Hydrodynamics and transverse mixing in a rectangular channel with bed forms and bank vegetation

The analysis of contaminants mixing in natural channels is an important topic of Environmental Hydraulics. In the mid-field, the prediction of transverse mixing is complicated by other typical features of natural channels, such as bed forms and vegetation. The present experimental research investigated the effects of bedforms and banks vegetation on the structure of the turbulent flow field and on the transverse mixing process in a channel. A flume equipped with a set of ten 2D fixed dunes on the bed and rice stems at the banks was used. Detailed velocity measurements were performed using acoustic Doppler velocimetry. Mixing measurements were made using a NaCl solution as the tracer and measuring its concentration downstream the point of injection. The results showed that dunes and vegetation have a significant effect on the flow field and on transverse mixing coefficient Dt-y . Comparing with the flat bed conditions, the presence of dunes and vegetation at the channels banks significantly modified the flow field and increased the strength of secondary currents. Finally, the presence of vegetation on channel banks increased the transverse mixing as compared with dunes without vegetation.