Post-Flood Field Surveys in Vietnam and the Philippines

Introduction: When Floods Become a Shared Regional Reality

Extreme flooding has become an increasingly familiar reality across Southeast Asia. In recent years, the intensity, frequency, and spatial extent of flood events have grown, driven by a combination of climate change, rapid urbanization, watershed degradation, and complex river system interactions. While floods are often analyzed through models and satellite imagery, their actual impacts and the mechanisms that shape them can only be fully understood on the ground. In January 2026, the Socio & Eco-Environmental Risk Management Laboratory(Kantoush Lab) of the Disaster Prevention Research Institute (DPRI), Kyoto University,conducted a coordinated post-flood field survey in two major river basins in Southeast Asia: the Vu Gia–Thu Bon River Basin in Vietnam and the Cagayan River Basin in the Philippines. These surveys were undertaken following a season marked by record-breaking rainfall, successive typhoons, and widespread socio-economic disruption. The field mission was led by Sameh Ahmed Kantoush, Professor at DPRI–Kyoto University,with technical leadership from Mohamed Saber Mohamed Sayed Ahmed, Associate Professor at DPRI. The research team includedMiki Yamai, MS Student, Jerome G.Gacu, and Czarimah Singson-Valdez, both PhD students at the same Laboratory. The mission aimed to move beyond headline flood statistics and examine how floods actually unfolded, how communities and institutions responded, and what lessons could be drawn to improve flood risk management in a changing climate.

Why Vietnam and the Philippines? A Shared Flood Context

Vietnam and the Philippines share striking similarities in their flood risk profiles. Both countries are highly exposed to tropical cyclones and extreme monsoon rainfall. Their major cities and agricultural areas are concentrated along floodplains and downstream river reaches. Upstream reservoirs support irrigation, hydropower, and water supply,while also playing a critical role in flood regulation. At the same time, rapid urban expansion and land-use change have altered natural drainage and sediment processes. What makes the comparison particularly valuable is that both basins experienced extreme flood events within the same season, yet exhibited different flood outcomes and response capacities. This provided a rare opportunity to study comparable hazards under different governance, operational, and technological conditions.

Central Vietnam: Record Rainfall and Rapid Flood Escalation

In late October 2025, Central Vietnam experienced one of its most severe flood events in recent history. An intense atmospheric river system, formed by the interaction of cold northeast winds and warm, moisture-laden air from the southeast, produced unprecedented rainfall. At Bach Ma Station near Hue, rainfall exceeded 1,700 mm in a single day, setting a new national record (Figure 1).

Downstream cities such as Da Nang and the historic town of Hoi An were rapidly inundated. During the field survey, the research team documented flood marks reaching several meters above ground level, extensive riverbank erosion, and large volumes of sediment deposited in residential and commercial areas. In many locations, floodwater rose by nearly two meters within a few hours, leaving little time for response. Field observations revealed that the impacts were not driven by rainfall alone. Rapid river response, short forecast lead times, and uncoordinated reservoir releasessignificantly amplified flood severity. Urban drainage systems were quickly overwhelmed, while multiple flood peaks reduced the system’s ability to recover between events. These compound factors transformed extreme rainfall into a large-scale urban disaster.

Northern Philippines: Successive Typhoons and Basin-Wide Flooding

Only weeks later, Northern Luzon in the Philippines faced its own flood crisis. TheCagayan River Basin, the country’s most extensive river system, was struck by an unusual sequence of three tropical cyclones within a short period. Each storm added rainfall to already saturated soils, progressively increasing runoff and river levels across the basin.

Downstream cities such as Da Nang and the historic town of Hoi An were rapidly inundated. During the field survey, the research team documented flood marks reaching several meters above ground level, extensive riverbank erosion, and large volumes of sediment deposited in residential and commercial areas. In many locations, floodwater rose by nearly two meters within a few hours, leaving little time for response. Field observations revealed that the impacts were not driven by rainfall alone. Rapid river response, short forecast lead times, and uncoordinated reservoir releasessignificantly amplified flood severity. Urban drainage systems were quickly overwhelmed, while multiple flood peaks reduced the system’s ability to recover between events. These compound factors transformed extreme rainfall into a large-scale urban disaster.

