Extreme Surge Research Articles

Effect of surge uncertainty on probabilistically computed dune erosion

To assess the flood protection capacity of dunes in The Netherlands, a semi-probabilistic dune-erosion prediction method is currently in use in which uncertainties in input parameters of an empirical dune erosion model were taken into account, with the exception of the uncertainty in the extreme surge distribution. Previous research has shown that the surge is by far the most influential parameter affecting erosion in the currently used erosion model, which is due both to the influence of the surge level itself and to the conditional dependence of the wave height and period on the surge level in the probabilistic model used for the assessment. Furthermore, the distribution of extreme surge levels has been shown to contain large statistical uncertainty. The inclusion of uncertainty in input variables into probabilistic models results in more extreme events (in this case erosion) for the same exceedance probability, largely due to the incorporation of higher values of the input variables. The goal of the research described in this paper was to determine the impact of the inclusion of uncertainty in the extreme surge distribution on the estimate of critical erosion (erosion associated with an exceedance frequency of 10 − 5 per year). The uncertainty in the surge distributions was estimated and parameterized, and was incorporated into the probabilistic model. A reduction in uncertainty was subsequently imposed to estimate what value a reduction in uncertainty can offer, in terms of the impact on critical erosion. The probabilistic technique first-order reliability method (FORM) was applied to determine the relative contribution of the uncertainty in the surge distribution (as well as the remaining stochastic variables) to the critical erosion. The impact of the inclusion of uncertainty in the surge distribution on the critical retreat distance was found to be substantial with increases ranging from 34% to 93% of the original estimate at five locations along the Dutch coast. The reduced uncertainty showed a more subtle impact, with increases in critical retreat distance ranging from 10% to 26% of the original estimate. The relative importance analysis showed that the uncertainty in the surge distribution has a strong influence, with the relative importance ranging from 10% to 23% for an exceedance frequency of 10 − 5 per year.

Regional frequency analysis of extreme storm surges along the French coast

Abstract. A good knowledge of extreme storm surges is necessary to ensure protection against flood. In this paper we introduce a methodology to determine time series of skew surges in France as well as a statistical approach for estimating extreme storm surges. With the aim to cope with the outlier issue in surge series, a regional frequency analysis has been carried out for the surges along the Atlantic coast and the Channel coast. This methodology is not the current approach used to estimate extreme surges in France. First results showed that the extreme events identified as outliers in at-site analyses do not appear to be outliers any more in the regional empirical distribution. Indeed the regional distribution presents a curve to the top with these extreme events that a mixed exponential distribution seems to recreate. Thus, the regional approach appears to be more reliable for some sites than at-site analyses. A fast comparison at a given site showed surge estimates with the regional approach and a mixed exponential distribution are higher than surge estimates with an at-site fitting. In the case of Brest, the 1000-yr return surge is 167 cm in height with the regional approach instead of 126 cm with an at-site analysis.

Statistics of fluctuations with a [formula omitted] spectrum at phase transitions in a spatially distributed system

A study has been made on the statistics of fluctuations in a spatially distributed system, which describes the interaction of nonequilibrium phase transitions. It is shown that at a certain intensity of external white noise acting on phase transitions, the time and spatial spectra of fluctuations have power laws S(f)∼f−α and S(k)∼k−γ. The dependence of the exponents α and γ on the value of the diffusion coefficient determining the spatial interaction of fluctuations have been found. Extreme low-frequency fluctuations has been revealed and distribution functions of their duration P(τ)∼τ−β and size P(s)∼s−ν have been determined. It has been found that the exponent α of the frequency dependence of time spectra and the exponent β of the distribution function of the fluctuation duration are related by the relation α+β=2. The exponent of the spatial spectrum γ and the exponent of the size distribution function ν are related by a similar relation: γ+ν=2.The results of experimental studies on fluctuations in a typical nonequilibrium phase transition such as transient modes of water boiling on a wire heater are presented. It was demonstrated that the critical exponents, which describe the power dependence of power spectra of fluctuations and the amplitude distribution of extreme surges, are related by the relation α+β=2 both in experiments and in the theoretical model of interacting heterogeneous phase transitions.

