Observations Of Suspended Sediment Dynamics In Chinese Coastal Seas By Acoustic Instruments

A better understanding of sediment dynamics in coastal waters has particular significance on ecological studies, coastal engineering, harbor and fishery management, especially in Chinese coastal seas which are well known for their high turbidity. Successful modeling of sediment transport patterns heavily depends on some critical parameters describing sediment resuspension and settling processes, such as critical bottom stress and settling velocity. However, the complex and highly variable marine environment makes the quantification of these parameters a formidable problem. A combination of novel devices and methods for in-situ monitoring and quantifying sediment resuspension and settling processes is imperative in Chinese seas.The objective of this paper is to report several pilot and comprehensive field campaigns conducted in Chinese coastal seas spanning from Western Yellow Sea to Yangtze Estuary. The observational scheme includes bottom-mounted quardropods and shipboard profiling measurements. The quardropods are equipped with Acoustic Doppler Current Profiler (ADCP), Acoustic Doppler Velocimeter (ADV), Optical Backscatter Sensor (OBS), and CTD, while shipboard instrumentation includes Laser In-Situ Scattering & Transmissometer (LISST-100) and multiple-parameters CTD profiler fitted with OBS. The combinations of instrumentations yield the temporal and spatial distributions of tidal currents, turbulent kinetic energy (TKE), suspended sediment concentration (SSC) and particle size distributions with fine resolutions.High-frequency current velocities measured by ADCP and ADV are carefully processed to give the mean tidal currents and turbulence quantities in the water column and close to the seabed. Optical and acoustic techniques, as well as bottle samples, are combined to determine the SSC distributions. Different methods or tools are utilized to clarify the mechanisms underlying the sediment resuspension and settling processes in different hydrodynamical settings (tides and waves) and turbulent mixing levels (i.e., well-mixed, strongly stratified water columns, or mechanically dissipated water by raft-culture) from a'macroscale'or'microscale'perspective.In the tidal boundary layer, tidally-induced bottom shear stress is the main mechanism that acted upon the seabed to stir up the sediments. Besides, observations in Jiaozhou Bay also found that the sediment could be aroused by high-frequency current oscillations (Seiche). During the ebb tide, the amplitude of seiche-induced oscillations and ebb tidal flows were of similar magnitude, the interaction between them led to multiple flow reversals and enhanced turbulence mixing in the water column, which subsequently aroused the benthic fluffs. A new method based on the Rouse profile fitted by LISST-100 volume concentration and Stokes settling law is first proposed are found to be effective to distinguish different types of suspended sediment.In East Yangtze Estuary, a pulse-coherent ADCP with a binsize of 0.02 m was first deployed at 0.8 mab to measure near-bed profiles of current velocities and suspended sediment concentration. Two novel approaches for estimating profiles of Reynolds stress and the rate of turbulent kinetic energy dissipation were verified. It is found that the sudden shifts of tidal current magnitudes could induce high-Reynolds-stress and high-SSC events, thus the acceleration of tidal currents was used to explain sediment resuspension. When tidal current reached the strongest value during the measurement, it is unexpected that the bottom stress and SSC was relatively low. Strong tidal currents resulted in more effective erosion and a reduction in bed roughness and drag coefficient at sediment-water interface and hence smaller bottom stress and sediment resuspension. Comparson reveals that in the well-mixed tidal BBL, the shear production of turbulent kinetic energy is locally in equilibrium with TKE dissipation.Waves are a powerful mechanism to stir up sediments. The boundary shear stress associated with the wave motion may be an order of magnitude larger than the shear stress associated with a current of comparable magnitude. Thus waves are capable of entraining significant amounts of sediment from the seabed when a current of comparable magnitude may be too weak even to initiate sediment motion. On the other hand, waves are an inefficient transporting mechanism, and to the first order, no net transport is associated with the wave motion over a wave period. Waves acting as a stirring mechanism making sediment available for transport by a weak current are a convenient conceptualization of combined waves and currents. In Sungou Bay, wave-induced bottom stress solely controlled the sediment entrainment, however, negligible vertical transfer of sediment is observed since tidal currents were extremely weak and no stable tidal BBL was formed.Actually, bottom shear velocity is an'artificial'velocity parameter, it is defined to describe in a time-averaged manner the cumulative effect of momentum tranfer in the BBL through the ejection/sweep etc. From a microscale perspective, the real mechanism that controls sediment resuspension process is the intermittent bursting of coherent structures near the seabed. Several methods are used to verify this theory, and a possible suspension mechanism is proposed. In the wave-dominated environment, the powerful wave motions act upon the seabed directly to entrain sediment. Within a wave period, the water motion were divided into four phases (wave crest, wave trough, up-crossing and down-crossing), and the TKE production and SSC in each phase were statistically obtained. Based on the statistical results, the most possible mechanism that accounts for sediment resuspension in wave boundary layer is proposed.The vertical diffusion of suspended sediment in the water column is strongly affected by mixing and stratification. In well-mixed boundary layer like Jiaozhou Bay in winter, the continuous input of wind energy, the forced convection induced by heat loss, and strong tidal mixing led to the rapid upward diffusion of suspended sediment by energetic turbulent eddies with a speed of 60 m/hr approximately. While in highly-stratified Yellow Sea Cold water region, turbulent mixing was strongly suppressed by buoyancy dissipaton. The speed of vertical transfer of sediment is about 3 m/hr, far less than that in well-mixed water. Observations showed that even an extremely weak stratification (N ~ 10-4– 2.5×10-4 s-1) could induce the suppression of sediment diffusion. In Sungou bay with high-density raft-mariculture, the drags caused by the existence of floats and rafts results in the considerable reduction in tidal current, wave orbital velocity, and strong dissipation of turbulent kinetic energy. The suspended sediment advected from Yellow Sea quickly settled down and acts as a material supply mechanism in Sungou Bay.The presence of a concentration gradient of suspended sediment in a flow provides stable stratification. It acts as a sink for TKE, which results in a diminished eddy viscosity and sediment diffusivity compared with an unstratified flow of the same mean velocity. In Sungou bay, because of the suppression of turbulence by near-bed stratification, the bottom shear velocity deceased 40% and the turbulent dissipation rate deceased 4 times. In bays with obvious biological deposition, a benthic fluff layer with loosely aggregated and fragile flocs is formed. When hydrodynamics is strong, these flocs are disturbed and torn by energetic turbulent eddies.Settling velocity of suspended sediment is also influenced by many factors, among which sediment properties (e.g., grain size, density, composition, cohesiveness, etc.) and ambient turbulence are most important. Several methods to estimate settling velocity based on the up-to-date devices (LISST-100, ADV, ADCP) have been reported to be successfully applied to Jiaozhou Bay with moderate sediment concentration. However, due to the limitations of each method and different principles behind them, careful inter-comparisons are indispensable to yield more reliable estimates.Based on the ADCP echo intensity and LISST-100 particle size spectra, the diel vertical migration of micro-zooplankton was captured in Yellow Sea Cold Wate region. The possible factors that affected the migration are preliminarily discussed. The vertical swimming speed of zooplankton is carefully estimated.