Tropical Storm Basyang unleashed heavy rainfall that exceeded Northern Mindanao’s 100-year return period, according to the Philippine Atmospheric, Geophysical and Astronomical Services Administration (PAGASA).

PAGASA said flooding in Iligan City was primarily caused by moderate to heavy rains that overwhelmed rivers and drainage systems.

“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 in a post on Saturday.

The Global Facility for Disaster Reduction and Recovery (GFDRR) explained that a 100-year flood refers to a 1-in-100 probability that a similar or more severe event could occur in any given year.

While such an event may happen two years or more in a row, GFDRR said this remains extremely rare.

“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,” GFDRR said.

“The most common misconception is that a 100-year flood will only occur once per century, but that is not true. There is a small probability that such an intense event could occur every year,” it added.

PAGASA said Basyang dumped more than 350 millimeters of rain in some upstream areas, triggering severe flooding that devastated communities, displaced thousands of residents, and claimed lives in Northern Mindanao.

The agency released the following 24-hour rainfall accumulation data from ground-based stations recorded on Friday:

Hindang ARG (automatic rain gauge): 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)
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,” PAGASA said.

The Office of Civil Defense (OCD) reported that Basyang has so far caused at least 12 deaths.

Iligan City was placed under a state of calamity on Friday due to the effects of intense rainfall and flooding, with authorities reporting that at least 3,800 families were displaced.

(With reports Jiselle Anne Casucian, GMA Integrated News)