Search

A comprehensive understanding of the processes of storm surge and flooding induced by typhoons is crucial for the development of effective emergency plans and risk management strategies. This study employed the three-dimensional coastal ocean circulation model FVCOM to simulate the storm surge in the Pearl River Estuary (PRE) during Typhoon Hato in 2017. In addition, the storm surge predictions are integrated with the InfoWorks 2D-1D hydrologic network drainage model to study urban flooding in the Inner Harbor district in Macao for the first time. Typhoon's wind and pressure fields were reconstructed based on parametric models, and numerical experiments were conducted to investigate the effects of tide-surge interaction. A thorough comparison between observed and simulated wind and water levels demonstrated a high degree of agreement. The results reveal that nonlinear tide-surge interactions are not uniformly distributed across the PRE. In the Lingdingyang waters, the impact of tide-surge interaction on the total water level ranges from 1% offshore to 30% upstream. In the coastal waters near Macao, this impact varies from 5% to 25%, with a peak impact of 25% observed near Hac Sa Bay in Macao, indicating a significant influence of tide-surge interaction in this area. Predicted city flood levels during HATO are well-validated against field data. The Inner Harbor of Macao is identified as the zone most severely affected by flooding induced by storm surge, with the highest flood depths recorded at its northern and southern ends. [ABSTRACT FROM AUTHOR]

Storm surges events are investigated using the ECHAR method, which identifies and quantifies the different dynamical structures of a typical storm surge event. In the North Atlantic, analysis of 65 tide gauges revealed that storm surge events display two major and two minor structures, each of them corresponding to specific ocean dynamics. The two major structures refer to a slow‐time Gaussian structure, lasting around 24 days, associated with the impact of the atmospheric pressure and a fast‐time Laplace structure, lasting around 1.4 days, mainly wind‐driven. The absence of the Gaussian structure along the North America coasts is explained by storms of smaller spatial extension, compared to Europe. Concerning the minor structures, a negative surge of around 6 cm just after the peak surge is observed over North America only. Such a sudden drop of the sea level is explained by the turning winds during the storm event, favored by the smaller spatial extension of storms. Finally, high frequency oscillations, with amplitude typically of 3 cm and up to 25 cm, are observed at some tide gauges. These oscillations refer to tide‐surge interactions and they are often maximum at a specific phase of the tide and/or enhanced because of resonant basins. Plain Language Summary: The storm surges refer to the extreme values of the sea level time series, after the removal of tides and the mean sea level. The storm surges are due to extreme weather systems such as storms and are responsible for coastal flooding. Rather than focusing on the maximum values of the sea level during storm events, we consider the full dynamics of the events from 20 days before to 20 days after each maximum. Doing so helps to properly describe the different types of storm surge events and explain the atmospheric and ocean dynamics leading to such extremes. Key Points: The new ECHAR method decomposes storm surge events in two major and two minor structures each of them reflecting specific ocean dynamicsIn North America a large drop of the sea level 1.5 days after the peak surge is explained by turning winds and storms of small extentObserved oscillations with amplitude up to 10 cm are the signature of tide‐surge interaction [ABSTRACT FROM AUTHOR]

Coastal evolutions are expected to have a significant impact on storm tides, disproportionately aggravating coastal flooding. In this study, we utilize a nested storm tide model to provide an integrated investigation of storm tide responses to changes in coastline and bathymetry along the Zhejiang coasts. We selected coastline and bathymetry data from 1980 and 2016, as well as data from three typical typhoon events (i.e., Winnie, Haikui, and Chan-hom) for simulating the storm surge processes. The results indicate that changes in the coastline and bathymetry from 1980 to 2016 have resulted in an increase in storm tides in the northern part and a decrease in the central part of Zhejiang. Specifically, storm tides in Hangzhou Bay have increased significantly, with an average increase of about 0.3 m in the maximum storm tides primarily attributed to coastline changes. On the contrary, in smaller basins like Sanmen Bay, while reclamation itself has reduced peak storm surges, rapid siltation has consequently exacerbated the storm surge. By decomposing storm tides into astronomical tides and storm surges, we discovered that the change in tidal levels was twice as significant as the surge change. Moreover, the nonlinear tide–surge interaction was nearly four times that of the pure surge, significantly contributing to storm surge variation. Alterations in the momentum balance reveal that the water depth-induced bottom friction and wind stress increase contributes to the local enlargement of storm tides at the bay head, while the coastline changes exaggerate nearshore storm tides through an increase in the advection term. [ABSTRACT FROM AUTHOR]

Coastal evolutions are expected to have a significant impact on storm tides, disproportionately aggravating coastal flooding. In this study, we utilize a nested storm tide model to provide an integrated investigation of storm tide responses to changes in coastline and bathymetry along the Zhejiang coasts. We selected coastline and bathymetry data from 1980 and 2016, as well as data from three typical typhoon events (i.e., Winnie, Haikui, and Chan-hom) for simulating the storm surge processes. The results indicate that changes in the coastline and bathymetry from 1980 to 2016 have resulted in an increase in storm tides in the northern part and a decrease in the central part of Zhejiang. Specifically, storm tides in Hangzhou Bay have increased significantly, with an average increase of about 0.3 m in the maximum storm tides primarily attributed to coastline changes. On the contrary, in smaller basins like Sanmen Bay, while reclamation itself has reduced peak storm surges, rapid siltation has consequently exacerbated the storm surge. By decomposing storm tides into astronomical tides and storm surges, we discovered that the change in tidal levels was twice as significant as the surge change. Moreover, the nonlinear tide–surge interaction was nearly four times that of the pure surge, significantly contributing to storm surge variation. Alterations in the momentum balance reveal that the water depth-induced bottom friction and wind stress increase contributes to the local enlargement of storm tides at the bay head, while the coastline changes exaggerate nearshore storm tides through an increase in the advection term.

