The core insight of the study is deceptively simple. Tides are not random. They are governed by the gravitational interplay between the Earth, the Moon and the Sun, producing regular cycles of rising and falling water that can be calculated years in advance. In many coastal locations, the tide determines whether a given storm surge is sufficient to push water levels above established flood thresholds. When a moderate surge coincides with a high tide, flooding follows; when the same surge arrives at low tide, water levels may remain safely below the threshold. As Haigh explains, even storm-driven flooding often inherits the timing of the tides, because the tide acts as the gatekeeper that decides whether the surge crosses the critical line.

What makes the new work distinctive is its focus on the hour of the day rather than the season or the year. Previous research on flooding has concentrated heavily on seasonal patterns, examining, for example, why river floods cluster in certain months or why coastal floods are more frequent during particular times of the year. Haigh notes that surprisingly little attention has been paid to what hour of the day flooding actually occurs. Part of the reason, he argues, lies in how flooding is measured. Flood impacts are commonly summarized as the number of flood days per year, and once an event is compressed into a single day, the information about whether the flooding happened at 3 a.m., 8 a.m. or 3 p.m. simply disappears. By returning to the underlying records and preserving the temporal detail, the research team was able to recover patterns that conventional flood statistics had erased.

The regional results are striking in their specificity. In Boston, floods occur predominantly around noon and midnight, mirroring the timing of that harbor’s high tides. In Southern California, flooding is more common in the morning, again tracking the local tidal cycle. In Florida, the picture is more varied, because tidal behavior differs substantially from one stretch of coastline to another. Along the Atlantic Coast and down toward the Florida Keys, coastal flooding is moderately predictable in its timing, while along the Gulf Coast, where tidal ranges are smaller and more complex, flooding is less predictable. These differences matter, because they show that the daily fingerprint of flooding is not a universal pattern but a local one, shaped by the specific tidal regime of each coastline.

The study also establishes that the phenomenon is not confined to places with semidiurnal tides, the familiar pattern of two high tides and two low tides each day that characterizes much of the U.S. East Coast and the U.K. The researchers found that predictable daily timing also emerges in locations with diurnal tidal systems, which experience only one high tide and one low tide per day. The reason lies in the way different tidal signals interact. Tides are generated by multiple astronomical constituents, each with its own period, and their superposition produces the observed water-level cycle. Whether a coastline experiences two highs a day or one, the resulting pattern remains regular enough that flood events, which require water levels to cross a threshold, cluster around the recurring peaks of that cycle.

The technical implication is that flood risk is not evenly distributed across the hours of a day, even when the weather is. A storm system that lingers offshore for twelve hours does not pose a uniform threat throughout its passage. Its most dangerous moments are those that coincide with high tide, and in many locations those moments can be identified in advance with the same precision used to publish tide tables. This reframing has consequences for how flood forecasting is constructed. Traditional forecasts emphasize the magnitude of expected water levels, but the timing of when those levels are most likely to be exceeded is equally important for operational decisions, from closing roads to repositioning emergency assets.

Haigh argues that this matters because timing influences impacts. The same flood can have very different consequences depending on whether it occurs during the morning commute, during business hours or in the middle of the night. A flood that peaks at rush hour can strand commuters, snarl traffic and endanger drivers who attempt to cross inundated roadways. A flood of identical height at 3 a.m. may cause comparable physical damage but far fewer casualties and disruptions, simply because fewer people are exposed. Understanding these temporal patterns, the study suggests, could improve flood forecasting, emergency planning, infrastructure operations and long-term coastal adaptation. Utilities, hospitals, schools and transit agencies could all incorporate the known tidal windows into their contingency planning, aligning preparedness measures with the hours when flooding is statistically most likely.

The research arrives at a moment when the baseline for coastal flooding is shifting. As sea levels continue to rise, coastal flooding is increasingly occurring without storms to spur it on, a phenomenon often described as nuisance or sunny-day flooding, in which exceptionally high tides alone are enough to inundate low-lying streets. Haigh points out that higher mean sea levels mean it takes only a relatively small weather contribution, or sometimes simply a very high astronomical tide, to cross a flood threshold. In such conditions, the timing of the tides becomes even more decisive, and understanding exactly when those critical tidal windows occur could become an increasingly important component of coastal flood forecasting and preparedness.

The practical payoff of the work could be substantial. Haigh says the findings could make existing flood forecasts more predictable, providing more warning before disasters occur. That added lead time can help residents, businesses and emergency managers anticipate disruption and plan accordingly, whether that means scheduling deliveries around high-tide windows, warning drivers before morning inundations or pre-positioning pumps and barriers. Because the tidal component of flood timing is deterministic, it can be computed far in advance, offering a layer of foresight that weather-driven forecasting alone cannot match. The uncertainty in a flood forecast then narrows to the meteorological contribution, while the tidal contribution is already known.

The study, titled Coastal flooding at predictable hours and published in Nature Communications on 28 July 2026, represents a collaboration among international researchers and reflects Haigh’s long career in sea level and coastal impacts. A professor who joined the University of Central Florida in 2025 as director of the Center for Integrated Coastal Research, Haigh previously held the chair in sea level and coastal impacts at the University of Southampton and directed the Center for Doctoral Training for Resilient Flood Futures. His team’s work suggests a quiet but consequential shift in coastal science: the hazard that communities have treated as random now reveals a schedule, and learning to read that schedule may prove one of the most practical tools available as rising seas make the ocean’s high-water moments an ever more frequent visitor to the world’s coastlines.

Subject of Research: Predictable daily timing of coastal flooding driven by tidal cycles

Article Title: Study shows coastal flooding occurs at predictable times

Article References: Study shows coastal flooding occurs at predictable times. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: coastal flooding, tides, sea level rise, flood forecasting, storm surge, tidal cycles, Nature Communications, University of Central Florida, emergency planning, nuisance flooding, tide gauges, coastal adaptation

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