Tide Gauges Reveal 77 Years of Hidden Storm Surges Across the Philippines

When Super Typhoon Haiyan tore across the central Philippines in November 2013, it pushed a wall of water through Leyte Gulf that killed thousands of people and rewrote the country’s understanding of coastal disaster risk. Yet for all the devastation that Haiyan and other typhoons have inflicted on the archipelago, no one had ever systematically combed through the nation’s instrumental sea level records to build a comprehensive, nationwide inventory of historical storm surge events. A new study published in the journal Natural Hazards has now done exactly that, mining nearly eight decades of tide gauge observations to reconstruct the Philippines’ storm surge history from 1947 to 2024, and the results offer both a sobering catalogue of past danger and a powerful new tool for forecasting the floods of the future.

The research, led by Anjela A. Ilagan of the Philippine Atmospheric, Geophysical and Astronomical Services Administration (DOST-PAGASA) and the University of the Philippines Diliman, together with Olivia C. Cabrera and Marcelino Q. Villafuerte II, tackles a persistent blind spot in Philippine hazard science. Most previous investigations of the country’s storm surges have concentrated either on individual catastrophic events or on numerical simulations of specific tropical cyclones. What was missing was a systematic, observation-based reconstruction spanning the whole archipelago and many decades. Because the Philippines is made up of more than seven thousand islands with an extraordinarily long and complex coastline, local bathymetry, shelf width, and coastal orientation can dramatically alter how high the sea rises when a typhoon makes landfall. A national picture assembled from actual measurements, rather than models alone, is therefore an essential foundation for hazard assessment.

The team’s approach hinged on a deceptively simple but technically demanding manipulation of tide gauge data. Tide gauges record the total water level at the coast, which is a mixture of the predictable astronomical tide, longer-term sea level variations, and the meteorological contribution from winds and pressure changes associated with storms. To isolate the storm signal, the researchers first removed the tidal component using harmonic analysis with the UTide software package, which fits the known tidal constituents to the observed record and allows the tide to be subtracted out. What remains is the residual sea level, the non-tidal part of the water level that carries the fingerprint of storm-driven water pile-up. The raw residuals were then smoothed with a low-pass filter to suppress high-frequency noise, ensuring that short-lived fluctuations unrelated to genuine surge events would not contaminate the detection.

Identifying storm surge events from residuals required a detection threshold, and here the researchers made a methodologically important choice. Rather than applying a single fixed cutoff across the entire country and record, they used an annually varying threshold, which accounts for the fact that baseline sea levels and seasonal conditions differ from year to year and from station to station. This approach, consistent with skew surge methods used by established sea level monitoring facilities, allows the detection algorithm to remain sensitive to genuine surge events even in years or locations where the background water level is naturally higher. Applying this threshold to the de-tided records across multiple coastal stations, the team identified 133 distinct storm surge events over the 77-year study period, the most comprehensive instrument-based storm surge inventory ever assembled for the Philippines.

The statistics that emerged from this inventory carry a message with direct implications for coastal safety. The majority of detected storm surges, measured as pure surge residuals, reached heights of only 20 to 50 centimeters, and only about 5 percent of events exceeded one meter. On its face, that might suggest that surges in the Philippines are generally modest. But the picture changes sharply when the researchers examined storm tide, the combined water level produced when the storm surge coincides with the astronomical tide. In approximately 39 percent of events, the storm tide exceeded one meter, nearly eight times the proportion seen for surge alone. The lesson is unambiguous: the timing of a typhoon’s arrival relative to the tidal cycle can transform a manageable rise in water into a genuinely dangerous coastal flood. A 40-centimeter surge arriving at high tide can inundate areas that the same surge at low tide would leave untouched.

The single largest surge in the record was measured at Pasacao, in the province of Camarines Sur, during Typhoon Molave in 2020, when the water level anomaly reached 211 centimeters, more than two meters above the expected tide. Molave, known locally as Quinta, swept across the Bicol region in late October 2020, and the Pasacao observation stands as a stark reminder of what even a storm that is not among the most intense on record can deliver when its winds align favorably with coastal geography. The study’s case analyses of Molave and Super Typhoon Goni, which struck the same general region just days later in early November 2020, highlighted how the orientation of the coastline relative to the storm’s track and the phase of the tide jointly determine the severity of coastal impacts. Two storms hitting within a week of each other, with different tracks and tidal timings, produced markedly different surge outcomes.

