Eleven years after Ondoy, the same neighbourhoods flooded and a whole northern valley went under. Here is why that repetition is the point.
In November 2020, Typhoon Ulysses (international name Vamco) crossed Luzon. In Metro Manila, the Marikina River rose above the levels of 2009 in places. Rescues repeated in the same barangays that had been rescued eleven years earlier.
Then the water moved north. The Cagayan Valley, drained by the country's largest river, flooded on a scale that surprised even people who live in a floodplain. Tuguegarao, Ilagan, Alcala, Amulung and dozens of towns were under water, some for days.
Ondoy in 2009 was described at the time as unprecedented. Eleven years later, the same neighbourhoods flooded again, in some places worse.
That is not bad luck. It is what a floodplain does. A 100-year flood is a description of average frequency over a long period, not a promise of spacing. Two of them can arrive eleven years apart, or two years apart, and neither outcome contradicts the label. The mechanics are in flood return periods explained.
The practical version: the fact that a place flooded recently is not evidence that it is now safe for a while. It is evidence that it is the kind of place that floods.
The northern flooding raised a second question that is still argued about: how much of it came from rain, and how much from water released from Magat Dam as the reservoir filled.
The honest answer is that both were involved, and the argument is a distraction from the useful point for a buyer. A large reservoir upstream changes the timing and the peak of what arrives downstream. It is part of the water system you are buying into, and it is not a hazard layer on any screening.
If your property is downstream of a major dam, ask the operating agency and the provincial disaster office how release warnings work, who receives them, and how much lead time they give. The general version is in living downstream of a dam.
The Marikina-specific version is in buying in Marikina, and the northern one in buying in Tuguegarao or Cagayan and buying in Isabela.
In areas affected in 2020, people can usually give you exact heights, because they marked them.
A hazard screening reports mapped susceptibility, elevation, slope, and distances at your point from public national datasets. It does not model dam operations, forecast rainfall, or account for works completed since the mapping. Those belong to the operating agency, PAGASA, and the local engineering office.
CheckHazard does not replace a professional geotechnical or engineering survey.