A numerical study based on FVCOM model was carried out to investigate the storm surge behavior induced by Typhoon Mangkhut in and around Lingdingyang Bay, Pearl River Estuary. Through elaborate calibration, the simulation results matched well with the measurements on the tidal-gauge stations. The validated Typhoon Mangkhut simulation was used as a case to study temporal–spatial behavior of the storm surge. Specifically, hourly spatial distribution of surge evolvement before the landfall of Typhoon Mangkhut and the time series of the surge from the twelve points in and around Lingdingyang Bay were analyzed. Beyond that, a series of artificial typhoons were designed through revising some parameters of the original typhoon including the landfall time, central pressure deficit, landfall location and moving direction, to study the influence caused by typhoon uncertainties. Results show that the influence of landfall time is dominated by the rule of tide–surge interaction, namely the lowest surge on the high tidal level and vice versa. The results also illustrate that the response of the surge to the same shift of the pressure deficit for a typhoon is uneven in space. The response of surge is more sensitive near shallow coastal regions than that in deep sea. Through investigating the surge in response to the track for the original typhoon, we find that a west shift of the track makes the ascending surge advanced and an east shift of the track makes the receding surge delayed in Lingdingyang Bay. A decrease of the angle between the coast and the direction of movement enlarges the maximum surge and an increase of the angle induces the opposite result. This study could improve our understanding about how uncertainties of a northwesterly moving typhoon would induce the storm surge response along the coastal regions such like Lingdingyang Bay, in Pearl River Estuary. [ABSTRACT FROM AUTHOR]

This work implements a dynamical downscaling approach, based on a set of nested two-dimensional hydrodynamic models, to quantify the expected changes in the total sea level climate and its components for the Uruguayan coast, using surface wind and sea level pressure projections from global climate models generated during the Phase 5 of the Coupled Models Inter-Comparison Project, considering three time horizons: historical period (1986-2005), short term (2027-2045) and long term (2082-2100), and the future scenarios RCP 4.5 and RCP 8.5. It is concluded that the main contribution to the projected changes in the area is the regional mean sea level rise, followed in importance by the effect that the increase in the water depth has on the amplitude of the tidal components. Moreover, it is concluded that changes in the meteorological residuals (or surges), associated with potential changes in the atmospheric circulation patterns, are negligible in the study area. The obtained results reinforce the need to resort to dynamic downscaling for projecting total sea level changes in areas characterized by wide and shallow continental shelfs and estuaries, as this approach allows to resolve the interactions that may arise between tides, surges and the mean sea level rise, something that cannot addressed with an approach based solely on statistical downscaling.

In this study, the characteristics and mechanisms of tide-surge interaction in the Pearl River Estuary (PRE) during Typhoon Hato in August 2017 are studied in detail using a 3D nearshore hydrodynamic model. The wind field of Typhoon Hato is firstly reconstructed by merging the Holland parametric tropical cyclone model results with the CFSR reanalysis data, which enables the model to reproduce the pure astronomical tides and storm tides well; in particular, the distinctive oscillation pattern in the measured water levels due to the passage of the typhoon has been captured. Three different types of model runs are conducted in order to separate the water level variations due to the astronomical tide, storm surge, and tide-surge interactions in the Pearl River Estuary. The results show the strong tidal modulation of the surge level, as well as alteration of the phase of surge, which also changes the peak storm tidal level, in addition to the tidal modulation effects. In order to numerically assess the contributions of three non-linear processes in the tide-surge interaction and quantify their relative significance, the widely used “subtraction” approach and a new “addition” approach are tested in this study. The widely used “subtraction” approach is found to be unsuitable for the assessment due to the “rebalance” effect, and thus the new “addition” approach is proposed along with a new indicator to represent the tide-surge interaction, from which more reasonable results are obtained. Detailed analysis using the “addition” approach indicates that the quadratic bottom friction, shallow water effect, and nonlinear advective effect play the first, second, and third most important roles in the tidal-surge interaction in the estuary, respectively.

Typhoons Soudelor (2015) and Dujuan (2015) were two of the strongest storms to affect the Taiwan Strait in 2015. This study investigated the response of the waters on the western bank of the Taiwan Strait to the passage of Soudelor and Dujuan. This included an investigation of the resonant coupling between the tide and storm surge, typhoon wave variation caused by the storm tide, and wave-induced water level rise. Analyses conducted using numerical model simulations and observations from tidal stations and buoys, obtained during the passage of both Soudelor and Dujuan, revealed that resonant coupling between the astronomical tide and storm surge in the Taiwan Strait was prominent, which resulted in tidal period oscillation on the storm surge and reduced tidal range. The tide wave arrived earlier than the predicted astronomical tide because of the existence of the storm surge, which was attributable to acceleration of the tidal wave caused by the water level rise. Wave height observations showed that the storm tide predominantly affected the waves, which resulted in wave heights that oscillated within the tidal period. Numerical experiments indicated that both the current and the water level affected wave height. Waves were affected mainly by the current in the middle of the Taiwan Strait, but mostly by water level when the water level was comparable with water depth. Wave setup simulations revealed that wave setup also oscillated within the tidal period, and that local bathymetry was the most important influencing factor of wave setup distribution. [ABSTRACT FROM AUTHOR]

The numerical method of lines (MOLs) in coordination with the classical fourth-order Runge‒Kutta (RK(4, 4)) method is used to solve shallow water equations (SWEs) for foreseeing water levels owing to the nonlinear interaction of tide and surge accompanying with a storm along the coast of Bangladesh. The SWEs are developed by extending the body forces with tide generating forces (TGFs). Spatial variables of the SWEs along with the boundary conditions are approximated by means of finite difference technique on an Arakawa C-grid to attain a system of ordinary differential equations (ODEs) of initial valued in time, which are being solved with the aid of the RK(4, 4) method. Nested grid technique is adopted to solve coastal complexities closely with least computational cost. A stable tidal solution in the region of our choice is produced by applying the tidal forcing with the major tidal constituent M2 (lunar semi-diurnal) along the southern open-sea boundary of the outer scheme. Numerical experimentations are carried out to simulate water levels generated by the cyclonic storm AILA along the coast of Bangladesh. The model simulated results are found to be in a reasonable agreement with the limited available reported data and observations. [ABSTRACT FROM AUTHOR]

