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1.
There is little information on the performance of vegetative filter strips (VFS) in filtering high‐concentration sediment from subcritical overland flow. Flume experiments on simulated grass strips were conducted using combinations of three slope gradients (3°, 9° and 15°), five 1‐m‐wide slope positions (from upslope to downslope), two flow rates (60 and 20 L min‐1 m‐1) and sediment concentrations of 100–300 kg m‐3 under simulated rainfall and non‐rainfall conditions. The results showed that sediment deposition efficiency increased with VFS width as a power function. Rainfall significantly reduced sediment deposited within VFS. Higher sediment concentration corresponded to a larger sediment deposition load but reduced deposition efficiency. Flow rate had a negative effect on deposition efficiency but no effect on deposition load. Sediments were more easily deposited at the upper slope position than downslope, and the upper slope position had a higher percentage of coarse sediments. The deposited sediment had significantly greater median diameters (D50) than the inflow sediment. A greater proportion of coarse sediments larger than 25 µm in diameter were deposited, and particles smaller than 1 µm and of 10–25 µm had a better deposition performance than particles of 1–10 µm. Rainfall reduced the deposited sediment D50 at a slope gradient of 3° and had no significant influence on it at 9° or 15°. A higher sediment concentration led to a smaller D50 of the deposited sediment. Rainfall had no significant effect on overland flow velocity. Both the deposited sediment load and D50 decreased with increasing flow velocity, and flow velocity was the most sensitive factor impacting sediment deposition. The results from this study should be useful to control sediment flowing into rivers in areas with serious soil erosion. Copyright © 2013 John Wiley & Sons, Ltd.  相似文献   

2.
The acceleration of saltating grains by overland flow causes momentum to be transferred from the flow to the grains, thereby increasing flow resistance and bed roughness. To assess the impact of saltating sediment on overland flow hydraulics, velocity profiles in transitional and turbulent flows on a fixed sand-covered bed were measured using hot-film anemometry. Five discharges were studied. At each discharge, three flows were measured: one free of sediment, one with a relatively low sediment load, and one with a relatively high sediment load. In these flows from 83 to 90 per cent of the sediment was travelling by saltation. As a result, in the sediment-laden flows the near-bed velocities were smaller and the velocity profiles steeper than those in the equivalent sediment-free flows. Sediment loads ranged up to 87·0 per cent of transport capacity and accounted for as much as 20·8 per cent of flow resistance (measured by the friction factor) and 89·7 per cent of bed roughness (measured by the ratio of the roughness length to median grain diameter). It is concluded that saltating sediment has a considerable impact on overland flow hydraulics, at least on fixed granular beds. Saltation is likely to have a relatively smaller effect on overland flow on natural hillslopes and agricultural fields where form and wave resistance dominate. Still, saltation is generally of greater significance in overland flow than in river flow, and for this reason its effect on overland flow hydraulics is deserving of further study. © 1998 John Wiley & Sons, Ltd.  相似文献   

3.
Effects of sediment load on hydraulics of overland flow on steep slopes   总被引：6，自引：0，他引：6  
Eroded sediment may have significant effects on the hydraulics of overland flow, but few studies have been performed to quantify these effects on steep slopes. This study investigated the potential effects of sediment load on Reynolds number, Froude number, flow depth, mean velocity, Darcy–Weisbach friction coefficient, shear stress, stream power, and unit stream power of overland flow in a sand‐glued hydraulic flume under a wide range of hydraulic conditions and sediment loads. Slope gradients were varied from 8·7 to 34·2%, unit flow rates from 0·66 to 5·26×10?3 m2 s?1, and sediment loads from 0 to 6·95 kg m?1 s?1. Both Reynolds number (Re) and Froude number (Fr) decreased as sediment load increased, implying a decrease in flow turbulence. This inverse relationship should be considered in modeling soil erosion processes. Flow depth increased as sediment load increased with a mean value of 1·227 mm, caused by an increase in volume of sediment‐laden flow (contribution 62·4%) and a decrease in mean flow velocity (contribution 37·6%). The mean flow velocity decreased by up to 0·071 m s?1 as sediment load increased. The Darcy–Weisbach friction coefficient (f) increased with sediment load, showing that the total energy consumption increased with sediment load. The effects of sediment load on f depended on flow discharge: as flow discharge increased, the influence of sediment load on f decreased due to increased flow depth and reduced relative roughness. Flow shear stress and stream power increased with sediment load, on average, by 80·5% and 60·2%, respectively; however, unit stream power decreased by an average of 11·1% as sediment load increased. Further studies are needed to extend and apply the insights obtained under these controlled conditions to real‐world overland flow conditions. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   

