Effect of Floodplain Submerged and Non-submerged Vegetation on the Transverse Mixing Coefficient of Pollutants

Floodplain vegetation affects the flow and pollutant transport in waterways. In this paper, the effect of submergence of floodplain vegetation on the transverse mixing coefficient (ε) in a compound channel with asymmetric section is investigated experimentally. The experiments were carried out with different vegetation densities and relative flow depths. Tracer concentration was measured using MATLAB’s image processing toolbox in three sections downstream of the injection point. The transverse mixing coefficients were calculated by standard method of moments according to the variations of variance of tracer concentration. The results showed that ε increases in the emergent vegetation compared to the bare conditions up to 127 and 114% in the main channel and floodplain, respectively, while these values are 43 and 37% for the submerged vegetation. Also, ε increases with the flow relative depth for both submerged and emergent vegetation conditions. As the relative depth increases from 0.15 to 0.25, the lateral mixing coefficient in the floodplain and main channel increases to 97 and 45% for submerged and to 91 and 42% for non-submerged vegetation, respectively. For a specific relative depth, ε increases with the vegetation density. Finally, it was found that as the vegetation density increases, the difference between ε values for submerged and emergent vegetation conditions increases too. In general, the results showed that both submerged and non-submerged floodplain vegetation affect the lateral mixing coefficient significantly.