Won Chul Cho

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.

Analysis of Coastal Erosion in Sanpo-Ri, Uljin-Gun (South Korea) using Field Survey and Measurement Data

ABSTRACT Kim, H.J.; Park, W.K., and Cho, W.C., 2017. Analysis of coastal erosion in Sanpo-Ri, Uljin-Gun (South Korea) using field survey and measurement data. 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. 344–348. Coconut Creek (Florida), ISSN 0749-0208. Several parts of the coastal road in Sanpo-Ri, Uljin-Gun, located in the eastern part of South Korea, have subsided due to erosion and scouring at the bottom of the front face of revetments, caused by frequent high wave attacks. In this study, we carried out field surveys and measurements (including waves, current, water depth, seabed material, beach profile, and shoreline) along the Sanpo-Ri coast and in Namdae-River and Wangpi-River, located in the northern part of Sanpo-Ri, in the summer and winter seasons, in order to analyze and determine the erosion process along the Sanpo-Ri coast. In addition, we used 8 aerial photographs, taken from 1988 to 2013, to analyze long-term coastline changes. The erosion of Sanpo-Ri coast was initiated and progressed not only due to the decreased width of the beach by the construction of coastal roads and the increase of reflected waves at vertical revetments, but also due to the 35 small dams installed in Namdae-River and Wangpi-River since 2000 that have interrupted the sand transport from the rivers to the coast. We performed numerical analyses on the generation of the long-shore current, sand transport, and shoreline change based on field data, and analyzed the erosion process along the Sanpo-Ri coast. These numerical monitoring data and results will be useful to establish coastline maintenance and protection plans on the Sanpoi-Ri coast and on other coasts with erosion problems.

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.

The Application of a Rip Current Warning Decision-Process System, Haeundae Beach, South Korea

ABSTRACT Lee, J.; Cho, W.C., and Lee, J.L., 2016. The Application of a Rip Current Warning Decision-Process System, Haeundae Beach, 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. 1167 - 1171. Coconut Creek (Florida), ISSN 0749-0208. Rip currents have the potential to pull swimmers into deep water, beyond offshore sand bars, posing a significant danger to beach goers worldwide. At Haeundae Beach in Busan, a south eastern region of South Korea, many rip current events have recently occurred, resulting in a number of people requiring rescue from these fast moving, seaward currents in 2014. The main objective of this study was to provide a warning decision-process for the occurrence of rip currents, to aid in the protection of human life in the areas where these rip currents occur. The rip current warning decision-process system developed in this work consists of both the NERiPS-H (Nearshore Rip current Prediction System for Haeundae Beach) and HAECUM (HAEundae Current Model) systems. By combining the predictive index model (NERiPS-H) and the numerical model for rip current forecasting (HAECUM), rip currents were forecasted using wave and wind data, along with forecasting information. Using this system, it possible to provide a 72-hour rip current forecast, which was made available on the KMA website. Four warning levels were chosen for forecasting of rip current occurrences by the KMA.

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.

Rip Currents Generation by Geomorphological Change in Gyeongpo Beach, South Korea

ABSTRACT Cho, W.C.; Kim, J.H.; Hur, D.S., and Kim, I.H., 2014. Rip currents generation by geomorphological change in Gyeongpo Beach, South Korea. Most beaches in Gangwon-do, South Korea have topographic characteristics that are mainly influenced by waves and seabed conditions. Gyeongpo Beach is one of the most famous beaches as a tourist attraction in the east coast of South Korea, and many people have visited and enjoyed sea-bathing in this area during the summer holiday season. However, rip currents are often source of danger for the swimmers because rip currents drag the swimmers away from the beach and put them to death by drowning. As Gyeongpo Beach has a straight shoreline, it is directly exposed to high waves that induce a significant change of bottom topography. Sandbars are usually formed in the processes of topographical changes from the swash zone to the surf zone and increase the possibility of rip current occurrence. Therefore, the monitoring of the topographic changes in this region is important to understand the mechanism of rip current generation. In this study, field observations were performed to measure foreshore slopes, water depths, and cross-sectional changes of bottom topography from the swash zone to the surf zone from December, 2013 through February, 2014. In order to trace nearshore currents, a GPS mounted on buoy-type equipment was designed, considering the minimization of wind effect. A single beam echo sounder equipped with RTK GPS (Real Time Kinematic GPS) was also used to observe the formation and movement of sandbar. The field observation results were used to understand the correlation between the sandbar movement and the rip current generation. According to the analysis results of two different field observation events in Gyeongpo Beach, a kind of salient beach is formed between O-ri Rock and the beach. Also, the results show that both arc sandbars and straight sandbars are mixed in the entire area. Finally, the field investigation results in this study show that the mechanism of rip current generation is closely related to the shape and location of sandbars.