Experimental study on influence of different patterns of patch on the flow field and scour/deposition process in the wake region

Flume experiments were conducted to comprehend the impact of different patterns of a patch on the flow field and the scour process in natural rivers. Velocity measurements, flow visualization, and scour tests were undertaken around different patch patterns, which were simulated inspiring from expansion process of a typical instream vegetation. The patches were idealized in which a circular patch of rigid emergent stems becomes elongated due to positive and negative feedback. Three characteristically different patches with a gradual decrease in density towards the downstream side were examined. This variation in density and length was assumed to occur due to sediment/seedling deposition, and the establishment of new plants. The expansion of patch was considered to occur in three stages: (a) stage 1, (b) stage 2, and (c) stage 3. These stages were replicated individually by only increasing in density and elongating the patch. In this way, two processes (i.e., elongation and increase in permeability), which usually have hydrodynamically opposite effects on flow fields, were simulated at the same obstruction. Despite generally elongated obstacles being streamlined bodies, the morphometric analysis of obtained by laser scanner revealed that streamlined elongation of permeable patches amplifies global scour and enhances localization of the local scour hole. This situation implies as the patch expands, in the wake region, the steady-wake region becomes shorter, turbulence diminishes, lateral shear stress enhances, and deposition cannot occur far from the patch. Consequently, as the patch expands, the hydrodynamic consequences may restrict further patch expansion after a certain length/density.