Sungwon Shin

Optical Measurements of Tsunami Inundation and Debris Movement in a Large-Scale Wave Basin

AbstractThis paper presents optical measurements of debris movement and tsunami inundation over an unobstructed beach in a laboratory wave basin. The debris consisted of rectangular boxes and was placed unconstrained on a flat section raised above the basin floor with no still water on the raised section. Debris movement was measured using two overhead video cameras and a novel object-tracking algorithm. Two standard optical techniques, wave edge detection and particle image velocimetry, were used to compare optical and in situ measurements of fluid velocity. The debris motion (position, velocity) in the onshore direction was found to be repeatable, but the offshore motion varied between trials because of the irregular nature of the flow field during the return. For debris in free translation, as the number of debris specimens increased, the peak average velocity decreased and the onset of the peak was delayed in the onshore direction. In the offshore direction, the velocity was lower by a factor 4–6 and ...

Spatiotemporal Characteristics of Near-Bed Pressure Gradients on a Barred Beach

Observations of the cross-shore variations of pressure gradients measured on a fixed barred beach in a large-scale laboratory wave flume are presented in this paper. The wave-induced components (0.1–1.0 Hz) of the free surface elevation, near-bottom vertical and cross-shore velocities, and pressure gradient are used in the analysis. The cross-shore variation of the pressure gradient showed the maximum value in the area of wave breaking over the bar, and the pressure gradients were considered to be influenced by the fluctuation of the water surface elevation, which are shown to be a rough proxy for the water surface slope. The cross correlation related to the vertical velocity showed no correlation in the wave breaking area. Surprisingly, although several measuring points were located in the wave breaking area or in the inner surf zone, the spatial variation of the pressure gradient and water surface elevation had a correlation coefficient greater than 0.8 with time lags approximately 0.1 of the wave perio...

STATISTICAL MODELING OF PRESSURE GRADIENTS ON A BARRED BEACH

This paper reports the cross-shore variations of a pressure gradient observed on a fixed barred beach in a large-scale laboratory wave flume. The data sets include measurements of the free water surface elevation, the near-bottom velocity and the near bottom pressure and the pressure gradient for one irregular wave and three regular wave cases. The cross-shore variation of the pressure gradient showed that the maximum value of pressure gradient appeared in the area of wave breaking, and the pressure gradients were influenced by the fluctuation of water surface elevation. The exceedance probabilities of pressure gradient were analyzed using irregular wave case data. Comparison of the onshore and offshore directions of the exceedance probability showed that the onshore direction dominant in this experiment; its values were generally large over the bar. The distribution of the exceedance probability of pressure gradient could be evaluated by the Weibull distribution.

A study on the optimal shape of wave energy conversion system using an oscillating water column

ABSTRACT Shin, S., Lee, K.-H., Kim, D.-S., Kim, K.-H., and Hong, K., 2013. A study on the optimal shape of wave energy conversion system using an oscillating water column With the intention of diversifying energy sources, the use of natural and renewable energy sources such as sunlight, wind, tides, geothermal heats and waves is attracting favorable attention. Furthermore, current environmental issues, including global warming, ozone depletion and nuclear problems, promote development of renewable energy technologies. Many renewable energy technologies are however still under progress. Among these, the wave energy conversion system using an oscillating water column (OWC) is nearing commercial stage. OWC use the air flow induced by the vertical motion of water column in the air chamber as a driving force of turbine. Although it is well known that OWC is one of the most efficient devices to harness wave power, there is still much uncertainty about the relationship between the optimal shape and its hydrodynamic performance under the confined wave conditions. In this study, we propose a new computational fluid dynamics solver using on the immiscible two-phase (gas and water) flow model to simulate an OWC system in a two and three dimensional numerical wave tank. The numerical experiments focus on air flow velocity directly related to the working of turbine to survey the optimal shape. In order to validate the developed numerical model, laboratory experiments for the simplified OWC are carried out in a wave tank under regular wave conditions. The comparisons between the results of the developed numerical model and experimental data reveal a favorable agreement between the air flow velocity as well as the water surface profiles. Based on the validated numerical model, the effects of the inlet and chamber shapes including length, height, width and slope on the maximum air flow velocity are numerically investigated. As a result, in case the non-dimensional chamber width normalized by the incident wave length is in the range of 0.12 to 0.43, the maximum air flow velocity occurs with the increase of the inlet height and shortness of the inlet length.

Effects of Obliquely Incident Waves on Overtopping for Vertical Walls

ABSTRACT Kim, Y.T.; Shin, S.W.; Choi, J.W., and Lee, J.I., 2016. Effects of oblique waves on overtopping for vertical walls. 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. 1357 - 1361. Coconut Creek (Florida), ISSN 0749-0208. In determination of the crest height of a vertical structure against attacking of obliquely incident waves, most of existing studies have suggested to use the overtopping reduction factor due to incident angles. However, they have not considered the amplification of wave heights and the spatial distribution of wave overtopping. In this study, a spatial distribution of wave overtopping along a vertical structure is experimentally investigated and also the wave overtopping reduction factors (γ) for incident wave angle is suggested. When the incident angle becomes larger, the wave overtopping reduction factor decreases almost linearly. EurOtop (2007) suggested the constant γ for β>45°, however the decreasing γ was calculated from this study.

