MANILA, Philippines?Too much rain fell on Marikina City during the Sept. 26 drenching by Tropical Storm ?Ondoy? (international codename: Ketsana) but more residents could have been saved had they been warned of flooding upstream nine hours before it hit.

That was the time it took for the floodwaters to travel from the watershed in San Isidro, Rizal, to J.P. Rizal Street in Marikina City.

?They had nine hours to evacuate, but there was no mechanism in place in which they could be warned of the coming waters,? said C.P. David, a geology professor at the University of the Philippines.

In a study presented at a forum organized by the UP National Institute of Geological Sciences on Tuesday, David said that the peak flood happened earlier upstream in the Marikina River and later subsided upon reaching low-lying areas of Marikina City.

?It?s like a big bulk of water rushing downward,? David explained.

David and his team interviewed residents from several points along the Marikina River beginning from the San Isidro watershed to J.P. Rizal Street.

Based on data from the Philippine Atmospheric, Geophysical and Astronomical Services Administration (PAGASA), the heaviest rainfall happened between 3 a.m. and 6 a.m., David said.

He noted how San Isidro residents had reported inundation in their area at around 9 a.m. to noon, and those in areas further downstream much later.

At Wawa in Montalban, David said residents noted that the torrent swept in at around 10 a.m. to noon, and at the San Mateo Bridge, around 2 a.m. to 6 p.m.

?In Barangay Nangka, the residents said there was flooding at around 4 p.m. to 6 p.m., and at the last point, J.P. Rizal, flooding began at around 6 p.m. to around midnight,? David explained.

David added that the flooding in the higher areas, such as San Isidro, subsided soon after two hours while the lowest point on J.P. Rizal Street was under water even after 12 hours.

Early warning system

?We cannot stop the flooding because the volume of rain was really too much, but we could have done something if we had known the flooding in Wawa early on,? David said.

The geologist called for an early warning system to monitor flood and rainfall levels along river systems, which then would be useful in issuing warnings if the levels get critical. He also noted a lag time of four to 12 hours between the peak rainfall and actual flooding in several areas.

?There are such warning systems in Pampanga and Bicol, but here in Metro Manila, I guess we didn?t foresee that we would be flooded this bad,? David said, noting that Marikina is not below sea level, unlike Navotas and Taguig.

Not straight but meandering

Hydraulics engineer Guillermo Tabios, however, cited the obstruction along rivers and areas not previously flood-prone and issues like urbanization and climate change.

Tabios of the UP College of Engineering said he used a hydraulic simulation to see how the water flowed down to Marikina City during Ondoy.

Tabios explained that the Marikina River is a ?meandering river,? meaning it snakes from one bend to another.

?But in the instance of a flood coming from a higher area, the water will flow down in a straight path and may flow even onto the low-lying areas near the bends,? he said.

Tabios cited the areas of SM Marikina, where flooding was as high as eight meters. Flooding at Provident Villages reached five to six meters.

Tabios said certain areas such as Provident Villages were flood plains or low-lying areas where water from the river would overflow.

?You?re not supposed to build anything six times the width of the river. Instead of fighting the river, you should just let the river flow its course,? he said, noting that the floodwater increased its speed as it hit Marikina City.

Tabios cited the presence of obstructions along the Manggahan Floodway, which he said was originally designed to be 260 meters in width and a high water level of 14 meters.

?But during Ondoy, the elevation of the water went up to 15 meters higher, which is two meters higher than its original design,? he added.

Other obstructions come in the form of sediment, water lilies and shanties of informal settlers, he said.

Detention ponds proposed

Tabios suggested the use of ?detention ponds? near riverbanks where floodwaters can be temporarily stored and then eventually released to flow downstream.

Developers of subdivisions, he said, should conduct geological studies on their property, especially if the area is a flood plain.

?Otherwise, are they willing to invest in technologies to protect the people in their areas? Or maybe we have to rethink on building dikes that would withstand floods for a longer period of time,? Tabios said.