4.
Although numerous studies have acknowledged that vegetation can reduce erosion, few process-based studies have examined how vegetation cover affect runoff hydraulics and erosion processes. We present field observations of overland flow hydraulics using rainfall simulations in a typical semiarid area in China. Field plots (5 × 2 m2) were constructed on a loess hillslope (25°), including bare soil plot as control and three plots with planted forage species as treatments—Astragalus adsurgens, Medicago sativa and Cosmos bipinnatus. Both simulated rainfall and simulated rainfall + inflow were applied. Forages reduced soil loss by 55–85% and decreased overland flow rate by 12–37%. Forages significantly increased flow hydraulic resistance expressed by Darcy–Weisbach friction factor by 188–202% and expressed by Manning's friction factor by 66–75%; and decreased overland flow velocity by 28–30%. The upslope inflow significantly increased overland flow velocity by 67% and stream power by 449%, resulting in increased sediment yield rate by 108%. Erosion rate exhibited a significant linear relationship with stream power. M. sativa exhibited the best in reducing soil loss which probably resulted from its role in reducing stream power. Forages on the downslope performed better at reducing sediment yield than upslope due to decreased rill formation and stream power. The findings contribute to an improved understanding of using vegetation to control water and soil loss and land degradation in semiarid environments.  相似文献   

5.
The impact of vegetated filter strips (VFS) on sediment removal from runoff has been studied extensively in recent years. Vegetation is believed to increase water infiltration and decrease water turbulence thus enhancing sediment deposition within filter media. In the study reported here, field experiments have been conducted to examine the efficiency of vegetated filter strips for sediment removal from cropland runoff. Twenty filters with varying length, slope and vegetated cover were used under simulated runoff conditions with an average sediment concentration of 2700 mg/L. The filters were 2, 5, 10 and 15 m long with a slope of 2·3 and 5% and three types of vegetation. Three other strips with bare soil were used as a control. The experimental results showed that the average sediment trapping efficiency of all filters was 84% and ranging from 68% in a 2‐m filter to as high as 98% in a 15‐m long filter compared with only 25% for the control. The length of filter has been found to be the predominant factor affecting sediment deposition in VFS up to 10 m. Increasing filter length to 15 m did not improve sediment trapping efficiency under the present experimental conditions. The rate of incoming flow and vegetation cover percentage has a secondary effect on sediment deposition in VFS. Copyright © 2004 John Wiley & Sons, Ltd.  相似文献   

6.
Water erosion on hillslopes is a worldwide environmental problem, which is a rainfall‐induced process, especially extreme rainfall. The great intensity of extreme rainfall strongly enhances the power of overland flow to detach soil and transport sediment. Plant litter is one of the most important constituents of ecosystems that often covers the soil surface and can be incorporated into topsoil. However, little attention has been paid to its effect on flow hydraulics owing to the veiled nature. This study aimed to examine the effects of incorporated litter on the hydraulic properties under extreme rainfall condition. To reach this goal, six litter rates of 0, 0.05, 0.10, 0.20, 0.35, and 0.50 kg m?2 and four litter types collected from deciduous trees, coniferous trees, shrubs, and herbs were incorporated into topsoil. Then, simulated rainfall experiments were performed on five slope gradients (5°, 10°, 15°, 20°, and 25°) with an extreme rainfall intensity of 80 mm h?1. The results showed that Froude number and flow velocity of the overland flow decreased, whereas flow resistance increased exponentially with litter incorporation rate. Litter type had an influence on flow hydraulics, which can mainly be attributed to the variations in surface coverage of the exposed litter and the litter morphology. Flow velocity and Darcy–Weisbach coefficient increased markedly with slope gradient. However, the variation of slope gradient did not modify the relationships between flow hydraulics and incorporated litter rate. The random roughness, resulting from heterogeneous erosion due to the uneven protection of surface exposed litter, increased linearly with litter incorporated rate. As rainfall proceeded, flow hydraulics varied with incorporated litter rate and slope gradient complicatedly due to the increases in flow rate and coverage of the exposed litter and the modification of soil surface roughness.  相似文献   

