Rainfall and runoff energy results in soil erosion. This paper presents new the concepts of rainfall and runoff energy and analyzes the relationship of rainfall energy and runoff energy with sediment transport based o...Rainfall and runoff energy results in soil erosion. This paper presents new the concepts of rainfall and runoff energy and analyzes the relationship of rainfall energy and runoff energy with sediment transport based on the conversion theory of kinetic and potential energy using artificial rainfall and mechanical calculation. The results show that the ratio of sediment detachment in sloping fallow overland flow increases with the slope gradient,rainfall energy and runoff energy, while the sediment detachment ratio under raindrop impact are significantly higher than those under no raindrop impact. The sediment concentration increases with the slope gradient and rainfall energy; when the slope gradient and rainfall energy are constant, the sediment concentration decreases as the runoff energy increases. Rainfall disturbance coefficients have a logarithmic correlation with the rate of rainfall energy and runoff energy. On the same slope gradient,when the rainfall energy is constant, the disturbance coefficient decreases as the runoff energy increases,while when the runoff energy is constant, the disturbance coefficient increases as the rainfall energyincreases. Rainfall energy results in sediment detachment, and runoff energy is the transportation for erosion sediment. This showed that rainfall energy and runoff energy are important in the sediment detachment and transportation of shallow overland flow.展开更多
Roughness elements are various in a mountain area; they include gravel and ground surface vegetation that often result in surface friction drag to resist overland flows. The variation and characteristics of flow resis...Roughness elements are various in a mountain area; they include gravel and ground surface vegetation that often result in surface friction drag to resist overland flows. The variation and characteristics of flow resistance strongly impact the overland flow process and watershed floods. In view of the universal existence of natural vegetation, such as Chlorophytum malayense(CM) or Ophiopogon bodinieri(OB), and the sand-gravel bed of the river channel, it is important to understand the role of different types of roughness elements in flow resistance. This study was performed to investigate and compare through flume experiments the behaviors of overland flow resistance by the reaction of multi-scale configuration of different roughness elements. The result showed that the resistance coefficient gradually reduced versus the increase of flow rate in unit width and tended to be a constant when q = 3.0 l/s.m, Fr = 1.0, and Re = 4000 for slopes of 6 to 10 degrees. The gap of the vegetated rough bed and the gravel rough bed is limited to the same as the gap of the two types of vegetation, CM and OB. It was noted that the vegetation contributed to the increase in form resistance negatively and may lead to the mean resistance on decrease. To classify the flow pattern, the laminar flows were described by DarcyWeisbach's equation. In the study the f-Re equation of vegetated bed was developed with f ?5000 Re.The friction coefficient for laminar flows can be regarded as the critical value for identifying the transformation point of the flow pattern.展开更多
The breakdown of soil aggregates under rainfall and their abrasion in overland flow are important processes in water erosion due to the production of more fine and transportable particles and,the subsequent significan...The breakdown of soil aggregates under rainfall and their abrasion in overland flow are important processes in water erosion due to the production of more fine and transportable particles and,the subsequent significant effect on the erosion intensity.Currently,little is known about the effects of sediment load on the soil aggregate abrasion and the relationship of this abrasion with some related hydraulic parameters.Here,the potential effects of sediment load on soil aggregate abrasion and hydraulic parameters in overland flow were investigated through a series of experiments in a 3.8-m-long hydraulic flume at the slope gradients of 8.7 and 26.8%,unit flow discharges from 2×10^-3 to 6×10^-3 m^2 s^-1,and the sediment concentration from 0 to 110 kg m-3.All the aggregates from Ultisols developed Quaternary red clay,Central China.The results indicated that discharge had the most significant(P<0.01)effect on the aggregates abrasion with the contributions of 58.76 and 60.34%,followed by sediment feed rate,with contributions of 39.66 and 34.12%at the slope gradients of 8.7 and 26.8%,respectively.The abrasion degree of aggregates was found to increase as a power function of the sediment concentration.Meanwhile,the flow depth,friction factor,and shear stress increased as a power function along with the increase of sediment concentration at different slope gradients and discharges.Reynolds number was obviously affected by sediment concentration and it decreased as sediment concentration increased.The ratio of the residual weight to the initial weight of soil aggregates(Wr/Wi)was found to increase as the linear function with an increasing flow depth(P=0.008)or Reynolds number(P=0.002)in the sediment-laden flow.The Wr/Wi values followed a power function decrease with increasing friction factor or shear stress in the sediment-laden flow,indicating that friction factor is the best hydraulic parameter for prediction of soil aggregate abrasion under different sediment load conditions.The information regarding the soil aggregate abrasion under various sediment load conditions can facilitate soil process-based erosion modeling.展开更多
Better understanding of the role of vegetation and soil on hydraulic resistance of overland flow requires quantitative partition of their interaction. In this paper, a total of 144 hydraulic flume experiments were car...Better understanding of the role of vegetation and soil on hydraulic resistance of overland flow requires quantitative partition of their interaction. In this paper, a total of 144 hydraulic flume experiments were carried out to investigate the hydraulic characteristics of overland flow. Results show that hydraulic resistance is negatively correlated with Reynolds number on non-simulated vegetated slopes, while positively on vegetated slopes. The law of composite resistance agrees with the dominant resistance, depending on simulated vegetation stem,surface roughness, and discharge. Surface roughness has greater influence on overland flow resistance than vegetation stem when unit discharge is lower than the low-limited critical discharge, while vegetation has a more obvious influence when unit discharge is higher than the upper-limited critical discharge. Combined effects of simulated vegetation and surface roughness are unequal to the sum of the individual effects through t-test, implying the limitation of using linear superposition principle in calculating overland flow resistances under combined effect of roughness elements.展开更多
