Author(s): Maurizio Righetti; Paolo Gardiman; Aronne Armanini
Linked Author(s): Aronne Armanini
Keywords: Vegetated channels; Turbulence; Double Averaging Method
Abstract: The paper faces the problem of the resistance due to vegetation in a river characterized by fully submerged vegetation formed by colonies of bushes. The flow presents strong spatial variations between plants, that make unreliable the traditional approach based on time averaging of turbulent fluctuations. A more useful model, based on time and spatial averaging (Double Averaging Method) is applied. The vertical distribution of mean velocity and turbulent stresses intensities have been measured by means of a 3D Acoustic Doppler Velocimeter (ADV), in an open channel flow, where real bushes (salix pentandra) have been used as vegetation, for two different vegetation densities and different flow depths. The velocity measurements were completed with the measurements of the hydrodynamic resistance exerted to the flow by the bushes. The measurements confirm that the contribution of dispersive stresses to the momentum balance cannot be neglected in the balance equations of the flow. Moreover, the measurements allow to depict the fundamental characteristics of both the mean flow field and the turbulence. In particular the net upward turbulent momentum flux, evaluated with the methodology of Wei and Willmart (1987), emerge to be strongly damped by vegetation presence; this circumstance can rationally explain the reduction of the transport capacity typically registered in vegetated channels.