Plant patch hydrodynamics in streams: Mean flow, turbulence, and drag forces

This paper investigates the effects of a finite-size vegetation patch on flow turbulence, variations in drag forces experienced by individual plants within the patch, and flow-drag interrelations. The experiments were conducted in a 32 m long laboratory flume with a 2.8 m long artificial patch. The plants were arranged to form a staggered pattern and three flow scenarios were tested. Velocities were recorded with an acoustic doppler velocimeter, whereas specifically designed devices were used to simultaneously measure the drag forces acting on nine plants within the patch. For all studied cases, the results show zones of increased turbulent energy close to the leading edge and along the patch canopy top, where turbulence shear production is enhanced. Zones of negative Reynolds stresses -(u'w') over bar are found inside the patch and they reflect the influence of plant morphology, which affects the shape of the longitudinal velocity profile and associated turbulent fluxes. Modifications to the power spectral densities of velocity by the plants indicate the emergence of two plant-induced mechanisms of energy production, which are most likely related to the wake turbulence and shear layer turbulence. Drag fluctuations appear to be correlated with the velocity field, with this correlation being especially profound at the highest-studied flow rate. The data suggest that the highlighted correlation stems from flow-plant interactions at the top of the vegetation canopy, likely due to the presence of large three-dimensional coherent structures.