Pollutant transport in overland flow over surfaces with spatially varying microtopography,roughness,and infiltration was investigated using the diffusion wave equation and transport rate-based equation.The finite volu...Pollutant transport in overland flow over surfaces with spatially varying microtopography,roughness,and infiltration was investigated using the diffusion wave equation and transport rate-based equation.The finite volume method in space and an implicit backward difference scheme in time were employed in the numerical solution of the 2D governing equations.The developed model was first tested against an analytical solution and an experimental study involving overland flow and the associated pollutant transport,subsequently a series of numerical tests were carried out.Non-point source pollution was investigated under spatially varying microtopography,roughness,and infiltration.The simulation results showed that microtopography and roughness were the dominant factors causing significant spatial variations in solute concentration.When the spatially varying microtopography was replaced by a smooth surface,the result was an overestimation of the solute rate at the outlet of the upland.On the other hand,when the spatially varying roughness was replaced by the average roughness and spatially varying infiltration rate by the average infiltration rate,the pollutant discharge at the outlet of the upland was not significantly affected.The numerical results further showed that one cannot ignore the spatial variations of slope and roughness when investigating the local pollutant concentration distribution.展开更多
In this study, we investigated the origin of the overland flow roughness problem and divided the current overland flow roughness research into three types, as follows: the first type of research takes into account the...In this study, we investigated the origin of the overland flow roughness problem and divided the current overland flow roughness research into three types, as follows: the first type of research takes into account the effects of roughness on the volume and velocity of surface runoff, flood peaks, and the scouring capability of flows, but has not addressed the spatial variability of roughness in detail; the second type of research considers that surface roughness varies spatially with different land usage types, land-cover conditions, and different tillage forms, but lacks a quantitative study of the spatial variability; and the third type of research simply deals with the spatial variability of roughness in each grid cell or land type. We present three shortcomings of the current overland flow roughness research, including(1) the neglect of roughness in distributed hydrological models when simulating the overland flow direction and distribution,(2) the lack of consideration of spatial variability of roughness in hydrological models, and(3) the failure to distinguish the roughness formulas in different overland flow regimes. To solve these problems,distributed hydrological model research should focus on four aspects in regard to overland flow: velocity field observations, flow regime mechanisms, a basic roughness theory, and scale problems.展开更多
The runoff calculation scheme in the Weather Research and Forecasting(WRF)model is based on an infiltrationexcess surface runoff scheme,which likely leads to an overestimation of soil moisture and an underestimation o...The runoff calculation scheme in the Weather Research and Forecasting(WRF)model is based on an infiltrationexcess surface runoff scheme,which likely leads to an overestimation of soil moisture and an underestimation of surface runoff when heavy rainfall occurs in areas with complex terrain.To overcome this defect,we considered the overland flow process on the grid scale of the WRF model for the first time by coupling a two-dimensional diffusion wave equation into the WRF Noah land surface model(LSM),called the WRF_Overland Flow(WRF_OLF)model.The new WRF model was then utilized to simulate the extreme rainfall that occurred during 19–22 July 2021 near the city of Zhengzhou in central China,which led to an extreme flood event.The results showed that the new WRF model simulated well the convergence and accumulation of overland flow in low-lying areas,changing the distributions of surface runoff and soil moisture and thereby influencing the exchanges of heat and water vapor between the surface and the atmosphere.The local change in non-adiabatic heating at the surface contributed to a decrease in surface pressure and then affected the development of the weather systems associated with the heavy rainfall event.Relative to a remarkable underestimation of rainfall in the original WRF simulation,the maximum rainfall intensity and the cumulative rainfall in the simulation with the new WRF configuration increased by 54.7%and 49.5%,respectively,bringing them closer to their observations.Concurrently,the new WRF model increased the skill for flood prediction.The results of this study provide new insights into the mechanisms of interaction between the land surface and the atmosphere and their roles in helping to predict heavy rainfall and associated flooding in areas of complex topography.展开更多
An evaluation of the interactions between vegetation,overland and soil erosion can provide valuable insight for the conservation of soil and water.An experiment was conducted to study water infiltration,runoff generat...An evaluation of the interactions between vegetation,overland and soil erosion can provide valuable insight for the conservation of soil and water.An experiment was conducted to study water infiltration,runoff generation process,rate of sediment erosion,and hydrodynamic characteristics of overland flow from a sloping hillside with different draw-off discharges from alfalfa and control plots with 20°slope.The effect of alfalfa on runoff and sediment transport reduction was quantitatively analyzed.Alfalfa was discussed for its ability to reduce the overland flow scouring force or change the runoff movement.Compared to the bare-soil plots,alfalfa plots generated a 1.77 times increase in infiltration rate.Furthermore,the down-slope water infiltration rate for the bare soil plots was higher than in the up-slope,while the opposite was found in the alfalfa plots.In addition,alfalfa had a significant effect on runoff and sediment yield.In comparison to the control,the runoff coefficient and sediment transportation rate decreased by 28.3%and 78.4%in the grass slope,respectively.The runoff generated from the alfalfa and bare-soil plots had similar trends with an initial increase and subsequent leveling to a steady-state rate.The transport of sediment reduced with time as a consequence of the depletion of loose surface materials.The maximum sediment concentration was recorded within the first few minutes of each event.The alfalfa plots had subcritical flow while the baresoil plots had supercritical flow,which indicate that the capability of the alfalfa slope for resisting soil erosion and sediment movement was greater than for bare soil plots.Moreover,the flow resistance coefficient and roughness coefficient for the alfalfa plots were both higher than for the bare-soil plots,which indicate that overland flow in alfalfa plots had retarded and was blocked,and the flow energy along the runoff path had gradually dissipated.Finally,the ability to erode and transport sediment had decreased.展开更多
