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The shoal area of the lower Yellow River in China is not flooded with water during the dry season, so various plants can grow. When floods overflow the plains in the flood season, the complexity of water resistance is increased due to the resistance to water flow by vegetation, which directly affects flood discharge in the beach area. The drag force coefficient (CD), Manning's roughness coefficient (n), and Darcy-Weisbach resistance coefficient (f) are commonly used to characterize vegetation drag force. Such studies are commonly conducted in clear water, but flood water in the lower Yellow River is generally muddy. In order to study the effect of the same sediment content and different sedimentation thicknesses on the resistance of muddy waters containing vegetation, this study conducted experiments in a flume (length = 28 m, width = 0.5 m, and height = 0.5 m) under different deposition thicknesses. The results showed that the vegetation drag force coefficient (CD), vegetation roughness (nb), and Darcy-Weisbach drag coefficient (f) all decreased logarithmically with increasing Reynolds number (Re) and Froude number (Fr). When Re > 30,000, under the conditions of different siltation thicknesses of vegetation, the vegetation roughness tended to stabilize near its minimum value. When the Reynolds number of the water flow is large (Re > 20,000), the variation of the Darcy-Weisbach drag coefficient f slows down with the Reynolds number Re. Logarithmic functions were established for the above resistance coefficients and flow coefficients, and the corresponding correlation coefficients were high, indicating that the conclusions were reliable. [ABSTRACT FROM AUTHOR]

Planting performance is essential for wheat seeders and reliable seed germination. The seed-filling effect is also important in the sowing performance. In this study, a self-suction wheat shooting device was developed to improve the seed-filling performance. In specific, the shooting device structure was optimized, in which negative pressure was generated in the inner cavity of the device. A DEM-CFD coupling simulation experiment was conducted to investigate the seed-filling and shooting performance. The effect of three working parameters (rotational speed of shooting device, window length and window width of directional seed delivery wheel) on the coefficient of variation of the seed-filling performance (CVSF), average shooting speed (ASS), average shooting depth (ASD) and drag force (DF) were analyzed. Afterward, a validation experiment was conducted to investigate the shooting performance of the self-suction shooting device. The results showed that the shooting performance of the self-suction device under optimized operation parameters could fully meet the shooting requirements of winter wheat in the North China plain. In specific, the coefficient of variation of the seed amount (CVSA), ASD and ASS of self-suction shooting device were 7.1%, 30.9 mm and 33.5 m/s at the rotational speed, window length and window width were 1200 rpm, 35 mm and 60 mm, respectively. [Display omitted] • A "seed–air-soil" coupling model is simulated by DEM–CFD approach. • The seed motion characteristics and shooting performance are described. • The characteristic of drag force in self-suction device is conducted. • A comparison of self-suction and mechanical shooting device is conducted. [ABSTRACT FROM AUTHOR]

Use of mangrove ecosystems for coastal flood protection requires reliable predictions of mangrove wave attenuation, especially if this capacity lessens due to storm‐induced forest damage. Quantifying and understanding the variation in drag forces and mechanical properties of mangrove vegetation can improve assessment of mangrove protective capacity. We studied five mangrove species common in the subtropical Pearl River Delta, south China. The studied species range from typically landward‐occurring to more seaward‐occurring pioneer species. We sampled across seven sites in the delta to study the impact of salinity on mechanical properties. We quantified strength and flexibility of branches (branch strength and flexibility related to branch diameter, modulus of rupture and modulus of elasticity), leaf strength (leaf attachment strength related to leaf size, and leaf mass per area) and drag properties (drag force related to surface area and drag coefficient). For all tested species, larger branch diameters resulted in higher mechanical strength. Larger leaf size resulted in larger peak pulling forces and larger branch surface area resulted in stronger drag forces. Notably, species that generally occur lower in the intertidal zone, where exposure to wind and waves is higher, had relatively stronger branches but more easily detachable leaves. This may be regarded as a damage‐avoiding strategy. Across the seven field sites, we found no clear effect of salinity on mangrove mechanical properties. This study provides a mechanistic insight in the storm damage process for individual mangrove trees and a solid base for modeling storm (surge) damage at the forest scale. [ABSTRACT FROM AUTHOR]

