Study On The Time-varying Vertical Diffusion Characteristics Of Suspended Sand And The Process Of Bedload Transport In Surf Zone

In the nearshore surf zone,breaking waves will have a great influence on the sediment movement,causing the sediment at the bottom of the water to suspend or leap upward,thus causing changes in the coastal geomorphology,so it is very important to study the sediment movement law under breaking waves.By analyzing the dynamic bed experimental data of sediment suspension caused by large scale flume waves and combining with the theoretical analysis,the time course and time average values of 13 sediment concentration and flow velocity along the bathymetric section within the wave breaking zone are experimentally obtained.This paper analyzes the mechanism of suspended sand generation and the time-varying characteristics of vertical diffusion of regular waves and wave groups in the climbing zone,inner wave breaking zone,sand dam notch zone and sand dam crest zone,gives the theoretical results of the time-varying characteristics of suspended sand vertical diffusion,and is verified by the experimental results,and proposes a simple equation for the vertical distribution of time-averaged suspended sand concentration.A time-varying leap bedload transport rate model considering the effects of wave acceleration and bedload particle collision force and a time-varying slip bedload transport rate model considering the effects of wave acceleration and slip friction force are developed,and the influencing factors of the forces in the equation and the time-varying characteristics of the bedload motion velocity and sand load are analyzed,and the time-averaged and time-varying bedload sand transport rates in the surf zone are predicted by the weighted results.The results show that.(1)For the case of regular waves,the mechanism and vertical diffusion characteristics of suspended sand concentration generation in different regions within the surf zone are different.In the crest and climbing area of the dam,the fluctuation of suspended sand under water is mainly driven by the increase of velocity and turbulent kinetic energy at the crest of the breaking wave,and the vertical distribution of suspended sand fluctuation can be analyzed by the suspended sand diffusion theory;in the inner surf zone and the notch area,the fluctuation of suspended sand under water is mainly controlled by the turbulent burst motion of the bottom boundary layer,and the time-varying characteristics are transient patterns.It is not possible to describe theoretically.(2)For the wave group case,the wave breaking turbulent kinetic energy is the dominant driving force for the peak generation of suspended sand under water.The reasons for concentrated wave breaking in different areas of the surf zone are different: in the crest and climbing area of the sand dam,concentrated wave breaking is caused by the shallowing of the water depth and the increase of the wave height to water depth ratio caused by the low frequency long wave trough;in the inner surf zone and the notch area,concentrated wave breaking is caused by the increase of the wave height to water depth ratio caused by the large wave height in the wave swarm.Using the theory of suspended sand diffusion can give the law of the peak appearance of suspended sand fluctuation in each water depth layer,and the time lag of the theoretical forecast is consistent with the experimental results.The vertical attenuation of the suspended sand fluctuation amplitude given by the theoretical analysis can be applied to explain the experimental results.(3)The proposed equation for the vertical distribution of time-averaged suspended sand concentration is simple and effective,and can predict the exponential distribution of time-averaged suspended sand concentration in the surf zone under different wave conditions by the suspended sand concentration under water,which is consistent with the experimental results.(4)The time-varying sand load of the leap bedload is characterized by a main peak and smaller fluctuations in one cycle;the time-varying sand load of the slip bedload is characterized by two peaks of different amplitudes in one cycle.The wave of leap bedload velocity has a sharp peak and a flat trough,while the slip bedload velocity increases more slowly and decreases more rapidly,with a short pause.The time-varying characteristics of sand transport rate of leap bedload and sand transport rate of slip bedload are determined by the time-varying characteristics of sand load and the time-varying characteristics of velocity,which also show asymmetry.The time-varying sand transport models of leap bedload and slip bedload are considered together to predict the time-averaged sand transport rate of bedload at different locations in the surf zone,which is more consistent with the experimental results,and the sand transport of bedload to the shore in the surf zone.