Study on Rip Current Generated by Submerged Breakwaters: Field Observation and Numerical Simulation

ABSTRACT Kim, I.H.; Lee, W.D.; Shin, S.; Kim, J.H.; Hur, D.S., and Cho, W.C., 2016. The Study on Rip Current Generated by Submerged Breakwaters: Field Observation and Numerical Simulation. 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. 1352 - 1356. Coconut Creek (Florida), ISSN 0749-0208. Jetties and submerged breakwaters have been constructed at Gangmun beach, Korea, for the past several years in order to protect the beach and to secure the estuary channel. However, drowning accidents due to rip currents have often occurred in the gap between the jetty and the submerged breakwater. In this study, field observations and numerical simulations were performed to understand the mechanism of the rip current occurrences at Gangmun beach. The field investigation included the geomorphological change as well as the rip current occurrences. Three-dimensional numerical simulations were carried out to find out the occurrence mechanism of the rip currents and the countermeasure for the rip current reduction. The results showed that the mean water level difference between the gap and the area behind the submerged breakwater induced the rip currents. The numerical model results also showed that the drainage channel in the submerged breakwater can reduce the rip current magnitude.

WAVE TRANSFORMATION, IMPACT, AND OVERTOPPING ON A RUBBLE MOUND BREAKWATER: LARGE SCALE MEASUREMENTS AND NUMERICAL MODELING

A large-scale laboratory experiment was conducted to study wave transformation, impact, and overtopping and to evaluate numerical model capabilities and limitations. A numerical model based on the Volume-Averaged/Reynolds-Averaged Navier-Stokes (VARANS) equations was used to compare the results with the experimental data. The model predictions of the free surface elevation and the cross-shore variation of the significant wave heights agreed with the experimental observations. For undertow, the agreement of model predictions with observations improved with distance away from the rubble mound breakwater. The numerical simulation of the wave impact pressure qualitatively reproduced the data measured during the experiment. Comparisons of numerical and experimental results for wave overtopping and turbulence are in progress.

Physical and Numerical Modeling of Irregular Wave Transformation over a Fringing Reef

ABSTRACT Shin, S.; Kim, Y.T., and Lee, J.I., 2016. Physical and Numerical Modeling of Irregular Wave Transformation over a Fringing Reef. 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. 922–926. Coconut Creek (Florida), ISSN 0749-0208. Two-dimensional laboratory experiments were conducted to investigate cross-shore variations of wave transformation, setup, and breaking phenomena over a fringing reef. Various conditions of irregular waves were generated by changing a water depth, a wave period, and a wave height. A multi-layer Boussinesq model was used to predict the wave transformation and the results were compared to the experimental results. One-layer and two-layer model configurations were selected to figure out and compare the model capability. In the model and data comparisons, both models well predicted the wave transformation characteristics and setups when the nonlinearity was small. However, as the wave nonlinearity increased, two-layer model results were more stable in the wave breaking region than one-layer model results. The results also showed that the broken wave heights finally reached stable conditions on the reef flat section in all cases and the relative stable wave heights (the ratio of the stable wave height to the water depth) were found as approximately 0.56. This data set can be used to benchmark studies for numerical model developers.

Numerical Modeling of Surf Zone Hydrodynamics over Movable Bed

ABSTRACT Shin, S.; Cox, D., and Yoon, H.D., 2014. Numerical modeling of surf zone hydrodynamics over movable bed. Understanding of hydrodynamics over moveable bed is crucial for predicting rip current generation. The capability of a numerical model based on Reynolds-averaged Navier–Stokes equations, COBRAS (COrnell BReaking waves And Structures), to simulate hydrodynamics over a movable barred beach was verified. The numerical simulation results were validated with the data collected from a large-scale two-dimensional experiment, which was conducted at the Hinsdale Wave Research Laboratory at Oregon State University. In this study, numerical model results were compared with the experimental results in terms of significant wave heights, wave setup, time-averaged horizontal velocities, and turbulent kinetic energy in different cross-shore locations. The COBRAS model successfully predicted the significant wave heights and setup from the offshore boundary to the vicinity of the bar location, but it slightly overestimated the significant wave heights when broken waves propagated to the shoreline. The numerical simulation was able to predict time-averaged horizontal velocities and turbulent kinetic energy well for all measurement locations. Overestimation of the turbulent kinetic energy at bar trough is because of the limitation of the turbulence closure scheme. Overall, based on the results of this study, the COBRAS model can be used to predict hydrodynamics in barred beaches, but improvement of the turbulence closure scheme is still required.

Three-Dimensional Laboratory Experiments for Tsunami Inundation in a Coastal City

Laboratory experiments were conducted for tsunami inundation to an urban area with large building roughness. The waterfront portion of the city of Seaside which is located on the US Pacific Northwest coast, was replicated in 1/50 scale in the wave basin. Tsunami heights and velocities on the inundated land were measured at approximately 31 locations for one incident tsunami heights with an inundation height of approximately 10 m (prototype) near the shoreline. The inundation pattern and speed were more severe and faster in some areas due to the arrangement of the large buildings. Momentum fluxes along the roads were estimated using measure tsunami inundation heights and horizontal fluid velocities. As expected, the maximum momentum flux was near the shoreline and decreased landward. Inundation heights and momentum flux were slowly decreased through the road with buildings on each side. The results from this study showed that the horizontal inundation velocity is an important factor for the external force of coastal structures.