This study presents an investigation to quantify the aerodynamics of truss bridge-girders via wind tunnel test and kriging surrogate model. Currently, the conventional methods documented in design specifications only take into consideration the mean drag force at null attack angle. To gain an in-depth understanding on the aerodynamics of truss bridge-girders, experiments on simplified bridge-girder models with various geometric parameters were carried out in uniform flow. A total of 15 truss bridge-girder models with aspect ratio (the ratio of width to height) B/D = 1.0, 1.3, 1.6, 1.9, and 2.2, solidity ratio (the ratio of projected to envelope areas) Φ = 0.20, 0.25, 0.30, 0.35, and 0.40, and two typical truss topologies (Warren and Pratt trusses) were examined in the most concerned range of wind angle of attack α = [–6°, 6°]. These truss bridge-girder models cover most of the high-speed railway bridges widely used in China. Experimental results show that the truss topology has limited effects on the aerodynamics of truss bridge-girders, whereas the effects of α, B/D, and Φ are significant. Based on these wind tunnel results, the ordinary kriging surrogate model was utilized to approximate the aerodynamics of truss bridge-girders. In using this model, aerodynamic force values for test cases can be interpolated with zero variance and uncertainties in unsampled design zones where geometric parameters can be quantified with Gaussian variance. [ABSTRACT FROM AUTHOR]

In this study, an explicit depth-averaged 2D flow model that considers vegetation effects was established to investigate the interactions between tidal currents and vegetation in coastal wetland waters. Roe's approximate Riemann solver, coupled with the drying-wetting boundary technique, was proposed to evaluate the interface fluxes and track the moving shoreline. In addition, the vegetation-induced drag force was added to the momentum equations as an internal source. Model validation based on two laboratory experiments demonstrated that the model results were in good agreement with the measured results. The model was then applied to evaluate the interactions between vegetation and tidal currents in the Liao River Estuary, China. The simulation results showed that vegetation played a critical role in attenuating flow velocities in vegetated waters. In the Liao River wetland, the reductions in the amplitudes of the peak depth-averaged velocities in Phragmites austral (P. australis) wetlands were significantly higher than those of the Suaeda heteroptera (S. heteroptera) wetlands. Moreover, the flow velocity changes in the wetland waters also impacted the kinetic energy, creating a new flow field structure. The simulation results also indicated that higher vegetation densities resulted in a larger flow velocity attenuation rate in the vegetated areas. [ABSTRACT FROM AUTHOR]

Tillage on hillslopes may not only induce severe soil erosion, but may also cause bedrock erosion under certain conditions. Yet, little is known about bedrock erosion by tillage in a hilly agricultural landscape, southwest China. The aim of this study is to quantify the translocation of rock fragments derived from bedrock fragmentation by hoeing under different conditions, including slope gradient, hoeing depth and soil‐covered thickness using a gravel tracing method. The reliability of the gravel tracing method was confirmed by the bedrock dyeing tracing method. Hoeing depth is a significant factor affecting the translocation rate of rock fragments (Qr). Meanwhile, under the condition of overlying soil layers (0.06−0.10 m thick), the values of Qr were significantly smaller with a reduction of 20.7−25.6%, compared with rock fragmentation by hoeing for bare bedrock. However, slope gradient was found to have insignificant effects on Qr. Fractured bedrock moved as individual small fragments, which was mainly controlled by the hitting force of the hoe, while soil moved in the shape of lumps, which was dominated by both drag force of the hoe and gravity. This study suggests that hoeing into soil‐covered bedrock can diminish bedrock erosion while providing soil matrix for shallow soil layers. Our work presents a quantitative assessment of bedrock erosion by hoeing and an underlying insight into characteristics of bedrock erosion by tillage operations in hilly agricultural regions with mudstone and shale, southwest China. © 2020 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Abstract In an urban area, the dominated effect of buildings on wind environment is significantly influenced by the non-uniform buildings. In this work, the surface wind pressure and total drag force measurement on non-uniform buildings were performed, utilizing wind tunnel experiments with the aim to improve the understanding and evaluation accuracy about flow over buildings in the urban area. Buildings models were designed with four morphological parameters which are planar area density, frontal area density, building shape and buildings layout. Two pressure measurement schemes: measurements on multiple pressure modules roughly aligned and on three pressure modules roughly scattered in a line along the wind direction were compared to preliminarily explore the estimation method of total drag force from surface pressure measurements. The effect of reference velocity and non-uniform characteristics on the drag were also investigated. Experimental results indicate that the form drag dominates the total drag force of the non-uniform buildings in an urban area under fully developed turbulence. The critical friction Reynolds number (Re ∗, based on 0.1 H as roughness height) for keeping Reynolds number independent is around 650. Results also imply the measurement scheme on three pressure modules can depict the general drag distribution along the wind direction approximately; the shape and the volume of the most windward building determine the magnitude of its local drag force while the density and the layout determine the drag coefficient distribution of buildings. • Distributions of local drag coefficients for non-uniform buildings. • Measuring surface pressure and total drag on buildings via wind tunnel experiment. • Describing the critical friction Reynolds number of buildings for R e independency. • Three pressure modules measurement can roughly depict general drag distribution. [ABSTRACT FROM AUTHOR]

