Wave-current bottom shear stresses and sediment re-suspension in the mouth bar of the Modaomen Estuary during the dry season

On the basis of the measurement data pertaining to waves, current, and sediment in February 2012 in the mouth bar of the Modaomen Estuary, the Soulsby formulae with an iterative method are applied to calculating bottom shear stresses （BSS） and their effect on a sediment resuspension. Swell induced BSS have been found to be the most important part of the BSS. In this study, the correlation coefficient between a wavecurrent shear stress and SSC is 0.86, and that between current shear stresses and SSC is only 0.40. The peaks of the SSC are consistent with the height and the BSS of the swell. The swell is the main mechanism for the sediment re-suspension, and the tidal current effect on sediment re-suspension is small. The peaks of the SSC are centered on the high tidal level, and the flood tide enhances the wave shear stresses and the SSC near the bottom. The critical shear stress for sediment re-suspension at the observation station is between 0.20 and 0.30 N/m2. Tidal currents are too weak to stir up the bottom sediment into the flow, but a WCI （wave-current interaction） is strong enough to re-suspend the coarse sediment.

Effects of bottom shear stresses on the wave-induced dynamic response in a porous seabed:PORO-WSSI (shear) model

When ocean waves propagate over the sea floor,dynamic wave pressures and bottom shear stresses exert on the surface of seabed.The bottom shear stresses provide a horizontal loading in the wave-seabed interaction system,while dynamic wave pressures provide a vertical loading in the system.However,the bottom shear stresses have been ignored in most previous studies in the past.In this study,the effects of the bottom shear stresses on the dynamic response in a seabed of finite thickness under wave loading will be examined,based on Biot＇s dynamic poro-elastic theory.In the model,an ＂u-p＂ approximation will be adopted instead of quasi-static model that have been used in most previous studies.Numerical results indicate that the bottom shear stresses has certain influences on the wave-induced seabed dynamic response.Furthermore,wave and soil characteristics have considerable influences on the relative difference of seabed response between the previous model（without shear stresses） and the present model（with shear stresses）.As shown in the parametric study,the relative differences between two models could up to 10% of p0,depending on the amplitude of bottom shear stresses.

Simulation of Random Waves and Associated Laminar Bottom Shear Stresses

This work presents a new approach for simulating the random waves in viscous fluids and the associated bottom shear stresses. By generating the incident random waves in a numerical wave flume and solving the unsteady two-dimensional Navier-Stokes equations and the fully nonlinear free surface boundaiy conditions for the fluid flows in the flume, the viscous flows and laminar bottom shear stresses induced by random waves axe determined. The deterministic spectral amplitude method implemented by use of the fast Fourier transform algorithm was adopted to generate the incident random waves. The accuracy of the numerical scheme is confirmed by comparing the predicted wave spectrum with the target spectrum and by comparing the nanlerical transfer function between the shear stress and the surface elevation with the theoretical transfer function. The maximum bottom shear stress caused by random waves, computed by this wave model, is compared with that obtained by Myrhaug＇ s model （1995）. The transfer function method is also employed to determine the maximum shear stress, and is proved accurate.

Analytical solutions of turbulent boundary layer beneath forward-leaning waves

As a typical nonlinear wave,forward-leaning waves can be frequently encountered in the near-shore areas,which can impact coastal sediment transport significantly.Hence,it is of significance to describe the characteristics of the boundary layer beneath forward-leaning waves accurately,especially for the turbulent boundary layer.In this work,the linearized turbulent boundary layer model with a linear turbulent viscosity coefficient is applied,and the novel expression of the near-bed orbital velocity that has been worked out by the authors for forward-leaning waves of arbitrary forward-leaning degrees is further used to specify the free stream boundary condition of the bottom boundary layer.Then,a variable transformation is found so as to make the equation of the turbulent boundary layer model be solved analytically through a modified Bessel function.Consequently,an explicit analytical solution of the turbulent boundary layer beneath forward-leaning waves is derived by means of variable separation and variable transformation.The analytical solutions of the velocity profile and bottom shear stress of the turbulent boundary layer beneath forward-leaning waves are verified by comparing the present analytical results with typical experimental data available in the previous literature.

BOTTOM SHEAR STRESS UNDER WAVE-CURRENT INTERACTION

The present work adopts the COHERENS-SWAN model developed by the first author through coupling three-dimensional hydrodynamic model （COHERENS） and third-generation wave model （SWAN）. Inside the COHERENS-SWAN, the SWAN is regarded as a subroutine and the time- and space-varying current velocity and surface elevation are obtained from the COHERENS. Wave-enhanced bottom shear stress, wave induced surface mixing length and wave dependent surface drag coefficient have been introduced into the COHERENS. Secondly, as wave-enhanced bottom shear stress （＂bottom shear stress＂ described as BSS sometimes in this article） is concerned, a modified bottom shear stress Grant and Madsen model which introduces random wave field is given and introduced to COHERENS-SWAN. COHERENS-SWAN is also adopted to simulate three-dimensional flow in the Yellow River Delta with wave-current co-existing. Four numerical experiments were given to study the effects of wave-current interaction on enhancing bottom shear stress. The simulated current velocities, wave height and wave period match well with field measurement data. The simulated significant wave height and wave period for the case with considering the effects of current can give better agreement with measurement data than the case without involving the effects of current. The introduction of random wave generates lower the bottom shear stress than the case without introducing it. There are obvious differences between bottom shear stress of two way interaction and one way interaction. Velocity field obtained by the COHERENS-SWAN is reasonable according to previous studies and measurements.

Short-term variations and influencing factors of suspended sediment concentrations at the Heisha Beach,Guangdong,China

Knowledge of sediment variation processes is essential to understand the evolution mechanism of beach morphology changes.Thus,a field measurement was conducted at the Heisha Beach,located on the west coast of the Zhujiang River(Pearl River)Estuary,to investigate the short-term variation in suspended sediment concentrations(SSCs)and the relationship between the SSC and turbulent kinetic energy,bottom shear stress(BSS),and relative wave height.Based on extreme event analysis results,extreme events have a greater influence on turbulent kinetic energy than SSC.Although a portion of the turbulent kinetic energy dissipates directly into the water column,it plays an important role in suspended sediment motion.Most of the time,the wave-current interaction is strong enough to drive sediment incipience and resuspension.When combined,the wave-current interaction and wave-induced BSSs have a greater influence on suspended sediment transport and SSC variation than current-induced BSS alone.The relative wave height also has a strong correlation with SSC,indicating that the combined effect of water depth and wave height significantly impacts SSC variation.Water depth is mainly controlled by the tide on the beaches;thus,the effects of tides and waves should be conjunctively considered when analyzing the factors influencing SSC.