Junwoo Choi

Boussinesq Modeling of a Rip Current at Haeundae Beach

The rip current occurred at Haeundae beach was numerically investigated under directional random wave environment. The numerical simulation was performed using a fully nonlinear Boussinesq equation model, FUNWAVE which is capable of simulating nearshore circulation since it includes the effect of wave-induced momentum flux and horizontal turbulent mixing. The results of numerical simulation show the time-dependent evolution of the wave-induced nearshore circulation system (including rip current) that are caused by nonlinear transformation of directional irregular waves due to unique topography of Haeundae. From the results, it was found that rip current is well generated and developed where relatively lower wave height and relatively deeper water depth along the longshore direction, and sudden and strong events of rip current were observed.

Boussinesq modelling of a rip current at Haeundae Beach in South Korea

ABSTRACT Bae, J.S., Yoon, S.B., Choi, J., 2013. Boussinesq Modelling of a Rip Current at Haeundae Beach. The rip current occurring at Haeundae beach in Busan, South Korea was numerically investigated under directional random wave environment. The numerical simulations were performed using a fully nonlinear Boussinesq equation model, FUNWAVE which is capable of resolving phase interaction and simulating nearshore circulation since it includes the effect of wave-induced momentum flux and horizontal turbulent mixing. To examine the modelling, the rip current occurred on the 12th of June, 2011 at Haeundae beach was simulated by using observations. The results of numerical simulation showed the time-dependent evolution of the wave-induced nearshore circulation that was caused by transformation of random waves due to the unique topography of Haeundae beach. And, it was found that the rip current was well generated and developed through the region where relatively lower wave heights were caused by refraction and diffraction of waves due to the topographical characteristics (i.e., submerged shoal). In addition, we found that a narrow-banded spectral wave (i.e., a swell spectrum) close to a regular wave generates stronger rip current than a broad-banded spectral wave under an equivalent wave height and period condition.

Surface Roller Modeling for Mean Longshore Current over a Barred Beach in a Random Wave Environment

Abstract Choi, J.; Lee, J.-I., and Yoon, S.B., 2012. Surface roller modeling for mean longshore current over a barred beach in a random wave environment. The effect of surface roller on mean longshore current, which depends strongly on the wave-breaking process, was investigated by applying various surface-roller models to a random wave-induced current-model system, incorporating a nearshore circulation model, SHORECIRC, and a random wave model, SWAN. The investigation was performed based on simulations of the SandyDuck field experiments, employing observed wave spectra and two-dimensional topography. The topography is characterized as a longshore variation caused by scouring because of bridge piers and a barred beach on a steep foreshore slope linked to a relatively broad bar formation on a seaward gentle slope. The experimental data show that before the waves intensely broke from a decrease in water depth on the steep slope, some partial breaking of random waves occurred in the wave-breaking process. The chosen surface-roller model was modified using the classification of the partial wave-breaking and intense wave-breaking regions. The mean longshore currents simulated using the modified surface-roller model was compared with the measurements as well as the results from the simulations using conventional surface-roller models. The modified simulations agreed with the measurements better than did those of the simulations.

Study of Rip Current Warning Index Function Varied according to Real-time Observations

A rip-current warning index function, which is estimated from the likelihood of rip current quantified based on numerical simulations under various sea environments and is varied according to real-time buoy-observations, was studied to help protect against rip current accidents at Haeundae beach. For the quantification, the definition of likelihood of rip current, which proposed by Choi et al. (2011, 2012b), was employed and estimated based on Boussinesq modelling. The distribution of likelihood of rip current was evaluated by using various simulations according to scenarios established based on physical quantities(i.e., wave parameters) of buoy-observations. To index the likelihood of rip current, empirical functions were derived based on the distribution and adjusted to observational environments. In this study, the observations from June to September in 2011 at Haeundae beach were applied to the rip-current index functions, and its applications into the real events found based on CCTV images were presented and investigated. In addition, limitations and improvements of the rip-current index function were discussed.

A DISPERSION-CORRECTED FINITE ELEMENT MODEL FOR SIMULATION OF DISTANT TSUNAMIS

A dispersion-corrected finite element model is developed to simulate the propagation of distant tsunamis over a slowly varying topography. A linear Boussinesq wave equation is solved to consider the dispersion effect of tsunami waves by employing a linear triangular mesh and an explicit time integration scheme. The numerical dispersion associated with the explicit scheme is minimized by adjusting both the mesh size and the dispersion-correction parameter. In order to test the present model, numerical simulations for the propagation of an initial Gaussian hump over various constant depths are conducted, and the numerical results are compared with analytical solutions of the linear Boussinesq equations. The present model is also tested for the propagation of tsunamis over a submerged circular shoal and the numerical results are compared with the numerical solutions obtained using another Boussinesq model. The present model is shown to be efficient and considerably accurate.

A Rip Current Warning System Based on Real-Time Observations for Haeundae Beach, Korea

ABSTRACT Choi, J.; Lim, C.H., and Yoon, S.B., 2014. A rip current warning system based on real-time observations for Haeundae Beach, Korea. A real-time rip current warning system was studied to help mitigate against rip current accidents at Haeundae Beach. The system produces a real time sequence of index representing the risk and the likelihood of rip current based on the real time observations such as waves, currents, and tide. The index is estimated by using the rip current likelihoods derived from the results of various numerical simulations with various sea conditions, the parameters of which correspond to the observation quantities. The rip current warning system is operated by taking into account the effect of wave height, wave period, wave direction, wave spectrum (i.e., frequency-directional spreading), longshore currents, and tidal elevation on the rip current likelihood. The real-time wave and tidal observations at the Haeundae coast in 2012 were applied to the rip current warning system, and its performances at several real events observed from the video cameras were presented and analyzed.

A comparative cross-sectional study on cosmetic outcomes after single port or conventional laparoscopic surgery

A B S T R A C T Objectives: The primary objective was to compare cosmetic outcomes and patients’ satisfaction between women who underwent single port laparoscopic surgery (SP-LS) or conventional laparoscopic surgery (conventional-LS). The secondary objective was to evaluate the cosmetic effect of SP-LS in women with abdominal scars from prior surgeries. Study design: Patients who were followed up after SP-LS or conventional-LS for gynecologic disease were included. Cosmetic outcome was evaluated at least six months after surgery. Objective and subjective scar analysis were performed using the Vancouver scar scale, the patient and observer scar assessment scale (POSAS), and a visual analog scale (VAS). Overall satisfaction with the surgery was evaluated with VAS. Results: 130 women were included, with 65 women in each group. No difference in objective scar assessment for only umbilical scar was shown. In the subjective scar assessment and satisfaction with the surgery, SP-LS group was more satisfied. SP-LS group with abdominal scar had higher satisfaction in overall satisfaction with the scar. Conclusions: SP-LS offers better cosmesis and overall patient satisfaction with surgery in women with gynecologic disease. SP-LS also seems to have considerable cosmetic benefit in women with abdominal scars from prior surgeries.