The Method for Evaluating Cross-Shore Migration of Sand Bar under the Influence of Nonlinear Waves Transformation

Sand bar migration on the gently sloping sandy bottom in the coastal zone as a result of nonlinear wave transformation and corresponding sediment transport is discussed. Wave transformation on the intermediate depth causes periodic exchange of energy in space between the first and the second wave harmonics, accompanied by changes in the wave profile asymmetry. This leads to the occurrence of periodical fluctuations in the wave-induced sediment transport. It is shown that the position of the second nonlinear wave harmonic maximum determines location of the divergence point of sediment transport on the inclined bottom profile, where it changes direction from the onshore to the offshore. Such sediment transport pattern leads to formation of an underwater sand bar. A method is proposed to predict the position of the bar on an underwater slope after a storm based on calculation of the position of the maximum amplitude of the second nonlinear harmonic. The method is validated on the base of field measurements and ERA 5 reanalysis wave data.

Development of a Numerical Model to Analyze the Formation and Development Process of River Mouth Bars

An integrated sediment management approach that includes the recovery of the amount of declined sediment supply is effective as a fundamental solution to coastal erosion. During planning, it is essential to analyze the transfer mechanism of the sediments generated from estuaries (the junction between a river and sea) to assess the amount and rate of sediment discharge (from the river to sea) supplied back to the coast. Although numerical models that interpret the tidal sand bar flushing process during flooding have been studied, thus far, there has been no study focusing on the formation and development processes of tidal sand bars. Therefore, this study aims to construct wave deformation, flow regime calculation, and topographic change analysis models to assess the amount of recovered sediment discharge and reproduce the tidal sand bar formation process through numerical analysis for integrated littoral drift management. The tidal sand bar formation process was simulated, and the wave energy and duration of action concepts were implemented to predict the long-term littoral movement. The river flux and wave conditions during winter when tidal sand bars dominantly develop were considered as the external force conditions required for calculation. The initial condition of the topographic data directly after the Maeupcheon tidal sand bar flushing during flooding was set as the initial topography. Consequently, the tidal sand bar formation and development due to nearshore currents dependent on the incident wave direction were reproduced. Approximately 66 h after the initial topography, a sand bar formation was observed at the Maengbang estuary.

Development of Physics-Based Morphology Model with an Emphasis on the Interaction of Incoming Waves with Transient Bed Profile due to Scouring and Accretion using Dynamic Mesh

A physics-based morphology model [Seoul Foam] was developed using the dynamic mesh technique to explain the interaction between the sea bed, which undergoes deformation due to siltation and scouring, and the incoming waves. In doing so, OlaFlow, an Open Foam-based toolbox, was used as a hydrodynamic model. To verify the proposed physically-based morphology [Seoul Foam] in this study, numerical simulations of the shoaling process over the beach of the uniform slope were implemented. The numerical result shows that the formation process of a sand bar over the foreshore was successfully simulated. As can be easily anticipated, the size of the sand bar was closely linked to the nature of incoming waves, and in the case of a rough sea, the foreshore slope was rapidly deformed due to scouring. In mild seas, several sand waves were formed near the shoreline, and when the exposure time was the same, the size of the sand waves was not as large as in rough seas.

New Records of the Diatoms (Bacillariophyceae) from the Coastal Lagoons in Korea

Lagoons are natural bodies of water that are isolated from the sea due to the development of a sand bar or spit. Each lagoon has distinct ecological characteristics, and these sites also serve as popular tourist attractions because they are common habitats for migratory birds and are characterized by beautiful natural scenery. Lagoons also have distinct ecological characteristics from those of their associated estuaries, and there are active research efforts to classify, qualify, and quantify the high biodiversity of lagoons. The lagoons in Korea are primarily distributed in the East Sea, and are represented by Hwajinpo, Yeongrangho, and Gyeongpoho. Here, we report the discovery of 11 unrecorded diatom species (Diploneis didyma, Mastogloia elliptica, Cosmioneis citriformis, Haslea crucigera, Pinnularia bertrandii, Pinnularia nodosa var. percapitata, Gyrosigma sinense, Gomphonema guaraniarum, Gomphonema italicum, Navicula freesei, Trybionella littoralis var. tergestina) among samples collected from the Hwajinpo, Hyangho, Maeho, Gapyeongri wetland, Cheonjinho, and Gyeongpoho lagoons in Korea during a survey from 2018–2020. We present the taxonomic characteristics, ecological information, habitat environmental conditions, and references for these 11 species.

Process-Based Model Prediction of Coastal Dune Erosion through Parametric Calibration

Coastal dunes are important morphological features for both ecosystems and coastal hazard mitigation. Because understanding and predicting dune erosion phenomena is very important, various numerical models have been developed to improve the accuracy. In the present study, a process-based model (XBeachX) was tested and calibrated to improve the accuracy of the simulation of dune erosion from a storm event by adjusting the coefficients in the model and comparing it with the large-scale experimental data. The breaker slope coefficient was calibrated to predict cross-shore wave transformation more accurately. To improve the prediction of the dune erosion profile, the coefficients related to skewness and asymmetry were adjusted. Moreover, the bermslope coefficient was calibrated to improve the simulation performance of the bermslope near the dune face. Model performance was assessed based on the model-data comparisons. The calibrated XBeachX successfully predicted wave transformation and dune erosion phenomena. In addition, the results obtained from other two similar experiments on dune erosion with the same calibrated set matched well with the observed wave and profile data. However, the prediction of underwater sand bar evolution remains a challenge.

