Hydraulic impacts of vegetation management in a real drainage channel - evidences from field measurements

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 study, hydraulic experiments were set up in a real reclamation channel to assess the vegetation effects on flow resistance, velocity distribution and turbulence at the field scale. The study represents a step forward following a first research carried out in the same area (Errico et al., 2018). Four mobile pumps have been operated to control the flow rates under different scenarios of vegetation management. 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 are examined in order 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.

References

A. Errico, V. Pasquino, M. Maxwald, G.B. Chirico, L. Solari, F. Preti (2018) The effect of flexible vegetation on flow in drainage channels: Estimation of roughness coefficients at the real scale; Ecological Engineering, Vol. 120, 411-421, https://doi.org/10.1016/j.ecoleng.2018.06.018.