ABSTRACT:

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.

ABSTRACT:

Instream plants, which act as biological processors, have emerged as potentially advantageous eco-morphologic tools to be used in river management and restoration. Most of the study on the geomorphic role of vegetation as it interacts with flow and sediment has focused on local patch-scale dynamics. In this study, we present flume experiments aimed at improving our understanding of how vegetation patches link together to impact reach-scale hydraulics and suspended sediment transport and disposition. On the hydraulics side, we investigate a) the relationship between the imposed instream vegetation configuration and water surface slopes, b) the secondary flow patterns which arise because of the imposed vegetative obstructions, and c) the role of vegetative resistance on residence time within reach. We also examine how different vegetation arrangements determine the patterns, quantity, and caliber of suspended sediment deposited in the flume. A key result of vegetation within the channel is the overall non-uniformity in flow conditions it imposes upstream, within, and downstream of the vegetated reach. At steady-state, vegetated patches locally increase the resistance, leading to a backwater condition upstream of the patch and high water-surface slopes within the patch. In general, this leads to enhanced deposition of suspended sediment upstream of the patch and reduced deposition within the patch relative to a case with no vegetation at all.

ABSTRACT:

The present experimental study investigated how the growth of an isolated patch affected the local flow field and the scour pattern. The evolution of the vegetation patch was mimicked by considering an idealized case, in which a circular patch of rigid emergent stems becomes elongated, due to positive and negative feedbacks. The growth of the patch was simulated at three consecutive stages by aligning the different patches with the flow direction with a gradual decrease in density towards the downstream side. This variation in density was assumed to occur due to sediment and seedling deposition at different growth stages. The growth of the patch was considered to occur in three stages: (a) initial, (b) development, and (c) mature. Despite generally elongated obstacles are hydrodynamically favorable bodies, the morphometric analysis of the scoured bed obtained by laser scanner revealed that streamlined elongation of permeable patches amplifies global scour and it enhances localization of the local scour hole. Results also showed that as the vegetation patch grows, the steady-wake region of low velocity and suppressed turbulence that favors sediment entrapment becomes shorter, thus hindering the further expansion of the patch, indicating the existence of an ideal aspect ratio for vegetation patches. It was also seen that under the effect of the growth of the patch, in the wake region, vertical velocity distribution evidently deviates from the logarithmic velocity profile, and also near the bed region, flow recovers rapidly due to elevated bed-induced turbulence.