Nonlinear interactions between tides and storm surges, i.e. the tide-surge interactions (TSI), play a critical role in modulating extreme coastal water levels. However, the TSI effects are often omitted in probabilistic modeling of extreme water levels, due to the extensive computational efforts caused by simulating numerous combinations of storm timings and random tidal phases. In this study, we proposed a statistical approach for fast estimation of TSI effects with applications in predicting extreme water levels. Storm tides in the northern expanse of the South China Sea (SCS) during typhoon events from 1979 to 2020 are simulated, based on which contributions of TSI to the total water level are extracted and analyzed. The results reveal a predominant negative influence of TSI on the total storm tide levels. It was found that for a given coastal location, statistically significant correlations exist between TSI divided by the peak storm tide and surge divided by the peak storm tide. Based on the correlations, location-dependent, multiple regression models for predicting TSI effects on storm tide levels were established using predicted surge and tidal levels as input, which avoids modeling numerous scenarios of different tidal phases and storm timings. By comparing with long term history data from a tide gauge, the proposed approach was shown to be able to reproduce the TSI contributions to extreme storm tides accurately and efficiently. By superposing the estimated TSI effects with predicted surge and random tides, a method for calculating the extreme water level with certain return periods is developed. Finally, an application of the method was demonstrated by calculating the 50-year return period water levels along the northern coast of the South China Sea. • A statistical method has been proposed for quantifying the tide-surge interaction during extreme storm tide. • A framework has been proposed for predicting the return period of extreme storm tides. • The RP50 storm tide along the northern coastline of the South China Sea has been calculated. [ABSTRACT FROM AUTHOR]

Because of the special topography and large tidal range in the South Yellow Sea, the dynamic process of tide and storm surge is very complicated. The shallow water circulation model Advanced Circulation (ADCIRC) was used to simulate the storm surge process during typhoon Winnie, Prapiroon, and Damrey, which represents three types of tracks attacking the South Yellow Sea, which are, moving northward after landing, no landing but active in offshore areas, and landing straightly to the coastline. Numerical experiments were carried out to investigate the effects of tidal phase on the tide-surge interaction as well as storm surge. The results show that the peak surge caused by Winnie and Prapiroon occurs 2–6 h before the high tide and its occurring time relative to high tide has little change with tidal phase variations. On the contrary, under the action of Damrey, the occurring time of the peak surge relative to high tide varies with tidal phase. The variation of tide-surge interaction is about 0.06–0.37 m, and the amplitude variations of interaction are smooth when tidal phase changes for Typhoon Winnie and Prapiroon. While the interaction is about 0.07–0.69 m, and great differences exists among the stations for Typhoon Damrey. It can be concluded that the tide-surge interaction of the former is dominated by the tidal phase modulation, and the time of surge peak is insensitive to the tidal phase variation. While the interaction of the latter is dominated by storm surge modulation due to the water depth varying with tide, the time of surge peak is significantly affected by tidal phase. Therefore, influence of tidal phase on storm surge is related to typhoon tracks which may provide very useful information at the design stage of coastal protection systems. [ABSTRACT FROM AUTHOR]

The Radial Sand Ridges (RSRs) area in the southern Yellow Sea are subject to tropical and extratropical cyclone activities frequently, in which the special geometry feature and moving stationary tidal system result in complex storm-induced hydrodynamic processes, especially the tide-surge interactions. We studied a rare weather event influenced simultaneously by an extratropical cyclone EX1410 and Typhoon Vongfong as an example to investigate the characteristics of storm surges, wave-surge, and tide-surge interaction in the RSRs area, and applied a high-resolution integrally-coupled ADCIRC+SWAN model, in which the meteorological forcing inputs are simulated by the WRF-ARW model. The model is validated by records from 4 tide gauges and 2 wave buoys along the Yellow Sea coast. Results show that the tide-surge interactions are of considerable regional heterogeneousness. The surge curves at Lüsi (in south RSRs) and Jianggang (in middle RSRs) have abrupt falls near the time of low tide, where the peak occurrence time of interaction residuals tend to shift towards the mid-ebb period. Significant increase of bed shear stress in shallow waters was proved the dominant factor to affect the tide-surge interaction in broad tidal flats of the RSRs area. Differently, the interaction pattern in the Xiyang Trough (in north RSRs), showed a unique rising in mid-flood period due to the phase advances of real surge waves in relatively deep waters. Therefore, we suggested to the local flood risk management that the tide-surge interaction tends to alleviate the flooding risk in the RSRs area around the time of high tide, but aggravate the risk on the rising tide in the Xiyang Trough and on the falling tide in large-scale tidal flats of the southern RSRs area. [ABSTRACT FROM AUTHOR]

Tide–surge interaction plays an important role in the distribution of surges along the coast of China. A comprehensive understanding of tide–surge interaction will provide more accurate estimates of extreme sea levels and storm surges. This study applied a statistical method to hourly tide–surge data from 12 tide gauges located along the coast of China to examine the dependency of residual maxima on tidal phases. Statistical significance has been tested quantitatively for each site utilizing the chi-square test. Results show that significant tide–surge interaction exists at all 12 tide gauge sites, but the strength of the interactions differs according to location. The distributions of peak residuals are significantly different from the uniform distribution and can be divided into three types. In the first type, the residual generally reaches its peak value during the rising tide (2–4 h before high tide), whereas in the second type, the residual reaches its peak mostly during the ebb tide (2–4 h after the high tide). In the third type, the residual reaches peaks during both ebb and rising tides. The main causes of tide–surge interactions are the tidal phase alteration caused by surge and the modulation of surge due to tides at the tide gauge. Tide–surge interactions at Lianyungang and Xiamen are more significant than those at other gauge sites. At Kanmen and Keelung, the tide–surge interactions are weakest. Variations of the tide–surge interaction at Xiamen and Quarry Bay show that the interaction has not varied significantly during the past 40 years and that tide–surge interactions tend to be more stable with the strengthening of the interaction. [ABSTRACT FROM AUTHOR]

Abstract The interaction of tides and storm surges can significantly modify the final levels reached by storm surges that threaten coastal areas. In this paper, numerical experimental results are presented that examine tidal influence on storm surges. This is done using a two-dimensional ADCIRC (Advanced Circulation Model) applied to the effects of Typhoon Rammasun (July 2014) on Tieshan Bay, China. The results show that, without considering tidal forcing, there is an underestimation of positive surge levels, while negative surge levels are overestimated. It is also shown that the prevailing wind direction and shape of Tieshan Bay affect the distribution of storm surges. The nonlinear residual levels caused by tide-surge interaction can reach 0.94 m at the top of the bay when peak negative storm surge levels occur, with nonlinear levels increasing from the outside to the head of the bay. Through the derivation of mathematical terms, a direct relationship between the nonlinear residual levels and the dynamic influencing factors is established. Further, it is demonstrated that the combination of wind stress and bottom friction terms and advection terms play leading roles in the derivations, whereas terms related to local acceleration and Coriolis force contribute little to the nonlinear levels. The combination of wind stress and bottom friction terms and advection terms show complex spatial and temporal variation. Highlights • An unstructured-grid and high resolution tide-surge coupled model was applied in this study. • The storm prediction error and nonlinear residual levels characteristics due to tide-surge interaction are investigated. • Direct relationship between the nonlinear residual levels and influencing factors is established. [ABSTRACT FROM AUTHOR]

