Jeseon Yoo

Macrotidal Beach Processes Dominated by Winter Monsoon: Byunsan, West Coast of Korea

Abstract Lee, H.J. and Yoo, J., 2012. Macrotidal beach processes dominated by winter monsoon: Byunsan, west coast of Korea. Macrotidal beaches are scattered in the west coast of Korea mostly in small embayments of the ria-type coastline and islands. In contrast to well-known tidal flats, these beaches have been little studied from the hydromorphologic viewpoint. Byunsan Beach was chosen to investigate beach processes affected by winter monsoon, which creates strong waves and wind-generated currents. The multiyear (2005–2008) profiling of the beach, together with the intensive sampling of surface sediments, provided the details of morphologic characteristics of the beach. Deep water winter wave characteristics were based on the measurements of an offshore buoy. A benthic tripod was deployed on the beach to observe waves, currents, and suspended sediments during a winter storm. The estimated morphodynamic parameters suggest that Byunsan Beach be classified as an ultradissipative state during winter. The tendency of grain size toward systematically fining seaward could be explained by the lateral displacements across the beach of swash and surf zones owing to a high tidal range. In addition, the tripod observations show that a storm readily generated offshore return flows capable of considerable beach erosion. The storm waves also significantly contributed to the erosion. This may reflect that the macrotidal, open beaches in the west coast of Korea are seasonally susceptible to pronounced erosion by winter monsoon. The results of this study may be useful for evaluating man-made environmental impacts caused largely by seawalls and dykes on the natural beach state intrinsic to the west coast of Korea.

Experimental Investigation of the Hydraulic Performance of Caisson-Pile Breakwaters

Abstract Porous breakwaters using a caisson-pile group can increase the resistance of the vertical coastal structures in harbors to external heavy impacts, and also help facilitate the exchange of water. This article presents experimental results for the hydraulic performance of independent caisson-pile type breakwaters having three rows of vertical piles, both evenly and unevenly spaced between rows. Compared with even spacing, uneven, sparser spacing tends to have lower wave reflections and higher energy loss coefficients. Wave transmission, by contrast, appears not to be considerably influenced by spacing pattern. The transmission coefficient increases with wave steepness (S0,s) and decreases with the ratio of the (total) width of the three pile rows to the wavelength (W/L0,s).

Field Observations of Spatial Structure of Hydrodynamics Including Waves and Currents in the Haeundae Coast

Field observations were conducted to collect hydrodynamic and morphological data, which are needed to account for mechanisms of bathymetry changes caused by physical forcings, in Haeundae beach. In order to quantitatively describe characteristics of wave transformations and current patterns in space in winter and summer, in-situ sensors for measuring waves and current profiles were installed at three locations in the cross-shore direction and also three locations in the along-shore direction. As for the results of wave measurements, waves with main direction from the east dominate in winter while waves are incident from the S and the ESE in summer. Analysis of current data reveals that currents over the study domain are considerably influenced by a pattern of tidal motions, thereby, mainly oscillating in the direction of tidal currents, i.e., east-west directions, in both winter and summer. Currents tend to be influenced by local bathymetry in the shallow water region, with the direction changed along the depth contours and the magnitude reduced as they approach the shoreline. The results analysed from the hydrodynamic data through this study can be further combined with the morphological and bathymetry data, leading to the quantification of seasonal sediment transport rates and sand budget changes.

Field observation and modeling of wave set-up on a macrotidal beach: the Malipo Experiment

ABSTRACT Choi, J.-Y., Park, J.-Y., Cho, K.-H., Hyun, S.-K., Yoo, J., Lee, D.-Y. and Jun, K.-C., 2013 Field observation and modeling of wave set-up on a macrotidal beach: the Malipo Experiment. Korea Research Ocean & Development Institute (KORDI: now renamed the Korea Institute of Ocean Science & Technology, KIOST) conducted a field observation experiment (the Malipo Experiment) using a variety of wave gages and current meters simultaneously at the Malipo Beach, South Korea, from December 2009 to March 2010 in order to understand the coastal processes in a macrotidal environment. The Malipo Beach has a typical macrotidal beach environment characterized by a very gentle beach slope of 1/50 and strong tidal currents corresponding to the extreme tidal range of 7 m. As a part of this program, we observed the intertidal-zone wave set-up using a wave-rider buoy moored at 20 m water depth and seven pressure-type wave gages perpendicularly aligned to the shore at the intertidal zone for 17 days beginning from December 17th, 2009 in order to understand wave breaking and set-up characteristics in the macrotidal environment. Based on the observation data analysis, the relationship between the wave set-up and wave height at offshore (Hs,0) is η = 0.11Hs,0 showing that influence of alongshore tidal currents on the wave set-up was negligible. It was analyzed that, however, wave set-up might be influenced by shore-normal currents. The wave set-up at the study site was smaller than previous researches. It might result from its low breaking index and the wave characteristics of short wave period and wide-spreading directional spectra at the study site. Results of model simulation by combining a wave model with a circulation model showed that simulated wave set-up heights were generally lower than the measurements.