7.
The interactions between overland flow hydraulics and sediment yield were studied in flume experiments on erodible soil surfaces covered by rock fragments. The high erodibility of a non-cohesive fine sediment (D50 + 0·09mm) permitted the effects of local turbulence and scour on sediment yield to be examined. Overland flow hydraulics and sediment yield were compared for experiments with pebble (D50 + 1·5cm) and cobble (D50 + 8·6cm) rock fragment covers. Cover percentages range from 0 to 99 per cent. Rock fragment size strongly affects the relations between flow hydraulics and rock fragment cover. For pebbles spatially-averaged hydraulic parameters (flow velocity, flow depth, effective flow width, unit discharge, total shear stress, Darcy-Weisbach friction factor, percentage grain friction and grain shear stress) vary most rapidly within cover percentages at low covers (power functions). In contrast, for cobbles these parameters vary most rapidly within cover percentages at high covers (exponential functions). As the type of the function that describes the relation between flow hydraulics and cover percentage can be deduced from the ratio of rock fragment height to flow depth, the continuity equation can be employed to determine the actual coefficients of the functions, provided the regression of one hydraulic parameter (e.g. flow velocity) with cover percentage is known and a good estimate exists for two values of another hydraulic variable for a low and a high cover percentage. The variation of sediment yield with cover percentage is also strongly dependent on rock fragment size, but neither the convex-upward relation for pebbles, nor the positive relation for cobbles can be solely attributed to the spatially averaged hydraulics of sheet-flow. Rock fragments induce local turbulence that leads to scour hole development on the stoss side of the rock fragments while deposition commonly occurs in the wake. This local scour and deposition substantially affects sediment yield. However, scour dimensions cannot be predicted by spatially averaged flow hydraulics. An adjustment of existing scour formulas that predict scour around bridge piers is suggested. Sediment yield from non-cohesive soils might then be estimated by a combination of sediment transport and scour formulas.  相似文献   

8.
Validation of a vegetated filter strip model (VFSMOD)   总被引：2，自引：0，他引：2  
Vegetated filter strips (VFS) are designed to reduce sediment load and other pollutants into water bodies. However, adaptation of VFS in the field has been limited owing to lack of data about their efficiency and performance under natural field conditions. A number of models are available that simulate sediment transport and trapping in VFS, but there is a general lack of confidence in VFS models owing to limited validation studies and model limitations that prevent correct application of these models under field conditions. The objective of this study is to test and validate a process‐based model (VFSMOD) that simulates sediment trapping in VFS. This model links three submodels: modified Green–Ampt's infiltration, Quadratic overland flow submodel based on kinematic wave approximation and University of Kentucky sediment filtration model. A total of 20 VFS, 2, 5, 10 and 15 m long and with various vegetation covers, were tested under simulated sediment and runoff conditions. The results of these field experiments were used to validate the VFS model. The model requires 25 input parameters distributed over five input files. All input parameters were either measured or calculated using experimental data. The observed sediment trapping efficiencies varied from 65% in the 2‐m long VFS to 92% in the 10‐m long filters. No increase in sediment removal efficiency was observed at higher VFS length. Application of the VFS model to experimental data was satisfactory under the condition that actual flow widths are used in the model instead of the total filter width. Predicted and observed sediment trapping efficiencies and infiltration volume fitted very well, with a coefficient of determination (R2) of 0·9 and 0·95, respectively. Regression analyses revealed that the slope and intercept of the regression lines between predicted versus observed infiltration volume and trapping efficiency were not significantly different than the line of perfect agreement with a slope of 1·0 and intercept of 0·0. Copyright © 2001 John Wiley & Sons, Ltd.  相似文献   

