Bottom Boundary Layer Processes Influenced By High Suspended Sediment Concentration On Inner Continental Shelves

The bottom boundary layer(BBL),located at the frontier of flow-bed interactions,is of inherent importance in marine sediment dynamics,gravity-driven sediment transport is a special bottom boundary layer process.The scientific problem of this paper is “Bottom Boundary Layer Processes influenced by High Suspended Sediment Concentration on Inner Continental Shelves”,which has been of particular importance to the international academic community.Based on observation and model,we take two areas,Changjiang estuary and Jiangsu coast,as example,to study(1)The effects of density stratification caused by vertical suspended sediments concentration(SSC)gradient on calculation of BBL parameters,and(2)gravity-driven sediment transport.Four field observations of the Changjiang Estuary of 2011 and Xinyang coast of2015,which was carried out by predecessors,and Dafeng coast of 2018 and 2019,which was carried out by myself and my colleagues,were synthesized,which mainly used the bottom tripods for continuous observation,and profile observation also was conducted,providing the most original data for this study.Field three-layer water samples and laboratory experiment were used to calibrate the field turbidity instruments of profiles sampling and bottom tripod,respectively.The laser diffraction method was used for grain size analysis of suspended and seabed surface sediment samples.Field velocities within the ~2 m bottom layer observed outside the Changjiang estuary,China,were analyzed with the classical log-fit method,showing that Z0 varies drastically with time.This can be attributed to the stratification effects associated with the vertical SSC gradient when the vertical SSC gradient reaches O(0.1 kg m-4)in the BBL.The calculated Z0 by the extended log-fit method based on the lowest layer close to the bed(0.22-0.38 m asb)in the observation of Changjiang estuary of 2011)approaches the real value of ~2.00 × 10-4 m of muddy bed.Results of vertical onedimensional(1DV)k-? model suggested strong stratification in BBL reduced CD.Two ADV velocities at 0.2 and 1 m asb observed in the tidal bottom boundary layer outside the Jiangsu coast,China,were analyzed with the LP,COV,and TKE methods.The COV and TKE methods provided a reasonable estimation of U*,and pronounced overestimation was identified with the LP method.This overestimation can be attributed to the stratification effects associated with the vertical SSC gradient near the bottom.The ratio of U* estimated from the COV method and TKE is 0.20,suggesting the TKE method performs well with the constant C = 0.20.This result is consistent with the suggestion of previous study.The modified logarithmic velocity distribution models could correct the overestimation with the empirical constant α,β,and A.The values ofα,β,and A derived from the observation data were 3.16,2.99,and 3.98,respectively,which are smaller than previous estimates.The value of α in this study is 3.16,which is slightly smaller than that of 3.33 demonstrated by previous study.Gravity-driven sediment transport generated on a tidally dominated shallow slope of Jiangsu Coast,the source of gravity-driven sediment transport is probably from the fluid mud which are formed in tidal flat.Wave and slope play a key role in initiating and extinguishing the gravity-driven sediment flow modulated by tides,respectively.The observed gravity-driven sediment flows dynamics support the use of the theoretical buoyancy-friction model.The sediment stratification effects should be taken into account when the logarithmic velocity distribution be used to calculated BBL parameters.The modified logarithmic velocity distribution models can be applied in numerical simulations when sediment stratification is important.Gravity-driven sediment flows may occur more frequently and in a much wider range of shelves than previously thought and have more complicated dynamic behavior.