Sangyoung Son

Assessment of the tsunami‐induced current hazard

The occurrence of tsunami damage is not limited to events causing coastal inundation. Even without flooding, maritime assets are vulnerable to significant damage from strong currents and associated drag forces. While such impacts have been observed in the past, they have not been well studied in any context. Nearshore tsunami currents are governed by nonlinear and turbulent physics and often have large spatial and temporal variability making high-fidelity modeling particularly challenging. Furthermore, measured data for the validation of numerical simulations is limited, with few quality data sets appearing after recent tsunami events. In this paper, we present a systematic approach for the interpretation of measured tsunami-induced current impacts as well as a validation approach for simulation tools. The methods and results provided here lay the foundation for much needed efforts to assess tsunami hazards in ports and harbors.

CALCULATION OF IRREGULAR WAVE REFLECTION FROM PERFORATED-WALL CAISSON BREAKWATERS USING A REGULAR WAVE MODEL

In this paper we examine several methods tor calculating the reflection of irregular waves from a perforated-wall caisson breakwater using a regular wave model. The first method is to approximate the irregular waves as a regular wave whose height and period are the same as the root-mean-squared wave height and significant wave period, respectively, of the irregular waves. The second is to use the regular wave model, repeatedly, for each frequency component of the irregular wave spectrum. The wave period is determined according to the frequency of the component wave, and the root-mean-squared wave height is used for all the frequencies. The third method is the same as the second one except that the wave height corresponding to the energy of each component wave is used. Comparison with experimental data from previous authors shows the second method is the most adequate, giving reasonable agreement in both frequency-averaged reflection coefficients and reflected wave spectra.

Finite Element Solution of Linear Waves on a Sloping Bottom Boundary

ABSTRACT Jung, T-H.; Son, S., and Ryu, Y. 2017. Finite element solution of linear waves on a sloping bottom boundary. A new, finite-element solution of linear water waves, which can be applied to a nonvertical bottom boundary, is introduced in this study. The present solution can be applied to regions in which the water depth gradually approaches zero, such as coastlines. To obtain this solution, the entire domain is divided into three subregions. In the up-wave and down-wave subregions, analytical solutions are used. In the middle region, which occupies most computational domain, the standard Galerkin finite-element method is applied. The introduced numerical method is compared with an analytical solution to show its validity.

Lagrangian-like Volume Tracking Paradigm for Mass, Momentum and Energy of Nearshore Tsunamis and Damping Mechanism

There is a gap between model- or theory-based research outputs, which suggest that the runup and amplification of nonbreaking waves generally increase as the sea bottom slopes decrease, and field observations, which indicate that tsunami damage has been rarely reported in places with vast continental shelfs. To resolve this contradiction, we propose a Lagrangian-like volume tracking paradigm to describe the energy, mass, and momentum of travelling nearshore tsunamis and apply the paradigm to analyse the tsunami damping mechanism at typical geophysical scales. The results support the following conclusions: (i) The suggested paradigm is consistent with field observations; continental shelfs with long and mild slopes can effectively diminish tsunami impacts. (ii) Potential energy becomes significant due to the energy transformation process on steeply sloped bathymetries. (iii) On mild-sloped bathymetries, tsunami potential and kinetic energies are conserved until breaking occurs. After breaking, undular bores attenuate tsunami energies effectively. (iv) For extended continental shelf bathymetries, more of the tsunami mass is reflected offshore.

MULTIFRACTAL CHARACTERISTICS OF AXISYMMETRIC JET TURBULENCE INTENSITY FROM RANS NUMERICAL SIMULATION

A turbulent jet bears diverse physical characteristics that have been unveiled yet. Of particular interest is to analyze the turbulent intensity, which has been a key factor to assess and determine turbulent jet performance since diffusive and mixing conditions are largely dependent on it. Multifractal measures are useful in terms of identifying characteristics of a physical quantity distributed over a spatial domain. This study examines the multifractal exponents of jet turbulence intensities obtained through numerical simulation. We acquired the turbulence intensities from numerical jet discharge experiments, where two types of nozzle geometry were tested based on a Reynolds-Averaged Navier–Stokes (RANS) equations. The k-𝜀 model and k-ω model were used for turbulence closure models. The results showed that the RANS model successfully regenerates transversal velocity profile, which is almost identical to an analytical solution. The RANS model also shows the decay of turbulence intensity in the longitudin...

