Woo Dong Lee

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.

Effect of Power Cooling Water to the Ecosystem of Youngil Bay and Hyeongsan River Estuary in South Korea

ABSTRACT Yoon, J.-S.; Kim, T.-W.; Cho, W.C., and Lee, W.D., 2017. Effect of power cooling water to the ecosystem of Youngil Bay and Hyeongsan River estuary in South 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. 174–178. Coconut Creek (Florida), ISSN 0749-0208. The POSCO (Pohang Steel Company) discharges about 1,000,000 m3/day of 30 °C power cooling water to the estuary of Hyeongsan River, which is located in the southeastern region of South Korea. The thermal effluent is mixed with sea water and diffused to Youngil Bay. At the beginning of the 1990s, the thermal effluent affects brackish water zone of Hyeongsan River and Youngil Bay and becomes a main cause of red tide in this sea area, which starts to occur from January. Besides, the thermal effluent also affects coastal sea water quality and ecosystem raising the surrounding sea water temperature. In this study, we figured out the present circumstances and problems in the brackish water zone of Hyeongsan River estuary and Youngil Bay, which resulted from the thermal effluent in winter season. We also found out the cause of red tide on the basis of 8 measured categories on site, such as temperature, salinity, DO, COD, TN, TP, chlorophyll-a, and flow velocity, and performed a numerical analysis using the measured data to precisely investigate the occurrence mechanism of red tide. The results show that intrusion range of saline wedge in the lower layer of water depth is reduced in summer season because of increased river discharge, however, the river water and thermal effluent is diffused to Youngil Bay forming a typical estuary front with stratified condition. On the contrary, in winter season, a constant vertical density distribution, resulting from a strong mixing of the upper and lower water layers, is formed at downstream of the thermal effluent outlet. However, a thermohaline front, developing a less denser distribution than that developed in the strong mixing area, is formed at downstream of the strong mixing area. The formation of thermohaline front induces flow stagnation in Youngil Bay and subsequently obstructs river discharge to Youngil Bay. Thus, a mass of chlorophyll-a is bred by the thermal effluent and consequently a red tide.

Numerical Model Study on the Wave and Current Control by Coastal Vegetation

ABSTRACT Lee, W.D.; Cox, D.T., and Hur, D.S., 2017. Numerical model study on the wave and current control by coastal vegetation. 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. 219–223. Coconut Creek (Florida), ISSN 0749-0208. In this study, three-dimensional numerical simulations were conducted to investigate the effect of the coastal vegetation on the wave and current controls. The model was modified to apply three-dimensional Navier-Stokes solver based on porous body model for the direct simulation of the wave energy dissipation through the vegetation. The new formula of a vegetation drag coefficient based on the wave-vegetation interaction was proposed through the hydraulic model tests and the formula was implemented to the model. The modified numerical model was tested and the results were compared with the experimental results to verify the numerical model capability. The model results well reproduced the wave energy dissipation inside the vegetation zone. Moreover, the numerical modeling was also conducted for the hydrodynamics on permeable submerged breakwater and the model results were compared with the results of vegetation. Inside the vegetation zone, the wave energy slowly decreased and the water level behind the vegetation was not increased seriously. Since the vegetation reduces the wave energy gradually, the width of the vegetation zone is an important factor for the effective control of the wave energy. Because the water level behind the vegetation zone did not increase too much, the return flow was small compared with that in the gap between the submerged breakwater cases. Therefore, if we deploy coastal vegetation effectively, this may be a better measure than coastal structures in terms of effectiveness and economy.