Landfalling tropical cyclones (TCs) frequently occur with strong intensity in most coastal areas, and storm surges are likely to occur in response to extreme sea level (ESL) growth. However, the level of ESL growth under various wind conditions, coastline geometries and tide-surge interactions has not been clarified. In the Pearl River Estuary and Daya Bay, observations of landfalling TCs have indicated an increasing frequency of intense and rapid landfalls in the 2010s as compared to the 2000s, accompanied by a noteworthy increase in storm surge. Based on a large ensemble (∼0.5 million storm surge events with various tracks, maximum wind speeds, maximum wind radiuses, translation speeds and tidal conditions) obtained from well-validated model simulations, the ESL growth in the study area is further quantified as follows: (a) ESL growth is more sensitive to the acceleration effect of landfalling TCs than to the strengthening effect of landfalling TCs since the effect of low acceleration (+3 m s ^−1 ) is comparable to that under notable strengthening (+10 m s ^−1 ); (b) ESL growth is strongly modulated by coastline geometry, especially in flared or arching coastline areas. ESL growth mainly occurs along flared coastline areas when landfalling TCs strengthen into severe TCs or typhoons but can also occur along arching coastline areas for stronger landfalling TCs, such as severe typhoons or supertyphoons; and (c) ESL growth could be increased or decreased by approximately 10% under the effect of tide-surge interactions. Both the large-ensemble method and the above ESL growth characteristics are worthy of attention in risk assessment and rapid prediction of storm surges in shallow waters.

Typhoons Soudelor (2015) and Dujuan (2015) were two of the strongest storms to affect the Taiwan Strait in 2015. This study investigated the response of the waters on the western bank of the Taiwan Strait to the passage of Soudelor and Dujuan. This included an investigation of the resonant coupling between the tide and storm surge, typhoon wave variation caused by the storm tide, and wave-induced water level rise. Analyses conducted using numerical model simulations and observations from tidal stations and buoys, obtained during the passage of both Soudelor and Dujuan, revealed that resonant coupling between the astronomical tide and storm surge in the Taiwan Strait was prominent, which resulted in tidal period oscillation on the storm surge and reduced tidal range. The tide wave arrived earlier than the predicted astronomical tide because of the existence of the storm surge, which was attributable to acceleration of the tidal wave caused by the water level rise. Wave height observations showed that the storm tide predominantly affected the waves, which resulted in wave heights that oscillated within the tidal period. Numerical experiments indicated that both the current and the water level affected wave height. Waves were affected mainly by the current in the middle of the Taiwan Strait, but mostly by water level when the water level was comparable with water depth. Wave setup simulations revealed that wave setup also oscillated within the tidal period, and that local bathymetry was the most important influencing factor of wave setup distribution.

Abstract: We apply a coupled circulation‐wave model to simulate extreme sea levels induced by tropical cyclones at the basin scale. We quantify storm tides, surges, waves, and their interactions for historical tropical cyclones (1988–2015) in the western North Atlantic Ocean. Comparisons between model results and field observations along the U.S. East and Gulf Coasts indicate that the overall performance of the model is satisfactory, with a root‐mean‐square error, bias, and Willmott skill of, respectively, 0.31, −0.04 m, and 0.9 for storm tides and 1.94 m, −0.22 m, and 0.87 for significant wave heights. Model results show that the highest surge levels were generated in Western Long Island Sound and NY/NJ Harbor, along the coasts of the Carolinas and Southwest Florida, near the Mississippi River delta, and along the coasts of Louisiana and East Texas. We find that the nonlinear tide‐surge interaction is often significant, but its contribution to peak storm tides induced by the most extreme events (i.e., storms that caused storm tides larger than 2 m) was moderate (−12% to 5%). We find that the maximum wave setup was relatively large (tens of cm) in most coastal regions, but it did not necessarily coincide with the peak storm tide. The contribution of wave setup to peak storm tides induced by the most extreme events was less than 17%. Finally, we show that while some offshore storms did not affect the coastal areas with high storm surges or strong winds, they generated the largest historical wave setup at the coast. [ABSTRACT FROM AUTHOR]

Wind-induced sea level blowouts, measured as negative storm surge or extreme low water (ELW), produce public safety hazards and impose economic costs (e.g., to shipping). In this paper, we use a regional hydrodynamic numerical model to test the effect of historical environmental change and the time scale, direction, and magnitude of wind forcing on negative and positive surge events in the New York Harbor (NYH). Environmental sensitivity experiments show that dredging of shipping channels is an important factor affecting blowouts while changing ice cover and removal of other roughness elements are unimportant in NYH. Continuously measured water level records since 1860 show a trend towards smaller negative surge magnitudes (measured minus predicted water level) but do not show a significant change to ELW magnitudes after removing the sea-level trend. Model results suggest that the smaller negative surges occur in the deeper, dredged modern system due to a reduced tide-surge interaction, primarily through a reduced phase shift in the predicted tide. The sensitivity of surge to wind direction changes spatially with remote wind effects dominating local wind effects near NYH. Convergent coastlines that amplify positive surges also amplify negative surges, a process we term inverse coastal funneling.

