ABSTRACT
Large Wood (LW) has important ecological functions. LW behaves as a bluff body in the flow field. The hydrodynamics around bluff bodies have been abundantly investigated in the past by means of experiments and numerical modeling. Most investigations concerned laboratory configurations, which may not be representative of natural river configurations. A field investigation on the natural Plizska River in Poland revealed that LW on the riverbed and banks causes a high heterogeneity in bathymetry and substrate, as well as irregular banklines. Measurements of the three-dimensional velocity vector were performed with high spatial and temporal resolution in three streamwise-vertical plans enveloping a LW that spanned almost the entire width of the river. The three plans were situated over different types of substrate: gravel, sand, and organic matter, respectively. The mean flow field was characterized by the typical features of flow around bluff bodies: flow divergence upstream of the LW, convergence downstream of the LW, and a wake zone with return currents just downstream of the LW. Turbulence levels were high in the flow, and substantially higher that predictions based on theory for canonical shear layers. The results indicate that an important part of the turbulence is not induced by friction on the riverbed, but by LW on the riverbed and banks.
