DAVAO CITY, Philippines (PNA) -- The recent flooding of five barangays in Davao City was traced to degradation of some forest lands in Marilog District where headwaters of the Matina Pangi River came from.

Engr. Noel B. Angeles, regional head of the Mines and Geo-Sciences Bureau (MGB) mining environment and safety division, said "there is what we call a natural degradation aside from its history where during the '60s and '70s there were logging operations and later the continuing slash and burn of second generation trees in the area."

During the June 29 flooding, he said it only took 1 hour and 35 minutes for the headwaters from Marilog to travel to Pangi River or about 90 percent run off.

Based on MGB's computation using the rainfall record of the Philippine Atmospheric Geophysical and Astronomical Services Administration (Pag-ASA), Angeles said the rainfall during that particular time was abnormally high with about 20 millimeters per hour of rainfall or 393 cubic meters per second.

He said aside from the recorded rainfall they also got samples from six areas to include those in Bago Oshiro, Puan, Tamayong, Calinan, Bangkal, Matina Pangi.

The characteristic of the rainfall was accompanied with scattered thunderstorm where in some areas it recorded about 60 millimeters for about 3 hours.

He said the heavy downpour caused soil erosion where soils were brought down by the heavy rain towards the Matina Pangi River.

Angeles explained that quarrying of sand and gravel will not cause flooding but the quarrying of soil near river banks will as it causes soil erosion and silting of the river.

He also said the irregular shape of the river was caused by the solid particles like soil, stones or rocks causing obstructions that made the water flow to different directions while seeking its own level.

"Normally the river follows a straight shape but because of obstructions over the years, the volume of water flows on another route," Angeles said.

He noted, however, that there is no recorded history yet of flooding in Matina Pangi.