Quantifying thresholds for significant dune erosion along the Sefton Coast, Northwest England

Field and model hindcast data are used to establish a critical dune erosion threshold for the Sefton Coast (NW England). Events are classified as causing significant erosion if they result in: (a) a mean dune retreat along the entire study area of >2m; (b) a dune retreat of ≥5m along a coastal segment ≥2km in length; and (c) an eroded area ≥20,000m2. For the period 1996 to 2008, individual storms were characterised using hindcast results from a POLCOMS-WAM model and measured data from the Liverpool Bay Coastal Observatory. Results show that combined extreme surge levels (>1.5m) and wave heights (>4m), or tidal water levels above 9.0m Chart Datum (CD), do not always result in significant dune erosion. Evidence suggests that erosion is more likely to occur when wave heights are >2.6m, peak water level is >10.2m CD at Liverpool and when consecutive tidal cycles provide 10h or more of water levels above 9.4m CD. However, lower water levels and wave heights, and shorter events of sustained water levels, can cause significant erosion in the summer. While the return period for events giving rise to the most severe erosion in the winter is >50years, significant erosion in the summer can be caused by events with return periods <1year. It is suggested that this may be attributable to a known reduction in the mean dune toe elevation c. 30cm. Although the study shows it might be possible to characterise objectively storm events based on oceanographic conditions, the resultant morphological change at the coast is demonstrated to depend on the time and duration of events, and on other variables which are not so easy to quantify. Further investigation is needed to understand the influence of alongshore and seasonal variability in beach/dune morphology in determining the response to the hydrodynamic and meteorological conditions causing significant erosion. Improved monitoring pre- and post-storm of changes in beach/dune morphology is required to develop reliable proxies that can be used to establish early warning systems to mitigate the impacts of erosion and flooding in the future.

Application of regional frequency analysis to the estimation of extreme storm surges

[1] Traditionally, extreme value theory is applied to single-site series of surge observations in order to estimate the probability of occurrence of extreme events at that particular site. However, single-site analyses give uncertain estimation of extreme quantiles, mainly because of the limited duration of observation periods. In order to reduce this uncertainty, regional frequency analysis (RFA) approaches suggest collecting information not only from a single-site series but also from all (statistically) similar available series of observation. The use of RFA is widely increasing in geosciences, but few applications have been attempted yet for surge estimation. The aim of this study is to examine the applicability of RFA to extreme storm surges. The surge data observed at 18 French harbors, located on the Atlantic coast from the Spanish to Belgian borders, were collected. The series span a period of 30 years, on average, with the longest series going back to the 19th century. Stationary and independent samples of extreme surges (peaks over a given threshold) are extracted and their (statistical) homogeneity has been tested via heterogeneity and discordancy measures based on L moments. Homogeneous regions have been identified and, in order to merge information on frequency of occurrence of surges from all the sites, a surge index pooling method is defined. Finally, a regional frequency distribution has been estimated. The hypothesis and the applicability of RFA application are discussed, with some ideas for future developments in the research direction.

Interpreting Century-Scale Changes in Southern North Sea Storm Surge Climate Derived from Coupled Model Simulations

Abstract This paper describes numerical experiments using a climate–storm surge simulation system for the coast of the United Kingdom, with a particular focus on the southern North Sea and the Thames estuary in southeastern England. Time series of surges simulated in the southern North Sea by a surge model driven by atmospheric data from a regional climate model and surges simulated by the same surge model driven by atmospheric data from a global climate model are compared. A strong correspondence is demonstrated, and a linear scaling factor relating them is derived. This factor varies slowly with location. Around the Thames estuary, extreme surges are compared in the same way, and the linear scaling factor for the extremes is found to be similar to that for the full time series. The authors therefore assert that in seeking significant trends in surge at this location using this model arrangement, the regional model downscaling stage could be avoided, if observations were used to establish a suitable scaling factor for each location. The influence of the tide–surge phase relationship is investigated, and extreme sea levels at the mouth of the River Thames from regional-model-driven simulations are compared to the extreme event of 1953. Although the simulated levels are slightly lower, they are found to be comparable given the observational uncertainty. The assumption that time-mean sea level changes can be added linearly to surge changes is investigated at this location for large changes in time-mean sea level. The authors find that the primary effect of such an increase is on the speed of propagation of tide and surge, supporting the case for a simple linear addition of mean and extreme sea level changes.

