Observations on the formation and location of transient rip currents
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Lee, J.L.; Kim, J.W.; Kim, H.S.; Cho, W.C., and Lee, J., 2018. Development of prediction system for invasion of swell-like waves on the southern coast of Korea. In: Shim, J.-S.; Chun, I., and Lim, H.S. (eds.), Proceedings from the International Coastal Symposium (ICS) 2018 (Busan, Republic of Korea). Journal of Coastal Research, Special Issue No. 85, pp. 706–710. Coconut Creek (Florida), ISSN 0749-0208.Recently, several drowning accidents caused by rip currents were reported along Haeundae Beach, located at the southeastern corner of the Korean Peninsula. Several dozen people were swept away by rip currents, and subsequently rescued between August 7 and 10, 2012. The main cause of rip currents is swell waves that approach the shore. The rip current becomes more dangerous when a relatively long-period swell-like wave invades and is intensified, during the passage of typhoons far off shore. However, existing wind wave models require significant computational cost and time to obtain solutions to a set of partial differential equations, and have low accuracy for predicting the invasion of swell-like waves on coasts because they simultaneously calculate wind wave generation and wave propagation. In this study, we used a fractional step method to solve the wind wave generation and swell wave propagation components, in which the former is simulated using a typical spectrum approach and the latter is simulated using a Lagrangian approach. The results from the theoretical model were compared with field data collected during the dangerous rip current events in August 2012.
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In this paper, a hydraulic model experiment was conducted to investigate the mechanism of longshore sediment transport on a sloping beach of 1:7 using a circular wave tank equipped with a spiral wave maker. The experimental results revealed that cross-shore sediment transport was predominant up to the formation of equilibrium cross-section. Then the longshore sediment transport was actively dominant near the bar after the development of equilibrium topography. The wave-breaking positions and the characteristics of sand displacement in the vicinity of the wave-breaking zone were observed throughout the installation of fluorescent sands in different cross-sections under several wave conditions on the same terrain to presume the coastal erosion. From the outcomes, it was confirmed that the gradient of the average water level due to the obliquely incident waves breaking and the differences in breaking positions along the shoreline accelerated the generation of longshore currents and longshore sediment transport in the sloping beach profile. Moreover, the actively longshore sediment transport tendency throughout the breaking waves and the longshore current had been confirmed in this study where the gradually increasing wave heights influenced the stronger degree of wave breaking on the sloping beach, which enhanced the sediment transport procedure to deform the beach profile.
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In this thesis, we introduce a new numerical model able to describe wave transformation from the shoaling to the swash zones, including overtopping. This model is based on Serre Green-Naghdi equations, which are the basic fully nonlinear Boussinesq-type equations. These equations can accurately describe wave dynamics prior to breaking, but their application to the surf zone usually requires the use of complex parameterizations. We propose a new approach to describe wave breaking in S-GN models, based on the representation of breaking wave fronts as shocks. This method has been successfully applied to the Nonlinear Shallow Water (NSW) equations, and allows for an easy treatment of wave breaking and shoreline motions. However, the NSW equations can only be applied after breaking. In this thesis, we aim at extending the validity domain of the NSW model SURF-WB (Marche et al. 2007) to the shoaling zone by adding the S-GN dispersive terms to the governing equations. Local switches to NSW equations are then performed in the vicinity of the breaking fronts, allowing for the waves to break and dissipate their energy. Extensive validations using laboratory data are presented. The new model, called SURF-GN, is then applied to study tsunami-like undular bore dynamics in the nearshore. The model ability to describe bore dynamics for a large range of Froude number is first demonstrated, and the effects of the bore transformation on wave run-up over a sloping beach are considered. We finally present an in-situ study of broken wave celerity, based on the ECORS-Truc Vert 2008 field experiment. In particular, we quantify the effects of non-linearities and evaluate the predictive ability of several non-linear celerity models.
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New parameterizations for the spectra dissipation of wind-generated waves are proposed. The rates of dissipation have no predetermined spectral shapes and are functions of the wave spectrum and wind speed and direction, in a way consistent with observation of wave breaking and swell dissipation properties. Namely, the swell dissipation is nonlinear and proportional to the swell steepness, and dissipation due to wave breaking is non-zero only when a non-dimensional spectrum exceeds the threshold at which waves are observed to start breaking. An additional source of short wave dissipation due to long wave breaking is introduced to represent the dissipation of short waves due to longer breaking waves. Several degrees of freedom are introduced in the wave breaking and the wind-wave generation term of Janssen (J. Phys. Oceanogr. 1991). These parameterizations are combined and calibrated with the Discrete Interaction Approximation of Hasselmann et al. (J. Phys. Oceangr. 1985) for the nonlinear interactions. Parameters are adjusted to reproduce observed shapes of directional wave spectra, and the variability of spectral moments with wind speed and wave height. The wave energy balance is verified in a wide range of conditions and scales, from gentle swells to major hurricanes, from the global ocean to coastal settings. Wave height, peak and mean periods, and spectral data are validated using in situ and remote sensing data. Some systematic defects are still present, but the parameterizations yield the best overall results to date. Perspectives for further improvement are also given.
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The wind wave and the swell in the North Pacific Ocean have been visually observed by a voluntary ship on which the composite wave height of wind wave and swell using the micro wave height meter simultaneously measured for more than three years. The composed wave heights from visual data of wind waves and swell are mostly consistent with the measured data. This consistency suggest that the wave height data by visual observation is cosiderably reliable. This report focuses on the character of swell in the North Pacific Ocean. The results of analysis are that (1) the swell always remains in the ocean regardless of wind force at that time, (2) the average height of the swell is 2.7 m and the average period is 8.8 sec, (3) the heights and periods of the swell are mostly larger than thats of the wind wave, (4) the direction of the swell comes to coincide with the main direction of wind in proportion to the wind force, (5) the more the height of swell becomes larger, the more the ship has a tendency to expose her quater side to the swell.
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