» Research Note « Estimation of Longitudinal Dispersion Coefficient in Compound Channels with Two Rows of Rigid Vegetation (tree) over Floodplain

Document Type : Technical Note


Shiraz Agriculture University


Vegetation can be a significant factor for improving water quality and reducing the concentration of pollutants. In this study, the longitudinal dispersion coefficient and variations of the tracer concentration have been studied using Potassium permanganate (KMnO4) as a non-decayable tracer. The experiments were conducted in a straight rectangular compound cross section with 8 m length, 0.25 m width, and 0.6 m height. Two rows of metal cylinders were used over the floodplain for simulating rigid vegetation. Digital image processing technique with imaging from tracer cloud in Matlab software was used for measuring tracer concentration in three sections downstream of the injection. The results showed that shear flow between vegetation and floodplain wall was invigorated in the presence of two rows of vegetation. Consequently, this shear flow had a significant impact on longitudinal dispersion coefficient. Accordingly, two rows of trees increase the longitudinal dispersion coefficient up to 39.2% in specific relative depth (0.56) compared to non-vegetated case and causes reduction in the concentration of contaminants downstream of the injection section. The results of non dimensional longitudinal dispersion coefficient obtained in this study compared with the empirical relationship proposed by different researchers and the accuracy of each method for estimating the longitudinal dispersion coefficient of waterways combined with vegetation over the floodplain were also investigated. The results showed that the models of Elder (1959) and Deng et al. (2002) predict the longitudinal dispersion coefficient with two rows of vegetation over flood plain with higher accuracy.


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