The influence of infragravity waves on the safety of coastal defences: a case study of the Dutch Wadden Sea

Many coastlines around the world are protected by dikes with shallow foreshores (e.g. salt marshes and mudflats) that attenuate storm waves and are expected to reduce the likelihood and volume of waves overtopping the dikes behind them. However, most of the studies to date that assessed their effectiveness have excluded the influence of infragravity (IG) waves, which often dominate in shallow water. Here, we propose a modular and adaptable framework to estimate the probability of coastal dike failure by overtopping waves (Pf). The influence of IG waves on overtopping is included using an empirical approach, which is first validated against observations made during two recent storms (2015 and 2017). The framework is then applied to compare the Pf values of the dikes along the Dutch Wadden Sea coast with and without the influence of IG waves. Findings show that including IG waves results in 1.1 to 1.6 times higher Pf values, suggesting that safety is overestimated when they are neglected. This increase is attributed to the influence of the IG waves on the design wave period and, to a lesser extent, the wave height at the dike toe. The spatial variation in this effect, observed for the case considered, highlights its dependence on local conditions – with IG waves showing greater influence at locations with larger offshore waves, such as those behind tidal inlets, and shallower water depths. Finally, the change in Pf due to the IG waves varied significantly depending on the empirical wave overtopping model selected, emphasizing the importance of tools developed specifically for shallow foreshore environments.

Characteristics of Low-Frequency Horizontal Noise of Ocean-Bottom Seismic Data

Horizontal records of ocean-bottom seismographs are usually noisy at low frequencies (< 0.1 Hz). The noise source is believed to be associated with ocean-bottom currents that may tilt the instrument. Currently horizontal records are mainly used to remove the coherent noise in vertical records, and there has been little literature that quantitatively discusses the mechanism and characteristics of low-frequency horizontal noise. In this article, we analyze in situ ocean-bottom measurements by rotating the data horizontally and evaluating the coherency between different channels. Results suggest that the horizontal noise consists of two components, random noise and principle noise whose direction barely changes in time. The amplitude and the direction of the latter are possibly related to the intensity and direction of ocean-bottom currents. Rotating the horizontal records to the direction of the principle noise can largely suppress the principle noise in the orthogonal horizontal channel. In addition, the horizontal noise is incoherent with pressure, indicating that the noise source is not ocean surface water waves (infragravity waves). At some stations in shallow waters (<300 m), horizontal noise around 0.07 Hz is found to be linearly proportional to the temporal derivative of pressure, which is explained by forces of added mass due to infragravity waves.

Performance Accuracy of Surfbeat in Modeling Infragravity Waves Near and Inside a Harbor

Infragravity (IG) waves significantly affect the operational efficiency of ports. Therefore, an accurate prediction of IG waves inside a harbor is necessary. In this study, the accuracy of the wave-group-resolving model XBeach Surfbeat (XB-SB, Delft University of Technology, Delft, The Netherlands) in predicting the IG waves inside a harbor was assessed by comparing its results with field measurements. Field measurements were performed at Hambantota Port in southern Sri Lanka. Three acoustic waves and current sensors were used to observe the wave characteristics inside and outside the harbor. First, the model was validated against observations outside the port. Next, the performance accuracy of XB-SB in modeling the hydrodynamics in the harbor was evaluated by comparing its results with the values measured inside the port. The results of the numerical simulations indicated that both the nearshore short and IG wave heights can be accurately reproduced by XB-SB in an open domain without many obstacles. However, the short wave heights in the harbor are severely underestimated by XB-SB. The IG waves inside the harbor are overestimated most of the time. Moreover, the natural periods of Hambantota Port are well calculated by XB-SB. In general, XB-SB is a reliable tool for predicting nearshore IG waves. However, it requires further improvement to reproduce the hydrodynamics in a well-sheltered harbor, such as Hambantota Port.

The Effects of Tidal Translation on Wave and Current Dynamics on a Barred Macrotidal Beach, Northern France

Barred macrotidal beaches are affected by continuous horizontal displacements of different hydrodynamic zones associated with wave transformation (shoaling, breaker and surf zones) due to significant tide-induced water level changes. A series of wave and current meters, complemented by a video imagery system, were deployed on a barred beach of northern France during a 6-day experiment in order to characterize the spatial and temporal variability of wave-induced processes across the beach. Wave and current spectral analyses and analyses of cross-shore current direction and asymmetry resulted in the identification of distinct hydrodynamic processes, including the development of infragravity waves and offshore-directed flows in the breaker and surf zones. Our results revealed a high spatial variability in the hydrodynamic processes across the beach, related to the bar-trough topography, as well as significant variations in the directions and intensity of cross-shore currents at fixed locations due to the horizontal translation of the different hydrodynamic zones resulting from continuous changes in water level due to tides.

The Influence of Infragravity Waves on the Safety of Coastal Defences: A Case Study of the Dutch Wadden Sea

Many coastlines around the world are protected by coastal dikes fronted by shallow foreshores (e.g. saltmarshes and mudflats) that attenuate storm waves and are expected to reduce the likelihood of waves overtopping the dikes behind them. However, most of the studies to-date that assessed their effectiveness have excluded the influence of infragravity (IG) waves, which often dominate in shallow water. Here, we propose a modular and adaptable framework to estimate the probability of coastal dike failure by overtopping waves (Pf). The influence of IG waves on wave overtopping is included using an empirical approach, which is first validated against observations made during two recent storms (2015 and 2017). The framework is then applied to compare the Pf of the dikes along the Dutch Wadden Sea coast, with and without the influence of IG waves. Findings show that including IG waves results in 1.1 to 1.6 times higher Pf values, suggesting that safety may be overestimated when they are neglected. This increase is attributed to the influence of the IG waves on the design wave period, and to a lesser extent the wave height, at the dike toe. The spatial variation in this effect, observed for the case considered, highlights its dependence on local conditions – with IG waves showing greater influence at locations with larger offshore waves and shallower water depths. Finally, the change in Pf due to the IG waves varied significantly depending on the empirical wave overtopping model selected, emphasizing the importance of tools developed specifically shallow foreshore environments.

Extreme sea level oscillations in the Sea of Japan caused by typhoons Maysak and Haishen in September 2020

<p>Two hazardous typhoons, Maysak and Haishen, in September 2020 produced extreme sea level oscillations in the Sea of Japan. These typhoons generated three different types of sea level variations: 1) storm surges (with typical periods from several hours to 1.5 days), 2) extreme seiches (with periods from a few minutes to several tens of minutes), and 3) storm-generated infragravity waves (with periods up to 3-5 min). The data from eleven tide gauges on Russian, Korean, and Japanese coasts were used to examine the properties of these oscillations. The relative contribution of the three separate sea level components and their statistical characteristics (duration, wave heights, and periods) were estimated. The periods of the main eigen modes of individual bays and harbours in the Sea of Japan were estimated based on spectral analysis of longterm background records at the corresponding sites. The results of wavelet analysis show the frequency properties and the temporal evolution of individual sea level components. We found that high-frequency sea level oscillations at stations Preobrazheniye and Rudnaya Pristan have a “white noise” spectrum, caused by the dominance of infragravity waves. A high correlation was detected between the variance of high-frequency sea level oscillations at these stations and the significant wind wave height evaluated from ERA5 for this water area.</p>