An Observational and Numerical Study of Storm-Induced Morphologic Changes at Sanpo Beach, Korea

ABSTRACT Son, S.; Kim, J.; Yoon, H.-D.; Jung, T.-H.; Do, K., and Shin, S., 2017. An observational and numerical study of storm-induced morphologic changes at Sanpo Beach, Korea. In: Lee, J.L.; Griffiths, T.; Lotan, A.; Suh, K.-S., and Lee, J. (eds.), The 2nd International Water Safety Symposium. Journal of Coastal Research, Special Issue No. 79, pp. 334–338. Coconut Creek (Florida), ISSN 0749-0208. This study was conducted to investigate morphological changes at Sanpo Beach due to the storm through field observation and numerical simulation. Sanpo Beach located on the southeast coast of Korea had been significantly affected by typhoon Goni while some changes in beach profile was expected due to the strong storm-induced currents. Series of field observations on the beach topography, bottom bathymetries, shorelines, sand samples, and incident waves were carried out in order to investigate the morphological changes at Sanpo Beach during the typhoon season. In particular, Real Time Kinematic-Global Positioning System (RTK-GPS) and single-beam echo-sounder were deployed to collect beach topography and bottom bathymetry data, respectively. Observational results represented spatial and temporal changes of shoreline and sand bars during a storm event. For more close look at the process of morphologic changes derived by the sediment transport mechanism, field-scale numerical simulations were performed in parallel using Xbeach. Xbeach is one of the widely-used, open-source models which calculate sediment transports and morphologic changes by hydrodynamic processes of short (and/or long) waves, wave-induced setup and unsteady currents. Bathymetric evolutions, as well as shoreline changes predicted by the numerical results, revealed good agreements with observational data.

Field Observation and Numerical Modelling of Rip Currents within a Pocket Beach

ABSTRACT Shin, S.; Nam, J.; Son, S.; Kim, I.H., and Jung, T.-H., 2017. Field observation and numerical modelling of rip currents within a pocket beach. In: Lee, J.L.; Griffiths, T.; Lotan, A.; Suh, K.-S., and Lee, J. (eds.), The 2nd International Water Safety Symposium. Journal of Coastal Research, Special Issue No. 79, pp. 229–233. Coconut Creek (Florida), ISSN 0749-0208. Understanding the occurrence mechanism of rip currents are very important in terms of the sediment transport and the water safety issues. In order to investigate the rip current occurrence mechanism, field observations were carried out at Chunjin beach and a numerical model was employed to simulate rip currents by using the field observation data. The collected data set includes the beach topography, bottom bathymetries, shoreline, sand sample, and incident waves for each season. The field observation data showed the spatiotemporal variations of shoreline and sand bars. The results also showed that the rip currents were generated near the crescentic sand bar. For better understanding the mechanism of rip currents, numerical simulation using phase- resolving, three-dimensional 3D Non-Hydrostatic WAVE Model (NHWAVE) was also carried out. The model considered incoming wave conditions collected from the field observation to look at the nearshore current generation patterns. Through this numerical model, this study could have investigated vertical profiles of nearshore wave-induced current while the field observation data represent depth uniform currents. The numerical simulation showed a good agreement with the field observation and provided useful information for nearshore circulation.

