The importance of discharge siting upon contaminant dispersion in narrow rivers and estuaries

Abstract

Far downstream of a sudden contaminant release in a narrow channel the concentration depends on the cloud size. This is largely determined by the longitudinal shear dispersion and the time of travel of the cloud. Near the source the efficiency of the shear dispersion and the velocity of the cloud are strongly dependent upon the source location across the flow. The shear dispersion is greatest when there is both strong shear and strong turbulent mixing (i.e. away from either the centre-line or the banks), while the velocity is least and the time-lag maximized for a source on the banks. The quantitative influence far downstream can be characterized in terms of a deficit variance and a centroid displacement. In this paper exact results are derived for these quantities. It is shown that, except when the banks are extremely steep, the time-lag has the strongest effect and the concentration far downstream of a point discharge is minimized when the discharge is sited at the bank.