Numerical Study on Tsunami Hazard Mitigation Using a Submerged Breakwater

Most coastal structures have been built in surf zones to protect coastal areas. In general, the transformation of waves in the surf zone is quite complicated and numerous hazards to coastal communities may be associated with such phenomena. Therefore, the behavior of waves in the surf zone should be carefully analyzed and predicted. Furthermore, an accurate analysis of deformed waves around coastal structures is directly related to the construction of economically sound and safe coastal structures because wave height plays an important role in determining the weight and shape of a levee body or armoring material. In this study, a numerical model using a large eddy simulation is employed to predict the runup heights of nonlinear waves that passed a submerged structure in the surf zone. Reduced runup heights are also predicted, and their characteristics in terms of wave reflection, transmission, and dissipation coefficients are investigated.

Numerical Study of Rip Current Generation Mechanism at Haeundae Beach, Korea

ABSTRACT Ha, T.; Jun, K.; Yoo, J., and Park, K.S., 2014. Numerical study of rip current generation mechanism at Haeundae Beach, Korea. Haeundae Beach is one of the most famous tourist attractions in Busan, Korea. Every year, millions of people visit the beach. Recently, however, numerous rip current accidents have occurred along the middle of the beach. For example, in 2013, more than 500 people were swept away by a strong channel of water flowing seaward from the shore and were rescued after being dragged into deeper water. Fortunately, no casualties were reported, but the authorities said that, given the right weather conditions, rip currents could occur again at any time, not only at Haeundae Beach but also at other beaches. Some studies have been conducted to determine the rip current generation mechanism at Haeundae Beach. Previous research suggests that a known mechanism of rip current generation associated with the nodal line area of honeycomb-patterned wave crests was one of the significant factors in rip current occurrences at Haeundae Beach. In this study, we performed numerical experiments to identify the rip current generation mechanism based on the hypothesis proposed in the study mentioned. A well-known Boussinesq equation solver, FUNWAVE-TVD, was employed to simulate nearshore circulation at Haeundae Beach. The model was verified by generating a rip current for a simple beach for honeycomb-patterned incident wave conditions. The model was then used in numerical simulations of wave transformation to identify the rip current generation mechanism for Haeundae Beach, and wave transformation characteristics were examined in detail to identify the possible origins of rip currents. The original bathymetry was modified and applied to simulation of nearshore circulation to understand how reefs located off the shore of Haeundae Beach contribute to rip current generation. The numerical results corresponded to a slightly different rip current generation mechanism. It is thought that honeycomb-patterned incident waves may induce rip current event at Haeundae Beach but the other factors may contribute as well.

Remote sensing of wave runup over breakwater slope in field using optical imagery

ABSTRACT Yoo, J., Choi, J.-Y., Yoon, J.-J., Jun, K.-C., Shim, J.-S. Kim, S. and Min, I.-K., 2013. Measurements of wave runup over breakwater observed in field using optical imagery. In this paper, wave run-up on the seaward slope of a breakwater in field was measured remotely using optical video imagery and investigated through comparisons with an empirical formula. In order to extract properties of wave run-up and over-topping on the breakwater slope from field videos, individual image frames were sequentially sampled at 3 Hz from a video burst first. Then, the sampled sequential image frames were rectified into real world coordinates. A crossshore image timestack was generated by collecting an image pixel array, which was defined along the cross-shore transect on the seaward breakwater face of interest. Trajectory of the run-up on the seaward face evolving in time was extracted from the image timestack using a line detection method. The properties of the run-up estimated from field video bursts were combined to the measurements of incident waves, in order to derive an empirical relation between incident waves and run-up. The video-based relative wave run-up height (i.e. Ru,2% / Hs) at a typical armored breakwater with tetrapods was found to be about 2.15. This value is about in the range of the general formula, which was derived from laboratory experiments in earlier studies.

