Real Scale Experiments for Flow Dynamics and Turbulence Analysis in Drainage Channels Colonized by Common Reed (Phragmites Australis) under Different Management Scenarios

The study of flows in vegetated channels appears of crucial importance, given the great attention on aquatic ecosystems that characterized worldwide the last decades of water management policies. Most of the studies on vegetated flows were carried out on flumes, while just a few were carried out at the real scale, due to the intrisic higher complexity. With the present report, we presnent the results of hydraulic experiments set up in two real reclamation channels to assess the vegetation effects on flow resistance, velocity distribution and turbulence at the field scale. Mobile pumps have been operated to control the flow rates under different scenarios of vegetation management: 1) full vegetation cover; 2) vegetation only on channel embankments; 3) vegetation totally removed from the channel by means of machinery commonly employed in the study area. Vegetation was mainly composed by common reed (Phragmites australis) which properties were characterized by measuring the stem density, diameters, horizontal and vertical distribution of stems and leaves at sample plots. Hydraulic profiles were monitored by rods located at four cross-sections. Velocity distribution and turbulences were measured by means of a current meter and an acoustic Doppler velocimetry along five verticals of two cross-sections. The presence of a dense, mature reed canopy affected both flow velocity distribution and channel conveyance. The opening of a central channel, with the release of lateral buffers of vegetation, resulted in an increase of flow velocity in the open portion of the channel and an increase in turbulence at the boundaries between open flow and the buffers of emergent vegetation. Cleared channel resulted in the highest discharge capacity and most regular distribution of flow velocity profiles, but also with the highest velocities at the banks. These results were examined to identify good practices of in-channel vegetation management, providing new insights to river managers, in a perspective of progressive reduction of negative impacts on aquatic environments.