Experimental Study On The Mixing Coefficient Of Pollutant Transport

In coastal zone, the mechanism of pollutant motion is different from that of pure currentcase due to wave breaking and wave-driven longshore currents. At present, though lots ofnumerical models have been presented to study the convection and diffusion of pollutant incase of coexist of wave and current, it is hard to see the literature about experimentaldetermination of mixing coefficient in wave field except for several semi-rational formulaonly suited to pure wave case. This paper study the mixing coefficient in wave field, the resultis verified by the documented empirical formula.(1) The present paper presents a simple and analytical method to solve mixing coefficient,which employs an instantaneous point source solution of two-dimensional diffusion equationand combines CCD image pick-up of temporal series. Relative magnitude of random instantisoconcentration line is obtained by graphic analysis of CCD image pick-up of temporal series.While the period-mean mixing coefficient is dependent on relative magnitude of randominstant isoconcentration line.(2) According to the above method, the period-mean longitudinal and transverse mixingcoefficients in wave breaking zone are obtained for two beach slopes 1:100 and 1:40.Additionally, for the former, the related coefficients in offshore region are also obtained. Allof them are compared against other documented results.(3) The article analyzes the measured data and makes an interpretation of the pollutantmotion under different wave conditions. The period-mean longitudinal mixing coefficient inwave breaking in the presence of longshore current is obtained, which is 7 times that for purewave case.