Impacts Of Tidal Currents And Surface Waves On Turbulence And Sediment Transport In The Bottom Boundary Layer Of Coastal Seas

The energetic turbulence in the bottom boundary layer(BBL)of coastal seas is a main dynamic characteristic that differs from the BBL in deep oceans.It affects the transport of particulate matters,hence regulates the supplement of nutrients in the upper layer,and impacts the marine primary production.This study is based on analyzing the observed velocity and the inferred suspended sediment concentration(SSC)from measurements with the bottom mounted acoustic Doppler velocimetry(ADV),in Heini Bay located in the western Yellow Sea,and at various sites in the East China Shelf Seas.This dissertation research covers the mechanisms of turbulence and sediment transport in the BBL under the influences of tidal currents and surface waves,and the establishment of the parameterization scheme of bottom drag coefficient(Cd).Several new methods are applied to analyze the long-time,continuous,and high-frequency observational data in the BBL under the influences of tidal currents and surface waves.The Synchro squeezedsubrange based on the Kolmogorov“-5/3”law.The analysis results establish the dependencies of the inertial subrange on the tidal current and turbulence intensities,and proposed the parameter setting for using a new acoustic Doppler current profiler(AD2CP)to estimate the profiles of turbulent parameters.The wave-turbulence decomposition is carried Wavelet Transform Method is applied to decompose the wave and turbulence components of the velocity and SSC fluctuations.For the analyses of the turbulent kinetic energy spectra,a new method is developed for the automatic identification of the inertial out for the ADV data at eight stations.The contributions of tidal currents and surface waves on Cd are denoted as Cd_t and Cdw,respectively.The estimated Cd_t and Cdw are not constants.Variations of Cd_t have no significant relationship with water depth.Generally,Cd_t decreases with the strengthening tidal currents,and Cdw increases with the enhancing surface waves.The analyses reveal a scale relationship between Cd_t and the turbulent Reynolds number(Rer),and that between Cdw and the bottom wave orbital velocity(Uw).The scale formula is Cd=0.395Rer 0·518+0.010(Uw/Ur)1.622.In the strong dynamic conditions,the bottom tidal energy dissipation estimated using Cd=0.0025 is higher than that estimated using the new scale formula of Cd.Based on the field observations in Heini Bay,we study the impacts of raft aquaculture for kelps,tidal currents,surface waves,and SSC on turbulence in the BBL.It is found that the presence of raft aquaculture causes the reductions of the current velocity and the turbulent Reynolds stress,but has weak influences on the turbulent kinetic energy(TKE)and the dissipation rate of TKE.In the BBL,flow shear is the main cause of turbulence production,and surface waves do not have significant contribution to the turbulent Reynolds stress but increase the dissipation rate of TKE.The presence of high SSC in the BBL does not suppress turbulence.The production of TKE due to the tidal flow shear needs to be combined with the production of disturbance energy by action of the mean field against the wave stress to balance the dissipation in the BBL under the influences of tidal currents and surface waves.The horizontal transport and resuspension processes of suspended sediment in the Heini Bay are studied through analyzing the field and remote sensing observations.In the presence of SSC fronts,the horizontal advection by the reciprocating tidal flow causes a flood-ebb asymmetry of the SSC in a tidal cycle.The swells induce intensified sediment resuspension and a flood-ebb symmetry of the SSC.The presence of surface waves causes the large net fluxes of suspended sediment away from the coast to the deep waters,implying the possibility of coastal erosion.Under the conditions dominated by currents,the erosion and deposition critical stresses of cohesive sediment are estimated to be 0.208 N m-2 and 0.188 N m-2,respectively.