Search

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]

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]

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]

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]

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]

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]