Understanding the interactions between crescentic bars, human interventions and coastline dynamics at the East coast of South Korea

Abstract

Crescentic sandbars are a commonly observed nearshore feature in coastal zones that strongly influence the surfzone circulations and are connected with the occurrence of rip currents. Furthermore, their spatial characteristics have been associated with the shoreline position in the form of shoreline perturbations, indicating morphological coupling between them and in turn affecting the beach width. As changes in the beach width affect coastal infrastructure and user functions such as recreation, understanding the bar dynamics is crucial for coastal zone management. The present work intends to improve this understanding by means of a case study at Anmok beach located at the South Korean East coast. Data analysis is used to estimate the long-term changes of the sandbar characteristics in time, under the influence of the ambient environmental conditions and human interventions. Additionally, this study investigates how to account for these features in modelling frameworks.
Available in-situ bathymetric surveys and high resolution aerial photographs are complemented with freely available satellite images in order to extract an estimate of the horizontal position of the sandbar crest line at Anmok beach. The introduction of satellite imagery observations dramatically increases both temporal coverage and frequency of the dataset resulting in 27 years of bar observations. The sandbar characteristics are found to change mainly in response to high wave energy events, while the initial sandbar position seems to be an important factor e.g. the closer the sandbar to the shore the more prone to changes it is. Furthermore, the results indicate that the alongshore migration of the sandbar features and the alongshore component of the wave energy flux show an agreement between their long-term (5-10 years) trends. This highlights the potential of the alongshore wave generated current to migrate the sandbar patterns in the alongshore direction. Moreover, the magnitude of the crescentic length and amplitude in an area 600-700m away from the port decreased after its construction.
The use of coastline models that assume alongshore uniform bathymetry can be questioned in cases of pronounced alongshore morphological variability. To this end, the impact of the alongshore variability of the sandbar on net annual sediment transports is investigated with the process based morel Delft3D for a selection of schematized bathymetries, created based on the natural variability of the sandbar at Anmok. It appears that for the symmetrical shaped crescentic sandbars, which are mostly present at Anmok beach, the influence of the alongshore variability on the net sediment transports is not large (~10%). The shoreline response to different sandbar configurations is investigated by computing alongshore incoming and outgoing sand volumes in cells specified close to the shoreline. It is found that the cross-shore distance between the sandbar and the shoreline is critical for the intensity of the erosion and accretion patterns formed at the coastline. Moreover, according to the predominant direction of the wave climate the alongshore location of these shoreline patterns can vary.
After highlighting the importance of the sandbar location for the beach width, an attempt to model the sandbar mean cross-shore location in response to the wave forcing is realized in pursuit of predictive capability for future changing wave conditions. As process based numerical models have shown limitations in producing any predictive skill, two empirical models selected from literature are used. The calibration and validation of the models is unsatisfactory, possibly connected with the spatial accuracy and varying temporal resolution of the dataset or the simplification of the models themselves.
From the present work, the use of satellite imagery for studying of long-terms sandbar dynamics and the derivation of its characteristics is proven to be quite promising. It allowed us to increase the number of observations and hence, study the sandbar dynamics beyond the time-frame of in-situ surveys and aerial photography. Therefore, we could study the effects of human intervention on the long-term sandbar characteristics. This technique is believed to be useful for other sites with crescentic sandbars around the world.