9.
Particle selectivity plays an important role in clarifying sediment transport processes in vegetative filter strips (VFS). 10-m long grass strips at slopes of 5° and 15° were subjected to a series of sediment-laden inflows experiments with different particle sizes to investigate the sediment transport and its response to overland flow hydraulics. The inflow sediments came from local soil, river-bed sand, and mixed, with median particle size d50 of 39.9, 207.9 and 77.4 μm, respectively. Three independent repeated experiments were carried for each treatment. The results show that when the sediment trapping lasted for a certain length of time, the re-entrainment of some small-sized particles was greater than the deposition; that is, net loss occurred, which was not erosion of the original soil. Net loss of particles is mainly determined by the particle diameter. The coarser the inflow sediment particles and/or the steeper the slope, the coarser the particles can be net lost. Deposited sediment causes the VFS bed surface to become smooth and hydraulic resistance decrease exponentially. Unit stream power P is more suitable than shear stress τ of overland flow to be used to describe the process of sediment particle transport in VFS. The relationship between P and d50 of outflow sediment is very consistent with the form of power function with a constant term. These results are helpful to understand the physical process of sediment transport on vegetation hillslopes.  相似文献   

10.
Soil and water conservation practices have been promoted for a long time, in order to sustain agricultural activities and prevent environmental pollution. Vegetated filter strips (VFS) have been used to reduce sediment pollution into water bodies at or near the pollutant source. However, factors effecting VFS performance under natural conditions have not been well understood owing to the physical, time and financial limitations of field experiments. The use of well‐validated simulation models to understand the performance of VFS and factors affecting sediment deposition is highly justified. The objective of this research is to investigate sediment trapping in VFS and to study various factors affecting VFS performance using the simulation model VFSMOD, which was developed by researchers at University of North Carolina. Recently, VFSMOD has been validated successfully by using 21 filters with varying length, slope and vegetated cover. A wide range of five parameters was selected for the simulations, namely filter length, filter slope, manning roughness coefficient, soil type and characteristics of incoming sediment from adjacent fields. Computer simulations revealed that the length of filter is the most significant factor affecting sediment trapping in VFS. The relative increase in trapping efficiencies was not linearly related to an increase in filter length. Inflow sediment class also has a major influence on sediment trapping in VFS. The trapping efficiency of clay sediments in a 15 m length VFS was 47% compared with 92% for silt from incoming sediment. Manning roughness coefficient had a moderate effect on sediment trapping and was more significant in short filters. Land slope and soil type of VFS had a minor influence on the performance of VFS. Copyright © 2001 John Wiley & Sons, Ltd.  相似文献   

11.
Abstract

Sediment transport capacity is a key concept in determining rates of detachment and deposition in process-based erosion models, yet limited studies have been conducted on steep slopes. We investigated the effects of sediment size on transport capacity of overland flow in a flume. Unit flow discharge ranged from 0.66 to 5.26?×?10-3 m2 s-1, and slope gradient varied from 8.7 to 42.3%. Five sediment size classes (median diameter, d 50, of 0.10, 0.22, 0.41, 0.69 and 1.16 mm) were used. Sediment size was inversely related to transport capacity. The ratios of average transport capacity of the finest class to those of the 0.22, 0.41, 0.69 and 1.16 mm classes were 1.09, 1.30, 1.55 and 1.92, respectively. Sediment transport capacity increased as a power function of flow discharge and slope gradient (R2?=?0.98), shear stress (R2?=?0.95), stream power (R2?=?0.94), or unit stream power (R2?=?0.76). Transport capacity generally decreased as a power function of sediment size (exponent?=??0.35). Shear stress and stream power predicted transport capacity better than unit stream power on steep slopes when transport capacity was <7 kg m-1 s-1. Sediment transport capacity increased linearly with mean flow velocity. Critical or threshold velocity increased as a power function of sediment size (R2?=?0.93). Further studies with fine soil particles are needed to quantify the effects of sediment size on transport capacity of overland flow on steep slopes.

