Tide-surge Interaction Research Articles

Sensitivity analysis of the tidal representation in Singapore Regional Waters in a data assimilation environment

The Singapore Regional Waters (SRW) is one of the more complex tidal regions in the world. This complexity is caused by various factors including the interaction of the Indian and Pacific oceans with their mainly semi-diurnal and diurnal tide, respectively, complicated coastline geometry, small islands and sharply varying bottom topography. Tidal data analysis is hampered by the lack of reliable coastal stations with long-term water level records while numerical tidal modelling studies suffer from lack of accurate high-resolution bathymetry data and uncertainty in the prescription of the tidal open boundary forcing. The present study combines numerical modelling with available along-track satellite altimetry data and a limited set of reliable coastal stations. It proposes a structured approach to study the sensitivity of tidal propagation and interactions to parameters like the prescription of tidal forcing at the open ocean boundaries, local depth information and seabed roughness. To guide and facilitate this analysis, the open-source software environment OpenDA for sensitivity analysis and simultaneous parameter optimisation is used. In a user-controlled way, the vector difference error in tidal representation could so effectively be reduced by ~50%. The results confirm the benefit of using OpenDA in guiding the systematic exploration of the modelled tide and reducing the parameter uncertainties in different parts of the SRW region. OpenDA is also shown to reduce the repetitive nature of simultaneous parameter variation. Finally, the behaviour of the tide in the region and its sensitivities to changes in tidal boundary forcing and to local depth and friction variation in the narrow regions of the Malacca Strait is now much better understood. With most of the systematic errors reduced in the numerical model as a result of the sensitivity analysis, it is expected that the model can be applied to study tide-surge interaction and is much better suited for later application in combination with data assimilation techniques such as Kalman filtering for which systematic model errors should be minimal.

Assessing changes in extreme sea levels: Application to the English Channel, 1900–2006

A recently extended and spatially rich English Channel sea level dataset has been used to evaluate changes in extreme still water levels throughout the 20th century. Sea level records from 18 tide gauges have been rigorously checked for errors and split into mean sea level, tidal and non-tidal components. These components and the interaction between surge and tide have been analysed separately for significant trends before determining changes in extreme sea level. Mean sea level is rising at 0.8–2.3 mm/year, depending on location. There is a small increase (0.1–0.3 mm/year) in the annual mean high water of astronomical tidal origin, relative to mean sea level, and an increase (0.2–0.6 mm/year) in annual mean tidal range. There is considerable intra- and inter-decadal variability in surge intensity with the strongest intensity in the late 1950s. Storm surges show a statistically significant weak negative correlation to the winter North Atlantic Oscillation index throughout the Channel and a stronger significant positive correlation at the boundary with the southern North Sea. Tide–surge interactions increase eastward along the English Channel, but no significant long-term changes in the distribution of tide–surge interaction are evident. In conclusion, extreme sea levels increased at all of the 18 sites, but at rates not statistically different from that observed in mean sea level.

Storms and surges in Irish coastal waters

Abstract Mechanisms of storm surge formation and severity on the north European continental shelf and, in particular, in the Irish coastal waters are examined in this paper. A numerical surge model was developed and used in combination with meteorological forcing and hydrographic records to analysis aspects of surge mechanics. An attempt is made to estimate sea level response to meteorology and to quantify pressure and wind contributions as a driving force. Results show that the inverted barometer term contributes to more than 80% of surge residual in the Celtic Sea and to approximately 55% along the British coast in the Irish Sea. Local winds on the Irish Sea have less of an impact on storm surge heights than previously reported; the Irish Sea surges are predominantly caused by the propagation of external surges into the basin from the north and south channels. An analysis of sea level variability due to wind or pressure was undertaken to select regions of positive and negative interactions. Negative correlation between wind and pressure-driven surges found in the Celtic Sea explains generally low magnitudes of surges in this region, while medium level correlation coefficients could be responsible for generally high surges in the eastern Irish Sea and coast of Scotland. Following a regression analysis, it was determined that in these regions a drop of atmospheric pressure by 1 mbar implies a rise of sea level by 1.2 cm. Non-linear tide–surge interactions are also investigated. Relationships between surge and tidal phase are determined at numerous locations in the Irish coastal waters. Analysis shows that interaction characteristics vary from site to site due to shallow water dynamics and variations in topography. The regional pattern of tide–surge interactions is quite remarkable. Finally, a characterisation of the statistical properties of extreme surges is given. An efficient approach is adopted considering statistics of extremes. A hybrid GEV-EV1 statistical solution was used to estimate the 10, 50 and 200-years extreme surge in the Irish coastal waters.