Understanding Characteristics of Upward and Downward Flows in Coral Reef Region for Safety of Recreational Diving

ABSTRACT Jeong, Y.M.; Kim, S.G.; Lee, J.H.; Yoon, J.S., and Lee, W.D., 2017. Understanding characteristics of upward and downward flows in coral reef region for safety of recreational diving. 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. 35–39. Coconut Creek (Florida), ISSN 0749-0208. The goal of this study is to gain an understanding of the flow characteristics in reef regions through numerical simulations for safe recreational diving in the region. The applied numerical model was first validated to verify its feasibility and validity for carrying out the numerical simulations. Subsequently, an underwater survey was conducted in the Visayan Islands in the Philippines to model the reef region. Based on the underwater survey results, simulations were conducted in a three dimensional numerical wave tank with a model coral island. According to the numerical analysis results, stronger currents were generated if either currents or both currents and waves entered the low water level lagoon area of the coral island. In addition, an overall trend of stronger flow velocities was observed when both waves and currents were present than when only currents were present. Particularly strong upward and downward flows and strong vortices were generated near the reef crest. Thus, open water divers who are inexperienced in underwater swimming need to pay particular attention when swimming in these areas. Lastly, six tips for safe diving in reef regions were obtained from analysis of the numerical results.

Applicability of Multiple Submerged Narrow-Crested Breakwaters for Reduction of Mean Water Level in Rear Side and Flow Control

ABSTRACT Hur, D.S.; Cho, W.C.; Yoon, J.S.; Kang, C., and Lee, W.D., 2017. Applicability of multiple submerged narrow-crested breakwaters for reduction of mean water level in rear side and flow control. 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. 179–183. Coconut Creek (Florida), ISSN 0749-0208. In this study, to confirm the applicability of multiple submerged narrow-crested breakwaters (SNCBs), a 3-D numerical simulation was conducted. The numerical simulation used the numerical model developed for analyzing the 3-D flow structure around the submerged breakwaters. In addition, to verify the validity and effectiveness of the numerical model, the wave height distribution and mean water level distribution around impermeable submerged breakwater (ISB) and permeable submerged breakwater (PSB) were compared with the results of the numerical model experiment. As a result, the simulated results accurately realized the experimental values. The numerical simulation was conducted by applying the multiple SNCBs and common PSB for comparative analysis of the effectiveness on flow control and increased mean water level in the rear side, respectively. When the SNCBs are installed by more than two rows, the flow control was better than that achieved by installing PBS. However, the wave reflection was high, and the water level difference between onshore and offshore increased, owing to increased water level in the rear side. Based on this, an opening was installed on the multiple SNCBs, for the reduction of water level in the rear side. As a result, an outgoing flow toward the open sea was generated, which reduced the mean water level in the rear side. When W/Lr ≥ 0.2, the mean water level difference between onshore and offshore decreased, compared to the case of the PSB. Therefore, when appropriately installing the multiple SNCBs, whose applicability has been confirmed in this study, this can become one of the methods that can replace existing PBSs, which are bulky in size and expensive to construct.

Analysis of Beach Deformation according to Nourishing Sand in Haeundae Beach, Korea

ABSTRACT Lee, W.D., Kim, I.H., Yoon, J.S., Cho, W.C. and Hur, D.S., 2016. Analysis of beach deformation according to nourishing sand in haeundae beach, Korea. 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. 1372 - 1376. Coconut Creek (Florida), ISSN 0749-0208. This study has analyzed the beach deformation characteristics of Haeundae Beach, which is one of the most representative beaches of S.Korea, after beach nourishment through monitoring and performing a numerical simulation. This study was able to analyze the beach deformation characteristics after beach nourishment in depth (e.g. variations in coastline, beach profile and the area and volume of beach, and beach stabilization process) in line with the results of marine geophysical survey (Do et al., 2015). This study surveyed the marine morphological changes around Haeundae Beach caused by Typhoon Neoguri, but morphological changes did not show in the water level of over 3m. In addition, this study was not only able to identify the short-term advance and retreat of coastline in the beach profile due to Typhoon Neoguri, but also understand this phenomenon on the basis of the results of wave field analysis using Numerical Wave Tank (NWT). Furthermore, this study was able to find that NWT and the coupling calculation of Contour-line model simulated the changes of coastline due to Typhoon Neoguri almost similarly. NWT and the coupling calculation of Contour-line based on long-term monitoring are expected to greatly contribute to understanding the beach deformation characteristics due to beach nourishment in future.

