The challenge the researchers set out to solve is fundamentally statistical. Coastal flooding simulations typically rely on time series of offshore conditions—water levels, waves and atmospheric surge—that force hydrodynamic models at their seaward boundaries. When scientists want to explore rare, catastrophic scenarios, they need to generate synthetic time series that are more extreme than anything actually observed, yet statistically consistent with the record. The standard toolkit for this task is extreme value theory, which has been extended in recent years to functional data: entire curves or time series treated as single observations in an infinite-dimensional space. But this functional framework rests on two demanding assumptions. The time series must be independent and identically distributed, and they must exhibit so-called regular variation, which implies that their marginal distributions have heavy, Pareto-type tails.

Real meteoceanic data, it turns out, satisfy neither condition. Working with a hindcast database covering the years 1979 to 2016 at the Gâvres site in southern Brittany, the team extracted 26,652 surge time series, each spanning a half tidal cycle—the six-hour window within plus or minus three hours of high tide, sampled at ten-minute intervals to give 37 points per cycle. Autocorrelation analysis revealed that successive tidal cycles are correlated, with correlation levels around 0.10 persisting even at a lag of 50 cycles, and that cross-correlations link different time points across neighboring cycles. The data also show clear seasonality, with the largest surges concentrated in the winter months of September through March. More troubling still, diagnostics of the tail behavior told a decisive story: the Hill estimator, which converges only under heavy-tailed assumptions, showed no stable region, and both the moments and maximum-likelihood estimators confirmed that the shape parameter of the generalized Pareto distribution was not positive. The surge series are short-tailed, violating the regular variation hypothesis outright.

Rather than discarding the functional extreme value framework, Gorse and his colleagues devised a two-stage preprocessing strategy to bring the data back into its realm of validity. The first step, which they call whitening, tackles temporal dependence. After detrending the series, they fit autoregressive models that capture the dependence of each cycle on its predecessors, retaining only one observation out of every three cycles to reduce residual correlation. The residuals of these models form, by construction, a sequence of independent random variables, while preserving the shape of the surge evolution within each cycle. The second step addresses the tail problem through a marginal transformation: at each of the 37 time points, the empirical distribution of the residuals is approximated by a semiparametric mixture, using the empirical cumulative distribution function in the non-extreme region and a generalized Pareto distribution above a threshold set at the 90th percentile. Applying the inverse of the logarithmic transformation then yields residuals with unit Fréchet marginals—distributions with exactly the heavy, Pareto-type tails that regular variation demands.

With the preprocessed data now satisfying the framework assumptions, the team constructed a probabilistic model using a polar coordinate representation, a technique borrowed from recent theoretical work on regular variation in Hilbert spaces. Each extreme time series is decomposed into a radial component, given by its L2 norm, and an angular component that captures its shape. Asymptotically, these two components become independent, and the radial part follows a Pareto distribution whose simulation is straightforward. The angular component is the hard part. The researchers reduced its dimensionality using principal component analysis, finding that the first three eigenvectors explain 83 percent of the variance of the extreme observations. They then modeled the joint distribution of the three principal component scores using vine copulas—hierarchical constructions that assemble bivariate copulas into a tree structure capable of capturing complex multivariate dependence without imposing a rigid parametric form on the whole vector. A Student t copula was selected for most pairs, though the team switched to a rotated Tawn copula where diagnostics revealed asymptotic independence between the first two coordinates.

Simulation then proceeds in reverse. New angular components are drawn from the fitted copula model, combined with freshly sampled Pareto radii, and pushed back through the inverse marginal transformation to recover extreme residuals on the original scale. Finally, the autoregressive model is inverted: each simulated residual is added to a scaled version of an initial reference time series, producing a complete synthetic surge curve. Crucially, this inversion step gives the method a remarkable flexibility. Because the output depends on the chosen reference series, the level of extremeness can be tuned at will. Selecting a non-extreme reference produces data-like simulations that follow the same statistical law as the observed extremes, while choosing an extreme reference—such as the series recorded during Storm Johanna itself—generates sequences of consecutive extreme events, a scenario of obvious relevance for flood risk assessment.

Validating a generator of synthetic extremes is a subtle business, and the authors attacked it from several angles simultaneously. Comparisons of empirical percentiles showed that the simulated levels fall within bootstrap confidence bands constructed from the observed extremes. A Kolmogorov–Smirnov test on the first principal component of the normalized series found no significant difference between simulations and observations, though a difference emerged in the second dimension, hinting at residual imperfections in the shape modeling. An extremogram analysis, which quantifies the dependence between extreme values at different time points within a cycle, placed the simulated dependence structure squarely within the confidence band of the observations. Return level plots, expressed in terms of return periods assuming roughly seven extreme observations per year, showed that the highest simulated values constitute a plausible extrapolation of the observed data toward larger, unobserved magnitudes.

Perhaps the most striking validation comes from machine learning. The team framed the problem as a two-sample classification test: if the simulated surge curves are truly indistinguishable from real extreme observations, then classifiers trained to tell them apart should perform no better than chance. Support-vector machines, generalized linear models and random forests were each trained on hundreds of labeled series, with the procedure repeated a hundred times to quantify uncertainty. When given the raw time series as input, all three classifiers hovered near 50 percent accuracy—the signature of statistical indistinguishability. Only when fed the angular component alone did accuracy rise to around 60 percent, suggesting that the simulator reproduces the magnitude of extremes more faithfully than some finer aspects of their shape. The team also showed that this success depends on their conditional sampling of the reference series; an unconditional approach allowed the classifiers to detect the simulations, confirming that the dependence between neighboring cycles must be handled carefully.

The implications extend well beyond one town on the Breton coast. Atmospheric storm surge is, in many coastal environments, a larger contributor to the storm tide than wave set-up, making it the natural first target for this methodology. The approach is implemented in R with publicly available code and the surge dataset deposited on Zenodo, lowering the barrier for other coastal engineering groups to adopt it. The authors are candid about the limitations and the road ahead: the current framework handles a single variable, whereas real forcing conditions are multivariate, involving wave height, period and direction alongside surge and wind. Extending the method to the multivariate case, or replacing the copula-based angular model with Gaussian mixtures or generative deep learning approaches, are flagged as promising directions. For now, the study delivers something coastal flood modelers have long needed—a principled way to conjure statistically faithful extreme scenarios from stubbornly non-ideal data, and with them, a sharper picture of the worst the sea may one day deliver.

Subject of Research: Statistical simulation of extreme storm surge time series over tidal cycles for coastal flooding analysis

Article Title: Simulation of extreme functionals in meteoceanic data: application to surge evolution over tidal cycles

Article References: Gorse, N., Roustant, O., Rohmer, J., & Idier, D. (2026). Simulation of extreme functionals in meteoceanic data: application to surge evolution over tidal cycles. Advances in Statistical Climatology, Meteorology and Oceanography, 12(1), 123-148. https://doi.org/10.5194/ascmo-12-123-2026

Image Credits: AI Generated

DOI: 10.5194/ascmo-12-123-2026

Keywords: storm surge, coastal flooding, extreme value theory, functional data analysis, copulas, autoregressive models, tidal cycles, Gâvres, France, hydrodynamic modeling, principal component analysis, machine learning validation

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