| In recent decades,the rapid development of economic construction in coastal areas has also brought many offshore environmental and ecological problems,and the problem of pollutant transport and diffusion in offshore area has also become a research hotspot.Based on the scientific experiments of longshore currents on straight slopes,this paper studies the transport and diffusion of pollutants in the nearshore wave-current field along the cross-shore and the longshore direction,which is not only of great significance to the protection and restoration of the nearshore ecological environment,but also can provide theoretical basis for practical engineering such as nearshore pollutant forecasting,protecting,and siting selection of sewage facilities.This article focuses on the following aspects:(1)First,using MATLAB to perform grayscale processing,binarization,spatial conversion,distortion correction and other processing on all ink motion trajectory images collected under regular wave and irregular wave incident cases in physical experiments to obtain pollutant images in the world coordinate system.Based on this,a program is written to obtain the coordinates of the centroid points and the degree of dispersion of the pollutant clusters in each case,and track the changes in the position and dispersion of the centroid points of the pollutants in each image in the horizontal two-dimensional direction.(2)Secondly,in terms of pollutant transportability,the change trend of the centroid point position of pollutants with time under regular wave and irregular wave cases is analyzed,and the linear fitting calculation is performed to obtain the transport velocity of pollutants along the cross-shore and the longshore direction in each cases.Then the relationship between the transport velocity and the mean longshore current maximum is analyzed.In terms of diffusivity,the degree of dispersion of the pollutant clusters is fitted to a Gaussian distribution model to estimate the cross-shore diffusion coefficient.In addition,based on the above calculation results,the effects of regular and irregular incident wave parameters on the transport velocity,the mean longshore current maximum and the diffusion coefficient are compared and analyzed.The results show that the transport velocity of the cross-shore and the longshore direction are in the range of 0.008?0.03 m/s and 0.06?0.08 m/s under the incident of regular waves,and in the irregular waves are 0.0016?0.021 m/s and 0.017?0.06 m/s,it shows that the pollutants are mainly transported along the longshore in the surf zone,and the smaller transport has occured in the cross-shore direction is due to the vertical component of the radiation stress in the case of regular waves,but the latter is affected by Stokes drift.Regardless in the regular or irregular wave cases,the transport velocity of pollutants along the longshore is about 35%of the mean longshore current maximum at the corresponding cases,and the mean longshore current maximum increases with the incident wave height.Under the condition that the incident waves have the same period and the wave heights are similar,the transport velocity of pollutants along the longshore and the mean longshore current maximum under regular waves are greater than those of irregular waves.The diffusion coefficient in the cross-shore direction are within the range of 0.16×10-3?2.6×10-3 m2/s and 2.3×10-4?2.2×10-3 m2/s under regular and irregular waves,and which coefficients decrease as the incident wave height increases.(3)Finally,the evolution characteristics of the instantaneous eddies in some wave cases are analyzed.The processed experimental images are used to study the horizontal two-dimensional scale of eddy-driven pollutant and the relative relationship with the width of the surf zone.Then the effects of instantaneous eddy on the transport and diffusion of pollutants are analyzed by fitting the scale change and the displacement of the center point change of the pollutant.The results show that the shearing motion of the longshore currents in the cross-shore direction results in instantaneous eddies in the surf zone.During the evolution process,the eddies are mainly transported in the longshore direction,and whose evolution range was mainly constrained to about 0.2 times the width of the surf zone under irregular wave cases.The transport velocity of vortex-driven pollutants in the longshore direction is greater than that of corresponding wave conditions in that direction.In terms of diffusivity,the diffusion velocity of the instantaneous eddy-driven pollutants in the cross-shore and the longshore direction is basically the same in all cases.And in the cases of irregular wave incidence,the diffusion velocity increases with the wave period and wave height. |
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