On February 15, 1941, a storm caused one of the major natural disasters in the Iberian Peninsula in the past century. The storm made landfall in the north of Portugal, leading to a large surge in the Tagus estuary. Adverse meteorological conditions combined with a high spring tide led to extensive flooding of dry land, causing severe damage and casualties. A suite of regional and local scale models is developed to analyze the event and the relative contributions of the different forcing agents to the extreme water levels. Quantitative and qualitative validations show that the models adequately reproduce this type of events. The models are then used to assess the inundation in the upstream reaches of the estuary where extensive agricultural lands are protected by dikes. Results show that over 25 km 2 could be inundated today, a value that would increase threefold for a sea level rise of 0.5 m. Then, the relative importance of the different forcing mechanisms on the extreme water levels is investigated through numerical experiments. It is shown that the regional surge and the setup induced by swell are the two main drivers of the inundation. In particular, the modulation of the wave setup by tides induces a semi-diurnal signal which is amplified by resonance inside the estuary. [ABSTRACT FROM AUTHOR]

A triply-nested two-dimensional (2D) ocean circulation model along with observed sea level records are used to study tide-surge interaction along the east coast of the Leizhou Peninsula (LP) which is characterized by extensive mudflats, large tidal ranges and a complex coastline. The dependency of surge maxima on the water level and the phase of tide are respectively investigated using two statistical approaches. Results show that tide-surge interaction along the east coast of the LP is significant, where surges peak 3–6 h before or after the nearest high water. The triply-nested 2D ocean circulation model is used to quantify tide-surge interaction in this region and to investigate its physical cause. The largest amplitudes of tide-surge interaction are found in the shallow water region of the Leizhou Bay, with values up to 1 m during typhoon events. Numerical experiments reveal that nonlinear bottom friction is the main contributor to tide-surge interaction, while the contribution of the nonlinear advective effect can be neglected. The shallow water effect enhances the role of nonlinear bottom friction in determining tide-surge modulation, leaving the surge peaks usually occur on the rising or falling tide. It is also found that the relative contribution of local wind and remote wind is different depending on the storm track and storm intensity, which would finally affect the temporal and spatial distribution of tide-surge interaction during typhoon events. These findings confirm the importance of coupling storm surges and tides for the prediction of storm surge events in regions which are characterized by shallow water depths and large tidal ranges. [ABSTRACT FROM AUTHOR]

Storm surges and the resulting extreme high sea levels are among the most dangerous natural disasters and are responsible for widespread social, economic and environmental consequences. Using a set of 220 tide gauges, this paper investigates the temporal variations in storm surges around the world and the spatial coherence of its variability. We compare results derived from two parameters used to represent storm surge: skew surge and the more traditional, non-tidal residual. We determine the extent of tide-surge interaction, at each study site, and find statistically significant (95% confidence) levels of tide-surge interaction at 59% of sites based on tidal level and 81% of sites based on tidal-phase. The tide-surge interaction was strongest in regions of shallow bathymetry such as the North Sea, north Australia and the Malay Peninsula. At most sites the trends in the skew surge time series were similar to those of non-tidal residuals, but where there were large differences in trends, the sites tended to have a large tidal range. Only 13% of sites had a statistically significant trend in skew surge, and of these approximately equal numbers were positive and negative. However, for trends in the non-tidal residual there are significantly more negative trends. We identified 8 regions where there were strong positive correlations in skew surge variability between sites, which meant that a regional index could be created to represent these groups of sites. Despite, strong correlations between some regional skew surge indices, none are significant at the 95% level, however, at the 80% level there was significant positive correlation between the north-west Atlantic - south and the North Sea. Correlations between the regional skew surge indices and climate indices only became significant at the 80% level, where Nińo 4 was positively correlated with the Gulf of Mexico skew surge index and negatively correlated with the east Australia skew surge index. The inclusion of auto-correlation in the calculation of correlation greatly reduced their significance, especially in the short time-series used for the regional skew surge indices.

Using hourly sea level data from four tide gauges, the changes of the extreme sea level in the Bohai Sea were analyzed in this work. Three components (i.e., mean sea level, tide and surge) as well as the tide–surge interaction were studied to find which component was important in the changes of extreme sea levels. Significant increasing trends exist in the mean sea level at four tide gauges from 1980 to 2016, and the increase rate ranges from 0.2 to 0.5 cm/year. The mean high tide levels show positive trends at four tide gauges, and the increasing rate (0.1 to 0.3 cm/year) is not small compared with the long-term trends of the mean sea levels. However, the mean tidal ranges show negative trends at Longkou, Qinhuangdao and Tanggu, with the rate from about −0.7 to −0.2 cm/year. At Qinhuangdao and Tanggu, the annual surge intensity shows explicit long-term decreasing trend. At all four tide gauges, the storm surge intensity shows distinct inter-annual variability and decadal variability. All four tide gauges show significant tide–surge interaction, the characteristics of the tide–surge interaction differ due to their locations, and no clear long-term change was found. Convincing evidence implies that the extreme sea levels increase during the past decades from 1980 to 2016 at all tide gauges, with the increasing rate differing at different percentile levels. The extreme sea level changes in the Bohai Sea are highly affected by the changes of mean sea level and high tide level, especially the latter. The surge variation contributes to the changes of extreme sea level at locations where the tide–surge interaction is relatively weak.

An unstructured grid, two-dimensional hydrodynamic model was established and applied to the coast of Taiwan to investigate the tide-surge interaction. Tidal elevations at the open boundaries coupled with a global ocean tidal model and the meteorological conditions using a cyclone model are used to drive the model. The model was calibrated and verified with the observed tidal levels at six tidal stations for seven typhoon events to ascertain the capability and feasibility of the model. The results show reasonable agreement between the simulated and observed tidal levels. The validated model was then applied to probe the influence of tide-surge interaction on phase, water levels, and storm surge height. We found that the tide-surge interaction influenced both the magnitude and timing of the surge, which depended on the typhoon path. The storm surge heights at different tidal stations were significantly influenced by wind stresses and directions. The water level rise due to the storm surge during high tide was greater at neap tide than at spring tide. Changing tidal ranges altered the prediction of the surge enough to induce the changes in peak water levels. [ABSTRACT FROM AUTHOR]

