Longitudinal dispersion in waterways with vegetated floodplain

•We investigate the influence of flow relative depth and floodplain vegetation on the longitudinal dispersion coefficient in a compound channel.•Vegetation can affect the longitudinal dispersion in rivers significantly.•Performances of various empirical models are evaluated.

Floodplains have a vital role in biocomplexity in ecosystem structure and function. Because of their long-term spontaneous development, vegetation established on the floodplains can influence ecological and environmental conditions of floodplains. Accidental contaminant spills in rivers can affect the water quality over long distances. In this situation, one-dimensional approaches can be used to describe the mass transport phenomena when longitudinal dispersion is the primary mechanism responsible for spreading the tracer cloud and for reducing peak concentrations. The longitudinal dispersion coefficients in simple shape channels have been studied extensively by many researchers; however, the flow conditions in the more complex geometries are completely different than those of simple channels. In this study, longitudinal dispersion phenomenon in a compound open channel is investigated experimentally. Tracer tests were conducted for both smooth and vegetated floodplain and for different flow conditions. Concentration profiles were extracted from the images taken before and during a passage of the tracer cloud using the image processing technique. It was found that the longitudinal dispersion coefficient increases by implanting vegetation over the floodplain as well as increasing the relative depth. Also, an attempt was made to compare the results of the present study with those of the predicted by some of the well-known empirical relationships available in the literature. Using three statistical parameters including NRMSE, DR and ME, the most accurate models for predicting the longitudinal dispersion coefficient in compound channels are proposed.