Investigation of Wave Breaking Turbulence in Morphodynamic Modelling

ABSTRACT Yoon, H.-D.; Cho, M., and Son, S.-Y., 2016. Investigation of wave breaking turbulence in morphodynamics modelling. In: Vila-Concejo, A.; Bruce, E.; Kennedy, D.M., and McCarroll, R.J. (eds.), Proceedings of the 14th International Coastal Symposium (Sydney, Australia). Journal of Coastal Research, Special Issue, No. 75, pp. 942–946. Coconut Creek (Florida), ISSN 0749-0208. The effect of wave breaking turbulence on morphodynamics was investigated using Xbeach. To achieve this goal, the results from Xbeach simulation were compared with a large-scale laboratory experiment (CROSSTEX) and a numerical model (COBRAS) which is based on Reynolds-Averaged Navier-Stokes (RANS) equation. To incorporate the impact of wave breaking turbulence, a new near-bed turbulence estimation was suggested in addition to the existing turbulence options in Xbeach. Hydrodynamics results from Xbeach were generally in a good agreement with CROSSTEX and COBRAS. However, morphodynamics from Xbeach did not match well with CROSSTEX nor COBRAS, especially in the vicinity of a longshore bar. Most bathymetric changes occurred in the inner surf zone or swash zone, showing dune erosion. The possible causes of disagreement can be related to the limitation of the depth-averaged advection-convection equation used for sediment transport in Xbeach. Also, long wave motions dominates morphologic changes near the dune area. It is concluded that more investigation is still required to better simulate morphodynamics.

Vertical Structure of Rip-currents in the Nearshore Circulation

ABSTRACT Son, S.; Jung, T.H., and Shi, F., 2016. Vertical structure of rip-currents in the nearshore circulation. In: Vila-Concejo, A.; Bruce, E.; Kennedy, D.M., and McCarroll, R.J. (eds.), Proceedings of the 14th International Coastal Symposium (Sydney, Australia). Journal of Coastal Research, Special Issue, No. 75, pp. 1402 - 1406. Coconut Creek (Florida), ISSN 0749-0208. A number of numerical models have been proposed over the past few decades to investigate rip currents in nearshore circulation zones. Rip currents are the offshore-directed flow generated mainly by wave breaking and momentum transfer over varying topography and they are important in relation to the management of coastal preservation, maintenance, and development. To date, a number of rip current studies have been conducted using depth-integrated two-dimensional (2D) or quasi-three-dimensional (3D) models based on wave-averaged formulations. However, compared with 3D hydrodynamic numerical models, these models are not capable of providing the accurate vertical profiles of current velocity. In this study, we examine the vertical variation of velocities induced by rip-currents using a phase-resolving, 3D Non-Hydrostatic WAVE model (NHWAVE) where an idealized rip channel is used to generate nearshore circulation. The vertical variation of velocities for several locations is investigated and compared with a depth-integrated numerical model for better understanding of depth-dependent hydrodynamics.

A Comprehensive Sensitivity Analysis of Tsunami Model System to the Parametric and Input Uncertainties

ABSTRACT Jung, T.-H.; Son, S. and Lynett, P., 2016. A Comprehensive Sensitivity Analysis of Tsunami Model System to the Parametric and Input Uncertainties. In: Vila-Concejo, A.; Bruce, E.; Kennedy, D.M., and McCarroll, R.J. (eds.), Proceedings of the 14th International Coastal Symposium (Sydney, Australia). Journal of Coastal Research, Special Issue, No. 75, pp. 1117 - 1121. Coconut Creek (Florida), ISSN 0749-0208. In the present study, it is examined the tsunami modelling sensitivity to the various input factors based on the simulation results of 2004 Indian Ocean Tsunami. The numerical tests were implemented using coupled shallow water equation model, COMCOT(Liu et al., 1998) and Boussinesq model(Son et al., 2011) on the multi-grid system. Four different configurations(CASE 1 through 4) where three factors were controlled independently had been set up and simulated. From the numerical results, it was examined that some of defined factors may have control over the level of prediction accuracy in tsunami impacts on the nearshore area. It was shown that hydrodynamic performances calculated by Boussinesq model(Son et al., 2011) were very distinct from those by COMCOT-only. Most importantly, this study also suggests that more thorough and dedicated investigations on the diffusive errors embedded inherently in numerical models are required for the future study.