The eastern coast of Korea is characterized by beach environments that are naturally well developed by open-sea waves from the East Sea (Sea of Japan). During the last decades, however, a large portion of the beaches have undergone artificial influences mostly resulting in remarkable erosion. Accordingly, the Korean government has been strongly in need of practical, efficient means to prevent and mitigate erosion. A comprehensive research project was thus launched in 2013 with a few core goals including reliable prediction and economic mitigation of erosion based on extensive in-situ observations. For this purpose, a test-bed site, Anmok Beach, was chosen to investigate into natural beach processes and the effects of an underwater dike on the beach. The beach is fronted by well-defined crescentic bars in the surf zone, a sedimentary bedform that commonly occurs in the eastern coast of Korea.
Self-recording instruments were deployed at a number of locations to monitor wave, current and suspended sediment concentrations near the seabed of the surf zone. These instruments included Aquadopp, AWAC and Vector, among others. The deployment in each field campaign was continued for a month covering two winter and one summer seasons in total. In addition, surf-zone bathymetric surveys were conducted with a single-beam echo sounder before and after each deployment. The main objective was to understand the interaction between the sand transport and the migration of crescentic bars in various hydrodynamic conditions. In general, the suspended sand transport was found to be much stronger in winter than in summer from these measurements.
On the basis of the wintertime observational data, a morphodynamical three-case scenario can be suggested as follows. When waves are less than 2 m high, sands move onshore at all locations and crescentic bars may be stationary or minutely move onshore. As wave heights increase in the range of 2-4 m, two different sub-cases take place. In sub-case 1 where waves approach normally to the shoreline, the rip circulation creates such that flows are onshore over the horn area but offshore over the bay area of the bars. However, this circulation becomes destroyed and instead a prominent longshore current develops over the entire surf zone in sub-case 2 with obliquely-incident waves. In these sub-cases, the crescentic bars may be more sharply defined and laterally displaced, respectively. Lastly, extreme waves greater than 4 m generate pronounced offshore flows all over the surf zone. This beach-wide offshore flow may be caused by a significant wave setup on the shoreline. A strong longshore currents could also be generated with a wave direction even slightly oblique (1-3o) to the shore-normal. The crescentic bars markedly move, offshore or obliquely offshore, according to the sand transport.
The above scenario is currently in just a working state. Additional measurements and analyses are required to put a narrower constraint on the range of wave height in each case, and to suggest the corresponding possible cause-and-effect interpretations. The underwater dike has been proven to perform its initial purposes, a recovery of the shoreline. However, the very adjacent segment of the shoreline began to retreat at the expense of the advance of the shoreline behind the dike. As another man-made structure, a small fishing port at the end of the study area is also expected to influence the sand transport in the study area. This has therefore added to the inventory of interesting research topics in this study.