The Phoenician settlement of Cerro del Villar (Málaga, southern Spain) and its natural vulnerability

<p>Tsunamis and other extreme wave events draw a severe threat to coastal populations today and in historic times. The ancient settlement of <em>Cerro del Villar </em>located in present-day Málaga, southern Spain, was built by Phoenicians around the second quarter of the 8<sup>th</sup> century BCE on a small sand bar (island) in the wide estuary of the Guadalhorce River. Later, the sand bar connected to the southern river bank and alluvial plane. Due to the low height above mean sea level, the site has been prone to river floodings, as well as extreme wave events of the Mediterranean Sea. In order to understand the palaeoenvironmental evolution and settlement history, as well as its vulnerability, it is important to analyse the nature of the events by dating and interpretation of the sedimentary record.</p><p>Here, we present first results of a short field campaign carried out in October 2019 at the western end of the Guadalhorce River palaeo-estuary, outside the boundaries of the archaeological zone. Two sediment cores (MAL-CV-1; <em>ca.</em> 3.70 m length and MAL-CV-2, <em>ca.</em> 4.69 m) were drilled southwest of the Phoenician site. A total of eight non-invasive ground-penetrating radar (GPR) profiles were carried out in the surroundings of the cores, and additional GPR profiles close to the beach were taken to understand the changes in the depositional environment along the coast. The cores cover a stratigraphy of three different sediment units: a basal sand unit representing a palaeo-beach, followed by a large silt and clay unit developed in a lagoon environment, and topped by another silt and clay unit representing floodplain conditions. At MAL-CV-1 two possible high-energy event units (Ey and Ez) interrupt the low-energy silt and clay units. At MAL-CV-2 event unit Ey is preserved as well, the other event unit Ez is concealed by an anthropogenic unit rich in ceramic, brick and glass fragments. GPR profiles show the same stratigraphy and allow a lateral continuation of the different units and event deposits. With the help of these GPR profiles, event unit Ez can be traced in-between the anthropogenic unit of MAL-CV-2. In terms of chronology, two radiocarbon dates establish the transition between the basal palaeo-beach and the lagoon at 4352-4325 cal. BC (6274-6301 cal. BP) and the anthropogenic layer to be younger than 2201-2126 cal. BC (4075-4150 cal. BP). The establishment of coastal freshwater lagoons with plentiful Hydrobia gastropods and ostracods resembles the last stage of post-glacial sea level rise in the Mediterranean. In the future, these promising first results will be extended by additional radiocarbon dates and a palynological study to better understand the climate and palaeoenvironmental evolution.</p>

Origen y distribución de depósitos de tsunami en la marisma de Chaihuín (40° S/73,5° O), Chile

At Chaihuín marsh, south of Valdivia (39°56’ S/73°33’ W), a sand bed was deposited during the 1960 earthquake. The aim of this study is to map the 1960 tsunami deposit in detail and to associate earlier sand layers with past tsunamis. Geologic field mapping by means of stratigraphic sections constructed using 111 cores in the marsh revealed the existence of three sand layers. The source of these sand layers was determined by a statistical comparison of their sedimentological and mineralogical signatures with modern depositional environments. The results show that tsunami waves probably transported the sand layers found in the marsh. It is inferred that these sand layers were deposited in the marsh by tsunamis that followed subsidence associated with the great historical megathrust earthquakes of 1575, 1737 or 1837, and 1960. However, the three layers are different from each other in terms of lateral distribution and source, which we interpret as either changes in the sand bar associated with human occupation or differences in coseismic slip distribution resulting in variable accommodation space provided by coseismic subsidence as well as in tsunami wave height.

Seasonal regulation of river discharge by the cascade reservoirs in the Lancang River and its effect on downstream freshwater and estuarine saltwater intrusion

This study assesses the seasonal regulation of river discharge by hydropower dam-induced cascade reservoirs in the Lancang River and its effect on downstream freshwater and estuarine saltwater intrusion. There are eight main reservoirs in the Lancang River, with a total regulation capacity of 25.67 billion m3, which regulates river discharge by conserving water in the flood season and releasing water in the dry season. River discharge during the dry season from 1960 to 2009 accounted for 21% of the annual discharge before the cascade reservoirs were constructed and increased to 33% from 2010 to 2015 after the cascade reservoirs were constructed at the Jinghong hydrological station, which is the lowermost station in the Lancang River. During the 2016 extreme drought in the lower Mekong River basin, the river discharge increased by 550, 367, 1283, 969, and 524 m3/s in January, February, March, April, and May, respectively, regulated by the cascade reservoirs at the Jinghong hydrological station. Considering runoff, tides, wind, and continental shelf currents, a high-resolution three-dimensional numerical model was used to simulate the effect of regulation of river discharge by the cascade reservoirs in the Lancang River on the saltwater intrusion in the Mekong River Delta (MRD). The simulation results show that the seasonal regulation of river discharge by the cascade reservoirs in the Lancang River weakens estuarine saltwater intrusion during the dry season, especially in the sand bar areas, which is much more significant in the extreme dry season of 2016. The seasonal regulation of river discharge by the reservoirs in the Lancang River makes the seasonal distribution of downstream river discharge more uniform, favoring downstream freshwater utilization and alleviating flood disasters and saltwater intrusion in the MRD.