Longitudinal dispersion coefficient in compound open channel with rigid vegetation on flood plain

Abstract

The longitudinal dispersion is a key element in determining the distribution of pollutant in rivers and waterways with vegetation on the flood plains. The focus of this study is on the effect of rigid vegetation characteristic (arrangement patterns) and different relative flow depths on longitudinal dispersion coefficient in a rectangular laboratory flume with compound cross section. Cylindrical piles with 5 mm diameter were attached on the flood plain to model rigid vegetation. Potassium permanganate (KMnO4) has been used as a conservative tracer. Digital image processing technique with imaging from tracer cloud in MATLAB was used for measuring of tracer concentration in three sections downstream of the injection. The results indicated that vegetation could be highly influential on longitudinal dispersion. As the tracer cloud moves downstream, the maximum tracer concentration decreases up to 66% for a specific relative depth (Dr=0.56).The longitudinal dispersion values in tandem and patch arrangement were found to be 39.2% and 86.6% greater than those in non-vegetated tests. Also, for all vegetation conditions tested in this study, the longitudinal dispersion coefficient increases with increasing the relative depth. Moreover, for a specific relative depth the longitudinal dispersion coefficient increases in patchy arrangement when compared to tandem arrangement.