The hydrodynamic characteristics of the overland flow with a geocell slope are different from those of traditional flows because of its special structure. In this paper, a hydraulic flume with different slope gradient...The hydrodynamic characteristics of the overland flow with a geocell slope are different from those of traditional flows because of its special structure. In this paper, a hydraulic flume with different slope gradients is used to study the hydrodynamic characteristics of the overland flow with geocell. The differences of flow characteristics between the overland flow with the geocell slope and the traditional flows are studied, and the hydrodynamic characteristics are obtained, including the flow pattern, the flow velocity and the hydraulic friction factor for the slope flow with geocell under different flow rates and slope gradients. The results show that there is a positive power function relationship between the rill depth of the slope surface （h） and the drag coefficient of the Darcy Weisbach （f）. There is a positive logarithmic function relationship between the drag coefficient f and the Reynolds number Red, and there is a negative power function relationship between the drag coefficient f and the Froude number Fr.展开更多
The hydrodynamic characteristics of the overland flow on a slope with a three-dimensional Geomat are studied for different rainfall intensities and slope gradients. The rainfall intensity is adjusted in the rainfall s...The hydrodynamic characteristics of the overland flow on a slope with a three-dimensional Geomat are studied for different rainfall intensities and slope gradients. The rainfall intensity is adjusted in the rainfall simulation system. It is shown that the velocity of the overland flow has a strong positive correlation with the slope length and the rainfall intensity, the scour depth decreases with the increase of the slope gradient for a given rainfall intensity, and the scour depth increases with the increase of the rainfall intensity for a given slope gradient, the overland flow starts with a transitional flow on the top and finishes with a turbulent flow on the bottom on the slope with the three-dimensional Geomat for different rainfall intensities and slope gradients, the resistance coefficient and the turbulent flow Reynolds number are in positively related logarithmic functions, the resistance coefficient and the slope gradient are in positively related power functions, and the trend becomes leveled with the increase of the rainfall intensity. This study provides some important theoretical insight for further studies of the hydrodynamic process of the erosion on the slope surface with a three-dimensional Geomat.展开更多
Estimating sediment transport capacity of overland flow is essential to the development of physically based soil erosion models.Correlation analysis indicates that stream power is a dominant factor for sediment transp...Estimating sediment transport capacity of overland flow is essential to the development of physically based soil erosion models.Correlation analysis indicates that stream power is a dominant factor for sediment transport in overland flows and a new sediment transport capacity equation is proposed based on dimensional analysis.The coefficients of the new equation are calibrated using the published laboratory data,and rainfall impact is taken into consideration by adding an empirical factor on the dimensionless critical stream power.The new sediment transport capacity equation is a function of stream power,rainfall impacted critical stream power and slope.The new equation is applied in a one-dimensional soil erosion model to simulate field data of a runoff plot and the simulation results are reliable.展开更多
The evolution of the overland flow velocity along the distance downslope on smooth and granular beds in different cases is investigated by means of the electrolyte tracer via flume experiments. The results demonstrate...The evolution of the overland flow velocity along the distance downslope on smooth and granular beds in different cases is investigated by means of the electrolyte tracer via flume experiments. The results demonstrate that a non-uniform flow regime and a uniform flow regime exist in the development process of the overland flow. Owing to the different attributes of beds＇ roughness, the position of those zones with different flow regimes varies correspondingly：（1） the overland flow on granular beds enters into the uniform regime much sooner, additionally, the roll waves tend to appear because of the presence of the proper flow resistance impa-rted by the roughness （coarse sands）, and large slopes （20o and 25o） which makes the flow velocities and depths to undulate spatially. Furthermore, the flow resistance of the overland flows with different roughness elements, that is the non-sands, the fine sands and the coarse sands, is calculated. A quadratic interpolation method of the third order accuracy is employed in the calculation of the longitu-dinal flow resistance. The results show that it is rational to use the bed slope to approximate the hydraulic energy slope over a rela-tive small roughness （the present roughness）, however on the other hand, if the mean flow velocities and depths rather than the local parameters are used to calculate the flow resistance, a considerable error will be induced within the non-uniform regime of the over-land flows, including the acceleration zone and the roll-wave zone.展开更多
Overland flow resistance plays an important role in predicting hillslope hydrological and erosion pro-cesses on hillslopes.Soil surfaces covered with grass and/or gravel may increase hydraulic resistance to overland f...Overland flow resistance plays an important role in predicting hillslope hydrological and erosion pro-cesses on hillslopes.Soil surfaces covered with grass and/or gravel may increase hydraulic resistance to overland flow,but there is little information on the determination on these resistance components in a composite slopeland.In this study,the smooth and sand-bed surfaces covered with different gravel or/and grass(cover degree¼2.5%,5%,10%,15%,20%)under five slope gradients(3,6,9,12,15)were subjected to inflows to investigate the hydraulic resistance and validate the sum law of resistance components.The results showed that,the overland flow mainly belonged to transition or turbulent and supercritical flow regime with 150<Re<2050 and 0.4<Fr<6.9.As the gravel or grass cover increased,Darcy-Weisbach resistance coefficient(f)significantly increased.The resistance can be well predicted by flow discharge,gravel or grass cover using a power equation.The flow discharge had a greater influence on flow resistance than gravel or grass cover,and the impact of flow discharge weakened with the increasing bed slopes.The total resistance was generally greater than the sum of resistance components,and there existed a positive additional resistance for the composite surfaces.The additional resistance accounted for 37.4%,4.2%and 16.6%to the total resistance for the sand surfaces covered with gravel,grass and both of them,respectively.These results can help to understand the rainfall-runoff processes and improve the design of soil and water conservation measures in the rocky and mountainous areas.展开更多