The hydrodynamic flows in the Singapore regional waters (SRW) are the result of a complex mix of tide, seasonal and meteorological effects. The study of non-tidal effects or sea level anomalies (SLA) in this region has shown that it is possible to model some of these anomalies. The present study addresses the non-tidal barotropic water levels and currents in the Singapore region in detail. This analysis includes a multi-scale approach, and addresses amongst others hydrodynamic model grid resolution and the importance of resolving non-linear tide–surge interaction. The results show that the water level and current anomalies phenomena in a complex region like SRW can be effectively modelled using an approach combining non-tidal barotropic and multi-scale numerical modelling. A detailed investigation of the levels of non-linear tide–surge interaction is carried out by simulating SLA events in the Singapore regional waters during North-East (positive SLA) and South-West (negative SLA) monsoons based on ECMWF numerical weather forcing conditions. The results of combining both approaches suggest that the finer grid resolution improves the accuracy of water level and current anomalies simulations. Furthermore, the results also indicate that for the simulations of non-tidal barotropic flows in this area, non-linear tide–surge interaction is important and should be taken into account. Finally, the behaviour of the non-tidal barotropic flow in the region and its changes with tidal variation in the shallow region of the Singapore Strait is now much better understood. Therefore, for reliable operational forecast of sea level, the inclusion of non-linear tide–surge interaction should be required in numerical models to reproduce both tides and surges with improved accuracy in this region. [ABSTRACT FROM AUTHOR]

With recent advances in measurement and information technology, there is an abundance of data available for analysis and modelling of hydrodynamic systems. Spatial and temporal data coverage, better quality and reliability of data modelling and data driven techniques have resulted in more favourable acceptance by the hydrodynamic community. The data mining tools and techniques are being applied in variety of hydro-informatics applications ranging from data mining for pattern discovery to data driven models and numerical model error correction. The present study explores the feasibility of applying mutual information theory by evaluating the amount of information contained in observed and prediction errors of non-tidal barotropic numerical modelling (i.e. assuming that the hydrodynamic model, available at this point, is best representation of the physics in the domain of interest) by relating them to variables that reflect the state at which the predictions are made such as input data, state variables and model output. In addition, the present study explores the possibility of employing ‘genetic programming’ (GP) as an offline data driven modelling tool to capture the sea level anomaly (SLA) dynamics and then using them for updating the numerical model prediction in real time applications. These results suggest that combination of data relationship analysis and GP models helps to improve the forecasting ability by providing information of significant predicative parameters. It is found that GP based SLA prediction error forecast model can provide significant improvement when applied as data assimilation schemes for updating the SLA prediction obtained from primary hydrodynamic models. [ABSTRACT FROM AUTHOR]

Abstract: Hourly tide-gauge data at Chennai, Visakhapatnam and Paradip along the east coast of India and at Hiron Point, at the head of Bay of Bengal, were analysed statistically for tide-surge interaction. Dependency of surge maxima on magnitude and phase of the tide were examined separately and interaction was quantified for each site. Along the east coast of India, interaction is found to be less, but significant. The degree of interaction is highest at Hiron Point, where surges peak more frequently about 4 h before the high tide. Application of a simple model shows the observed surge peak distribution at Hiron Point can be explained in terms of phase alteration of tide due to surge and surge modulation by tide. The degree of interaction tends to increase northward as the tidal range and surge height increases. [Copyright &y& Elsevier]

The aim of this study is to investigate tide-surge interaction in narrow channels with complex and relatively shallow topography. A high-resolution depth-integrated tidal and storm surge model has been implemented for the Tjeldsund channel which is an important sailing lane in northern Norway. A horizontal grid resolution down to 50 m is applied in order to represent the complex bottom topography and the formation of jets and small-scale eddies. Two typically storm surge events in December 2004 have been examined in detail. The tide-surge interaction is found to influence the generation of higher harmonics and the formation of eddies in the current field. In some cases, the magnitude of storm surge currents may reach the same magnitude as the tidal currents enhancing the formation of jets and eddies. [ABSTRACT FROM AUTHOR]

In real-time operational coastal forecasting systems for the northwest European shelf, the representation accuracy of tide-surge models commonly suffers from insufficiently accurate tidal representation, especially in shallow near-shore areas with complex bathymetry and geometry. Therefore, in conventional operational systems, the surge component from numerical model simulations is used, while the harmonically predicted tide, accurately known from harmonic analysis of tide gauge measurements, is added to forecast the full water-level signal at tide gauge locations. Although there are errors associated with this so-called astronomical correction (e.g. because of the assumption of linearity of tide and surge), for current operational models, astronomical correction has nevertheless been shown to increase the representation accuracy of the full water-level signal. The simulated modulation of the surge through non-linear tide-surge interaction is affected by the poor representation of the tide signal in the tide-surge model, which astronomical correction does not improve. Furthermore, astronomical correction can only be applied to locations where the astronomic tide is known through a harmonic analysis of in situ measurements at tide gauge stations. This provides a strong motivation to improve both tide and surge representation of numerical models used in forecasting. In the present paper, we propose a new generation tide-surge model for the northwest European Shelf (DCSMv6). This is the first application on this scale in which the tidal representation is such that astronomical correction no longer improves the accuracy of the total water-level representation and where, consequently, the straightforward direct model forecasting of total water levels is better. The methodology applied to improve both tide and surge representation of the model is discussed, with emphasis on the use of satellite altimeter data and data assimilation techniques for reducing parameter uncertainty. Historic DCSMv6 model simulations are compared against shelf wide observations for a full calendar year. For a selection of stations, these results are compared to those with astronomical correction, which confirms that the tide representation in coastal regions has sufficient accuracy, and that forecasting total water levels directly yields superior results. [ABSTRACT FROM AUTHOR]

A fairly undeveloped barrier island along the mid-Atlantic coast, Onslow Beach, was exposed to two differing, yet sequential meteorological events in the fall of 2008. The response of the barrier island differed significantly enough to warrant investigation into the causes of aberrant overwash locations. Tropical Storm Hanna generated high significant wave heights for a short period of time and caused overwash events along the southern portion of Onslow Beach. The Nor'easter and subsequent wind shift after TS Hanna generated significant wave heights that were lower than during TS Hanna, yet more locations of overwash were recorded along the beach. Data from NOAA wave buoys and a nearshore deployed AWAC were analyzed to understand the underlying physics behind the recorded differences in barrier island response. These data were also used to validate a coupled hydrodynamic (ADCIRC) and waves (SWAN) model to investigate the alongshore variability. Low frequency variability, on the order of days, and tidal timing of shoreward high significant wave heights contributed to the recorded variability. [ABSTRACT FROM AUTHOR]

