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Possible origin of coastal sands and their long-term distribution along the high slope-gradient, wave-dominated eastern coast, Korea

2022-03-24T07:12:06Z

Title: Possible origin of coastal sands and their long-term distribution along the high slope-gradient, wave-dominated eastern coast, Korea Authors: Hee Jun Lee; Chanhong Park; Chang Woong Shin; Ha, Ho Kyung; Jeseon Yoo; Eui Young Jeong; Sang Hoon Lee; Jun-Yong Park Abstract: This study aims to investigate possible source and distributional patterns of coastal sands in the high slope-gradient, wave-dominated coast with no large rivers, eastern Korea. Chirp (2？ 7 kHz) seismic profiles show that the coastal sand deposits, the uppermost transparent layer, are up to 7.1 m thick off small, mountainous stream mouths, and thins laterally, showing a radial distribution from the stream mouths with a slightly SE-skewed elongated (i.e., alongshore) geometry. This suggests that the terrigenous coarse sediments emptied by the streams during the summer flash flooding season have been a major source to the adjacent coastal sands. Hydrodynamic measurements with benthic tripod systems indicate that alongshore currents occurring during episodic storms play a significant role in the displacement of the coastal sands toward the southeast, resulting in the slightly SE-skewed elongated (i.e., alongshore) distributional geometry of the sands.al sand deposits, the uppermost transparent layer, are up to 7.1 m thick off small, mountainous stream mouths, and thins laterally, showing a radial distribution from the stream mouths with a slightly SE-skewed elongated (i.e., alongshore) geometry. This suggests that the terrigenous coarse sediments emptied by the streams during the summer flash flooding season have been a major source to the adjacent coastal sands. Hydrodynamic measurements with benthic tripod systems indicate that alongshore currents occurring during episodic storms play a significant role in the displacement of the coastal sands toward the southeast, resulting in the slightly SE-skewed elongated (i.e., alongshore) distributional geometry of the sands.

High-resolution comparison of sediment dynamics under different forcing conditions in the bottom boundary layer of a shallow, micro-tidal estuary

2022-03-24T07:11:41Z

Title: High-resolution comparison of sediment dynamics under different forcing conditions in the bottom boundary layer of a shallow, micro-tidal estuary Authors: K. Park; Ha, Ho Kyung Abstract: Data for high-resolution profiles of current velocity and suspended sediment concentration (SSC) were collected in bottom boundary layer (BBL). The data were used to study the vertical and temporal varaibility in SSC under various forcing conditions of tide, wind and freshwater discharge. During the winter stormy season, the background SSC was low (0.015？0.03 g l- 1). An episodic storm-induced erosion/resuspension was responsible for the short-lasting high SSC in BBL. During the spring flooding period, the background SSC was relatively high (0.04？0.07 g l- 1) likely due to the large amount of suspended sediment from the fluvial input and bed softening, and the contribution of wind forcing to sediment resuspension was somewhat enhanced by the destratification in BBL. When the freshwater discharge was extremely high (> 5000 m3 s-1), the entire water column in shallow areas was influenced by freshwater input. Therefore, the thermohaline anomaly's contribution to the stratification considerably weakened, while the SSC's contribution strengthened. When the freshwater discharge was relatively low (< 5000 m3 s- 1), a critical wind stress for sediment erosion (0.08？0.1 Pa) was observed to abruptly increase the SSC.ing conditions of tide, wind and freshwater discharge. During the winter stormy season, the background SSC was low (0.015？0.03 g l- 1). An episodic storm-induced erosion/resuspension was responsible for the short-lasting high SSC in BBL. During the spring flooding period, the background SSC was relatively high (0.04？0.07 g l- 1) likely due to the large amount of suspended sediment from the fluvial input and bed softening, and the contribution of wind forcing to sediment resuspension was somewhat enhanced by the destratification in BBL. When the freshwater discharge was extremely high (> 5000 m3 s-1), the entire water column in shallow areas was influenced by freshwater input.

Estimation of high-resolution sediment concentration profiles in bottom boundary layer using pulse-coherent acoustic Doppler current profilers

2022-03-24T07:12:10Z

Title: Estimation of high-resolution sediment concentration profiles in bottom boundary layer using pulse-coherent acoustic Doppler current profilers Authors: J.P.-Y Maa; Ha, Ho Kyung; Y.H. Kim; K. Park Abstract: The capability of two acoustic profilers ？SonTek's 1.5-MHz pulse-coherent acoustic Doppler profiler (PC-ADP) and RDI's 600-kHz acoustic Doppler current profiler (ADCP) with pulse-coherent mode ？was evaluated for estimating high-resolution suspended sediment concentration (SSC) profiles in bottom boundary layer. In the laboratory measurements with a PC-ADP, two types of sediments were tested to study acoustic responses to grain size. A natural sediment sample from Clay Bank, a mixture of clay and very fine sand, showed a good linear relationship between range-corrected volume scattering ( ) and backscattered strength (E) until SSC increased up to about 10 g l-1. In contrast, a commercially available kaolinite exhibited earlier signal saturation and non-unified linear regressions between and E, most likely because the particle size is much smaller than the transmitted acoustic wavelength. Using a pulse-coherent ADCP, the field measurement results from Mobile Bay, Alabama showed that the acoustically-derived SSC profiles were well matched with the optically-derived outcomes although slight discrepancies were noted. The overestimation of acoustically-derived SSC near the bed may be related to the side lobe interference near the bed and the enhanced acoustic sensitivity by coarser particles and denser aggregates eroded from the bed. Mean absolute error of acoustic estimates was within 4.1？7.3% of the optically-derived SSC range, which is attributable to the different acoustic and optical scattering responses to given sediment size spectra. Despite some error sources in acoustic inversion, the results from laboratory and field experiments suggest that the pulse-coherent acoustic profiler is able to reveal the evolution of in-situ near-bed SSC profiles with high vertical and temporal resolution.

Hydrographic variability on a coastal shelf directly influenced by estuarine outflow

2022-03-24T07:12:06Z

Title: Hydrographic variability on a coastal shelf directly influenced by estuarine outflow Authors: F. Hernandez; W. Graham; Ha, Ho Kyung; K. Park; B. Dzwonkowsik; S. Powers Abstract: Hydrographic variability on the Alabama shelf just outside of Mobile Bay, a major source of river discharge into the Gulf of Mexico, is examined using time series of water column temperature and surface and bottom salinity from a mooring site with a depth of 20 m in conjunction with a series of across-shelf CTD surveys. The time series data show variability in a range of time scales. The density variation is affected by both salinity and temperature, with its relatively strong annual signal mostly determined by temperature and its year to year variability mostly determined by salinity. Seasonal mean structures of temperature, salinity, and density show a transition from estuarine to shelf conditions in which three regions with distinct seasonal characteristics in their horizontal and vertical gradient structures are identified. Correlation analysis with the available forcing functions demonstrates the influence of Mobile Bay on the variability at the mooring site. At low frequencies, river discharge from Mobile Bay has a varying influence on salinity, which is absent during the periods with unusually low discharge. At shorter synoptic time scales, both the estuarine response to the crossshelf wind stress and the shelf response to the along-shelf wind stress are significantly correlated with temperature/salinity variability: the former becoming important for the surface layer during winter whereas the latter for the bottom layer during both winter and summer. These forcing functions are important players in determining the estuarine？shelf exchange, which in turn is found to contribute to the shelf hydrographic structure.