The hydrodynamic response of overland flow to vegetation coverage on convex slopes remains inadequately quantified despite it is critical for soil erosion control in terrains dominated by such topography.This study sy...The hydrodynamic response of overland flow to vegetation coverage on convex slopes remains inadequately quantified despite it is critical for soil erosion control in terrains dominated by such topography.This study systematically investigated the influence of varying vegetation coverage(0%,1.08%,3.24%,4.69%and 9.81%)on the hydrodynamic characteristics of convex slopes through indoor flume experiments under diverse flow discharges(5.5-13.5 m^(3)/h)and slopes(5°-25°).The results revealed three key hydrodynamic mechanisms:(1)Flow retardation and energy dissipation:Increasing vegetation coverage significantly reduced overland flow velocity and promoted higher flow depth,thereby enhancing water retention and energy dissipation.Both stream power(Ω)and unit stream power(ω)declined by 13.9%-30.1%compared to bare slopes.(2)Flow Regime Transition:Froude number(Fr)decreased with increasing vegetation coverage,promoting the transition from supercritical to subcritical flow.The Reynolds number(Re)consistently exceeded 500,indicating the absence of laminar flow.(3)Modification of flow resistance:Vegetation resistance increased nonlinearly with coverage.Maximum bed shear stress was observed at 4.69%coverage(23.5%higher than bare slopes).However,Manning’s(n)and Darcy-Weisbach(f)coefficients did not correlate clearly with Re,indicating that vegetation coverage and slope type feedback significantly change flow resistance mechanisms.展开更多
The grid drop concept is introduced and used to develop a micromechanism-based methodology for calculating watershed flow concentration. The flow path and distance traveled by a grid drop to the outlet of the watershe...The grid drop concept is introduced and used to develop a micromechanism-based methodology for calculating watershed flow concentration. The flow path and distance traveled by a grid drop to the outlet of the watershed are obtained using a digital elevation model (DEM). Regarding the slope as an uneven carpet through which the grid drop passes, a formula for overland flow velocity differing from Manning's formula for stream flow as welt as Darcy's formula for pore flow is proposed. Compared with the commonly used unit hydrograph and isochronal methods, this new methodology has outstanding advantages in that it considers the influences of the slope velocity field and the heterogeneity of spatial distribution of rainfall on the flow concentration process, and includes only one parameter that needs to be calibrated. This method can also be effectively applied to the prediction of hydrologic processes in un-gauged basins.展开更多
Water chemistry changes when it flows through different pathways.This study aims to characterize the differences of water(including rainwater,overland flow,soil water,groundwater,and stream water)chemistry of five kin...Water chemistry changes when it flows through different pathways.This study aims to characterize the differences of water(including rainwater,overland flow,soil water,groundwater,and stream water)chemistry of five kinds of water in Obagbile Catchment in Southwest Nigeria,determine the changes in water chemistry that occur as the water moves from one pathway to another,and identify the factors responsible for the water chemistry changes.To do these,we collected 50 water samples from 10 heavy storms that received equal to or more than 10 mm of rain within an hour to test the changes of water chemical properties across various pathways in this study.The results show that overland flow had the highest p H and electrical conductivity(EC)and rainwater had the lowest values of the two parameters.Ca^(2+),Mg^(2+),Cl^(ˉ),and HCOO^(ˉ) were found to have their highest concentrations in stream water;meanwhile,NO_(3)^(–),NH_(4)^(+),and SO_(4)^(2–) were found to have almost the same low concentrations in all the water samples.K+was only dominant in stream water;while dissolved organic carbon(DOC)was lowest in rainwater,same in overland flow,soil water,and groundwater samples,and highest in stream flow.Principal component analysis(PCA)showed that for all the water samples from different pathways,two factors mainly accounted for the total variances.The two factors were related to the crustal and anthropogenic sources in rainwater,suggesting that the high loadings of major cations(e.g.,Ca^(2+) and Mg^(2+)) in rainwater samples are soil-derived.The PCA for the overland flow and soil water showed strong correlations among pH,EC,and the concentrations of Na^(+),Mg^(2+),HCOO^(-),and CH_(3)COO^(-),while the high loadings of all the parameters and the strong correlations among each other were evident in the stream water.In conclusion,the chemical constituents found in water are also the components of pathways through which the water flows.The major factors responsible for the change in the chemical properties of water in Obagbile Catchment are weathering and anthropogenic activities.展开更多
Effectively managing floods in urban regions requires effectively designed and well-maintained runoff collection system. The absence of such a system and intense rainfall event will have the potential to disrupt the u...Effectively managing floods in urban regions requires effectively designed and well-maintained runoff collection system. The absence of such a system and intense rainfall event will have the potential to disrupt the urban life and cause significant economic loss to properties. Grated inlets, which are a key component in urban drainage network, are used to capture the runoff. In this work, a three dimensional CFD model was developed based on open-source CFD tool, OpenFOAM®, to model flow over a grated inlet. An incompressible, transient, multiphase flow, Volume of Fluid (VOF) simulation was performed to predict the water flow rate through the grate inlet. The predicted flow rates are compared with the HEC-22 monograph values. The close agreement between the results shows the potential of using CFD modeling approach to test the reliability of existing drainage inlets for different flow scenarios.展开更多
基金supported by the National Natural Science Foundation of China(41571262)the Chinese Ministry of Water Resources Science and Technology Promotion Program(TG1308)
文摘Rainfall and runoff energy results in soil erosion. This paper presents new the concepts of rainfall and runoff energy and analyzes the relationship of rainfall energy and runoff energy with sediment transport based on the conversion theory of kinetic and potential energy using artificial rainfall and mechanical calculation. The results show that the ratio of sediment detachment in sloping fallow overland flow increases with the slope gradient,rainfall energy and runoff energy, while the sediment detachment ratio under raindrop impact are significantly higher than those under no raindrop impact. The sediment concentration increases with the slope gradient and rainfall energy; when the slope gradient and rainfall energy are constant, the sediment concentration decreases as the runoff energy increases. Rainfall disturbance coefficients have a logarithmic correlation with the rate of rainfall energy and runoff energy. On the same slope gradient,when the rainfall energy is constant, the disturbance coefficient decreases as the runoff energy increases,while when the runoff energy is constant, the disturbance coefficient increases as the rainfall energyincreases. Rainfall energy results in sediment detachment, and runoff energy is the transportation for erosion sediment. This showed that rainfall energy and runoff energy are important in the sediment detachment and transportation of shallow overland flow.