An 11-year validation of wave-surge modelling in the Irish Sea, using a nested POLCOMS–WAM modelling system

In the future it is believed that extreme coastal flooding events will increase (in frequency and intensity) as a result of climate change. We are investigating the flood risks in the eastern Irish Sea posed by extreme storm events. Here, an 11-year simulation (01/01/1996–01/01/2007) including wave–current interaction has been validated. These data can then be used to investigate the potential for coastal flooding in the study area. To accurately model a storm event in the eastern Irish Sea both wave effects and the influence of the external surge need to be considered. To simulate the waves, we have set up a one-way nested approach from a 1° North Atlantic model, to a 1.85 km Irish Sea model, using the state-of-the-art 3rd-generation spectral WAve Model (WAM). This allows the influence of swell to be correctly represented. The Proudman Oceanographic Laboratory Coastal-Ocean Modelling System (POLCOMS) has been used to model the tide–surge interaction. To include the external surge we have set up a one-way nested approach from the 1/9° by 1/6° operational Continental Shelf surge model, to a 1.85 km Irish Sea model. For the high resolution Irish Sea model we use a POLCOMS–WAM coupled model, to allow for the effects of wave–current interaction on the prediction of surges at the coast. Using two classification schemes the coupled model is shown to be good and often very good at predicting the surge, total water elevation and wave conditions. We also find the number of low level surge events has increased in the study area over the past decade. However, this time period is too short to determine any long-term trends in the wave and surge levels.

Tide and surge energy budgets for Eastern Canadian and Northeast US waters

A depth averaged barotropic model is used to quantify the effect of tides and storm surges on the energy budget for the coastal waters bordering the Northwest Atlantic. The model is first shown capable of producing realistic predictions of both tides and surges through comparison with independent sea level observations. In particular, it is shown that the model can predict tidal range with a typical RMS error of 20 cm, and realistic semi-diurnal to diurnal amplitude ratios. Storm surge hindcasts are also shown to be predicted with typical RMS errors less than 8 cm. Following successful validation of the model we produce, for the first time, spatial maps of maximum speed due to combined tide and surge (due to atmospheric forcing) current, tidal only current, and surge only current. The model is next used to examine the energy budget of the study area. It is shown that the energy flux vectors for the semi-diurnal and diurnal tides are very different. O 1 vectors tends to be more aligned with the bathymetry than the M 2 vectors. Most dissipation was found to occur in the high tide region of the Bay of Fundy. It is also shown that in the mean (i.e. over a season or a year), surges and tide–surge interaction do not play an important role in the energy budget. However, during a large surge event this is not true; surge and tide–surge interaction can have an order one impact on the energy balance. We discuss how such infrequent and unusually large events impact the nearshore environment.

Storm surge computations in estuarine and near-coastal regions: the Mersey estuary and Irish Sea area

An unstructured grid storm surge model of the west coast of Britain, incorporating a high-resolution representation of the Mersey estuary is used to examine storm surge dynamics in the region. The focus of the study is the major surge that occurred during the period 11–14 November 1977, which has been investigated previously using uniform grid finite difference models and a finite element model of the west coast of Britain. However, none of these models included the Mersey estuary. Comparison of solutions in the eastern Irish Sea with those computed with these earlier models showed that, away from the Liverpool Bay region, the inclusion of the Mersey estuary had little effect. However, at the entrance to the Mersey, its inclusion did influence the solution. By including a detailed representation of the Mersey estuary within the model, it was possible to conduct a detailed study of storm surge propagation in the Mersey, which had never previously been performed. This detailed study showed for the first time that the surge’s temporal variability within the estuary is influenced by surge elevation at its entrance. This varies with time as a function of spatial and temporal variations of wind stress over the west coast of Britain. Within the Mersey, calculations show that the spatial variability is mainly determined by changes in bottom topography, which had not been included in earlier finite difference models. However, since water depth is influenced by variations in tidal elevation, this, together with tide surge interaction through bottom friction and momentum advection, influences the surge. The ability of the finite element model to vary the mesh in near-shore regions to such an extent that it can resolve the Mersey and hence the impact of the Mersey estuary upon the Liverpool Bay circulation shows that it has distinct advantages over earlier finite difference models. In the absence of detailed measurements within the Mersey at the time of the surge, it was not possible to validate predicted surge elevations within the Mersey. However, significant insight into physical processes influencing the surge propagation down the estuary, its reflection and spatial/temporal variability could be gained.

