The estimation of a critical shear stress based on a bottom tripod observation in the southwest off Jeju Island, the East China Sea

摘要

The resuspension and deposition of sediment within a bottom boundary layer(BBL) is the main dynamic processes that control the fate of the suspended sediment in shelf seas.The numerical study of sediment transport patterns relies on the knowledge of some critical parameters that describe sediment erosion and deposition.A critical shear stress is estimated based on field observations at the edge of a mud area southwest off Jeju Island,the East China Sea.On the basis of the simultaneous observation of velocity and suspended sediment concentrations within the BBL by means of acoustic instruments including an acoustic Doppler velocimeter and an acoustic Doppler current profiler,the settling velocity is estimated by turbulent oscillations of the SSC under the assumption of inertial-dissipation balance.This method gives a mean value of 0.91 mm/s and standard deviation of 0.20 mm/s,which is an order of magnitude larger than the value obtained by an empirical method of Soulsby and by an in situ submersible particle size analyzer.The difference is possibly due to the distinct natures of two methodologies,the inertial-dissipation method is more indicative of the BBL dynamics and is thus believed to provide reasonable in situ estimates of the settling velocity,whereas Soulsby's method is usually suitable for still water.A novel method for estimating the critical stresses of erosion and deposition based on statistical analyses of the temporal variability of the SSC(which is defined as a derivative of the depth-averaged SSC with respect to time) and the corresponding bottom shear stress is proposed.Both critical stresses of erosion and deposition vary between 0.11 Pa and 0.25 Pa with corresponding median values of 0.20 Pa and 0.16 Pa,respectively,which confirms that the critical stresses of erosion is somewhat higher than the critical stresses of deposition.Another method of estimating the critical shear stress by means of the settling velocity is also employed,which yields reasonable critical shear stress values of 0.06-0.17 Pa.