Study On The Features Of Longshore Currents On Mild Beaches And Boundary Rip Current Systems

Wave breaking and wave-induced currents(such as longshore currents and rip currents)are the main driving forces for the sediment transport,costal morphology evolution and tracer dispersion.Therefore,it is of theoretical value and application significance to study the wave breaking and wave-induced currents.The present study establishes a calculation model for the multiple wave breaking on the very gentle beaches,illustrates the features of longshore currents induced by the multiple wave breaking,and investigated the features of boundary rip current systems on the barred beach.The research was carried out mainly by the physical experiments,and the numerical calculation and theoretical analysis are also adopted.Very gentle beaches are very common and their bottom slopes are very small,ranging from 1:100 to 1:1000 or less.Multiple wave breaking and wave recovery processes are typical phenomena of wave motions observed on very gentle coastal beaches and have not been accounted for in the previous models for the calculations of wave height and longshore currents.The present study presents such models by establishing a new formula for the wave energy dissipation in the wave energy equation and by introducing a new criteria to judge the processes of multiple wave breaking and wave recovery.The established new formula has a small energy dissipation over the wave recovery region and can properly describe the wave recovery motion in this region.The model is validated by comparing the calculated results to the laboratory experiment data of cross-shore wave height variation and longshore current profile on planar and barred beaches with different slopes,and it is shown to be applicable for the cases with or without the presence of the wave recovery process.The present model can reproduce the wave height variation and M-type longshore current profile caused by a second wave breaking observed on a planar beach with slope 1:100,and can predict the stepped variation of wave height and wave setup of multiple wave breaking on a very gentle beach and the corresponing longshore current profile with a chain of narrow peaks.The linear and nonlinear instability features of longshore currents with the M-type profile and multi-peak type and descrete pulse-like type profile on mild slopes are investigated.There are usually sandbars,groin and headlands on sandy beaches.Wave reflection can occur on the groins or headlands and leads to alongshore variability of wave heights.The longshore current can be obstructed by these rigid lateral boundaries.So rip current systems controlled by these lateral boundaries are the important subject in the study of rip currents.The boundary rip current systems on a barred beach with the effects of rip channels and intersecting waves are stuided by the laboratory experiment.The measured data are used to analyze the mass conservation of each rip current unit and it is found that the lateral currents between rip current units make a contribution to the rip current transport in the boundary rip current systems.The numerical simulation is used to analyze the generation mechanism of the lateral currents between rip current units,onshore currents and rip currents.The interaction between the rip current systems in the reflection area and the longshore current outside the reflection area is also illustrated.By comparing the cases with and without rip channel,the effects of rip channel on the features of boundary rip current systems are studied.In the studying of mass transport balance in rip current units,it is necessary to determine the contribution of Stokes drift transport and surface roller transport,but there are not suitable calculation methods for them.Therefore,the present study also carried physical model experiment to provide the calculation formula for them.For the unbroken waves outside the surfzone,the present study analyzes the role of Stokes drift in the advective diffusion equation of wave-avergaed tracer concentration.