To investigate the effect of drag force on stability of residual soil slopes, a modified approach based on the limit equilibrium method is proposed by establishing a nonlinear coupled mathematical model to calculate the seepage velocities in soils and the runoff velocities at a fluid-solid interface. In this mathematical model, the Navier-Stokes equation and Brinkman-extended Darcy equation are used to describe the incompressible surface runoff and seepage in soils, respectively. Then, the drag force can be derived by adopting Newton's law of internal friction, and the factor of safety (FOS) of residual soil slopes considering drag force can be calculated based on the limit equilibrium method. The modified approach was applied to analyse the stability of rainfall-induced landslides in residual soils in Nanjiang, Sichuan, China, 2011. We found that the runoff depth can significantly control the FOS of the residual soil slopes and cause slope failures. The results of sensitivity analysis highlight the significance for considering drag force due to surface runoff when calculating the FOS of the residual soil slope. The FOS obtained from the modified approach is more reasonable, and this modified approach also can be used for evaluating or predicting rainfall-induced slope instability in residual soils. [ABSTRACT FROM AUTHOR]

Frequently, large-scale typhoon-induced casualties and financial losses in global mega-delta cities can be buffered to a large extent by the presence of mangrove wetlands occurring in tropical coastal zones. However, there is little information about how mangrove wetland dynamics induce wave energy damping. Here, a series of biomorphodynamic field data was acquired during a storm period induced by Typhoon Sinlaku over a mangrove wetland in the Nanliu Delta, the largest delta of the northern Beibu Gulf in China; these data revealed the wave attenuation process of native Aegiceras corniculatum (AC). The results indicated that the wave damping coefficient of AC was higher during the storm period than during normal weather conditions. Sapling and adult AC were densely distributed in the field and had a greater effect on reducing wave height than seedlings, which were sparsely distributed, even though seedlings might have a stronger damping ability. Moreover, wave height was linearly attenuated with landward wave propagation distance along a transect of AC plots of different ages. Our work further indicated that the slopes and intercepts of the linear fits between wave height and landward wave propagation distance under storm and normal conditions were closely related to incident wave height, the water level and submerged vegetation volume. These results highlight the role of AC trees of different ages and densities under distinct weather conditions in the wave attenuation process. • Wave damping was determined based on mangrove structures and water level. • Mangroves of different ages produced distinctive drag forces. • Mangrove band design for wave attenuation depends on local ecohydrodynamic conditions. [ABSTRACT FROM AUTHOR]

• Aerodynamic characteristics of five surfaces undergoing wind erosion differ. • Pulsation intensity, turbulence intensity, and drag coefficient increase with z 0. • Consumption of roughness elements on wind force affects the surface stability. • τ R /τ increases with increasing wind speed for flexible roughness elements but decreases for rigid roughness elements. Aerodynamic characteristics is a crucial factor influencing soil wind erosion, which is closely related to the properties of surface roughness elements. In the expansive arid and semi-arid northern China, grassland, farmland, mobile sandy land, and gobi are the main land surface types that suffer wind erosion to varying degrees. To investigate the aerodynamic characteristics of different surfaces, we used three-dimensional ultrasonic anemometers to observe the near-surface wind speed above grassland, farmland, mobile sand, and two types of gobi surfaces in areas of northern China (a total of five observation sites). We compared the aerodynamic characteristics of the five surfaces and their causes (i.e., the surface roughness length, wind speed pulsation, turbulence intensity, and drag coefficient), and analyzed how surface roughness elements weakened the airflow. The roughness length was greatest for grassland, followed by farmland, sand, fine gobi, and black gobi. The pulsation intensity of near-surface wind speed increased linearly with increasing wind speed at all sites. At a given wind speed, increasing roughness length increased the pulsation intensity of wind speed. The probability distributions for the drag coefficient and turbulence intensity of the five surfaces followed a positively skewed distribution. The drag coefficient and turbulence intensity were greatest for grassland, followed by farmland, sand, fine gobi, and black gobi surfaces. The drag force exerted on the roughness elements increased with increasing wind speed, but the ratio of drag force exerted on the roughness elements to the total drag force (τ R / τ) for flexible roughness elements (i.e., plants) increased as the wind speed increased; for rigid roughness elements, τ R / τ decreased with increasing wind speed. This confirms that the shelter capability of flexible roughness elements against soil wind erosion increases with the increase of wind speed, while that of rigid roughness elements is opposite. [ABSTRACT FROM AUTHOR]

