Search

Understanding of bottom sediment erodibility is necessary for the sustainable management and protection of coastlines, and is of great importance for numerical models of sediment dynamics and transport. To investigate the dependence of sediment erodibility on degree of consolidation, we measured turbidity, waves, tidal currents, intratidal bed-level changes, and sediment properties on an exposed macrotidal mudflat during a series of tidal cycles. We estimated the water content of surface sediments (in the uppermost 2 cm of sediment) and sub-surface sediments (at 2 cm below the sediment surface). Bed shear stress values due to currents ( τ c ), waves ( τ w ), and combined current–wave action ( τ cw ) were calculated using a hydrodynamic model. In this study, we estimate the critical shear stress for erosion using two approaches and both of them give similar results. We found that the critical shear stress for erosion ( τ ce ) was 0.17–0.18 N/m 2 in the uppermost 0–2 cm of sediment and 0.29 N/m 2 in sub-surface sediment layers (depth, 2 cm), as determined by time series of τ cw values and intratidal bed-level changes, and values of τ ce , obtained using the water content of bottom sediments, were 0.16 N/m 2 in the uppermost 2 cm and 0.28 N/m 2 in the sub-surface (depth, 2 cm) sediment. These results indicate that the value of τ ce for sub-surface sediments (depth, 2 cm) is much greater than that for the uppermost sediments (depth, 0–2 cm), and that the τ ce value is mainly related to the water content, which is determined by the extent of consolidation. Our results have implications for improving the predictive accuracy of models of sediment transport and morphological evolution, by introducing variable τ ce values for corresponding sediment layers, and can also provide a mechanistic understanding of bottom sediment erodibility at different sediment depths on intertidal mudflats, as related to differences in the consolidation time. [ABSTRACT FROM AUTHOR]

An understanding of the dynamics and behaviors of suspended sediments is vital in analysis of morphological, environmental, and ecological processes occurring in coastal marine environments. To study the mechanisms of maintaining high suspended sediment concentrations (SSCs) on a tide-dominated offshore shoal, we measured water depths, current velocities, SSCs, wave parameters and bottom sediment compositions in the southern Yellow Sea. These data were then used to calculate bottom shear stresses generated by currents ( τ c ), waves ( τ w ), and wave–current interactions ( τ cw ). SSCs time series exhibited strong quarter-diurnal peaks during spring tides, in contrast to the semidiurnal signal during neap tides. A Fourier analysis showed that suspended sediment variations within tidal cycles was mainly controlled by resuspension in most stations. There existed relatively stable background SSCs (maintaining high SSCs among tidal cycles) values at all four stations during both windy (wind speed > 9.0 m/s) and normal weather conditions (wind speed < 3.0 m/s). The background SSCs had strong relationship with spring/neap-averaged τ cw , indicating background SSCs were mainly controlled by mean bottom shear stress, with a minimum value of 0.21 N/m 2 . On account of the strong tidal currents, background SSCs of spring tides were greater than that of neap tides. In addition, on the base of wavelet, statistics analyses and turbulence dissipation parameter, background SSCs during slack tide in the study area may be maintained by intermittent turbulence events induced by a combined tidal current and wave action. [ABSTRACT FROM AUTHOR]

An understanding of the dynamics and behaviors of suspended sediments is vital in analysis of morphological, environmental, and ecological processes occurring in coastal marine environments. To study the mechanisms of maintaining high suspended sediment concentrations (SSCs) on a tide-dominated offshore shoal, we measured water depths, current velocities, SSCs, wave parameters and bottom sediment compositions in the southern Yellow Sea. These data were then used to calculate bottom shear stresses generated by currents ( τ c ), waves ( τ w ), and wave–current interactions ( τ cw ). SSCs time series exhibited strong quarter-diurnal peaks during spring tides, in contrast to the semidiurnal signal during neap tides. A Fourier analysis showed that suspended sediment variations within tidal cycles was mainly controlled by resuspension in most stations. There existed relatively stable background SSCs (maintaining high SSCs among tidal cycles) values at all four stations during both windy (wind speed > 9.0 m/s) and normal weather conditions (wind speed < 3.0 m/s). The background SSCs had strong relationship with spring/neap-averaged τ cw , indicating background SSCs were mainly controlled by mean bottom shear stress, with a minimum value of 0.21 N/m 2 . On account of the strong tidal currents, background SSCs of spring tides were greater than that of neap tides. In addition, on the base of wavelet, statistics analyses and turbulence dissipation parameter, background SSCs during slack tide in the study area may be maintained by intermittent turbulence events induced by a combined tidal current and wave action. [ABSTRACT FROM AUTHOR]

Growing evidence has suggested that human activities could cause significant changes in marine sedimentation processes. The spatio-temporal response of sedimentary environment to human activities, however, remains unclear. Here, we present new evidence of sedimentary changes in the East China Sea over the past three decades (i.e., 1984–2016 CE) by using the mean grain size of sensitive components and geochemical parameters (i.e., Si/Ti and Fe/Al) from two sedimentary records from the Zhejiang-Fujian mud belt (ZFMB). Comparison with a compilation of published data shows that natural climate oscillations were the dominant factors that controlled the sedimentation processes before 2005 CE. Variations in the mean grain size of sensitive components over the ZFMB were mainly controlled by the East Asian Winter Monsoon before 2005 CE. The Si/Ti and Fe/Al ratios in the northern and southern of the ZFMB before 2005 CE were controlled by the Pacific Decadal Oscillation/Kuroshio Current and the East Asian Summer Monsoon, respectively. However, our synthesis shows a significant shift in the sedimentation processes between the north and south of the ZFMB since 2005 CE, i.e., after acceleration of the human imprint on sedimentation (e.g., dam construction). The mean grain size of sensitive components and the Si/Ti and Fe/Al records in the north and south of the ZFMB since 2005 CE are mainly controlled by human activities (e.g., human-induced catchment changes in the Changjiang River) and natural climate oscillations (e.g., the East Asian Winter Monsoon and Pacific Decadal Oscillation/Kuroshio Current), respectively. Our study reveals a human-induced asynchronous sedimentary signal in the East China Sea since 2005 CE, and may be applicable to river-dominated marginal seas worldwide during the human-impacted period, providing new evidence to better characterize the Anthropocene epoch. • Natural climate oscillations control the sedimentation processes of the Zhejiang-Fujian mud belt before 2005 CE. •Asynchronous sedimentation processes exist between the north and south of the Zhejiang-Fujian mud belt since 2005 CE. •Human activities cause this asynchronous sedimentary response in space since that time. [ABSTRACT FROM AUTHOR]

Abstract: A sea survey was conducted in May 2001 using CTD together with direct-reading current meter and water sampling to investigate the sediment dynamics off the Yangtze River mouth. Data obtained from five observational sites reveal density flows characterized by the halocline, thermocline and associated sharp current velocity gradients that prevail in the study area, especially at the freshwater plume front. The interaction of the Yangtze freshwater plume, littoral current and tidal current has generated these density flows. Particularly, the anomalous high speed (>140cms−1) current recorded at the freshwater plume front during the early flood stage leads to the resuspension of the sediment (SSC reaching 0.5gl−1) off the river mouth during that time period. The suspended sediment flux measured at B2, the ideal site for recording frontal processes in the estuary, indicates the main trend of southward transport of fine-grained sediment up to 60kgm−2 s−1 at early flood stage and 1.32kgm−2 s−1 as net flux during two tidal periods. It is inferred that the density flow at the plume front plays key roles in dispersing the Yangtze fine suspended sediments southward during the non-flood season. [Copyright &y& Elsevier]