Hydraulic and Environmental Stability Analysis in the Estuary of Gahwa River and Sacheon Bay by the Change of Discharge of Namgang Dam in South Korea

ABSTRACT Kim, T.W., Lee, W.D., Hur, D.S., Lee, J.L and Yoon, J.S., 2016. Hydraulic and Environmental Stability Analysis in the Estuary of Gahwa River and Sacheon Bay by the Change of Discharge of Namgang Dam in South Korea. 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. 213–217. Coconut Creek (Florida), ISSN 0749-0208. Namgang dam is located in the southern part of South Korea. Flow discharged from Namgang dam for a flood control runs Kahwa river and finally reaches Sacheon bay. In a rainy or typhoon period, the discharged flow can inundate and affect the hydraulic stability and safety of estuarine areas of Kahwa river, Sacheon river, Jungseonpo river, and Jukcheon river. In this study, we performed a 3-dimensional hydraulic experiment and numerical analysis to analyze the level of water rise in the estuarine areas and environmental effect to Sacheon bay by the change of discharge of Namgang dam. A hydraulic model, based on bathymetric and aerial LIDAR survey data, was made in the scale of 1/50. The experimental and numerical results were compared with the observational results. From the experimental and numerical results, it reveals that as the amount of discharge from Namgang dam increases, water levels in the estuarine areas of Jungseonpo river and Sacheon river and the industrial area around Sacheon bay significantly increases by the effect of backwater. In addition, analyzing the measured salinity concentration, we also find out that the range of diffusion of discharged water into Sacheon bay and it takes about 20 days to recover the original salinity concentration.

Numerical Studies on Rip Current Control Using Scene-friendly Structures at Haeundae Beach, Korea

ABSTRACT Lee, W.D.; Shin, S.W.; Jeong, Y.M., and Hur, D.S., 2014. Numerical studies on rip current control using scene-friendly structures at Haeundae Beach, Korea. The sporadically occurring rip currents put swimmers in risk during the season of every summer vacation. It is known that the rip current generation is affected by the characteristics of incident waves and bottom topography but the mechanism of the rip current occurrence is still not clear. In order to understand the mechanism of the rip current and to establish countermeasures, 3D numerical simulation was performed based on the realistic bathymetry of Haeundae Beach by using LES-WASS-3D. The numerical simulation results showed that the wave heights vary parallel to the shore line by inequality of bottom bathymetry (submerged shoal and rocks) and this wave height variation induces the water level difference. The model results also showed that the rip currents were generated through the region of relatively deep water and low water level that is caused by the water level difference. The present study suggested scene-friendly structures such as submerged breakwaters, drainage layers, and gabions as the countermeasures of rip current control. These structures successfully reduced rip current by controlling the energy unbalance in the region of rip current occurrence.

Control Technologies in Reducing Rip Currents around the Open Inlet between Two Submerged Breakwaters

ABSTRACT Hur, D.S.; Cho W.C.; Yoon, J.S.; Kim, I.H., and Lee, W.D., 2014. Control technologies in reducing rip currents around the open inlet between two submerged breakwaters. In the present study, we investigated the flow generated around the open inlet between two submerged breakwaters installed at Songdo Beach, Busan, Korea. We also performed the numerical analysis on the characteristics of the rip current generated around the open inlet with various sizes and arrangements of the submerged breakwaters. Furthermore, based on the numerical results of this study, we suggested three technologies to control the rip current and applied these technologies around the submerged breakwaters to find out the effectiveness of the control technologies. From the numerical analysis, we found out that RCCT-3 (Rip Current Control Technology-3), which installs a drainage channel inside the submerged breakwater to reduce water surface elevation at the rear side of the submerged breakwater, is most effective in reducing the rip current around the open inlet.