Determination of the longitudinal dispersion coefficient in lowland streams with occurrence of dead zones

Abstract

Paper describes the field tracer experiments – determination of the coefficient of longitudinal dispersion in streams of lowland type (channels, small slopes and velocities) with expected occurrence of dead zones. Tracer experiments were carried out on three different streams in south Slovakia. The evaluation of field measurements confirmed in all cases the presence of dead zones, i.e. stream sections and regions with the appearance of small velocities that were formed due to the extensive presence of vegetation in the stream. These areas capture part of the transported substance (tracer) and then gradually release the substance and incorporated it back into the stream, creating a significant distortion of the tracer concentration time course. Strong influence of the dead zones raises the question of the adequacy using standard analytical solutions, whether for determining the coefficient of longitudinal dispersion or for modelling the dispersion of pollution or other substances carried by the stream.