基金support from the authorities of the National Natural Science Foundation of China (Grant No. 41171016)Sichuan Province Science and technology support program (Grant No. 2014SZ0163)the Open Foundation of State Key Laboratory of Hydraulics and Mountain River Engineering, Sichuan University (Grant No. SKHL1309 and SKHL1418)
文摘Roughness elements are various in a mountain area; they include gravel and ground surface vegetation that often result in surface friction drag to resist overland flows. The variation and characteristics of flow resistance strongly impact the overland flow process and watershed floods. In view of the universal existence of natural vegetation, such as Chlorophytum malayense(CM) or Ophiopogon bodinieri(OB), and the sand-gravel bed of the river channel, it is important to understand the role of different types of roughness elements in flow resistance. This study was performed to investigate and compare through flume experiments the behaviors of overland flow resistance by the reaction of multi-scale configuration of different roughness elements. The result showed that the resistance coefficient gradually reduced versus the increase of flow rate in unit width and tended to be a constant when q = 3.0 l/s.m, Fr = 1.0, and Re = 4000 for slopes of 6 to 10 degrees. The gap of the vegetated rough bed and the gravel rough bed is limited to the same as the gap of the two types of vegetation, CM and OB. It was noted that the vegetation contributed to the increase in form resistance negatively and may lead to the mean resistance on decrease. To classify the flow pattern, the laminar flows were described by DarcyWeisbach's equation. In the study the f-Re equation of vegetated bed was developed with f ?5000 Re.The friction coefficient for laminar flows can be regarded as the critical value for identifying the transformation point of the flow pattern.
基金financially supported by the National Natural Science Foundation of China(41771304)the National Key Research and Development Program of China(2017YFC0505404)。
文摘The breakdown of soil aggregates under rainfall and their abrasion in overland flow are important processes in water erosion due to the production of more fine and transportable particles and,the subsequent significant effect on the erosion intensity.Currently,little is known about the effects of sediment load on the soil aggregate abrasion and the relationship of this abrasion with some related hydraulic parameters.Here,the potential effects of sediment load on soil aggregate abrasion and hydraulic parameters in overland flow were investigated through a series of experiments in a 3.8-m-long hydraulic flume at the slope gradients of 8.7 and 26.8%,unit flow discharges from 2×10^-3 to 6×10^-3 m^2 s^-1,and the sediment concentration from 0 to 110 kg m-3.All the aggregates from Ultisols developed Quaternary red clay,Central China.The results indicated that discharge had the most significant(P<0.01)effect on the aggregates abrasion with the contributions of 58.76 and 60.34%,followed by sediment feed rate,with contributions of 39.66 and 34.12%at the slope gradients of 8.7 and 26.8%,respectively.The abrasion degree of aggregates was found to increase as a power function of the sediment concentration.Meanwhile,the flow depth,friction factor,and shear stress increased as a power function along with the increase of sediment concentration at different slope gradients and discharges.Reynolds number was obviously affected by sediment concentration and it decreased as sediment concentration increased.The ratio of the residual weight to the initial weight of soil aggregates(Wr/Wi)was found to increase as the linear function with an increasing flow depth(P=0.008)or Reynolds number(P=0.002)in the sediment-laden flow.The Wr/Wi values followed a power function decrease with increasing friction factor or shear stress in the sediment-laden flow,indicating that friction factor is the best hydraulic parameter for prediction of soil aggregate abrasion under different sediment load conditions.The information regarding the soil aggregate abrasion under various sediment load conditions can facilitate soil process-based erosion modeling.
基金supported by the Fundamental Research Funds for the Central Universities (Grant No. 2016ZCQ06)supported by the National Natural Science Foundation of China (Grant No. 51309006)
文摘Better understanding of the role of vegetation and soil on hydraulic resistance of overland flow requires quantitative partition of their interaction. In this paper, a total of 144 hydraulic flume experiments were carried out to investigate the hydraulic characteristics of overland flow. Results show that hydraulic resistance is negatively correlated with Reynolds number on non-simulated vegetated slopes, while positively on vegetated slopes. The law of composite resistance agrees with the dominant resistance, depending on simulated vegetation stem,surface roughness, and discharge. Surface roughness has greater influence on overland flow resistance than vegetation stem when unit discharge is lower than the low-limited critical discharge, while vegetation has a more obvious influence when unit discharge is higher than the upper-limited critical discharge. Combined effects of simulated vegetation and surface roughness are unequal to the sum of the individual effects through t-test, implying the limitation of using linear superposition principle in calculating overland flow resistances under combined effect of roughness elements.