Citation Zhang, G.-H., Wang, L.-L., Tang, K.-M., Luo, R.-T. & Zhang, X.C. (2011) Effects of sediment size on transport capacity of overland flow on steep slopes. Hydrol. Sci. J. 56(7), 1289–1299.  相似文献   

12.
The slope effects on sediment trapping process in vegetative filter strips (VFS) are usually neglected in current modelling practices for VFS operation, which hamper the VFS design and performance evaluation, especially on steep slopes. To fill the knowledge gap, 12 laboratory experiments of sediment trapping in VFS were conducted with three different inflow discharge (80, 100, and 120 ml s−1) and four slope angles (5,10, 15, and 20°). The experimental results show that, on steep slopes (10, 15, and 20°), a part of trapped sediment particles in VFS can be eroded again and then dragged to the downstream as bed load, whilst they do not move on gentle slope (5°). To describe the complex processes, a simple and effective modelling framework was developed for sloped VFS by coupling the slope infiltration, runoff, and modified sediment transport model. The model was tested against the experimental results and good agreements between the modelled and observed results were found in both runoff and sediment transport processes for all cases. On steep slopes, the sediment trapping performance of VFS decreases significantly because the erosion of deposited sediment particles can account for more than 60% of the sediment load in the outflow. The slope effect on sediment trapping efficiency of VFS varies greatly with soil, VFS, and slope properties. The model was compared with previous sediment transport equation and found that both methods can satisfactorily predict the sediment trapping of VFS on gentle slopes, but previous sediment transport equation is likely to overestimate the sediment trapping efficiency in VFS on steep slopes. This model is expected to provide a more realistic and accurate method for predicting runoff and sediment reduction in VFS on sloping surfaces.  相似文献   

13.
Interrill erosion processes on gentle slopes are affected by mechanisms of raindrop impact, overland flow and their interaction. However, limited experimental work has been conducted to understand how important each of the mechanisms are and how they interact, in particular for peat soil. Laboratory simulation experiments were conducted on peat blocks under two slopes (2.5° and 7.5°) and three treatments: Rainfall, where rainfall with an intensity of 12 mm h?1 was simulated; Inflow, where upslope overland flow at a rate of 12 mm h?1 was applied; and Rainfall + Inflow which combined both Rainfall and Inflow. Overland flow, sediment loss and overland flow velocity data were collected and splash cups were used to measure the mass of sediment detached by raindrops. Raindrop impact was found to reduce overland flow by 10 to 13%, due to increased infiltration, and reduce erosion by 47% on average for both slope gradients. Raindrop impact also reduced flow velocity (80–92%) and increased roughness (72–78%). The interaction between rainfall and flow was found to significantly reduce sediment concentrations (73–85%). Slope gradient had only a minor effect on overland flow and sediment yield. Significantly higher flow velocities and sediment yields were observed under the Rainfall + Inflow treatment compared to the Rainfall treatment. On average, upslope inflow was found to increase erosion by 36%. These results indicate that overland flow and erosion processes on peat hillslopes are affected by upslope inflow. There was no significant relationship between interrill erosion and overland flow, whereas stream power had a strong relationship with erosion. These findings help improve our understanding of the importance of interrill erosion processes on peat. Copyright © 2017 John Wiley & Sons, Ltd.  相似文献   

14.
MODELING EPHEMERAL GULLY EROSION FOR CONSERVATION PLANNING   总被引：9，自引：0，他引：9  
1INTRODUCTIONEphemeral gully erosion,which is caused by concentrated flow within cultivated farm fields,is distinct from rill erosion.Ephemeral gully erosion is also distinct from gully erosion in permanent,deep,incised channels,formed by headcuts moving upstream.Ephemeral gully erosion is often overlooked.It is not estimated with rill-interrill erosion prediction technology such as the Revised Universal Soil Loss Equation(Renard et al.,1997),and it is often not measured in field survey…  相似文献   

