A comparative study of longitudinal dispersion models in rigid vegetated compound meandering channels.

Releasing and mixing of pollutants in rivers can cause serious threats for downstream users. Longitudinal dispersion is an important factor in describing pollutant transport process in rivers. The focus of this study is on the effect of floodplain rigid vegetation characteristics including its arrangement, density and relative depth (the ratio of the flow depth in the main channel to that over floodplain) on flow and longitudinal dispersion coefficient in a meandering channel with compound cross section. Digital image processing technique was used to measure the tracer concentration. Sequential images were continuously captured at seven sections downstream of release point. PVC cylinders were installed over the floodplain to simulate rigid vegetation in tandem and random arrangements. Acoustic Doppler Velocimeter (Micro-ADV) was used to measure 3-D velocity components. The results showed that roughening the flood plain with stems increases the longitudinal flow velocity and Reynolds shear stress in the main channel, while these parameters decrease in the flood plain compared to non-vegetated cases. As a result, in the presence of rigid vegetation, the travel time decreases in the main channel up to 20% compared to non-vegetated conditions. As the relative flow depth increases, the dimensionless longitudinal dispersion coefficient, K/U∗H, decreases up to 74%. Also, the dimensionless longitudinal dispersion coefficient increases up to 38% with stem density in a specific relative flow depth of Dr = 0.11. Finally, a comparison has been made between laboratory data and some well-known previously reported models. Later, the most accurate equations are presented for predicting the longitudinal dispersion coefficient in rigid vegetated compound meandering channels. The results of the present study reveal that rigid vegetation over flood plain can be used as an alternative method in river rehabilitation projects for environmental management purposes.