Dynamic issues in the SE South America storm surge modeling

The southeastern coast of South America is located in one of the most cyclogenetic areas in the Southern Hemisphere. Occasionally, those systems may lead to disastrous flooding. Strong and persistent along-shore winds on the shelf may also result in some coastal flooding of smaller magnitude. Nested depth-averaged numerical models are used for the simulation of the storm surge. This study describes the storm surge dynamics in the area and the consequences of different regimes to the storm surge prediction. The large extension and limited depth of the Argentinean Shelf permits a smooth transition of the storm surge to the Rio de la Plata and a relatively long forecast horizon for its innermost zone. On the other hand, it is demonstrated that currents in the navigable channels of the Bahia Blanca estuary are the main cause for a strong tide–surge interaction. There, the relative phase between the meteorological forcing and the tide can shift the peak of the storm surge several hours.

A numerical model study of barotropic subtidal water exchange between estuary and subestuaries (tributaries) in the Chesapeake Bay during northeaster events

Northeasters are storms that affect the Chesapeake Bay area more frequently, last for longer periods and impact larger areas than hurricanes. Their impacts on storm surge development and the water exchange between estuary and subestuaries (tributaries) in the Bay vary from one event to another. In this study, three different northeaster events were selected based on their tracks when passing through the Chesapeake Bay area. An unstructured grid finite volume model ELCIRC was utilized to examine the response of the water level of the Chesapeake Bay to three selected northeasters, and the barotropic subtidal water exchanges between the tributaries and the estuary in the Bay. Model sensitivity tests were conducted to examine various effects induced by, for example, tide–surge interaction, open boundary condition, river inflow, wetting-and-drying of the low-lying land area and the usage of 2-D or 3-D mode. The results show that excluding tide–surge interaction did not deteriorate the model performance in the lower Bay but it increased the model inaccuracy in the upper Bay and in the tributaries; using radiation boundary condition decreased the sea level variation in the Bay without appropriately specifying incoming wave; excluding wetting-and-drying of low-lying land area reduced the volume flux by approximately 5%; and using 3-D mode generally increased the water level variation in the Bay. The model predicted storm surges well for three northeaster events. Further diagnostic experiments show that the relative importance of the local and remote winds in generating storm surges in the Bay varied with different northeasters. The inverse barometeric effect played an important role in inducing storm surges for two selected northeasters. The interaction between the tributaries and the Bay proper is considerable. The impacts of the remote wind and Bay wind can be much larger than that of the tributary wind and, thus, control the hydrodynamics and mass transport in the tributaries. The Bay wind and tributary wind effects are largely affected by the wind direction and wind phase, and geographic locations of the tributaries in the Bay. The tributary wind can be dominant over the remote wind and Bay wind effects when the local wind stress and barometric pressure changes are large.

Extreme Sea Levels in the English Channel: Calibration of the Joint Probability Method

The probability of extreme sea levels along the coasts has always been statistically estimated from the series of local observations. The inference is never conclusive, and an attempt is given here to improve the methods already used with reference to the area of the English Channel. The joint probability method (JPM) is the starting point: In most cases it underestimates the return times (or overestimates extreme levels at a fixed time). The proposed extension is based on a more careful use of observed extremes by fitting a coefficient Cc deduced from the data set, which requires that the maximum record height be in agreement with the return period of the record length. This correction calibrates the whole series of extreme estimations to the observed maximum. Likewise an attempt to roughly explain this correction is given that explores the tide–surge interaction and seasonal dependence. The parameters are specifically computed for 15 tide-gauge stations, and the comparison is extended to other known methods, like the Gumbel one (in most cases overestimating the levels) and GEV simulations (which appear much better). Finally extreme levels with estimated return times of 10, 50, and 100 years, respectively, are proposed for each site, and a test for validity was performed by splitting certain long records into small samples, thus checking the spread of the results.

