Abstract. Beach erosion at the unprotected downdrift end of a groin is common with waves approaching the structure obliquely. This phenomenon has often occurred on the downdrift side of natural groins on the east coast of South Korea during high waves in winter months. The resulting planform assumes a distinctive crenulate shape with a maximum indentation point landward of the erosion. An analytical model is employed to study the beach erosion at the downdrift end of a natural rock groin at Jeongdongjin Beach in Korea, using mathematical equations derived from the parabolic model for headland-bay beaches in static equilibrium, to predict the downdrift control point and maximum indentation of the eroded shoreline. These equations are solved using the prevailing wave height, wave angle at breaking and wave direction derived from analyzing NOAA's wave data over 40 years and the longshore sediment transport rate calculated from the wave data. The location of the calculated maximum indentation is also verified using shoreline video monitoring data and compared with the result of a one-line numerical model for shoreline change. The limitation of the proposed analytical model is discussed as is the effect of sediment bypassing the groin.

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In the vicinity of the coast, there is a risk of complex disasters in which inland flooding, wave overtopping, storm surge, and tsunami occur simultaneously. In order to prepare for such complex disasters, it is necessary to set priorities for disaster preparedness through risk assessment and establish countermeasures. In this study, risk assessment is carried out targeting on Marine city, Centum city, and Millak waterside parks in Busan, where complex disasters have occurred or are likely to occur.<br/>For risk assessment, inundation prediction map constructed by the Ministry of Public Administration and Security in consideration of sea level rise, rainfall and storm surge scenarios and authorized data on social and economic risk factors were collected. The socioeconomic risk factors selected are population, basements, buildings, sidewalks, and roads, and the risk criteria for damage targets are set for each risk factors. And it was assessed considering the maximum inundation depth and maximum flow velocity of the inundation prediction map. Weights for each factor were derived through expert questionnaires. The risk assessment index that was finally evaluated by calculating the risk index for each element and applying weights was expressed as a risk map by different colors into four levels of attention, caution, alert and danger.

Abstract. Downdrift coastal erosion has occurred at natural or man-made groynes on Korea’s eastern coast, caused by oblique high waves in winter months. The resulting shoreline planform has a sagging shape with a maximum indentation point within the eroded shoreline. This study focused on solving the frequent and severe coastal erosion problem of this type at the Jeongdongjin review of wave data over 40 years from the National Oceanic and Atmospheric Administration (NOAA), as well as analyzing shoreline monitoring images for identifying the yielding line of maximum indentation points. An analytical method was developed to verify the eroding shoreline in a sagging shape and its maximum indentation by applying the conservation principle of sediment transport and the empirical model of equilibrium shoreline. To examine how well the empirical formula is suitable for the Jeongdongjin coast, the annual directional spectrum of the observed wave data was applied to the simple diffraction wave model for the gamma breakwater, and satisfactory agreement was obtained by comparing it with the shoreline results. Breaking wave height and angle, duration, longshore sediment transport coefficient, and protruding length of the groyne were the inputs. The theoretical results are in good agreement with those of the shoreline monitoring program. The factors mitigating downdrift coastal erosion of this type were identified by applying the obtained theoretical solution, and the engineering solutions were examined via factor analysis.