In this study, a reduction in sediment budget due to the development of a river watershed, resulting in coastal erosion, was reviewed, and the rate of background erosion was calculated through an examination of the loss of coastal sediment into the open sea. The west coast of the Korean peninsula is severely impacted by the intercept of inflowing sediments from rivers, owing to the watershed development. However, the effects have not fully propagated into the entire coastal area, and thus, the long-term coastal erosion remains insignificant. However, a serious and irrevocable disaster may occur once the coastal erosion begins. Therefore, an analysis of the coastal erosion resulting from changes in the sediment budget, due to the development of the watershed, was conducted on Janghang Songrim Beach. A littoral cell of the Geum River was selected for a quantitative analysis of the decrease in the sediment budget from the watershed development. The rate of coastal sediment loss offshore, which reflects the characteristics of the Janghang Songrim Beach, and the future rate of coastal erosion were calculated. Then, the results were verified by employing geometrically corrected satellite photographs from previous years. This will enable us to predict the time of coastal erosion in the future due to a reduction in the sediment budget and watershed development, and prepare for future disasters resulting from the coastal erosion. Based on research into the components constituting the coastal development, the present study presents theoretical formulae allowing the prediction of the sediment budget and providing a practical contribution to the prevention of coastal erosion, for which additional reliable studies need to be conducted.
Abstract. In many parts, coastal erosion is severe due to human-induced coastal zone development and storm impacts, in addition to climate change. In this study, the beach erosion risk was defined, followed by a quantitative assessment of potential beach erosion risk based on three components associated with the watershed, coastal zone development, and episodic storms. On an embayed beach, the background erosion due to development in the watershed affects sediment supply from rivers to the beach, while alongshore redistribution of sediment transport caused by construction of a harbor induces shoreline reshaping, for which the parabolic-type equilibrium bay shape model is adopted. To evaluate beach erosion during storms, the return period (frequency) of a storm occurrence was evaluated from long-term beach survey data conducted four times per year. Beach erosion risk was defined, and assessment was carried out for each component, from which the results were combined to construct a combined potential erosion risk curve to be used in the environmental impact assessment. Finally, the proposed method was applied to Bongpo–Cheonjin Beach in Gangwon-do, South Korea, with the support of a series of aerial photographs taken from 1972 to 2017 and beach survey data obtained from the period commencing in 2010. The satisfactory outcomes derived from this study are expected to benefit eroding beaches elsewhere.
Abstract. Coastal erosion is more severe due to human-induced coastal zone development in addition to natural climate change. Anthropogenic development affecting coastal erosion is divided into three areas; watersheds, coastal waters, and coastal land areas. In this study, the ultimate effect of anthropogenic development on changes in the amount of sand, changes in the littoral drift, and changes in shoreline variability in these three planar areas is expressed as quantitative risk potential of beach erosion damage, defined as a change in the planar surface of the sand beach. The change in the amount of sand is due to the law of conservation of matter, and the littoral drift characteristic of sand is interpreted as a change in the main crest line at the breaking point, and the response characteristics of shoreline position is interpreted as change in the erodibility and recovery characteristics of beach sand. This quantitative method was applied to Bongpo-Cheonjin Beach of erosion grade D (frequency of erosion damage within 5 years) in Gangwon-do, Korea to identify the cause of erosion and evaluate the detailed applicability of this method. It was interpreted using a series of aerial photographs taken from 1972 to 2017 and survey data obtained from the erosion rating project started in 2010. In the erosion rating project, the GPS shoreline survey of 4 times per year and the sand sampling at the swash zones of base line at 150 m intervals are mainly carried out. We showed the feasibility of methodology evaluating the risk potential for beach erosion proposed in this study, and it can be expected that this method will be applicable to eroded beaches elsewhere.
Recently, studies have been conducted that long-term changes in shoreline position can be sufficiently interpreted using an ordinary differential equation that includes only erosion and recovery processes. Here, the erosion process term is given as a function of the breaking wave energy, which causes the shoreline to retreat to the ultimate erosion position by the incoming wave energy. The recovery process term is given as a function of the concentration of suspended sediment and allows it to recover to its shoreline position. Therefore, in this study, we propose a numerical technique that simulates long-term changes in the beach profile by extending the ordinary differential equation to be applied to the change in seabed constituting the beach profile by applying the parabolic equation of the equilibrium beach profile of the surf zone. This model also consists of a term that allows the beach profile to converge to the equilibrium beach profile due to the breaking wave energy and another term that allows it to converge back to the linear shoaling profile when the wave is extinguished. Therefore, it is possible to simulate the repeated formation and disappearance of scarp and berm whenever a storm wave passes, and it can also be applied to the morphological change at the beach with a large tidal range. The validity of the proposed methodology was verified by comparing the long-term shoreline observation data of Tairua Beach, New Zealand, where the tidal difference is about 2 m, with the results of the long-term beach section convergence model of this study. In addition, short-term observation data were also compared and analyzed to investigate the ability to simulate morphological changes due to episodic erosion and recovery processes. The results of this study are expected to be applied not only to the beach profile but also to the three-dimensional morphology change of the beach, and it is expected that it will serve as a cornerstone for a more detailed topographic change prediction study due to sea level rise.
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