Direct measurement of vegetation resistance in prototype scale

The hydrodynamic force exerted by water stream on series of isolated willows of several size and foliage at full scale is experimentally analysed. The plants were both fully and partially submerged by the flow. The hydrodynamic resistance was measured by means of a properly designed force transducer. The plants drag coefficient is evaluated considering the willows equivalent to a series of rigid isolated cylinders; the estimated values of the drag coefficient are close to the theoretical values expected for the isolated cylinder and, due to the willows flexibility, decreases with the plant's Reynolds number. The measurements allow to evaluate the effect of plant stiffness on drag: a linear relation between force and the square of velocity is determined for the tallest, partially submerged, rigid willows; a linear increase of drag force with flow velocity is observed for the flexible plants in completely submerged conditions, due to streamlining (bending of trunk and shrinking of branches) and consequent decrease of the drag coefficient and of the momentum absorbing area. Finally it is clearly found out the great contribution given by the foliage to the global resistance of the plant; this contribution reaches the 40% of the overall drag in present experiments.