Cities such as Tuguegarao, Cauayan, and Ilagan experienced widespread flooding, with large portions of urban and agricultural land submerged (Figure 2). Despite pre-emptive water releases from major reservoirs, including Magat Dam, floodwaters still overtopped riverbanks due to the sheer volume of inflow from uncontrolled tributaries. During the field survey, researchers documented flood depths, sediment deposition in urban streets and farmlands, and extensive riverbank erosion. In some areas, agricultural zones effectively became temporary flood storage areas, functioning as extensive floodplains or “flood lakes” during peak flows. These observations highlighted the basin-scale nature of the flooding and the limits of localized structural measures. During the field survey, researchers documented flood depths, sediment deposition in urban streets and farmlands, and extensive riverbank erosion. In some areas,agricultural zones effectively became temporary flood storage areas, functioning as extensive floodplains or “flood lakes” during peak flows. These observations highlighted the basin-scale nature of the flooding and the limits of localized structural measures.

What the Team Did in the Field

The post-flood surveys were designed to systematically capture physical, operational, and social dimensions of flooding. Activities included:

1. Flood mark documentation along rivers, streets, and buildings to reconstruct flood depths and spatial extent;

2. Observation of sediment deposits in urban and agricultural areas, providing insight into erosion and transport processes;

3. Assessment of river morphology and bank conditions, particularly in erosion- prone reaches;

4. Site visits to flood-prone infrastructure, including bridges, levees, drainage outlets, and reservoir outlets.

5. Engagement with local officials and communities, gathering firsthand accounts of flood timing, warnings, evacuation, and recovery These ground observations serve as essential validation data for hydrologic, hydraulic, and sediment transport models currently being developed by the research team.

Listening to Communities and Institutions

Beyond physical measurements, the surveys emphasized human experience and governance processes. Meetings were held with local government units, disaster risk reduction offices, universities, and community representatives (Figure 3). These discussions revealed common challenges across both countries: limited warning time, fragmented data systems, and the difficulty of coordinating decisions during rapidly evolving flood events.

In the Philippines, a Graduate School Colloquium on Integrated Water Resources Management provided a platform for sharing preliminary findings and engaging students, researchers, and practitioners. In Vietnam, exchanges with local experts highlighted ongoing efforts to integrate smart technologies into flood management. Across both settings, a consistent message emerged: effective flood resilience depends as much on information flow and coordination as on physical infrastructure.

Comparative Insights: Why Outcomes Differed

While both basins experienced extreme hydrometeorological forcing, their outcomes differed in meaningful ways. In Vietnam, particularly in Da Nang, the presence of integrated monitoring platforms and centralized decision-support systems enabled faster situational awareness and response (Figure 4). Real-time integration of rainfall data, river levels, traffic conditions, and community reports allowed authorities to prioritize evacuations and emergency actions despite short lead times.

In contrast, flood response in the Cagayan River Basin relied more heavily on manual monitoring, fragmented reporting, and reactive coordination. While local efforts were substantial, the absence of a unified, basin-wide operational picture limited the effectiveness of response under successive extreme events. These contrasts do not point to failure or success in isolation, but rather illustrate how institutional capacity and data integration can significantly influence flood outcomes, even when infrastructure and hazard levels are similar.

Key Lessons from the Post-Flood Surveys

Several overarching lessons emerged from the comparative field investigations:

1. Extreme floods are compound events - Flood impacts result from the interaction of rainfall, dam operations, river hydraulics, sediment dynamics, urban drainage,and governance structures, not from rainfall alone.

2. Field evidence remains indispensable - Flood marks, sediment deposits, and community observations provide critical insights that cannot be captured by remote sensing or models alone.

3. Smart governance enhances resilience - Integrated monitoring, real-time data sharing, and coordinated decision-making significantly improve response capacity during extreme events.

4. Communities are active agents, not passive victims - Local knowledge, preparedness practices, and communication channels play a vital role in reducing loss of life and supporting recovery.

Looking Forward: From Observation to Action

The findings from these post-flood surveys will directly support ongoing research and capacity-building activities of the Kantoush Lab. Field data will be used to validate and refine hydrologic–hydraulic and sediment models, improve flood hazard mapping, and develop integrated flood and sediment management strategies tailored to Southeast Asian river basins. More broadly, the mission reinforces the importance of learning across borders. The suggest highlights the three pillars based on the assessment shown in Figure 6. What happened in Central Vietnam can happen in Northern Philippines, and vice versa. By sharing experiences, data, and solutions, river basin managers and researchers can move toward more adaptive, people-centered flood risk management.

As climate extremes intensify, flood resilience will not be built by infrastructure alone. It will depend on knowledge, coordination, trust, and the ability to act on timely information—grounded in both science and lived experience.