Sudden pollutant discharge in vegetated compound meandering rivers

Abstract

In this study, the effects of different vegetation densities and different relative flow depths on the longitudinal dispersion coefficient are investigated in a compound meandering channel. Simulated vegetation with three different densities was placed over the floodplain and tracer was released in line simultaneously and equally in the main channel and flood plain. Digital image processing technique was used to measure the tracer concentration along the channel by analyzing a series of sequential images of the tracer cloud. Acoustic Doppler Velocimeter was used to measure 3-D velocity components. The results showed that the depth-averaged longitudinal velocity increases in the main channel due to the presence of vegetation but it declines in the floodplain. The maximum amount of the turbulent kinetic energy and the dimensionless longitudinal dispersion coefficient (K/U⁎H) were observed at the bend apex. Moreover, as the relative flow depth increases, K/U⁎H declines in the compound meandering channel for all the vegetated cases. Additionally, the longitudinal dispersion coefficient increases up to 59% in the main channel and decreases up to 42% in the floodplain by increasing the vegetation density in a specific relative flow depth.