基金Project supported by the National Natural Science Foundation of China (No. 51009120)the Research Fund for the Doctoral Program of Higher Education of China (No. 20090101120065)the State Key Laboratory of Soil Erosion and Dryland Farming on the Loess Plateau of China (No. 10501-243)
文摘Pollutant transport in overland flow over surfaces with spatially varying microtopography,roughness,and infiltration was investigated using the diffusion wave equation and transport rate-based equation.The finite volume method in space and an implicit backward difference scheme in time were employed in the numerical solution of the 2D governing equations.The developed model was first tested against an analytical solution and an experimental study involving overland flow and the associated pollutant transport,subsequently a series of numerical tests were carried out.Non-point source pollution was investigated under spatially varying microtopography,roughness,and infiltration.The simulation results showed that microtopography and roughness were the dominant factors causing significant spatial variations in solute concentration.When the spatially varying microtopography was replaced by a smooth surface,the result was an overestimation of the solute rate at the outlet of the upland.On the other hand,when the spatially varying roughness was replaced by the average roughness and spatially varying infiltration rate by the average infiltration rate,the pollutant discharge at the outlet of the upland was not significantly affected.The numerical results further showed that one cannot ignore the spatial variations of slope and roughness when investigating the local pollutant concentration distribution.
基金supported by the National Natural Science Foundation of China(Grants No.41471025 and 40971021)the Natural Science Foundation of Shandong Province(Grant No.ZR2014DM004)
文摘In this study, we investigated the origin of the overland flow roughness problem and divided the current overland flow roughness research into three types, as follows: the first type of research takes into account the effects of roughness on the volume and velocity of surface runoff, flood peaks, and the scouring capability of flows, but has not addressed the spatial variability of roughness in detail; the second type of research considers that surface roughness varies spatially with different land usage types, land-cover conditions, and different tillage forms, but lacks a quantitative study of the spatial variability; and the third type of research simply deals with the spatial variability of roughness in each grid cell or land type. We present three shortcomings of the current overland flow roughness research, including(1) the neglect of roughness in distributed hydrological models when simulating the overland flow direction and distribution,(2) the lack of consideration of spatial variability of roughness in hydrological models, and(3) the failure to distinguish the roughness formulas in different overland flow regimes. To solve these problems,distributed hydrological model research should focus on four aspects in regard to overland flow: velocity field observations, flow regime mechanisms, a basic roughness theory, and scale problems.
基金Supported by the National Key Research and Development Program of China(2023YFF0805300)。
文摘The runoff calculation scheme in the Weather Research and Forecasting(WRF)model is based on an infiltrationexcess surface runoff scheme,which likely leads to an overestimation of soil moisture and an underestimation of surface runoff when heavy rainfall occurs in areas with complex terrain.To overcome this defect,we considered the overland flow process on the grid scale of the WRF model for the first time by coupling a two-dimensional diffusion wave equation into the WRF Noah land surface model(LSM),called the WRF_Overland Flow(WRF_OLF)model.The new WRF model was then utilized to simulate the extreme rainfall that occurred during 19–22 July 2021 near the city of Zhengzhou in central China,which led to an extreme flood event.The results showed that the new WRF model simulated well the convergence and accumulation of overland flow in low-lying areas,changing the distributions of surface runoff and soil moisture and thereby influencing the exchanges of heat and water vapor between the surface and the atmosphere.The local change in non-adiabatic heating at the surface contributed to a decrease in surface pressure and then affected the development of the weather systems associated with the heavy rainfall event.Relative to a remarkable underestimation of rainfall in the original WRF simulation,the maximum rainfall intensity and the cumulative rainfall in the simulation with the new WRF configuration increased by 54.7%and 49.5%,respectively,bringing them closer to their observations.Concurrently,the new WRF model increased the skill for flood prediction.The results of this study provide new insights into the mechanisms of interaction between the land surface and the atmosphere and their roles in helping to predict heavy rainfall and associated flooding in areas of complex topography.
基金This work was supported by the“111”Project of the Ministry of Education in China(No.111-2-16)Specialized Research Fund for the Doctoral Program of Higher Education(No.20100204120017)National High Technology Research and Development(863 Program)of China(No.2011AA100503).
文摘An evaluation of the interactions between vegetation,overland and soil erosion can provide valuable insight for the conservation of soil and water.An experiment was conducted to study water infiltration,runoff generation process,rate of sediment erosion,and hydrodynamic characteristics of overland flow from a sloping hillside with different draw-off discharges from alfalfa and control plots with 20°slope.The effect of alfalfa on runoff and sediment transport reduction was quantitatively analyzed.Alfalfa was discussed for its ability to reduce the overland flow scouring force or change the runoff movement.Compared to the bare-soil plots,alfalfa plots generated a 1.77 times increase in infiltration rate.Furthermore,the down-slope water infiltration rate for the bare soil plots was higher than in the up-slope,while the opposite was found in the alfalfa plots.In addition,alfalfa had a significant effect on runoff and sediment yield.In comparison to the control,the runoff coefficient and sediment transportation rate decreased by 28.3%and 78.4%in the grass slope,respectively.The runoff generated from the alfalfa and bare-soil plots had similar trends with an initial increase and subsequent leveling to a steady-state rate.The transport of sediment reduced with time as a consequence of the depletion of loose surface materials.The maximum sediment concentration was recorded within the first few minutes of each event.The alfalfa plots had subcritical flow while the baresoil plots had supercritical flow,which indicate that the capability of the alfalfa slope for resisting soil erosion and sediment movement was greater than for bare soil plots.Moreover,the flow resistance coefficient and roughness coefficient for the alfalfa plots were both higher than for the bare-soil plots,which indicate that overland flow in alfalfa plots had retarded and was blocked,and the flow energy along the runoff path had gradually dissipated.Finally,the ability to erode and transport sediment had decreased.