Numerical Simulation of Winter Waves and Currents in the Haeundae Coast using 2DH Model

In order to investigate characteristics of waves and currents varying in space in the Haeundae coast in winter, numerical simulations by using a 2-D spectral wave model(SWAN) and 2-DH hydrodynamic model(Delft3D) were carried out in this study. The results of numerical simulations were validated with the field data collected at several different locations in the study area in February, 2014. From the numerical simulations, it was found that waves and currents were significantly influenced in terms of direction and magnitude by bottom topography characterized by straggling rock crops covered with sea grasses. The coupling of SWAN and Delft3D models also revealed that alongshore currents directing from the east to the west were developed in the nearshore, due to the influence of larger waves with the main incident direction from the east.

Laboratory Investigations on Effects of Water Level Change on Surf-zone Processes

ABSTRACT Yoo, J.; Shin, S.; Do, K.D.; Shim, J.S.; Ha, T., and Jun, K.C., 2014. Laboratory investigations on effects of water level change on surf-zone processes. Tidal level and current often influence the rip current and the morphological changes in the macro-tidal beaches. Effects of water level change on surf zone processes were investigated through laboratory experiments conducted in a two dimensional wave flume. A movable beach was installed using a find sand in the flume starting with 1/50 slope from the offshore and ending with 1/20 slope near the shore. Irregular waves were generated by a piston type wave maker with active absorption. In the meantime, water level was changed gradually by using a pump and drain system to consider waves and tide simultaneously. Herein, three different tidal phases (i.e. flood tide, full tide and ebb tide) were conditioned with a gradual depth change of about 10 ~ 15 cm in the offshore during a period of about 30 minutes per phase except the full tide, while only one wave condition was kept targeting a significant wave height of 12 cm and significant wave period of 1.7 s. Measurements of wave attenuations in the wave breaking zone influenced by the tidal change were made using in-situ sensors and an image-based analysis method. The rates of wave attenuation (i.e. the ratio of wave height to water depth) measured during the both of flood and ebb tides were found to be larger, ranging from about 0.8 to 1.4, compared to those recorded during the full tide with little depth change. In addition, the attenuation coefficient of the flood tide tends to be larger than that of the ebb tide.

Transformation of small-scale meteorological tsunami due to terrain complexity on the western coast of Korea

ABSTRACT Ha, T., Choi, J.-Y., Yoo, J., Chun, I., Shim, J., 2014. Transformation of small-scale meteorological tsunami due to terrain complexity on western coast of Korea. In: Green, A.N. and Cooper, J.A.G. (eds.), Proceedings 13th International Coastal Symposium (Durban, South Africa), Journal of Coastal Research, Special Issue No. 70, pp. 284–289, ISSN 0749-0208. On the 4th of May, 2008, a meteorological tsunami occurred in the Yellow Sea and abnormal waves were observed at Jukdo Island, located on the western coast of Boryeong, Korea. The event attracted the attention of many engineers and scientists because it caused extreme waves only at Jukdo Island, unlike the meteorological tsunami event in 2007, which caused abnormal extreme waves along the western coast of Korea. In general, a meteorological tsunami can be forecasted using large-scale meteorological observation data and climate modeling systems. However, a small-scale meteorological tsunami like the event that occurred in 2008, which can cause abnormal extreme waves only in specific coastal areas, should be analyzed using a high-resolution modeling system because water motions can be affected by local terrain. In this study, numerical experiments were conducted to identify the generation and amplification mechanisms of the meteorological tsunami that hit Jukdo Island in 2008. To achieve this objective, we generated virtual meteorological tsunamis using atmospheric pressure disturbances observed in 2008 and simulated the propagation and run-up of the meteorological tsunamis over real topographies by varying specific characteristics of the local terrain. A three-dimensional hydrodynamic model, MOHID, and a Boussinesq-type wave model, FUNWAVE-TVD Version 2.0, were employed to simulate the generation and transformation of the meteorological tsunamis, respectively. The numerical models were first validated by comparing the numerical results with the available tidal records observed during the event, and then used to identify the generation and amplification mechanism of the meteorological tsunami that hit Jukdo Island. Finally, the transformation of a small-scale meteorological tsunami due to terrain complexity is discussed in detail.