15.
Near‐surface airflow over a morphologically simple, vegetated, 8 m high foredune with a small wave‐cut scarp was measured for onshore to oblique‐onshore conditions during a low‐moderate (5–6 m s‐1 ) wind event and a high velocity (11–18 m s‐1) sand‐transporting gale event. Flow across the foredune was characterized by significant flow compression and acceleration up and across the foredune during both events. During the gale, a pronounced jet (speed bulge) developed at the foredune crest, which increased in magnitude with increasing wind speed. The vertical (W) velocity component of the 3D flow field was positive (upwards) across the stoss slope under low wind conditions but negative (downwards) during gale wind conditions, with upslope acceleration. During the low velocity event, there was speed‐down within the vegetation canopy, as would be expected for a porous roughness cover. During the strong wind event there was speed‐up in the lower portion of the vegetation canopy, and this was found up the entire stoss slope. Sediment transport during the gale force event was substantial across the beach and foredune despite the moderate vegetation cover and minimum fetch. Aeolian suspension was evident in the lee of the dune crest. The observations presented herein show that strong storm winds are an effective mechanism for translating sediment landwards across a high vegetated foredune, contributing sediment to the stoss slope, crest and leeward slopes of the foredune and backing dunes. Copyright © 2013 John Wiley & Sons, Ltd.  相似文献   

16.
The Manning equation is one of the most widely used formulae for calculating the velocity of shallow overland flow in hydrological and erosion models. Precise estimation of the Manning's friction coefficient (n) is critical to determining overland flow and soil erosion processes. Few studies have been conducted to quantify the effects of sediment load on Manning's n on steep slopes. This study was conducted to investigate the potential effects of sediment load on Manning's n in a flume with a fixed bed, under wide ranges of hydraulics and sediment loads. Slope gradient varied from 8·7 to 34·2%, unit flow rate from 0·66 to 5·26 × 10?3 m2 s?1, and sediment load from 0 to 6·95 kg m?1 s?1. The Reynolds number ranged from 350 to 5899. Results showed that Manning's n varied in both sediment‐free and sediment‐laden flows ranging from 0·012 to 0·055. The apparent Manning's coefficients of sediment‐laden flow were much greater than those of sediment‐free flow. The mean Manning coefficient of sediment‐laden flow was 51·27% greater than the mean value of sediment‐free flow. For sediment‐laden flow, Manning's n could be estimated with a power function of unit flow discharge and sediment content. Further studies are needed to quantify the potential effects of sediment load on the Manning's n on erodible beds and in fields. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   

17.
Changing fire regimes and prescribed‐fire use in invasive species management on rangelands require improved understanding of fire effects on runoff and erosion from steeply sloping sagebrush‐steppe. Small (0·5 m2) and large (32·5 m2) plot rainfall simulations (85 mm h–1, 1 h) and concentrated flow methodologies were employed immediately following burning and 1 and 2 years post‐fire to investigate infiltration, runoff and erosion from interrill (rainsplash, sheetwash) and rill (concentrated flow) processes on unburned and burned areas of a steeply sloped sagebrush site on coarse‐textured soils. Soil water repellency and vegetation were assessed to infer relationships in soil and vegetation factors that influence runoff and erosion. Runoff and erosion from rainfall simulations and concentrated flow experiments increased immediately following burning. Runoff returned to near pre‐burn levels and sediment yield was greatly reduced with ground cover recovery to 40 per cent 1 year post‐fire. Erosion remained above pre‐burn levels on large rainfall simulation and concentrated flow plots until ground cover reached 60 per cent two growing seasons post‐fire. The greatest impact of the fire was the threefold reduction of ground cover. Removal of vegetation and ground cover and the influence of pre‐existing strong soil‐water repellency increased the spatial continuity of overland flow, reduced runoff and sediment filtering effects of vegetation and ground cover, and facilitated increased velocity and transport capacity of overland flow. Small plot rainfall simulations suggest ground cover recovery to 40 per cent probably protected the site from low‐return‐interval storms, large plot rainfall and concentrated flow experiments indicate the site remained susceptible to elevated erosion rates during high‐intensity or long duration events until ground cover levels reached 60 per cent. The data demonstrate that the persistence of fire effects on steeply‐sloped, sandy sagebrush sites depends on the time period required for ground cover to recover to near 60 per cent and on the strength and persistence of ‘background’ or fire‐induced soil water repellency. Published in 2009 by John Wiley & Sons, Ltd.  相似文献   

