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

The study, led by Somayeh Abdollahi of Ghent University together with colleagues from Flanders Hydraulics, the National Geographic Institute, and the Agency for Maritime and Coastal Services, harnessed twenty-three years of high-resolution water-level observations recorded between 2001 and 2023. Four tide gauges anchored the analysis: Nieuwpoort and Oostende on the open coast, the harbour station at Zeebrugge, and the offshore Westhinder platform, designated MP7, which stands sentinel over the shipping approaches far from land. For the first time in Belgian waters, the team systematically compared multiple analysis windows, reconstruction strategies, and tidal-constituent selection procedures, all within the widely used UTide harmonic-analysis framework developed by David Codiga at the University of Rhode Island.

Harmonic analysis is the classical engine of tidal science, tracing its lineage back to Arthur Doodson’s landmark 1921 decomposition of the tide-generating potential. The idea is elegant: the tide at any location can be represented as the sum of dozens of sinusoidal constituents, each with a precisely known astronomical frequency but an amplitude and phase that must be estimated from observations. Once those amplitudes and phases are locked in, the tide can be predicted forward or backward in time indefinitely. LAT is then defined as the lowest water level the astronomical tide alone can produce, excluding the chaotic contributions of storm surges and weather. In practice, however, the answer depends heavily on which constituents are included, how long a record is analysed, and how the reconstruction is performed.

The researchers confronted this methodological fragility head-on. They tested four period-selection approaches, labelled M1 through M4: a single long-record harmonic analysis spanning 2001 to 2019 with annual reconstructions; separate year-by-year analyses; annual analyses used to predict the following nineteen years; and a sophisticated complex averaging of annual harmonic coefficients that preserves the vector nature of tidal amplitudes and phases. Each method was applied at all four stations, and the spread in the resulting LAT estimates turned out to be far from trivial, underscoring that a single ‘correct’ LAT value is an illusion unless the analytical recipe is specified precisely.

Equally consequential was the choice of tidal constituents. The team compared three selection strategies: the default constituent set offered by UTide, a fixed set traditionally used by the Dutch hydrographic service, and a novel iterative procedure guided by the spectrum of the analysis residuals. In the residual-spectrum-guided approach, the researchers analysed the tide, examined the frequency content of what remained unexplained, and added constituents corresponding to prominent residual peaks, repeating the process until the spectrum was clean. This data-driven strategy, implemented entirely with UTide routines, emerged as the preferred configuration, capable of capturing shallow-water constituents that generic or borrowed constituent sets overlook in the complicated hydrodynamics of the southern North Sea.

The preferred configuration, based on the 2001–2019 input window, yielded deterministic LAT values of −0.648 metres at Nieuwpoort, −0.499 metres at Oostende, −0.251 metres at Zeebrugge, and −0.300 metres at the offshore Westhinder station, all expressed relative to the Belgian national height datum TAW/DNG. The pronounced spatial variation, with LAT nearly half a metre deeper at Nieuwpoort than at Zeebrugge, reflects the complex interplay of coastal geometry, bathymetry, and shallow-water tidal dynamics along the Belgian coast. No single conversion grid, the researchers caution, can faithfully represent such station-specific behaviour without careful local calibration.

To quantify how confident one can be in these numbers, the team propagated uncertainty in the harmonic parameters using 1000 Gaussian Monte Carlo realisations, perturbing the estimated amplitudes and phases according to their statistical uncertainties and recomputing the LAT minimum for each realisation. The resulting conditional standard deviations were remarkably tight, ranging from 0.0031 to 0.0034 metres, suggesting that once the harmonic model and constituent set are fixed, the LAT estimate is numerically robust. The authors are careful, however, to frame this spread as conditional on the chosen model rather than a complete uncertainty budget; it captures parameter uncertainty but not the full spectrum of errors that could affect an operational datum.

The Monte Carlo experiments also delivered a reassuring message about computational practicality. Comparing ensembles of 500 and 1000 realisations across the four period-selection methods produced maximum absolute differences of only 0.007 metres in the Monte Carlo mean, 0.009 metres in the conditional standard deviation, and 0.020 metres in one percentile-interval bound. For the preferred residual-spectrum-guided configuration, those differences shrank further to 0.003, 0.004, and 0.006 metres respectively. In other words, halving the ensemble size barely moves the answer, a finding that will ease the computational burden for hydrographic offices considering similar analyses elsewhere.

Throughout the study, the researchers treated existing operational products, including station values published by the Agency for Maritime and Coastal Services and the official LAT conversion grid, as consistency benchmarks rather than independent ground truth. This distinction matters: the operational values were not used to validate the new estimates in an absolute sense, but to check whether the updated analysis remains compatible with the datums that Belgian mariners and coastal engineers rely on today. The deterministic LAT, defined as the minimum of the unperturbed harmonic reconstruction, and the Monte Carlo mean, the average of minima across perturbed realisations, were deliberately reported separately, since the two quantities answer subtly different questions.

The broader significance of the work extends beyond Belgium’s short coastline. The 18.61-year nodal cycle, in which the lunar orbit’s precession modulates tidal amplitudes by up to tens of centimetres, means that short analysis windows can bias LAT estimates substantially, a phenomenon documented in tide-gauge records worldwide. By demonstrating the value of a nineteen-year input window, roughly one full nodal cycle, and by providing a reproducible framework for constituent selection and uncertainty propagation, the Belgian team has offered a template that other hydrographic services can adapt. As sea levels rise and extreme water levels evolve, the low end of the tidal envelope deserves the same scrutiny as the high end, and this study shows exactly how that scrutiny should be applied. The tide-gauge data underpinning the analysis remain freely available through the Meetnet Vlaamse Banken portal, inviting researchers everywhere to test, refine, and extend the approach.

Subject of Research: Assessment of Lowest Astronomical Tide at Belgian North Sea tide gauges using long-term harmonic analysis and residual-spectrum-guided constituent selection

Article Title: Assessment of lowest astronomical tide at belgian north sea tide gauges using long-term harmonic analysis and residual-spectrum-guided constituent selection

Article References: Abdollahi, S., Vanlede, J., Verbeurgt, J., Verstraeten, J., De Kuyper, A., Gurdebeke, P., & De Wulf, A. (2026). Assessment of lowest astronomical tide at belgian north sea tide gauges using long-term harmonic analysis and residual-spectrum-guided constituent selection. Ocean Dynamics, 76(9), Article 96. https://doi.org/10.1007/s10236-026-01850-3

Image Credits: AI Generated

DOI: 10.1007/s10236-026-01850-3

Keywords: Lowest Astronomical Tide, Belgian North Sea, harmonic analysis, tide gauges, tidal constituents, UTide, Monte Carlo uncertainty, vertical datum, ocean dynamics, hydrography, coastal sciences, residual spectrum