Hydrological connectivity is an important characteristic of wetlands that maintains the stability and functions of an ecosystem. This study investigates the temporal variations of hydrological connectivity and their driving mechanism in Baiyangdian Lake, a large shallow wetland in North China, using a time series of open water surface area data derived from 36 Landsat 8 multispectral images from 2013–2019 and in situ measured water level data. Water area classification was implemented using the Google Earth Engine. Six commonly used indexes for extracting water surface data from satellite images were compared and the best performing index was selected for the water classification. A composite hydrological connectivity index computed from open water area data derived from Landsat 8 images was developed based on several landscape pattern indices and applied to Baiyangdian Lake. The results show that, reflectance in the near-infrared band is the most accurate index for water classification with >98% overall accuracy because of its sensitivity to different land cover types. The slopes of the best-fit linear relationships between the computed hydrological connectivity and observed water level show high variability between years. In most years, hydrological connectivity generally increases when water levels increase, with an average R2 of 0.88. The spatial distribution of emergent plants also varies year to year owing to interannual variations of the climate and hydrological regime. This presents a possible explanation for the variations in the annual relationship between hydrological connectivity and water level. For a given water level, the hydrological connectivity is generally higher in spring than summer and autumn. This can be explained by the fact that the drag force exerted by emergent plants, which reduces water flow, is smaller than that for summer and autumn owing to seasonal variations in the phenological characteristics of emergent plants. Our study reveals that both interannual and seasonal variations in the hydrological connectivity of Baiyangdian Lake are related to the growth of emergent plants, which occupy a large portion of the lake area. Proper vegetation management may therefore improve hydrological connectivity in this wetland. [ABSTRACT FROM AUTHOR]

• Field experiments were conducted in a floodplain dominated by unidirectional current. • Flow structures influenced by vegetation varied with submergence conditions of vegetation. • Both the mean velocity and turbulent kinetic energy within vegetation were reduced. • Vegetation enhanced sediment deposition and decreased its concentration over water depth. To understand the influence of aquatic vegetation on flow structures and sediment deposition, field experiments were conducted in a floodplain with water movement dominated by unidirectional current. Flow velocity, suspended sediment concentration, and sediment deposition were measured within and without vegetation at four sites covered by Phragmites communis (reed), Artemisia selengensis , Carex cinerascens , and a mix of them. Results show that the presence of vegetation reduced the time-averaged velocity and turbulent kinetic energy (TKE) and altered vertical distributions of the time-averaged velocity and TKE. For emergent conditions, both the time-averaged velocity and the TKE were uniformly distributed through the entire water column. Two layers were separated at the top of vegetation for submerged conditions. Within vegetation both the time-averaged velocity and TKE were decreased and uniformly distributed, similar to observations under emergent conditions. The discontinuity of drag force at the top of vegetation generated a region with rapidly increased velocity near the vegetation interface. TKE reached its maximum near the top of vegetation and decreased gradually toward both bed bottom and water surface. In present study, canopy with larger frontal area caused bigger reduction of time-averaged velocity and TKE within vegetation. By reducing the mean and turbulent velocities, the presence of vegetation decreased the depth-averaged suspended sediment concentration by 30–50% and increased the thickness of sediment deposited over one flood season by as high as 190% when compared with bare bed. [ABSTRACT FROM AUTHOR]

A submerged tidal energy device with contra-rotating Diffuser Augmented Tidal Turbines (DATTs) has been investigated in this paper. The device is moored to the seabed with a single mooring line, which limit it to operate at mean water depth, but otherwise allows it to float freely with the tidal current, like a kite in the wind, to harness tidal current energy. This research focuses on the evaluation of stability and power generation of the submerged tidal energy device based on 1:5th scaled model tests and the full-scaled prototype sea trials. A 1:5th scaled model had been manufactured and tested in a circulating water channel to observe the power generation performance and working attitude around the designed inflow velocity. A full-scaled prototype was manufactured and tested near the CHU Island in Shandong Province, China. The results show that the device can change its direction automatically to make the DATTs face the tidal inflow, as the tidal current changes direction. The device has a good stability in pitch and roll motions. But the device's stability in yaw motions is worse than the other two, which will significantly affect the power generation performance and introduce more demanding structural requirements. • A submerged tidal energy system with contra-rotating DATTs is presented in this paper. • The connection point position is determined by the drag forces of the DATTs and the buoyancy body. • From results of model tests, the device has a relatively good stability to support the contra-rotating DATTs working. • The full-scale device sea trials seem to prove the feasibility and reliability of the system. • From results of full-scaled prototype sea trials, the full-scaled device has good stability in pitch and roll motions. • In yaw motions, the full-scaled device can orient itself to face the tidal current direction. [ABSTRACT FROM AUTHOR]