Study On The Longshore Currents And Rip Currents On Barred Beaches

The characteristics of longshore current and rip current on barred beach is studied experimentally and numerically for slopes 1:100 and 1:40.Which include the bimodal feature of longshore current,the horizontal distribution and vertical distribution of rip current by intersecting wave and their instabilities.The intersecting wave trains in the experiment were created by reflecting of obliquely incident waves on a vertical wall which stands perpendicular to the coastal line.The two intersecting waves are with the same amplitude and frequency,but opposite angle.This will form the standing wave in the longshore direction,which causes periodic variation of wave height and set-up in the longshore direction.Both in situ instruments and drifter measurement methods are used to measure the horizontal spatial variation of rip current circulation patterns for each wave condition.An experimental and numerical study of bimodal velocity profile of longshore currents over barred beaches for slope 1:100 and 1:40 under regular wave and irregular wave cases is studied.The uniformity of mean longshore current in the alongshore direction induced by passive flow circulation system is disscussed by measuremental results of ADV located in alongshore direction.The research focus is on the locations,values and ratios of two velocity peaks for the cases of mild slope beach.The dependence of its bimodal feature on wave height and period is discussed.Numerical simulation corresponding to experiment is performed to study the dependence of distribution features on the factors of numerical model.An experimental and numerical study of rip currents over barred beaches for slope 1:100 and 1:40 and barred beaches with rip channel for slopes 1:100 and 1:40 is studied.The wave field is measured by 60 capacitance wave gauges placed in the position nodes/antinodes,which were normal to shoreline.Both in situ instruments and drifter measurement methods are used to measure the horizontal spatial variation of rip current circulation patterns for each wave condition.So as to obtain the characteristics of rip current:nearshore circulation system,offshore current and vortex characteristics.The factors influencing the distribution of measured rip currents are presented,which include the wave period,wave height,rip channel and beach slope.The numerical simulation of the experimental results based on the POM model is carried out.An experimental and numerical study of vertical distribution of rip currents over barred beaches with rip current for slope 1:40 is studied.According to the complete self-similarity assumption and incomplete self-similarity assumption,the logarithmic and distribution model are advised to predict the vertical distribution of rip currents.These models are applied to calculate the vertical distribution of rip currents which were observed in laboratory experiements.The numerical simulation of the experimental results based on the POM model is carried out.The influence of sandbar on the instability of longshore current is studied,which is represented by the velocity profile of longshore currents with two characteristics.One is the second peaks of the velocity profile of longshore currents,and the other is the maximum peak position of the velocity profile.Based on the linear instability theory and the maximum entropy spectral analysis method,the instability characteristics of longshore currents along the beach are analyzed.The different growth patterns of longshore currents instability due to different fitting curve of mean longshore currents at two slope 1:100 and 1:40 are given by linear instability analysis.The spectral analysis results of the time series of the longshore current for slope 1:100 and 1:40 to show the characteristics of the longshore current instability is given,which compare with the results of the linear shear instability model.The instability of the longshore current is reflected by the comparison of the instability wavelengths calculated by the ink movement and the instability wavelength calculated by the linear theory.The spectral analysis results of the time series of the rip current over barred beaches for slope 1:100 and 1:40 and barred beaches with rip channel for slopes 1:100 and 1:40 are used to show the characteristics of the rip current instability.