Abstract: We used a two-dimensional, barotropic, numerical model to examine the complex behaviour of the tides in Bass Strait. Bass Strait, located in south-east Australia between the Australian continent and Tasmania, has the appropriate dimensions to create a half-wave tidal resonance for the M 2 tides. The M 2 tidal wave (amplitude < 0.4 m) enters the strait from both strait openings, increasing the M 2 tidal amplitude towards the central northern Tasmanian coast to a maximum of ∼1.1 m. The tidal phases and current ellipses in the strait showed the M 2 tides resembled a half-wavelength resonance in a curved, open basin. The semidiurnal, S 2, and diurnal, K 1 and O 1, tidal amplitudes were comparatively small (<0.2 m), progressive in nature and mostly constant through the strait. The model simulations revealed that when only open boundary tidal (OBT) forcing was included in the model, the model over predicted the M 2 amplitudes by ∼10–15% in the strait''s central region; when OBT and direct gravitational tidal (DGT) forcing were included, the model accurately reproduced the observed tidal amplitudes. DGT forcing was used to generate resonantly amplified M 2 tides (amplitudes of 0.1–0.3 m) locally, with destructive interference between the OBT-forced and DGT-forced tides occurring in the centre of the strait. The model simulations also revealed that storm systems propagating west to east attenuated the M 2 tidal amplitudes in the strait. The greatest decrease in the tidal amplitudes occurred along the central northern Tasmanian coast and was attributed to tide surge interaction and resonance behaviour in the strait. [Copyright &y& Elsevier]

Abstract: A regional two-dimensional hydrodynamic model using the MIKE-21 software and data from a pre-operational forecasting system of the English Channel is described and applied to the Solent–Southampton Water estuarine system. The regional model was able to predict surge heights with a root mean squared error (RMSE) accuracy of 0.09 m during a three month hindcast in the winter of 2009, comparing closely with accuracy assessments from other independent systems. However, consistent underprediction of tidal harmonic constituent amplitudes was present throughout the region leading to errors in the prediction of the total water level elevations. Despite the complex nature of the Solent tidal regime, interpolation of tidal elevations from harmonic analysis at fixed tide gauge locations was shown to be effective in reducing this uncertainty at gauged and un-gauged sites. The degree to which tide–surge interactions were taking place was examined. Of particular interest was the quantification of the sensitivity of the predicted surge to the levels of uncertainty expected in the prediction of the tide within a complex nearshore region such as the Solent. The tide–surge interaction during three surge events was shown to be greatest in the Western Solent and Southampton Waters regions, where the tidal uncertainty was greatest. Interaction between the tide and surge resulted in a change of up to 0.3 m (11%) in the predicted total peak water level when the surge was added to the harmonic analysis-based tidal prediction. Despite the significant effect of removing the tide–surge interactions, tests indicated that the error in tidal range expected in the regional models tidal prediction altered the prediction of the surge only enough to induce changes in peak total water elevations by up to 0.03 m during an event on 10th March 2008. The findings suggest that the current tidal predictions in complex estuarine systems, such as the Solent, are of high enough quality to reproduce the majority of the tide–surge interactions taking place and that the error in the surge due to uncertainties in the predicted tide are expected to be relatively small. [Copyright &y& Elsevier]

A three-dimensional finite volume unstructured mesh model of the west coast of Britain, with high resolution in the coastal regions, is used to investigate the role of wind wave turbulence and wind and tide forced currents in producing maximum bed stress in the eastern Irish Sea. The spatial distribution of the maximum bed stress, which is important in sediment transport problems, is determined, together with how it is modified by the direction of wind forced currents, tide-surge interaction and a surface source of wind wave turbulence associated with wave breaking. Initial calculations show that to first order the distribution of maximum bed stress is determined by the tide. However, since maximum sediment transport occurs at times of episodic events, such as storm surges, their effects upon maximum bed stresses are examined for the case of strong northerly, southerly and westerly wind forcing. Calculations show that due to tide-surge interaction both the tidal distribution and the surge are modified by non-linear effects. Consequently, the magnitude and spatial distribution of maximum bed stress during major wind events depends upon wind direction. In addition calculations show that a surface source of turbulence due to wind wave breaking in shallow water can influence the maximum bed stress. In turn, this influences the wind forced flow and hence the movement of suspended sediment. Calculations of the spatial variability of maximum bed stress indicate the level of measurements required for model validation. [ABSTRACT FROM AUTHOR]

Abstract: 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 20cm, and realistic semi-diurnal to diurnal amplitude ratios. Storm surge hindcasts are also shown to be predicted with typical RMS errors less than 8cm. 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. vectors tends to be more aligned with the bathymetry than the 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. [Copyright &y& Elsevier]

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 Río 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 Bahía 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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Abstract: We use a novel statistical approach to analyse changes in the occurrence and severity of storm surge events in the southern and central North Sea over the period 1955–2000, using 1) output from a numerical storm surge model and 2) in situ data on surge levels at sites for which sufficiently long observational records are available. The methodology provides a robust diagnostic tool for assessing the ability of models to reproduce the observed characteristics of storm surges, in a fashion that properly accounts both for variability within a single long run of the model and for recording error in observational data on sea levels. The model re-analysis data show strong positive trends in the frequency and severity of storm surge events at locations in the north-eastern North Sea, whilst trends at locations in the southern and western North Sea appear to be dominated by decadal variability. Trends in in situ data for sites along the North Sea coast are fairly synchronous with corresponding trends in the model re-analysis data, with the most serious discrepancies attributable to known problems in the observational record. [Copyright &y& Elsevier]

Abstract: During the passage of Hurricane Floyd, a unique water level fluctuation occurred in the Chesapeake Bay with double high peaks of storm tide in the lower Bay and a moderate set-down in the upper Bay. The response of water level inside the Bay to the hurricane has been studied through analysis of observations and model simulations. The ADvance CIRculation (ADCIRC) model was applied to the Chesapeake Bay for this study. A high-resolution tide and storm surge model capable of storm surge prediction has been developed for the Bay. The study shows that the water level variation during the hurricane inside the Bay can be well explained by the superposition of two distinct physically driven mechanisms: storm tide that developed in the offshore regions propagating into the Bay and surge generated by local wind. The double peaks of storm tide that occurred in the lower Bay regions are mainly caused by the tidal component, which modulates the incoming surge wave generated in the offshore regions. The model experiments show that the incoming storm tide can propagate to the head of the estuary without much attenuation. Consequently, the superposition of the surge generated by the local northeasterly and northwesterly winds and the incoming storm tide results in an enhanced set-up in the lower Bay region, while the set-down in the upper Bay regions is reduced. The offshore water level variation influences the lower Bay regions more significantly. The storm tide propagating into the Bay accounts for most of the water level variation in the lower Bay regions during the storm. [Copyright &y& Elsevier]

