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Extreme value statistics applied to positive storm surges (PSS) is a necessary tool for coastal management adaptation and decision-making. Generally, when extreme values of PSS are calculated, the maximum height of PSS or the skew surge (SK) for each event is used. In this paper, a comparison of the results obtained by applying the generalized extreme value (GEV) distribution to series of maximum PSS and SK (GEV-MPSS and GEV-MSK, respectively), observed in two port areas with different hydrodynamic and tidal characteristics, is performed for the period from 1956 to 2012. The sites chosen are Buenos Aires and Mar del Plata. The first one is located at the Río de la Plata Estuary, with PSS that tripled the height of the astronomical tide. The second is located on the Atlantic coast of Buenos Aires Province, with PSS that often reach the amplitude of the astronomical tide. The GEV distribution parameters are calculated by the maximum likelihood method, and estimated heights are obtained using MPSS and MSK for return periods up to 500 years. Results show that estimated heights calculated using GEV-MPSS or GEV-MSK differ from one to another for both locations. GEV-MPSS appears to overestimate the meteorological effect in some events, while GEV-MSK appears to underestimate this effect on others. It is also shown that both methodologies are not applicable to all events at the same location. An alternative to obtain a more representative dataset of meteorological action would be using both methodologies, selecting the MSK or the MPSS that identify the maximum wind effect for each event. [ABSTRACT FROM AUTHOR]

Based on previous studies of tide‐gauge records from locations with semidiurnal tidal regimes, extreme skew surges are always assumed independent of the high water (HW). However, differences in water depth between HW peaks of semidiurnal tidal regimes can be much lower than those in mixed semidiurnal regimes, where one daily HW is higher than the other. We statistically analyze tide‐gauge records of 15 sites worldwide with a mixed semidiurnal regime and find that for approximately half of these sites extreme skew surges occur more often during smaller HWs. This dependence is not caused by seasonality effects, and the places that show dependence are all located in areas with a shallow continental shelf, thus indicating possible tide‐skew surge interactions. In those cases where dependence exists, using the skew surge does not seem to offer any advantages over the nontidal residual. Plain Language Summary: The water level during storms (i.e., storm surges) in coastal areas largely depends on the level of the tide. This effect can be more noticeable in places with a mixed semidiurnal regime, which is characterized by two high water peaks per day, with one high water peak being larger than the other. Knowledge of this effect can help forecasting and prevention of flooding events in these regions. We statistically analyze water level records of 15 sites worldwide with a mixed semidiurnal regime and find that in around half of these sites storm surges occur more often at lower high water levels. These places are all located in areas with a shallow continental shelf, such as estuaries or closed seas. Key Points: Extreme skew surges are not independent of the high water level in some sites with mixed tidal regimeExtreme skew surges occur more often during lower high tidal waters in the sites that show dependenceThe dependence observed between extreme skew surges and high waters does not arise from seasonality effects [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.

• The skew surge is more suitable than the residual water level for surge risk assessment frameworks. • The residual water level may be a better metric for capturing the relationship between storm surges and climate changes. A better understanding of the storm surge phenomenon is essential for coastal protection, future planning, and coastal ecosystem conservation. Using hourly sea level data from 15 tide gauges along the coastline of China the metrics, skew surge and residual water level, which are used to represent storm surges, are compared in this study. The relationship between residual water level, skew surge and predicted high tide were analyzed using two statistical measurements. The results showed that unlike the annual maximum residual water level, the annual maximum skew surge and the astronomical high tide tended to coincide more frequently (more than 50% of the annual maximum skew surges occurred at high tide). Thus, the skew surge was more suitable than the residual water level for use in risk-based coastal planning frameworks. Using the monthly maximum value and indices of the storm surge (storm surge number, duration and intensity), we compared the performances of skew surge and residual water level in expressing changes of storm surges. These two metrics can both capture changes in storm surge activities on seasonal, decadal and long-term scales, and correlation coefficients of storm surge intensity between residual water level and skew surge were greater than 0.80 at 11 tide gauges. At most tide gauges, the skew surge was significantly correlated with 4–5 nearby tide gauges while the residual water level was correlated with only 1–2 tide gauges, the skew surge may be a better metric for risk-based coastal planning frameworks if the interpolation method is used. The El Niño-Southern Oscillation (ENSO) can affect the storm surge frequency along the coastline of China. The effects of the ENSO were more obvious when the residual water level was selected. In conclusion, the skew surge can be selected for use in risk-based coastal planning frameworks, while the residual water level may be a better metric for capturing the relationship between storm surges and climate changes. [ABSTRACT FROM AUTHOR]