Numerical modeling of tide-surge interaction along Orissa coast of India

The Orissa coast of India is one of the most vulnerable regions of extreme sea levels associated with severe tropical cyclones. There was extensive loss of life and property due to the October 1999 super cyclone, which devastated large part of the Orissa coast. The shallow nature of the head bay, presence of a large number of deltas formed by major rivers of Orissa such as Mahanadi and Dhamra, and high tidal range are responsible for storm surge flooding in the region. Specifically, rising and falling tidal phases influence the height, duration, and arrival time of peak surge along the coast. The objective of the present study is to evaluate the tide-surge interaction during the 1999 Orissa cyclone by using nonlinear vertically integrated numerical models. The pure tidal solution for the head bay region of the Bay of Bengal provides the initial condition for the fine resolution nested grid Orissa model. However, the feedback from the Orissa model does not affect the head bay model as the study provides a one-way interaction. Numerical experiments are performed to study the tide-surge interaction by considering various relative phases of the tidal waves with the surge-wave produced by 1999 Orissa cyclone. The comparison, although utilizing only the limited estimates of tidal data, appears adequate to assert that the principal features are reproduced correctly.

Influence of non-linear effects upon surge elevations along the west coast of Britain

A two-dimensional vertically integrated hydrodynamic finite-element model of the west coast of Britain is used to examine the response of the region to extreme meteorological forcing. The extent to which tide–surge interaction modifies the computed surge elevation and current distributions is examined in detail. The nature of the finite-element model with its ability to refine the mesh in nearshore regions is ideal for examining the influence of non-linear effects upon surges in these regions. Calculations using spatially uniform orthogonal wind stresses show that the surge elevation and current in shallow water are particularly sensitive to the method used to remove the tide as a result of the highly non-linear nature of the tide–surge interaction in these regions. The most accurate means of de-tiding the solution is by subtracting a tide derived by harmonic analysis of the tide and surge time series at the time of the surge. Subtracting a tide-only solution (the usual approach) leads to tidal energy leaking into the surge solution. Calculations show that this arises because the surge modifies the tidal amplitude and phase in shallow-water regions to such an extent that they are appreciably different to those found in the tide-only calculation. Results suggest that this problem becomes more important, as nearshore meshes are refined in an attempt to improve surge prediction. This suggests that in the future, highly accurate fine-mesh models will be required to compute total water levels without the present linear separation into tidal and surge signal used in operational surge prediction.

Extreme sea levels in two northern Mediterranean areas

The ‘calibrated’ Joint Probability Method (JPM) is used to estimate extreme sea levels at four stations in the northern Adriatic (Trieste, and three observatories in the lagoon of Venice area) and three stations in the Gulf of Lions (Port-Vendres, Sete, and Grau-de-la-Dent in the Rhone delta). The JPM, that involves empirical evaluation of the probability of tide and surge separately, can also be used for short records, but tends to overestimate return levels. A correction is therefore necessary. The correction coefficient Cc adopted is deduced from the dataset, by imposing to the maximum record height to coincide with the return period of the record length. Cc, which appears linked to tide-surge interaction and seasonal effects, is generally less than unit. It calibrates the whole series of extreme estimations to the observed maximum. If Cc is >1, it provides evidence that exceptional surges have occurred during the period considered. In this case its application would lead to overestimation of extreme sea levels and other methods of calculation should be preferred. This is the case, for example, for the Lagoon of Venice. Finally, extreme levels with estimated return times of 2, 10, 50 and 100 years, respectively, are proposed for each site.

Recent evolution of surge-related events and assessment of coastal flooding risk on the eastern coasts of the English Channel

