Tuesday night torrent swells Buhisan dam

THE torrential rain that inundated streets and low-lying barangays in Metro Cebu on Tuesday night, Oct. 13, 2020, also swelled garbage-clogged drainage, creeks and rivers, killed three persons, and, yet somehow, it poured a tad of good news for the Metropolitan Cebu Water District (MCWD)—it overflowed Buhisan dam in Cebu City.

Since then, the dam has been producing 4,000 cubic meters of water per day (cmd) from only 3,500 cmd last week, according to Edna Inocando, MCWD’s public information division manager.

The increase, however, is still below the dam’s normal production of 5,000 to 6,000 cmd.

Inocando said the water supply production rate in Buhisan dam is affected by silt accumulated at the bottom of the dam. A silt is made up of rock and mineral particles that are larger than clay but smaller than sand.

A pileup of silt reduces a dam’s storage volume, and it causes the water to overflow fast during a heavy downpour.

However, MCWD has already applied desilting in its two dams in Barangay Buhisan and Barangay Jaclupan, Talisay City.

Inocando said desilting of dams is regularly done every dry season, but this activity was postponed in the summer months this year because of the Covid-19 pandemic.

The Jaclupan facility’s production has decreased to 31,500 cmd from 33,000 cmd.

Aside from siltation, Inocando said turbidity, or when the water loses its transparency due to the presence of suspended particles, also affects the amount of water supply the dam produces.

Tuesday downpour

The Tuesday night downpour produced 81 millimeters of rainwater, equivalent to 405,000 barrels of rain per square kilometer, according to Al Quiblat, chief of the Philippine Atmospheric, Geophysical and Astronomical Services Administration (Pagasa)-Mactan station.

According to Quiblat, the rainfall that lasted for one hour and 30 minutes was also equivalent to a month’s worth of rain that the Pagasa-Mactan station had recorded last February.

The heavy downpour was an indirect effect of Tropical Depression Ofel, which along with the southwest monsoon (habagat), created thunderstorm clouds last Tuesday.

Quiblat said a torrential rain dumped more than 35 millimeters of rainfall in an hour on a certain area.

The Pagasa official has called for continuous declogging activities in canals, waterways and drainage systems.

The La Niña and monsoon-induced weather systems could severely affect Central Visayas in the last two months of the year.

The Pagasa announced on Oct. 2 the onset of the La Niña phenomenon, a weather pattern that causes increased rainfall. (WBS)