Mapping the geographic distribution of the 133 events revealed a pronounced regional pattern that reflects the underlying physics of surge generation. High storm surges clustered along the coasts of Catanduanes, Camarines Sur, Eastern Samar, Cagayan, Batanes, and Palawan. What these disparate locations share is a shallow and wide continental shelf, a bathymetric configuration that favors surge amplification. When storm winds push ocean water across a broad, shallow shelf, friction with the seabed constrains the water’s vertical escape and forces it to pile up at the coast, a mechanism that deep water adjacent to steep coasts does not permit. Eastern Samar’s presence in this list will resonate with anyone familiar with Haiyan, whose catastrophic surge in Leyte Gulf was amplified by exactly this kind of shelf-driven shoaling. Equally telling is where surges were largely absent: Mindanao, the large southern island, registered few significant events, a pattern consistent with its exposure to a less typhoon-frequented portion of the western North Pacific basin.

Beyond its scientific findings, the database itself may prove to be the study’s most consequential product. A validated, observation-based catalogue of historical surge events spanning nearly eight decades provides exactly the kind of ground truth that storm surge forecasters, hazard mappers, and model developers need. Numerical surge models, from the high-resolution unstructured-mesh systems used in operational hurricane forecasting to global reanalyses of extreme sea levels, depend on historical observations for calibration and validation. Extreme value statistics, the branch of statistics used to estimate the probability of rare events such as hundred-year surges, likewise require long, homogeneous observational records to produce trustworthy return-period estimates. By anchoring such analyses in real tide gauge measurements from Philippine waters, the new inventory reduces the reliance on extrapolations from other coastlines and improves the credibility of national hazard assessments, evacuation planning, and coastal infrastructure design.

The study also arrives at a moment when the stakes of surge forecasting in the Philippines are rising. Mean sea level is climbing globally, and even a modest long-term rise raises the baseline upon which every future storm surge builds, effectively converting yesterday’s moderate surge into tomorrow’s damaging flood. The researchers’ use of an annually varying threshold partly reflects this shifting baseline, and their 1947-to-2024 record offers a valuable window into how surge occurrences have unfolded across a period of both changing climate and changing observation technology. The authors acknowledge the National Mapping and Resource Information Authority’s Hydrography Branch for providing the tide gauge data, and the work was carried out without dedicated external funding, a testament to the value of careful analysis of existing national observational assets.

For a country that sits squarely in the most active tropical cyclone basin on Earth, with roughly twenty storms entering its area of responsibility in a typical year, the ability to look back through 77 years of measured sea level data is more than an academic exercise. It is a way of letting the ocean itself testify about which coasts are most exposed, how badly the tide can conspire with the wind, and what the worst-case scenarios look like when the next Molave or Goni forms over the warm waters of the Pacific. The 133 events now catalogued in this inventory are, in effect, 133 rehearsals for future disasters, and the researchers hope that forecasters, planners, and coastal communities across the archipelago will use them to be better prepared when the next rehearsal becomes the real thing.

Subject of Research: Historical storm surge events in the Philippines reconstructed from tide gauge observations

Article Title: Investigating historical storm surge occurrences in the Philippines from tide gauge observations

Article References: Ilagan, A. A., Cabrera, O. C., & Villafuerte, M. Q., II (2026). Investigating historical storm surge occurrences in the Philippines from tide gauge observations. Natural Hazards, 122(18), Article 625. https://doi.org/10.1007/s11069-026-08360-x

Image Credits: AI Generated

DOI: 10.1007/s11069-026-08360-x

Keywords: storm surge, storm tide, Philippines, tide gauge, tropical cyclone, Typhoon Molave, Super Typhoon Goni, sea level, coastal flooding, hazard assessment, UTide, Natural Hazards