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

Abstract

Vegetation plays a significant role in preventing desertification, conservation of soil and water. However, nowadays, there is still uncertainty regarding the mechanisms of flow resistance caused by dissimilar vegetation covers. To address this gap, a series of these was conducted in this study to investigate the influence of synthetic grass and stems on the variation of resistance on overland flows. Thirty vegetation configurations were selected (combining five synthetic grass coverage options and six synthetic stem coverage ones), and tested against five unit discharges (flow range = 0.28–2.22 L m−1 s−1) and four slope gradients (3.49%–20.78%). The results obtained showed that the unit discharge is the key driving factor for the transition from laminar to transitional flow. Furthermore, the relationship between the resistance coefficient (f) and the Reynolds number (Re) was not monotonically increasing or decreasing, but behaviours observed were specifically linked to each vegetation coverage. However, a critical coverage threshold was identified, and it corresponded to 2.72% when the slope gradient tested was 3.49%. This threshold decreased with the increase of the slope gradient. In addition, when the vegetation coverage was less than the critical threshold, the f was negatively correlated with Re, otherwise if the vegetation coverage was higher than the critical threshold identified the f–Re relation was positively correlated. The total flow resistance under synthetic grass and stem cover was unequal to the linear superposition of grain resistance and form resistance caused by synthetic stem and grass, which meant that the linear superposition approach is not applicable to overland flows. Finally, a model was developed to predict the flow resistance by applying the π‐theorem and the multiple nonlinear regression analysis and it has been validated against the experimental results confirming its accuracy and high performance (adj.R2 = 0.99, NSE = 0.94).

Estimating response times, flow velocities and roughness coefficients of Canadian Prairie basins

“…This may be why the equation of Langridge et al (2021), which includes representations of the climate and the peak discharge, performed relatively well in this study. Cen et al (2022) found that unit discharge was the most important factor in determining the transition from laminar to transitional flows, in flume experiments using synthetic vegetation.…”

Rigid vegetation affects slope flow velocity

“…Vegetation characteristics such as shape, stiffness, degree of submergence, and arrangement, in uence the hydrodynamic properties of slope ow [20]. Cen et al [21] studied the hydrodynamic properties of slope ow under combinations of exible and rigid vegetation and found that different types of vegetation can differentially affect ow resistance. Serio et al [20] also studied the hydraulic characteristics of owing rivers under the in uence of rigid and exible vegetation and found that the density and rigidity of vegetation affected the spatiotemporal distribution of the average ow velocity to different degrees.…”

“…Teng et al [23] reported that the distribution of ow velocity under different arrangements of rigid vegetation was lower close to the channel bottom and in the inner layers of the vegetation, with ow velocities along the water depth displaying S-shaped and inverse S-shaped pro les. Most simulation studies on vegetation set the growth direction of vegetation perpendicular to the slope (BS) [21,24,25]; however, in nature, vegetation can also grow perpendicular to the horizontal plane (BH). As there is little research that focuses on the effects of vegetation growth direction on average ow velocity, the impacts of these two types of vegetation on slope ow velocity must be measured independently.…”