This paper is based on statistical analysis of hourly tide measurements for some 285 equivalent full years from the stations of Weymouth, Bournemouth, Portsmouth, Newhaven, Dover and Sheerness in the UK, and of Cherbourg, Le Havre, Dieppe, Boulogne, Calais and Dunkirk in France. For each tidal value, surge heights have been determined and correlated with hourly or three-hourly wind and air pressure data from nearby meteorological stations. Major surges in the area are generally produced by storms associated with wind from north-west or south-west that tend to push oceanic water into the Channel. Recent medium-term climate evolution does not seem to increase the flooding risk at French stations, where surge-related winds tend to decrease in frequency and speed (Cherbourg, Dieppe and Boulogne) or show little change (Le Havre). However, the long-term risk of flooding will increase through the loss in land elevation due to a continuation of the local relative sea-level rise, especially if this effect will be enhanced by an acceleration in the global sea-level rise predicted by climatic models. The northern side of the Channel (Weymouth, Bournemouth and Portsmouth) is mainly exposed to southerly winds that show variable trends. It is also apparently affected by strong subsidence trends during the last two decades. If lasting, such trends can only increase long-term flooding risk. The flooding risk has not increased near the eastern end of the Channel. The duration of significant cyclonic events tends to decrease near Cherbourg but tends to increase near Weymouth, with no conclusive trends in other stations (Portsmouth, Calais and Dunkirk), where extreme surges may occur also in relatively high-air-pressure situations. In conclusion, medium-term coastal flooding risk seems to increase especially at Weymouth, Bournemouth and Portsmouth, and also, but less so, at Le Havre and Sheerness. In addition, few extreme surges occurred during the last decades at the time of spring high tide, which would seem to be a fortunate coincidence or, in some cases, an effect of tide–surge interaction. The risk of occurrence of less favourable random events in the near future is therefore of concern, and flood potential would greatly increase if the global sea-level rise expected in the near future is also considered.

Seiches induced by storms in the English Channel

Investigation of an extreme storm surge event revealed the presence of an oscillation in sea level with a period of 2–6 h. The English Channel is well known for its quarter- and sixth-diurnal tides, and it was expected that these oscillations were simply the result of tide-surge interactions. However, results of model experiments permitted the reproduction of oscillations from wind forcing alone. The forcing used was from an extreme wind event in December 1989. In this article we describe these oscillations, and in particular their spatial characteristics. It is shown that the dominant response is a transverse mode of the English Channel, with minimum amplitude in the central Channel and maximum amplitude in the Baie de Seine and the Golfe de St Malo.

Dependence between sea surge, river flow and precipitation in south and west Britain

Abstract. Estuaries around Great Britain may be at heightened risk of flooding because of the simultaneous occurrence of extreme sea surge and river flow, both of which may be caused by mid-latitude cyclones. A measure especially suited for extremes was employed to estimate dependence between river flow and sea surge. To assist in the interpretation of why flow-surge dependence occurs in some areas and not in others, the dependence between precipitation and surge and between precipitation and river flow was also studied. Case studies of the meteorological situations leading to high surges and/or river flows were also carried out. The present study concerns catchments draining to the south and west coasts of Great Britain. Statistically significant dependence between river flow and daily maximum sea surge may be found at catchments spread along most of this coastline. However, higher dependence is generally found in catchments in hilly areas with a southerly to westerly aspect. Here, precipitation in south-westerly airflow, which is generally the quadrant of prevailing winds, will be enhanced orographically as the first higher ground is encountered. The sloping catchments may respond quickly to the abundant rainfall and the flow peak may arrive in the estuary on the same day as a large sea surge is produced by the winds and low atmospheric pressure associated with the cyclone. There are three regions where flow-surge dependence is strong: the western part of the English south coast, southern Wales and around the Solway Firth. To reduce the influence of tide-surge interaction on the dependence analysis, the dependence between river flow and daily maximum surge occurring at high tide was estimated. The general pattern of areas with higher dependence is similar to that using the daily maximum surge. The dependence between river flow and daily maximum sea surge is often strongest when surge and flow occur on the same day. The west coast from Wales and northwards has slightly stronger flow-surge dependence in summer than in winter, whereas dependence is stronger in winter than in summer for the southern part of the study area. Keywords: : Britain, dependence, sea surge, river flow, precipitation, mid-latitude cyclone, seasonality, time lag

Sea-level oscillations with 6-h period in the North Sea 29?31 October 2000. An analysis of data from stations in the northern North Sea and along the western coast of Norway

During the storm event 29–31 October 2000 long regular trains of residual sea-level oscillations with period about 6 h occurred in the North Sea area. Crest-to-trough heights up to 0.8 m were observed along the western coast of Norway. Model simulations show that these oscillations were mainly due to spatial and temporal variations in the atmospheric forcing and were not the result of nonlinear tide–surge interaction. The oscillations propagated northwards along the Norwegian coast and the signal was amplified in some fjords in agreement with predictions by high-resolution regional models. In Skagerrak in the north eastern North Sea the main storm-surge signal appeared as amplitude-modulated oscillations with period about 12 h and crest-to-trough height up to 1.5 m. Here the tide–surge interaction contributed up to about 20% of the peak surge. On the coast of Scotland and in the southern part of the North Sea nonlinear effects produced oscillations with 6-h period at some stations.