18.
Sediment grains in a bedrock‐alluvial river will be deposited within or adjacent to a sediment patch, or as isolated grains on the bedrock surface. Previous analysis of grain geometry has demonstrated that these arrangements produce significant differences in grain entrainment shear stress. However, this analysis neglected potential interactions between the sediment patches, local hydraulics and grain entrainment. We present a series of flume experiments that measure the influence of sediment patches on grain entrainment. The flume had a planar bed with roughness that was much smaller than the diameters of the mobile grains. In each experiment sediment was added either as individual grains or as a single sediment pulse. Flow was then increased until the sediment was entrained. Analysis of the experiments demonstrates that: (1) for individual grains, coarse grains are entrained at a higher discharge than fine grains; (2) once sediment patches are present, the different in entrainment discharge between coarse and fine grains is greatly reduced; (3) the sheltering effect of patches also increases the entrainment discharge of isolated grains; (4) entire sediment patches break‐up and are eroded quickly, rather than through progressive grain‐by‐grain erosion; (5) as discharge increases there is some tendency for patches to become more elongate and flow‐aligned, and more randomly distributed across the bed. One implication of this research is that the critical shear stress in bedrock‐alluvial channels will be a function of the extent of the sediment cover. Another is that the influence of sediment patches equalizes critical shear stresses between different grain sizes and grain locations, meaning that these factors may not need to be accounted for. Further research is needed to quantify interactions between sediment patches, grain entrainment and local hydraulics on rougher bedrock surfaces, and under different types of sediment supply. Copyright © 2016 John Wiley & Sons, Ltd.  相似文献   

19.
Flume studies were conducted in order to evaluate the influence of slope, sediment size, discharge and inflow sediment concentration on sediment deposition by overland flow. Additionally, experiments were carried out to measure transport capacity of overland flow at low slopes, using a wide range of discharges. The experimental data show that the hydraulic conditions where net deposition occurs can be divided into two domains. The first domain is characterized by hydraulic conditions where transport capacity is not significant. In the second domain net deposition still occurs but transport capacity is significant. The size of the latter domain is dependent on the sediment size distribution, on the hydraulic roughness and on the inflow sediment concentration. The experiments clearly indicate the necessity of incorporating a threshold value in any deposition equation. These experiments demonstrate that shear stress is a valuable threshold for deposition modelling. Copyright © 1999 John Wiley & Sons, Ltd.  相似文献   

20.
Accelerated pluvial erosion on hillslopes modified by off-road vehicles (ORVs) is analysed using results from 50 rainfall simulation experiments conducted in the Mojave Desert, California. Sediment yield from 1 m2 hillslope plots subjected to intense, 20-minute rainfalls is typically increased 10 to 20-fold following ORV use. Salient effects of vehicle traffic, which reduce infiltration, increase runoff sediment transport efficiency, and enhance gully formation, are further studied by combining simple theoretical relations with experimental data. This analysis helps identify factors controlling erosion on natural desert hillslopes, as well as those used by ORVs. Erosion of natural or vehicle-used desert surfaces is heavily influenced by runoff hydraulics. Calculated Darcy-Weisbach friction factors decrease by an average of 13-fold following vehicular slope modification, whereas runoff Reynolds numbers increase by an average of 5 1/2-fold. The capacity of overland flow to transport sediment is related to runoff power and its degree of localization, which usually increase considerably following ORV activity; however, the ability of overland flow to move large grains (competency) is related to a combination of factors not always systematically influenced by ORV use. Kinematic runoff routing, which is used to extrapolate experimental results to longer slope lengths, leads to the suggestion that the hydraulic roughness of desert hillslopes strongly influences their erosional behaviour.  相似文献   

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