Interaction between neighboring vegetation patches: Impact on flow and deposition

Abstract

Flow and sedimentation around patches of vegetation are important to landscape evolution, and a better understanding of these processes would facilitate more effective river restoration and wetlands engineering. In wetlands and channels, patches of vegetation are rarely isolated and neighboring patches influence one another during their development. In this experimental study, an adjacent pair of emergent vegetation patches were modeled by circular arrays of cylinders with their centers aligned in a direction that was perpendicular to the flow direction. The flow and deposition patterns behind the pair of patches were measured for two stem densities and for different patch separations (gap widths). The wake pattern immediately behind each individual patch was similar to that observed behind an isolated patch, with a velocity minimum directly behind each patch that produced a well-defined region of enhanced deposition in line with the patch. For all gap widths (D), the velocity on the centerline between the patches (U c ) was elevated to a peak velocity U max that persisted over a distance L j . Although U max was not a function of D, L j decreased with decreasing D. Beyond L j , the wakes merged and U c decayed to a local minimum. The merging of wakes and associated velocity minimum produced a local maximum in deposition downstream from and on the centerline between the patches. If this secondary region of enhanced deposition promotes new vegetation growth, the increased drag on the centerline could slow velocity between the upstream patch pair, leading to conditions favorable to their merger.

A review of concentrated flow erosion processes on rangelands: Fundamental understanding and knowledge gaps

“…This vegetation driven spatial heterogeneity (VDSH) stems from differential soil development and evolution processes between areas under canopies and bare ground (e.g., Bhark and Small (2003), Caldwell, Young, McDonald, andZhu (2012), De Ploey (1984), and Nulsen, Bligh, Baxter, Solin, and Imrie (1986)) resulting in feedback mechanisms perpetuating or further accentuating the bare groundunder canopy soil dichotomy (Puigdefabregas et al, 1999). In addition, observations in semiarid rangelands suggest that deposition mounds form upstream of plant clumps as a result of energy losses and changes in transport capacity that accompany overland flow diversion by plant stems (e.g., Meire, Kondziolka, and Nepf (2014) and Rominger and Nepf (2011)). The entrapment of nutrients along with sediments in these mounds creates areas of nutrients concentration where plants thrive spatially alternated by bare or poorly vegetated zones of water and nutrient depletion, forming the premise of the "resource islands" or "vegetation island" concept (e.g., Li, Zhao, Zhu, Li, and Wang (2007) and Ridolfi, Laio, and D'Odorico (2008)).…”

Experimental Investigations of Scour Pools around Porous Obstructions

“…Based on analysis of the experimental results, a scour pool developed when the flow blockage 2aWv/W was approximately >5.0 because the flow accelerated through the contraction. A local scour pool did not develop when the ratio of the obstruction width to the channel width 2Wv/W was <0.4 because the flow induced by the obstructions was independent [16]. We found that the scoured region induced by acceleration of the flow moved further downstream as the flow blockage decreased.…”

“…When the flow blockage increases, the diverging flow from the porous obstructions increases due to higher drag, leading to greater magnitude of the velocity and turbulence at the center of the channel. In particular, the flow between paired obstructions is similar to a turbulent jet at higher flow blockage [16].…”

The Impact of Shrubby Floodplain Vegetation Growth on the Discharge Capacity of River Valleys

“…It can be assumed that vegetation defines the flow conditions in the channel during periods of high discharge. Flow conditions at various plant configurations were also investigated among others by [6][7][8][9][10]. Yang et al [11] studied the hydraulic characteristics of overland flow.…”