Experimental Study of Overland Flow through Rigid Emergent Vegetation with Different Densities and Location Arrangements

Abstract

The effect of vegetation density on overland flow dynamics has been extensively studied, yet fewer investigations have focused on vegetation arrangements with different densities and position features. Flume experiments were conducted to investigate the hydrodynamics of flow through rigid emergent vegetation arranged in combinations with three densities (Dense, Middle, and Sparse) and three positions (summit, backslope, and footslope). This study focused on how spatial variations regulated hydrodynamic parameters from two dimensions: direction along the slope and water depth. The total hydrodynamic parameters of bare slopes were significantly different from those of vegetated slopes. The relationship between Re and f illustrated that Re was not a unique predictor of hydraulic roughness on vegetated slopes. In the slope direction, all hydrodynamic parameters on vegetated slopes exhibited fluctuating downward/upward trends due to the clocking effect before the vegetated area and the rapid conveyance effect in the vegetated area, whereas constant values were observed on bare slopes. The performance of hydrodynamics parameters suggested that the dense rearward arrangement (SMD) was the optimal vegetation pattern to regulate flow conditions. Specifically, the vertical profiles of the velocity and turbulence features of the SMD arrangement at different sections demonstrated the significant role of vegetation density in identifying the velocity layers along the water depth. The maximum velocity and Reynolds Stress Number (RSN) indicated the position where local scour was most likely to occur, which would improve our basic understanding of the mechanisms underlying hydraulic and soil erosion processes.Thus, determining the optimum conditions for flow within vegetated slopes is vital for water and soil conservation and control.To understand the mechanisms underlying the hydraulic and soil erosion processes on vegetated hillslopes, hydrodynamic characteristics, which were mainly quantified by hydraulic parameters, e.g., water depth, mean velocity, flow pattern and resistance, kinetic energy and momentum distributions were considered as fundamental variables [6,7]. The hydrodynamic effects of emergent vegetation have been widely investigated in controlled laboratory experiments. In previous studies, well-defined rigid cylinder rods were assumed to represent an appropriate model for groups and trees or reeds and to generate accurate representations of the geometric characteristics [8]; thus, this method has been utilized as vegetation stems in numerous laboratory studies [7]. Generally, vegetation promotes the water depth [9], slows down the flow [10,11], enhances flow resistance [12], raises energy and momentum coefficients [13], attenuates the turbulence intensity [14], and promotes diffusion and deposition processes [15]. Dunkerley et al. [16] pointed out that plants could stabilize the flow state as laminar flow, whereas Hamilton et al. [17] stated that mixed flow states occurred on vegetated slopes. The definition of t...

Effect of rigid, bank vegetation on velocity distribution and water surface profile in open channel

“…Simulation of rigid vegetation is commone.g., in Stone & Shen 2002;James et al 2004;Meftah et al 2006;Kothyari et al 2009;Cheng & Nguyen 2011;Panigrahi 2015;Ahmed & Hady 2017;Chakraborty & Sarkar 2018;Wang et al 2018;and Tong et al 2019. (See Table 2).…”

Research on the Impact of Using a Combination of Rigid and Flexible Vegetation on Slope Hydrological Properties in Loess Regions

“…Rigid vegetation and flexible vegetation showed significant differences in their influence on the hydraulic characteristics of slope flow. First, for rigid vegetation, the resistance to slope flow was greater because of its structural stability, especially in the flow rate, Reynolds number, and water depth increase; the total energy in the longitudinal position compared to bare slopes showed a fluctuating downward trend [24], indicating a stronger buffer capacity, which is conducive to reducing slope erosion and enhancing soil and water conservation. Moreover, rigid vegetation stem diameter exhibited significant resistance to surface runoff [22], and the resistance increased with increasing stem diameter [23].…”

“…For example, the density of rigid vegetation is one of the key factors affecting the flow regime of slopes [20], and it directly determines the roughness of the slope surface: with the increase in vegetation cover, the slope velocity is correspondingly reduced, which decreases the water flow rate [21]. The stem diameter of rigid vegetation also exerts a significant drag effect on surface runoff [22], and the Darcy-Weisbach drag coefficient and vegetation stem diameter are proportional to the vegetation stem diameter, i.e., the larger the vegetation stem diameter, the greater the resistance to runoff [23], and rigid vegetation configured behind slopes is considered the best way to alter water flow conditions and reduce soil erosion [24]. When vegetation was submerged, the coefficient of water flow resistance was unusually sensitive to the change in flow regime [25], indicating that the presence of vegetation not only changed the physical characteristics of the slope but also significantly affected the kinetic characteristics of water flow.…”

Experimental study on the effect of simulated grass and stem coverage on resistance coefficient of overland flow

“…Furthermore, Zhang et al (2017) found that the relationship between f and Re was not constant, and there was a critical coverage that made them appear differently correlated. The above research shows that the f–Re relation is affected by vegetation types and underlying surface conditions, which indicates that Re is not the only factor to predict the resistance coefficient (Wang et al, 2018). In fact, f is also affected by vegetation coverage (Li et al, 2013; Shang et al, 2020), stem diameter (Zhang et al, 2018; Zhao et al, 2015), slope gradient (Zhang, Liu, et al, 2019), and water depth (Baptist et al, 2010).…”