A practical formulation for estimating the extreme vector excursion of a floating structure

In the design of a moored floating structure, the traditional approach is to assess the extreme surge and sway responses separately. This paper highlights the importance of considering the extreme lateral excursion, based on the vector sum of the surge and sway offsets, and an approximate formula is developed for this purpose. The method allows for first- and second-order platform motions, as well as mean offsets. The extreme vector excursion is expressed as a simple function of the maximum surge and sway responses, and the correlation coefficient between the two motions. The proposed approximation is benchmarked against more rigorous methods, including time domain simulation. It is found that provided the estimates for the surge and sway extremes are precise, the formula itself does not induce significant errors in the extreme vector excursion. Moreover, it is observed that the prediction tends to be conservative. The practicality of the proposed formulation makes it amenable for incorporation into an existing design code or analysis procedure.

An investigation of recent decadal‐scale storm events in the eastern Irish Sea

The Proudman Oceanographic Laboratory Coastal Modelling System coupled to the WAve Model (POLCOMS‐WAM) modeling system has been used to model combined tides, surges, waves, and wave‐current interaction in the Irish Sea on a 1.85 km grid. A method for data analysis is presented to determine what factors and interactions contribute to extreme conditions in a region of interest. An 11 year hindcast (1996–2006) has been performed to investigate the meteorological conditions that cause extreme surge and/or wave conditions in Liverpool Bay. A one‐way nested model approach was used. For waves, a 1° North Atlantic WAM model forces the boundary of the Irish Sea model, driven by ERA‐40 wind (∼1° resolution every 6 h). To capture the external surge generated outside of the Irish Sea, the (1/9° × 1/6°) Proudman Operational surge model extending to the continental shelf edge was run for tide and surge and was forced by Met Office mesoscale winds (∼12 km resolution every hour). The data implied that the largest surges at Liverpool are generally driven by winds from the south to the west while the largest waves are forced by winds from the west to the northwest. The worst storm conditions in Liverpool Bay result under southwesterly wind conditions that veer to the west. The large tidal range in the region acts to enhance the impact of the surge through tide‐surge interaction. Moreover, the highest water levels in Liverpool Bay are in response to southwesterly winds combined with high‐water spring tide. Even though no significant surge occurs at this time, the flood threat is at its greatest.

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.

Surge modelling in the eastern Irish Sea: present and future storm impact

It is believed that, in the future, the intensity and frequency of extreme coastal flooding events may increase as a result of climate change. The Natural Environment Research Council (NERC) Flood Risk from Extreme Events (FREE) project, Coastal Flooding by Extreme Events and EU FP7 Morphological Impacts and Coastal Risks Induced by Extreme Storm Events project are investigating the flood risks in the eastern Irish Sea, an area that includes most of England’s coastal types. Using a previously modelled and validated historical extreme surge event, in November 1977, we now investigate the changes in peak surge as a result of possible future climate conditions. In order to simulate the surge, we have set up a one-way nested approach, using the Proudman Oceanographic Laboratory Coastal Ocean Modelling System 3D baroclinic model, from a domain covering the whole NW European continental shelf, through to a 1.85 km Irish Sea model; both areas are forced by tides, atmospheric pressure and winds. We use this modelling system to investigate the impact of enhanced wind velocities and increased sea levels on the peak surge elevation and residual current pattern. The results show that sea level rise has greater potential to increase surge levels than increased wind speeds.

Reduction Method of Surge Voltage on AC Railcar's Body

Surge voltage is created on AC railcar bodies when a train raises its pantograph or when its main circuit breaker is switched. This surge voltage expands and vibrates at the end car as a result of the reflection effect in distributed-parameter circuit theory. Extreme surge voltage causes damage to and malfunction of on-board equipment, especially electronic devices such as train signaling and brake control systems. This paper outlines the mechanism behind the phenomenon of surge generation on AC railcar bodies, outlines the results of experiments for improving AC railcar ground systems, and discusses ideal ground systems for the prevention of extreme surge voltage.

Storm surge simulation using wind-wave-surge coupling model

Simulation of a storm surge caused by Typhoon 9918 in the Yatsushiro Sea, Kyushu, Japan was hindcasted by the synchronous coupled wind-wave-surge model composed of a Meso-scale meteorological model (MM5) for the wind and sea surface pressure, a spectral third-generation wind-wave model (Wavewatch III) for waves, and the coastal ocean model (Princeton Ocean Model). Inclusion of the whitecap wave breaking stresses (whitecap dissipation stress) in the coastal ocean model made it possible to reproduce the extreme surge height in the extremely shallow bay.

