Quantification of the effects on the flow velocity caused by gramineous plants in the loess plateau in North-Western China

Accurate prediction of the mean velocity of overland flow is the premise and foundation for establishing a soil erosion model, but it is difficult to accurately estimate the mean flow velocity with the presence of vegetation. To explore the variation law of the mean velocity of overland flow under the influence of gramineous plants typical in the Loess Plateau in North-Western China, indoor scouring tests with ten levels of vegetation coverage (9.42 %–94.25 %), seven unit discharges (0.278–1.667 L·m−1·s−1), and five slope gradients (4°–12°) were performed. The results showed that the mean flow velocity initially increased and then decreased with an increase in vegetation coverage, and the critical cover was affected by the unit discharge. For a slope of 4°, the mean flow velocity with a vegetation coverage of 94.25 % was only 21.6 %–32.0 % of that on a bare slope, indicating that vegetation can effectively reduce flow velocity. For each experiment conducted, with an increase in vegetation coverage, the overland flow gradually moved from laminar flow to transitional flow. Based on the principle of equivalent roughness and Manning's equation, a prediction model was also established in order to predict more accurately the mean velocity associated with overland flow, and it has been validated against the experimental results demonstrating a satisfactory agreement with the measured values (adj.R2 = 0.879, NSE = 0.867). These results provide further insights regarding the influence that the vegetation can have on the flow velocity and contribute to develop a better management of these environmental areas

Explicit Solution for Critical Depth in Closed Conduits Flowing Partly Full

Critical depth is an essential parameter for the design, operation, and maintenance of conduits. Circular, arched, and egg-shaped sections are often used in non-pressure conduits in hydraulic engineering, irrigation, and sewerage works. However, equations governing the critical depth in various sections are complicated implicit transcendental equations. The function model is established for the geometric features of multiple sections using the mathematical transform method and while considering non-dimensional parameters. Then, revised PSO algorithms are implemented in MATLAB, and the right solution’s formula for the critical depths in various non-pressure conduit sections is established through optimization. The error analysis results show that the established formula has broad applicability. The maximum relative errors of the formula for critical depths are less than 0.182%, 0.0629%, and 0.170% in circular, arched, and egg-shaped sections, respectively, which are more accurate than those of existing formulas; the form of the formula proposed in this work is also more compact than that of the existing formulas. The results of this research may be useful in design, operation, and maintenance in conduit engineering

Effect of gravel coverage on the hydrodynamic characteristics of overland flow on the Loess Plateau in China

In the Loess Plateau of China, the presence of gravel mulching is widespread, and investigating the dynamic changes in hydrodynamic properties induced by gravel coverage is crucial for optimizing soil and water conservation in this region. In this study, the effect of gravel coverage on the hydrodynamic parameters characterizing the overland flow was investigated through indoor artificially simulated scouring experiments, considering ten different gravel coverages, four slope gradients, and nine flow discharges. The results demonstrated significant differences in hydrodynamic parameters under various experimental conditions (P<0.01). The flow depth exhibited a linear increase with increasing gravel coverage. Compared to flow velocity observed on non-gravel-covered slope, the reduction percentage of flow velocity ranged from 13.62% to 72.4% on gravel-covered slopes. Furthermore, a prediction model was developed to quantify the impact of gravel coverage on the mean flow velocity of overland flow. The model achieved a high R2 of 0.858 and a low RME of 13.61%. The experiments revealed that the overland flow was distributed in the “virtual laminar”-subcritical and transitional-supercritical zones, with mutual restrictions between the influence of gravel coverage and slope gradient on the flow regime. Finally, it was observed that the Darcy-Weisbach resistance coefficient increased with an increase in gravel coverage. These findings provide a solid theoretical basis for optimizing soil erosion prediction models based on the hydrodynamic characteristics of overland flow, and offer guidance for the rational allocation of soil and water conservation measures for gravel-covered slopes

A calculation model of the mean flow velocity of overland flow considering a variety of grass covers and raindrop’s characteristics

The effect of vegetation distribution patterns, coverage, and raindrop impact on the overland flow velocity is highly intricate. To quantify these effects, a rigorous experimental campaign was conducted involving five rainfall intensities (ranging between 60 and 120 mm h−1), six vegetation patterns (diamond pattern - DP, random pattern - RP, checkerboard pattern - CP, vertical strip pattern aligned with the slope direction - VP, step strip pattern - SP, banded pattern perpendicular to the slope direction - BP), five vegetation coverage (ranging between 30% and 70%) and three slope gradients (ranging between 8.72% and 25.88%). The results obtained show that the BP configuration has the best flow velocity reduction effect, which can lessen the flow velocity by 58.68% - 69.27% compared with the bare slope, while the change for VP is only 4.80% - 6.30%. This indicates that BP yields significant soil and water conservation benefits. Furthermore, when the vegetation coverage is 30%, a concentrated flow formed between the vegetation patches under the RP and CP configurations, resulting in a higher overland flow velocity greater than the one recorded for the bare slope, which is unfavorable for soil and water conservation and should be avoided. Finally, a model was established to predict flow velocity and it was built by combining the equations of momentum and mechanical balance. After having calibrated the model and assessed its performance against research data available in literature, it was possible to confirm its reliability and consistency. These findings provide scientific guidance for assessing the soil and water conservation effectiveness of different vegetation patterns

Overland-Flow Resistance Characteristics of Nonsubmerged Vegetation

In order to discover the resistance characteristics of overland flow with vegetation cover, indoor drainage experiments with six slopes, seven flow discharges, and five degrees of vegetation cover are systematically studied based on the basic theories of hydrodynamics and hydraulics. The experimental results show that the overland flow index of nonsubmerged vegetation gradually increased from 0.333 to 0.845 as the degree of vegetation cover increased. Moreover, the relationship between the flow resistance, water depth, and Reynolds number did not present a monotonic increasing or decreasing tendency, but depended on the vegetation cover. When the degree of vegetation cover was lower than the critical value, the flow resistance coefficient decreased with increasing water depth and Reynolds number, whereas the opposite tendency was found when the degree of vegetation cover was greater than the critical value. The resistance coefficient gradually increased with increasing vegetation cover when the flow discharge was constant. Moreover, the increasing rate became steeper as flow discharge increased. The prediction formulae of average velocity and the resistance coefficient are proposed, and predicted results agree with corresponding experimental data. This conclusion is of scientific significance both in understanding the mechanisms of flow retardation and sand fixation by vegetation as well as in guiding the implementation of ecological restoration projects. (C) 2017 American Society of Civil Engineers