The water came quickly.

In a matter of minutes, roads across Laoag City and neighboring towns disappeared beneath murky floodwaters as severe tropical storm “Kiyapo” unleashed relentless rain over Northern Luzon. Vehicles stalled, neighborhoods were inundated and hundreds of families sought safer ground.

Then, almost as suddenly, the water was gone.

Hours after each heavy downpour eased, streets that had become impassable were once again open to traffic. Residents who had watched floodwaters creep into their homes found them steadily retreating before the next burst of rain arrived.

The cycle repeated itself throughout the storm.

It was an observation shared by many residents and acknowledged by local disaster officials: flooding developed rapidly during periods of intense rainfall, but in many areas, the water also subsided within hours after the rain weakened.

The contrast raises a question that goes beyond the storm itself.

If floodwaters could drain away so quickly, why did they accumulate just as fast?

The answer is not as simple as saying it rained heavily, nor can it be explained by the presence or absence of flood-control projects alone.

What happened was the result of several factors converging at the same time — an extraordinary volume of rainfall, the way modern cities shed water, the condition of drainage networks and waterways, and the physical limits of infrastructure designed to manage flooding.

Philippine Atmospheric, Geophysical and Astronomical Services Administration (PAGASA) recorded 299.9 millimeters of rainfall in Laoag City between Friday morning and Saturday morning, equivalent to nearly three weeks’ worth of the city’s average July rainfall falling within just 24 hours.

At the height of the storm, Ilocos Norte was placed under a Red Rainfall Warning as “Kiyapo” and the enhanced southwest monsoon produced intense and persistent rainfall across the province.

Rainfall of that intensity does not simply fill rivers. It saturates the ground, overwhelms drainage canals, sends runoff racing through streets and low-lying communities, and briefly creates more water than existing drainage systems can immediately carry away.

The rapid disappearance of floodwaters may seem contradictory, but hydrologists say it actually offers an important clue about what occurred.

Much of what residents experienced was likely urban or pluvial flooding — a type of flooding caused when rain falls faster than drainage systems can temporarily accommodate.

Unlike river flooding, which may linger for days as swollen waterways gradually return to normal levels, urban flooding often rises and falls with the rain itself. Once the intensity of the downpour decreases, canals, drainage lines and rivers begin catching up, allowing accumulated water to flow away.

The quick recession of floodwaters therefore should not be mistaken as evidence that the flooding was insignificant. Instead, it suggests that drainage systems were temporarily overwhelmed rather than permanently incapacitated.

As rainfall eased, the volume of incoming water dropped below what those systems could discharge, allowing floodwaters to recede almost as quickly as they had formed.

Flood-control infrastructure also has practical engineering limits. Drainage systems, floodwalls and dikes are designed using projected rainfall intensities and river flows based on engineering standards.

They reduce flood risk, but they are not built to prevent flooding under every possible weather scenario. When rainfall exceeds the conditions for which these structures were designed, temporary flooding can still occur even if the infrastructure itself remains structurally sound.

The changing landscape has likewise altered how rainwater behaves.

As Laoag City and surrounding municipalities have expanded, more land has been covered by roads, buildings and concrete surfaces that prevent water from soaking into the ground.

Instead, rainwater moves rapidly across these impervious surfaces and into drainage canals. Every new subdivision, commercial establishment and paved road increases the amount of runoff entering the drainage system during heavy rainfall.

Many drainage networks, however, were constructed decades ago when urban populations were smaller and paved areas were considerably fewer. As cities continue to grow, the amount of stormwater generated during extreme weather events grows as well, placing greater demands on infrastructure that may not have been designed for today’s conditions.

The condition of rivers and waterways also plays an important role. Over time, sediment naturally accumulates in river channels, while vegetation, debris and waste can restrict the flow of water through canals and drainage systems.

These factors reduce the capacity of waterways to convey large volumes of runoff during intense storms. PAGASA identifies regular clearing and maintenance of waterways as key measures to help improve water flow and lessen flood risks.

Rather than pointing to a single cause, “Kiyapo” demonstrated how flooding develops when several systems reach their limits simultaneously. Extreme rainfall arrived faster than the landscape could absorb it.

Drainage canals received more runoff than they could immediately discharge. Rivers were already carrying enormous volumes of water from continuous rainfall. For a period of time, the amount of water entering the system simply exceeded the amount leaving it.