Experimental And Numerical Study On Coastal Cross-shore Sand Transport And Sand Suspension

A significant feature of coastal sand suspension is the effect of wave breaking.Wave breaking make larger sand concentrations and beach evolution,so it is important to study the principle of sand moving under breaking waves.A series of large-scale wave flume experiments were conduct to investigate the spatial and temporal features of suspended sand concentrations for different wave types(regular waves,wave groups,and irregular waves),and the response of sand suspension to wave height variation in the wave series and temporal length of wave group.Formulae for the near-bed concentration Ca(reference concentration)and the sediment vertical mixing parameter m are established based on the large-scale wave flume experiments.The advantage of the new formulae is that the turbulent kinetic energy induced by wave-breaking can be taken into account.Based on the above new formulae,two new expressions for suspended sediment load v(x)and undertow-related suspended sand transport rate qsc are also established.A new method to calculate short-wave-skewness related suspended sand transport rate<qss>is proposed.Also,new formulae of bed-load sand transport rate qb and its short-wave-averaged value<qb>are established by analysis the forces imposed on the sand particle,which account the impacts of flow velocity,short-wave-skewness,longwave,flow acceleration,and bed slope.The sand transport sand bed profile evolution were also investigated,and the effects of roller were studied by experimental results and a numerical model.The results show that:(1)For the regular wave in the swash zone and the inner surf zone,the peaks of short-wave components of near-bed sand concentration and turbulent kinetic energy appear near the peaks of free water surface elevation ?(or flow velocity u),but not near the trough(offshore maximum)of ?(or u).This indicates that in these zones the main reason for peaks of sand concentration is wave breaking but not flow velocity(bottom friction).However,the above feature of sand concentration is not significant in the inner surf zone and at the bar trough.For wave groups,the wave group modulation parameter ? affects the temporal variation strength of amplitudes of sand concentration and turbulent kinetic energy.The larger the ? is,the stronger these quantities' temporal variations are.The wave group period TG will influence the temporal lag between peaks of short-wave-averaged sand concentration and short-wave-averaged turbulent kinetic energy at the bar trough.For the irregular wave,the amplitudes and periods of short-wave-averaged sand concentration and short-wave-averaged turbulent kinetic energy rely on the amplitude of waves with larger height and the temporal interval between them.(2)New formulae for the near-bed concentration and the sediment vertical mixing parameter are applicable for different types considered in this study.Because of the strong correlation between turbulent flow and suspended sand concentration,these new formulae presented by this paper are superior to other formulae based on wave or flow velocities.In addition,the new expressions for suspended sediment load v(x)and time-averaged undertow-related suspended sand transport rate qsc can also give reliable predicted results.(3)The results predicted by the new formulae for short-wave-averaged,short-wave-skewness related suspended sand transport rate<qss>coincide with the experimental results for different wave types considered in this study.The results predicted by the new formula for bed-load sand transport rate qb are generally in agreement with the experimental results.Furthermore,the predicted results given by it is more precise than those given by Meyer-Peter and Muller formula,Bailard formula,Nielsen formula,and enhanced Bailard formula.(4)The experimental and numerical results show that the roller is important to to sandbar migration.With smaller incident wave height,the roller energy at bar crest is weak and the offshore qsc is smaller than the onshore qba,so the sand bar migrates onshore.In contrast,when the incident wave height is larger,the roller energy at bar crest is strong and the offshore qsc is larger than the onshore qba,so the sand bar migrates offshore.