基金supported by the National Natural Science Foundation of China(Grant No.11072133)
文摘The hydrodynamic characteristics of the overland flow with a geocell slope are different from those of traditional flows because of its special structure. In this paper, a hydraulic flume with different slope gradients is used to study the hydrodynamic characteristics of the overland flow with geocell. The differences of flow characteristics between the overland flow with the geocell slope and the traditional flows are studied, and the hydrodynamic characteristics are obtained, including the flow pattern, the flow velocity and the hydraulic friction factor for the slope flow with geocell under different flow rates and slope gradients. The results show that there is a positive power function relationship between the rill depth of the slope surface （h） and the drag coefficient of the Darcy Weisbach （f）. There is a positive logarithmic function relationship between the drag coefficient f and the Reynolds number Red, and there is a negative power function relationship between the drag coefficient f and the Froude number Fr.
基金Project supported by the National Natural Science Foundation of China(Grant No.11372165)
文摘The hydrodynamic characteristics of the overland flow on a slope with a three-dimensional Geomat are studied for different rainfall intensities and slope gradients. The rainfall intensity is adjusted in the rainfall simulation system. It is shown that the velocity of the overland flow has a strong positive correlation with the slope length and the rainfall intensity, the scour depth decreases with the increase of the slope gradient for a given rainfall intensity, and the scour depth increases with the increase of the rainfall intensity for a given slope gradient, the overland flow starts with a transitional flow on the top and finishes with a turbulent flow on the bottom on the slope with the three-dimensional Geomat for different rainfall intensities and slope gradients, the resistance coefficient and the turbulent flow Reynolds number are in positively related logarithmic functions, the resistance coefficient and the slope gradient are in positively related power functions, and the trend becomes leveled with the increase of the rainfall intensity. This study provides some important theoretical insight for further studies of the hydrodynamic process of the erosion on the slope surface with a three-dimensional Geomat.
基金supported by the National Basic Research Program of China ("973" Program) (Grant No. 2007CB407202)
文摘Estimating sediment transport capacity of overland flow is essential to the development of physically based soil erosion models.Correlation analysis indicates that stream power is a dominant factor for sediment transport in overland flows and a new sediment transport capacity equation is proposed based on dimensional analysis.The coefficients of the new equation are calibrated using the published laboratory data,and rainfall impact is taken into consideration by adding an empirical factor on the dimensionless critical stream power.The new sediment transport capacity equation is a function of stream power,rainfall impacted critical stream power and slope.The new equation is applied in a one-dimensional soil erosion model to simulate field data of a runoff plot and the simulation results are reliable.
基金Project supported by the National Natural Science Foundation of China(Grant No.41171016)
文摘The evolution of the overland flow velocity along the distance downslope on smooth and granular beds in different cases is investigated by means of the electrolyte tracer via flume experiments. The results demonstrate that a non-uniform flow regime and a uniform flow regime exist in the development process of the overland flow. Owing to the different attributes of beds＇ roughness, the position of those zones with different flow regimes varies correspondingly：（1） the overland flow on granular beds enters into the uniform regime much sooner, additionally, the roll waves tend to appear because of the presence of the proper flow resistance impa-rted by the roughness （coarse sands）, and large slopes （20o and 25o） which makes the flow velocities and depths to undulate spatially. Furthermore, the flow resistance of the overland flows with different roughness elements, that is the non-sands, the fine sands and the coarse sands, is calculated. A quadratic interpolation method of the third order accuracy is employed in the calculation of the longitu-dinal flow resistance. The results show that it is rational to use the bed slope to approximate the hydraulic energy slope over a rela-tive small roughness （the present roughness）, however on the other hand, if the mean flow velocities and depths rather than the local parameters are used to calculate the flow resistance, a considerable error will be induced within the non-uniform regime of the over-land flows, including the acceleration zone and the roll-wave zone.
基金the National Natural Science Foundation of China Project(Grants 51779004,41530858,42077059)China National Science and Technology Major Project of Water Pollution Control and Treatment(2018ZX07101005-04).
文摘Overland flow resistance plays an important role in predicting hillslope hydrological and erosion pro-cesses on hillslopes.Soil surfaces covered with grass and/or gravel may increase hydraulic resistance to overland flow,but there is little information on the determination on these resistance components in a composite slopeland.In this study,the smooth and sand-bed surfaces covered with different gravel or/and grass(cover degree¼2.5%,5%,10%,15%,20%)under five slope gradients(3,6,9,12,15)were subjected to inflows to investigate the hydraulic resistance and validate the sum law of resistance components.The results showed that,the overland flow mainly belonged to transition or turbulent and supercritical flow regime with 150<Re<2050 and 0.4<Fr<6.9.As the gravel or grass cover increased,Darcy-Weisbach resistance coefficient(f)significantly increased.The resistance can be well predicted by flow discharge,gravel or grass cover using a power equation.The flow discharge had a greater influence on flow resistance than gravel or grass cover,and the impact of flow discharge weakened with the increasing bed slopes.The total resistance was generally greater than the sum of resistance components,and there existed a positive additional resistance for the composite surfaces.The additional resistance accounted for 37.4%,4.2%and 16.6%to the total resistance for the sand surfaces covered with gravel,grass and both of them,respectively.These results can help to understand the rainfall-runoff processes and improve the design of soil and water conservation measures in the rocky and mountainous areas.