Revising the paradigm of tidal analysis ? the uses of non-stationary data

Surface elevation and current records contain non-tidal variance, often dismissed as noise. The processes responsible for the non-tidal component may also modulate the tidal signal, altering its strength and frequency structure. Because of their manner of generation and propagation, internal tides are inherently irregular. The non-stationary character of these and other tidal processes provides an integral and useful property of tidal records, because it provides an opportunity to obtain insights into tidal dynamics and the interaction of tidal and non-tidal processes. It is, moreover, productive to use multiple approaches in analyzing coastal and estuarine tidal processes so that both the time-varying and average frequency content are determined. Only by confronting the causes of non-stationary behaviour in this way can some of the remaining challenges in tidal analysis and prediction be overcome, e.g. shelf and estuarine currents, river tides, internal tides, tide-surge interactions and tidally influenced ecological processes. Several examples illustrate the utility of non-stationary tidal analysis methods.

Tide–Surge Interaction in the Meghna Estuary: Most Severe Conditions

A numerical model with a (1/120)° resolution is used in studying 1) the effects of tide–surge interaction and 2) the effects of cyclone landfall on the Noakhali–Cox's Bazar coast of Bangladesh (i) on the time deviation of the surge arrival from the landfall time of a cyclone and (ii) on the duration of the maximum surge. The present study found that the tide–surge interaction in the Meghna estuary shows, in general, the progressive wave nature of the local tide. If the peak of the maximum surge coincides with the tidal peak near the landfall, the surge propagates toward the north faster than when the surge peak coincides with the tidal trough. Cyclones that make landfall before the arrival of the tidal peak produce higher but shorter duration surges than those that make landfall after the arrival of the tidal peak. If the landfall time of the cyclone is kept fixed, the surge peaks are found to arrive earlier and to be of shorter duration with (i) an increase in the propagation speed of the cyclone and (ii) a decrease in the radius of the maximum cyclonic wind. For the peak of the maximum surge, it may take about 3–4 h to propagate from the southern estuary to the northernmost estuary. The surge, which propagates northward, reaches the northernmost estuary earlier and becomes longer in duration with smaller angles of crossing of the cyclone with the coastline. A slow moving cyclone with a larger radius of cyclonic wind, making landfall in the northernmost area, with a small angle of crossing relative to the Noakhali–Cox's Bazar coastline, and landfall just before the high tide is found to be the most severe cyclone, capable of producing the highest surges in the vast and shallow estuary.

Numerical study of the influence of waves and tide-surge interaction on tide-surges in the Bohai Sea

The author’s combined numerical model consisting of a third generation shallow water wave model and a 3-D tide-surge model with wave-dependent surface wind stress were used to study the influence of waves on tide-surge motion. For the typical weather case, in this study, the magnitude and mechanism of the influence of waves on tide-surges in the Bohai Sea were revealed for the first time. The results showed that although consideration of the wave-dependent surface wind stresses raise slightly the traditional surface wind stress, due to the accumulated effects, the computed results are improved on the whole. Storm level maximum modulation can reach 0.4 m. The results computed by the combined model agreed well with the measured data.

The Effect of Non-Stationarity on Extreme Sea-Level Estimation

SUMMARY The sea-level is the composition of astronomical tidal and meteorological surge processes. It exhibits temporal non-stationarity due to a combination of long-term trend in the mean level, the deterministic tidal component, surge seasonality and interactions between the tide and surge. We assess the effect of these non-stationarities on the estimation of the distribution of extreme sea-levels. This is important for coastal flood assessment as the traditional method of analysis assumes that, once the trend has been removed, extreme sea-levels are from a stationary sequence. We compare the traditional approach with a recently proposed alternative that incorporates the knowledge of the tidal component and its associated interactions, by applying them to 22 UK data sites and through a simulation study. Our main finding is that if the tidal non-stationarity is ignored then a substantial underestimation of extreme sea-levels results for most sites. In contrast, if surge seasonality and the tide–surge interaction are not modelled the traditional approach produces little additional bias. The alternative method is found to perform well but requires substantially more statistical modelling and better data quality.