Performance of operational systems with respect to water level forecasts in the Gulf of Finland

This paper is devoted to the validation of water level forecasts in the Gulf of Finland. Daily forecasts produced by four setups of operational, three-dimensional Baltic Sea oceanographic models are analyzed using statistical means and are compared with water level observations at three Finnish stations located on the northern coast of the Gulf of Finland. The overall conclusion is that the operational systems were skillful in forecasting water level variations during the study period from November 1, 2003, to January 31, 2005. The factors causing differences between the water level forecasts of different models are discussed as well. An important task of operational sea level forecasting services is to provide accurate and early information about extreme water levels, both positive and negative surges. During the study period, two major winter storms occurred which caused coastal flooding in the region. According to our analysis, the operational models forecast the rise of water levels during these events rather successfully. Nowadays, operational forecasts can provide early warnings of extreme water levels at least 1 day in advance, which may be regarded as a minimum requirement for an operational forecasting system. The paper concludes that the models generally performed very well, with over 93% of the hourly water level forecasts found to be within the range of ±15 cm of the observed water levels, and with the timing of the water level peaks accurately predicted. Further discussion and studies dealing with the assessment of the skills of both operational meteorological and oceanographic forecasts, especially in connection with rare surge events, will be necessary. Skill assessment of operational oceanographic models would be relatively easy if acceptable error limits or a quality system was developed for the Baltic Sea operational models.

Analysis of dune erosion processes in large-scale flume experiments

Large-scale physical model tests were conducted with different wave periods to examine the physical processes driving dune erosion. The model tests have been carried out in a flume (2DV) with a sandy dune exposed to extreme surge and wave conditions [Van Gent, M.R.A., Van Thiel de Vries, J.S.M., Coeveld, E.M., De Vroeg, J.H. and Van de Graaff, J., 2008. Large-scale dune erosion tests to study the effect of wave periods. Coastal Engineering. doi:10.1016/j.coastaleng.2008.04.003.]. Detailed measurements in time and space of water pressure, flow velocities and sediment concentrations were performed in the near shore area. The data revealed that both short- and long waves are important to inner surf hydrodynamics. Depth averaged flows are directed offshore and increase towards the shore line. The corresponding mean sediment concentrations rise sharply towards the dune face (up to 50 g/l near the bed). The strong increase in the mean sediment concentration towards the dune face correlates well with the maximum wave surface slope which in turn is coupled to both the pressure gradient and the near-bed wave-breaking induced turbulence. Analysis shows that the pressure gradient is only partially coupled to the flow acceleration suggesting that the latter cannot always be used as a proxy for the first. Weak correlation is obtained with the near-bed flows related to the bed shear stress. Tests with a larger wave period resulted in a larger dune erosion volume. During these tests more wave energy (combined incident and infragravity waves) reached the dune face, but more importantly, this wave energy is dissipated by fewer waves resulting in more intense wave breakers and steeper wave fronts. It is therefore expected that the wave-breaking induced near-bed turbulence increases resulting in significantly higher (O(100%)) mean sediment concentrations. In addition the mean flow velocities are comparable, yielding a substantially larger offshore directed sediment transport capacity. This increase in offshore directed transport is only partially compensated by a concurrent increase in the wave related onshore transport capacity associated with intrawave processes, resulting in a net increase in the dune erosion rate.

Predictability of seasonal east coast winter storm surge impacts with application to New York's Long Island

AbstractThe characteristics of seasons with enhanced east coast winter storm (ECWS) and storm surge activity are identified from among a set of global atmospheric circulation indices and local land and sea surface temperature (SST) anomalies. Without regard for storm strength or surge potential, the most active ECWS seasons occur in association with El Niño events. There is also some indication that such seasons are preferred under the positive phase of the Pacific decadal oscillation (PDO). In terms of storm surge potential, forecasts of strong ECWS activity are more skillful than direct forecasts of the number of extreme surge events. In both cases, SSTs off the southeast US coast and in the Gulf of Mexico differentiate high seasonal activity from relatively inactive seasons. Warmer‐than‐normal SSTs in both regions during summer provide a measure of storm activity in the subsequent winter.The results provide a means of anticipating seasonal ECWS activity, and to some degree impacts, that is similar to widely used forecasts of tropical storm activity. From a predictive standpoint, forecasts of active strong storm seasons and low surge activity exhibit fairly high false alarm ratios. However, the false alarm rate for forecasts of low storm activity or high surge activity is less than 10%. Copyright © 2008 Royal Meteorological Society

