mobilized operations to have steel delivered for the fixing of the bridge, which sustained minor damage,’’ Dizon said, adding that the flood control structure will be repaired by the contractor since it is still incomplete. ‘‘We are continuing the repairs around the clock,’’ he said.