基金financially supported by the National Natural Science Foundation of China(Grant NO.52279056)Inner Mongolia open list project(Grant NO.2024JBGS0023)。
文摘The hydrodynamic response of overland flow to vegetation coverage on convex slopes remains inadequately quantified despite it is critical for soil erosion control in terrains dominated by such topography.This study systematically investigated the influence of varying vegetation coverage(0%,1.08%,3.24%,4.69%and 9.81%)on the hydrodynamic characteristics of convex slopes through indoor flume experiments under diverse flow discharges(5.5-13.5 m^(3)/h)and slopes(5°-25°).The results revealed three key hydrodynamic mechanisms:(1)Flow retardation and energy dissipation:Increasing vegetation coverage significantly reduced overland flow velocity and promoted higher flow depth,thereby enhancing water retention and energy dissipation.Both stream power(Ω)and unit stream power(ω)declined by 13.9%-30.1%compared to bare slopes.(2)Flow Regime Transition:Froude number(Fr)decreased with increasing vegetation coverage,promoting the transition from supercritical to subcritical flow.The Reynolds number(Re)consistently exceeded 500,indicating the absence of laminar flow.(3)Modification of flow resistance:Vegetation resistance increased nonlinearly with coverage.Maximum bed shear stress was observed at 4.69%coverage(23.5%higher than bare slopes).However,Manning’s(n)and Darcy-Weisbach(f)coefficients did not correlate clearly with Re,indicating that vegetation coverage and slope type feedback significantly change flow resistance mechanisms.
基金supported by the National Nature Science Foundation of China(Grant No.50609005)the Fok Ying-Tong Education Foundation for Young Teachers in the Higher Education Institutions of China(Grant No.101075)
文摘The grid drop concept is introduced and used to develop a micromechanism-based methodology for calculating watershed flow concentration. The flow path and distance traveled by a grid drop to the outlet of the watershed are obtained using a digital elevation model (DEM). Regarding the slope as an uneven carpet through which the grid drop passes, a formula for overland flow velocity differing from Manning's formula for stream flow as welt as Darcy's formula for pore flow is proposed. Compared with the commonly used unit hydrograph and isochronal methods, this new methodology has outstanding advantages in that it considers the influences of the slope velocity field and the heterogeneity of spatial distribution of rainfall on the flow concentration process, and includes only one parameter that needs to be calibrated. This method can also be effectively applied to the prediction of hydrologic processes in un-gauged basins.
文摘Water chemistry changes when it flows through different pathways.This study aims to characterize the differences of water(including rainwater,overland flow,soil water,groundwater,and stream water)chemistry of five kinds of water in Obagbile Catchment in Southwest Nigeria,determine the changes in water chemistry that occur as the water moves from one pathway to another,and identify the factors responsible for the water chemistry changes.To do these,we collected 50 water samples from 10 heavy storms that received equal to or more than 10 mm of rain within an hour to test the changes of water chemical properties across various pathways in this study.The results show that overland flow had the highest p H and electrical conductivity(EC)and rainwater had the lowest values of the two parameters.Ca^(2+),Mg^(2+),Cl^(ˉ),and HCOO^(ˉ) were found to have their highest concentrations in stream water;meanwhile,NO_(3)^(–),NH_(4)^(+),and SO_(4)^(2–) were found to have almost the same low concentrations in all the water samples.K+was only dominant in stream water;while dissolved organic carbon(DOC)was lowest in rainwater,same in overland flow,soil water,and groundwater samples,and highest in stream flow.Principal component analysis(PCA)showed that for all the water samples from different pathways,two factors mainly accounted for the total variances.The two factors were related to the crustal and anthropogenic sources in rainwater,suggesting that the high loadings of major cations(e.g.,Ca^(2+) and Mg^(2+)) in rainwater samples are soil-derived.The PCA for the overland flow and soil water showed strong correlations among pH,EC,and the concentrations of Na^(+),Mg^(2+),HCOO^(-),and CH_(3)COO^(-),while the high loadings of all the parameters and the strong correlations among each other were evident in the stream water.In conclusion,the chemical constituents found in water are also the components of pathways through which the water flows.The major factors responsible for the change in the chemical properties of water in Obagbile Catchment are weathering and anthropogenic activities.
文摘Effectively managing floods in urban regions requires effectively designed and well-maintained runoff collection system. The absence of such a system and intense rainfall event will have the potential to disrupt the urban life and cause significant economic loss to properties. Grated inlets, which are a key component in urban drainage network, are used to capture the runoff. In this work, a three dimensional CFD model was developed based on open-source CFD tool, OpenFOAM®, to model flow over a grated inlet. An incompressible, transient, multiphase flow, Volume of Fluid (VOF) simulation was performed to predict the water flow rate through the grate inlet. The predicted flow rates are compared with the HEC-22 monograph values. The close agreement between the results shows the potential of using CFD modeling approach to test the reliability of existing drainage inlets for different flow scenarios.
