TARLAC CITY — Three days after a portion of the Ninoy Aquino Bridge collapsed, questions are shifting from what happened to why the decades-old structure gave way, with authorities examining the possible impact of powerful river currents, erosion, and damage to its foundation.

A substantial section of the bridge collapsed shortly before midnight on 28 August after heavy southwest monsoon rains caused the Tarlac River to swell and its currents to intensify.

Witnesses said the affected portion, near the boundary of Barangays Mabini and Cut-Cut 1, began showing signs of instability at around 11:30 p.m. before collapsing at approximately 11:55 p.m.

The Department of Public Works and Highways (DPWH) and local authorities have yet to establish the exact cause. Their assessment is expected to determine whether the collapse was linked to river scouring, foundation damage, structural deterioration, the bridge’s age, previous repairs, or a combination of factors.

Scouring is among the immediate concerns when a bridge spans a swollen river. Fast-moving water can wash away soil and sediment surrounding bridge piers and foundations, potentially reducing their support and leaving structural components vulnerable to movement or failure.

But while the Tarlac River was running high and fast when the bridge collapsed, the presence of strong currents alone does not establish that this caused the failure.

The investigation has also drawn attention to possible quarrying activities along the Tarlac River, amid questions of whether changes to the riverbed may have contributed to erosion and weakened the bridge’s foundations.

Any connection, however, between quarrying and the collapse remains unproven and would have to be established through an official technical investigation.

The Aquino Bridge is a major crossing on the Romulo Highway connecting western Tarlac City with the central Poblacion area. It was closed immediately following the collapse, with barricades installed at both ends of the Tibag–San Isidro crossing.

The incident has underscored concerns about the vulnerability of aging bridges during extreme weather. A historical account posted by the Tarlac provincial government said the bridge was constructed in 1988, although this had yet to be verified with the DPWH.

The collapse disrupted a key transport link, with motorists directed to alternate routes. The Agana Bridge was designated as an alternate crossing, but a portion of that structure reportedly collapsed the following night amid the continuing rains and strong river current.

Authorities are considering a temporary steel structure while the damaged bridge undergoes assessment.

For now, the central question remains unanswered: Was the Aquino Bridge brought down by the swollen river, a compromised foundation, decades of structural wear — or several factors acting together?

The ongoing technical investigation is expected to provide the answer.