The impact of climate change on storm surges over Irish waters

The Regional Ocean Model System (ROMS) of Rutgers University is used to investigate the influence of anthropogenic climate change on storm surges over Irish waters, particularly on the extreme values. Two experiments were performed to confirm the validity of the approach in the current climate: the first focused on hindcasting the surge generated by a storm in early 2002 while the second provided surge statistics by running the model for the period 1990–2002; in both cases ROMS was driven with ERA-40 forcing fields. The results show that the model is capable of simulating both specific surge events and surge climate statistics with reasonable accuracy (order of 10 cm). Model outputs were also compared spatially against satellite altimetry data, corrected for long wavelength errors, from 1993 to 2001. The ROMS model consistently reproduces the sea level changes in the Irish Sea, and over the waters to the south and west of Ireland. For the investigation of the impact of the climate change on storm surges, the same configuration of ROMS was driven by atmospheric forcing fields downscaled from ECHAM5/OM1 data for the past (1961–1990) and future (2031–2060; SRES A1B greenhouse gas scenario); the downscaled data were produced using the Rossby Centre Regional Atmosphere model (RCA3). The results show an increase in storm surge events around Irish coastal areas in the future projection, except along the south Irish coast; there is also a significant increase in the height of the extreme surges along the west and east coasts, with most of the extreme surges occurring in wintertime.

Flood threat anomaly for the low coastal areas of the English Channel based on analysis of recent characteristic flood occurrences

In the English Channel, extreme surge heights did not occur at the time of extreme high tides during the last decades and maximum recorded heights usually do not exceed the maximum astronomical tide by more than a few decimetres. To understand whether this lack of coincidence may be due to specific phenomena or only to chance, we have studied hourly tide records lasting a few decades from nine English and nine French stations as well as air pressure and wind data from nearby meteorological observatories. Among the case studies of moderate flooding at several coastal stations occurring during spring tide, we have selected those of 24–25/10/1980 and of 30/01/1983 to 02/02/1983 as representative of a normal situation without any special chance. The third case study 26–28/02/1990 was potentially more dangerous because of the storm intensity and duration; however, by chance, surge peaks occurred near the low tide. Finally, the propagation of the surge peak of 15–16/10/1987, which reached the maximum height recorded during all the instrumental period at several stations, has been followed all along the English Channel, using the hourly records of 12 tide-gauge stations and of 16 meteorological stations. The surge peak of this great storm, probably the strongest in the last two centuries, occurred everywhere at high tide and spread with the same velocity of the tidal wave. Fortunately, no major flooding occurred because it was the day after a neap tide. In conclusion, some good fortune has saved the low coastal areas of the English Channel from major floods during the last decades. However, the occurrence of the peak of a strong storm surge arriving near the western entrance of the Channel at the time of a great astronomical high tide is a possible event that could be devastating along both sides of the Channel coasts.

Cherishing the Goose with the Golden Eggs: Trends in Migrant Remittances from Europe to Morocco 1970–2004

In contrast to earlier predictions, migrant remittances from Europe to Morocco have shown an increasing trend over the past decades. Remittances constitute a vital and relatively stable source of foreign capital. The so-called “euro effect” and concomitant money laundering can only explain part of the recent, extreme surge in remittances. The structural solidity of remittances is explained by the unforeseen persistence of migration to northwestern Europe; new labor migration toward southern Europe; and the durability of transnational and transgenerational links between migrants and stay-behinds. The stable economic-political environment and new “enlightened” policies toward migrants explain why Morocco has been relatively successful in channeling remittances through official channels.

Profiling Price Dynamics in Online Auctions Using Curve Clustering

Electronic commerce, and in particular online auctions, have received an extreme surge of popularity in recent years. While auction theory has been studied for a long time from a game-theory perspective, the electronic implementation of the auction mechanism poses new and challenging research questions. In this work, we focus on the price formation process and its dynamics. We present a new source of rich auction data and introduce an innovative way of modelling and analyzing price dynamics. We represent auctions as functional objects by accommodating the special structure of bidding data. We then use functional data analysis to characterize different types of auctions. Our findings suggest that there are several types of dynamics even for auctions of comparable items. By profiling these differences with respect to features associated with the auction format, the seller and the winner we find new relationships between dynamics and auction settings, and we tie these findings to the existing literature on online auctions.

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.