An open channel with one-sided rectangular vegetation patches: Regulatory effects and turbulence structure analysis

The arrangement of vegetation patches in rivers creates critical habitats, but the hydrodynamics of alternating patch-cavity systems are not fully understood. This study investigates the flow structure and turbulence in an open channel with one-sided rectangular vegetation patches of two densities (solid volume fraction ϕ = 0.10 and 0.05). Three-dimensional velocity data were collected using an acoustic Doppler velocimeter. We found that the downstream flow adjustment distance (Xeq) was negatively correlated with vegetation density ϕ. In the developed equilibrium region, the flow field consists of a low-velocity cavity and a high-velocity mainstream, separated by a turbulent mixing layer. Flow visualization revealed two distinct exchange mechanisms: weak internal circulation driven by shear-layer eddies in the dense case and suppressed eddy penetration due to strong “bleed flow” through the patch in the sparse case. At the cavity-mainstream interface, quadrant analysis showed that momentum exchange was dominated by ejection (Q2) and sweep (Q4) events with sweeps being the most intense contributors to Reynolds stress. These findings provide a quantitative basis for designing patch-cavity systems to create specific ecological functions, such as refugia or dispersal corridors, in river restoration projects.