The study explores storm floods in the Yangtze Estuary to examine the surge responses to the variation of storm tracks and storm timing under the combined effect of astronomical tides, storm surge, and runoff. A new method of typhoon scenario construction is developed based on a hypothesis of the influenced sphere of the Yangtze Estuary (ISYE) raised in the paper and two representative typhoon tracks, the direct landing typhoon (DLT) and the offshore northward typhoon (ONT), are studied by the worst cases with five scenarios of tidal range using FVCOM. The result shows that the surge threats of DLT and ONT concentrate on the upper estuary and inner estuary, respectively. The phase shift due to the acceleration of phase speed, resulting from the surge-tide interaction and the flood current, is proposed to be the primary factor increasing surge in the rising-tide. A transfer of the peak surge position is detected outside the estuary, which can well explain the abnormally small surge near the low-tide. Further, the larger tidal range tends to attenuate surge levels due to the enhanced depth in the inner and upper estuarine areas. • An improved indicator for assessing the effect of typhoons and surges on the limited scope of regions is applied. • A new method of typhoon scenarios construction is raised with respect to the prevailing wind controlling the waters. • Surge performance is different in different tracks of typhoons. • Different surge responses are explained by the phase shift. [ABSTRACT FROM AUTHOR]

Storm propagation speed (SPS) can noticeably impact coastal floods around semi-closed basins influenced by extratropical offshore storms. As a case study, the SPS impact on potential flood hazards due to extreme water levels along the UK east coast was studied using a numerical shelf sea model (FVCOM). The storm Xaver, which caused the largest North Sea surge over the past 60 years, was studied as a base scenario. Halving/doubling the SPS results in a smaller surge and a longer/shorter surge duration. Hence, the largest peak water level was found at actual speed, while the largest potential flood hazard occurred at half speed. Tide–surge interaction tends to reduce the M2 tide along the coast and advance its propagation for all SPS. A three-dimensional semi-analytical model, including a time-periodic wind forcing, was used to investigate the dominant mechanisms behind the surge dynamics, where wind duration is directly related to the SPS. Long wind durations correspond to small SPS, and vice versa. The semi-analytical model was applied to the North Sea. The model reproduces the spatial features of the North Sea surge and its dependence on SPS, confirming the surge induced by offshore storms is primarily associated with wind set-up. Model results suggest the SPS of Xaver is likely to have contributed greatly to the occurrence of the largest North Sea surge due to wind-generated resonance. The impact of the SPS on the surge and tide–surge interaction are of great importance to coastal flood hazard assessment. • Fast moving storms can cause large coastal floods due to basin resonance. • The propagation speed of the storm Xaver is responsible for its coastal impact. • Surge can advance the tidal propagation, which is more evident for slower storms. [ABSTRACT FROM AUTHOR]

Wind-induced sea level blowouts, measured as negative storm surge or extreme low water (ELW), produce public safety hazards and impose economic costs (e.g., to shipping). In this paper, we use a regional hydrodynamic numerical model to test the effect of historical environmental change and the time scale, direction, and magnitude of wind forcing on negative and positive surge events in the New York Harbor (NYH). Environmental sensitivity experiments show that dredging of shipping channels is an important factor affecting blowouts while changing ice cover and removal of other roughness elements are unimportant in NYH. Continuously measured water level records since 1860 show a trend towards smaller negative surge magnitudes (measured minus predicted water level) but do not show a significant change to ELW magnitudes after removing the sea-level trend. Model results suggest that the smaller negative surges occur in the deeper, dredged modern system due to a reduced tide-surge interaction, primarily through a reduced phase shift in the predicted tide. The sensitivity of surge to wind direction changes spatially with remote wind effects dominating local wind effects near NYH. Convergent coastlines that amplify positive surges also amplify negative surges, a process we term inverse coastal funneling. [ABSTRACT FROM AUTHOR]

Using hourly sea level data from four tide gauges, the changes of the extreme sea level in the Bohai Sea were analyzed in this work. Three components (i.e., mean sea level, tide and surge) as well as the tide–surge interaction were studied to find which component was important in the changes of extreme sea levels. Significant increasing trends exist in the mean sea level at four tide gauges from 1980 to 2016, and the increase rate ranges from 0.2 to 0.5 cm/year. The mean high tide levels show positive trends at four tide gauges, and the increasing rate (0.1 to 0.3 cm/year) is not small compared with the long-term trends of the mean sea levels. However, the mean tidal ranges show negative trends at Longkou, Qinhuangdao and Tanggu, with the rate from about −0.7 to −0.2 cm/year. At Qinhuangdao and Tanggu, the annual surge intensity shows explicit long-term decreasing trend. At all four tide gauges, the storm surge intensity shows distinct inter-annual variability and decadal variability. All four tide gauges show significant tide–surge interaction, the characteristics of the tide–surge interaction differ due to their locations, and no clear long-term change was found. Convincing evidence implies that the extreme sea levels increase during the past decades from 1980 to 2016 at all tide gauges, with the increasing rate differing at different percentile levels. The extreme sea level changes in the Bohai Sea are highly affected by the changes of mean sea level and high tide level, especially the latter. The surge variation contributes to the changes of extreme sea level at locations where the tide–surge interaction is relatively weak. [ABSTRACT FROM AUTHOR]

Newly exposed concepts of POT declustering (Bernardara et al., 2014) within the GPD-Poisson model are applied to the joint probability of tide and surge for determining extreme sea levels, as a variation of the Revised Joint Probability Method (RJPM, Tawn and Vassie, 1989). A mixture model is proposed for the meteorological residual (surge) component with a non-parametric (empirical) density for the bulk values and parametric models for both the lower and upper tails. In particular, a distinction is made between values observed at regular time steps, called sequential values, and the clusters of extreme values, or events, on which the statistical extrapolations are performed. The sea level distribution is obtained by convolution of the tide and surge density functions. Confidence intervals are also proposed. This model is applied to the case study of Brest, France using both hourly and high water values. Two methods for handling tide-surge interaction are presented and discussed and a comparison with a direct approach is made. [ABSTRACT FROM AUTHOR]