Experimental investigation of channel flow through idealized isolated tree-like vegetation

Riparian and floodplain tree-like emergent vegetation alter significantly the flow field and lead to complicated three-dimensional flow patterns, characterized by increased turbulence production, with the potential to induce morphological changes. The canopy presence in tree-like vegetation leads to the formation of lee wake vortices and can induce a strong subcanopy flow. The present experimental study employs artificial, rigid, tree-like emergent vegetation elements, with relatively simple structure, in order to investigate the canopy effects on the flow field. Specifically, the tree-like canopy is simulated by placing an element on top of a wooden rod simulating the trunk. Three elements with an equal encircling diameter of 16 cm are examined as canopy in tree-like vegetation, namely a circular cylinder and two hexagonal arrays comprising smaller circular cylinders with two different individual diameters. The experiments were conducted in a 26 m long laboratory flume and the velocity measurements were carried out with an acoustic Doppler velocimeter. The results show that the canopy porosity has a direct impact on the subcanopy flow intensity and on the required distance that the flow needs to recover. In addition, the subcanopy flow disrupts the formation of a steady wake region behind the entire porous element and inhibits the development of a recognizable von Karman vortex street.