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1.
ABSTRACTIn this study, the effect of single and double row piles for reducing scouring in a mild-curved river meander was studied experimentally. The experimental study focused on the effect of vegetation on bed topography in a mild-curved meander bend. The experimental tests were conducted in a laboratory flume under clear water flow conditions. A series of experimental tests were carried out with a fixed bed and non-vegetated and vegetated moveable beds with different vegetation patterns. Analysis of the flow characteristics indicated that when the bed was mobile with vegetation on the inner bank, the core of maximum streamwise velocity shifted towards the centreline of the bend. Additionally, the cross-sectional kinetic energy increased from 0.05% for the fixed-bed test to 4.30% for the test with a double row of vegetation. Furthermore, the presence of vegetation was found to increase the uniformity of the distribution of turbulence intensity and to reduce the Reynolds shear stress along the test section. Also, the mass fluxes increased from the outer bank to the inner bank and from the upstream towards the downstream of the bend. Finally, comparison of bed topography in vegetated and non-vegetated channels showed that the maximum scour depth at the bend apex was reduced by 77% and 62% for the cases with one row and two rows of vegetation, respectively. The results of this study were compared with previously proposed models for predicting the vertical distribution of the streamwise velocity at the bend apex. It was found that Johannesson and Parker’s model (JPM) gave the lowest value of standard error. The above findings are useful in river training works and, in particular, for restoration of meandering rivers.
Editor D. M.C. Acreman; Associate editor C. Cudennec 相似文献
2.
Influence of root characteristics and soil variables on the uprooting mechanics of Avena sativa and Medicago sativa seedlings 
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下载免费PDF全文 Katharina Edmaier Benoît Crouzy Roland Ennos Paolo Burlando Paolo Perona 《地球表面变化过程与地形》2014,39(10):1354-1364
The success of seedlings and rejuvenated woody debris growing on river bedforms depends on the resistance to uprooting by flow provided by their simple root architecture. Avena sativa and Medicago sativa seedlings were used in flume experiments as prototypes for juvenile riparian plants. Very little is known about the magnitude of root anchoring forces and the role of secondary roots of such simple root systems. We performed 1550 vertical uprooting experiments on Avena sativa and Medicago sativa seedlings grown in quartz sand. Seedlings were pulled up by direct traction using a wheel driven by a computer‐controlled motor and the force was recorded. Roots were scanned and architectural parameters (root length and number of roots) determined. Uprooting force and work (the integral of the applied force times the distance over which it is applied) were then related to root architecture and soil variables. Resistance to uprooting increased with decreasing sediment size and sediment moisture content. The initial response of the root–soil system to uprooting showed linear elastic behaviour with modulus increasing with plant age. While the maximum uprooting force was found to increase linearly with total root length and be mainly dependent on the length of the main root, uprooting work followed a power law and has to be related to the whole root system. Thus, for the young plants we considered, secondary roots are responsible for the ability to withstand environmental disturbances in terms of duration rather than magnitude. This distinction between primary and secondary roots can be of crucial importance for seedlings of riparian species germinating on river bars and islands where inundation is a main cause of mortality. Beyond clarifying the biomechanical role of soil and root variables, the uprooting statistics obtained are useful in interpreting and designing ecomorphodynamic flume experiments. Copyright © 2014 John Wiley & Sons, Ltd. 相似文献
3.
Flow in meandering bends is characterized by the formation of a large cross‐sectional central‐region circulation cell. The width‐to‐depth ratio is one of the most important parameters affecting the entity of the cross‐circulation motion. In steep outside bends, beside the central‐region cell, a counter‐rotating circulation cell often forms in the upper part of the outer‐bank. In spite of its practical importance, the evolving mechanisms of both the circulation cells and their role on boundary shear stress distribution in bends are not yet fully understood. The aim of the present paper is to gain some insight into how cross‐sectional flow motion evolves along meandering bends. Experiments have been carried out in a laboratory meandering channel of large amplitude, over a deformed‐rigid bed, for two values of the width‐to‐depth ratio. The three‐dimensional flow velocity field has been measured in detail at five cross‐sections, almost equally spaced along the channel reach between two consecutive apex sections. The measurements have been carried out on a fine grid by an acoustic Doppler velocity profiler. The distributions of the cross‐sectional flow (e.g. cross‐sectional flow velocity, net transversal flux) and turbulent kinetic energy are analyzed in each investigated section. Measurements show that the counter‐rotating circulation cell is evident only in the case of ‘small’ width‐to‐depth ratio. Such circulation cell begins at the bend entrance and it is fully developed at the bend apex; then it decays. At the bend apex, the core of maximum velocity is found near the bed at about the separation between the central and the outer‐bank circulation cells. Moreover, the presence of the counter‐rotating circulation cell allows the bank shear stress to maintain low values in the outer‐side of the bend. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
4.
Flow and sedimentary processes in the meandering river South Esk,Glen Clova,Scotland 总被引：1，自引：0，他引：1
Channel geometry, flow and sedimentation in a meander bend of the River South Esk were studied from bankfull stages (January–February) to low water stages (May) in 1974. Bed topography varied little over the study period, showing a typical pool and ripple geometry. Variation of mean depth and velocity with discharge differed from section to section around the bend, due primarily to locally varying flow resistance with stage. The flow pattern for all stages was dominated by a single spiral over the point bar, with a development zone at the bend entrance. Deviation of bed shear stress from the mean flow direction was in general accord with theory, especially for high stages. The use of a uniform longitudinal water surface slope in the calculation of bed shear stress is not justified because of a complicated water surface topography, also such calculated shear may not represent effective bed shear on grains, as it accounts also for energy losses associated with secondary flows. Dunes covered much of the bar at high stages, with increasing proportions of ripples, sand ribbons and lower phase plane beds at low stages. Local flow resistance generally decreases from dunes, diminished and ripple-backed dunes, ripples, sand ribbons to plane beds, and bed forms are predicted quite well by the stream power-grain size scheme. Mean size, sorting and skewness of sediment over the bed changes little with stage. In general, size decreases, sorting improves and skewness changes from positive to negative from the talweg to the inner bank, and in the downstream direction. Allen's (1970a, b) force balance equation for moving bed load particles is supported for bankfull stage, with some reservations, and textural characteristics are explained by progressive sorting in the direction of sediment transport. Large-scale trough cross stratification (with some flat bedding) formed at high stage by dunes (and lower phase plane beds) dominates the point bar sediments. Alternations of fine-medium sand (often cross-laminated) and vegetation-rich layers result from periodic deposition on the grassed upper bar surface. Fining upwards sequences produced by lateral channel migration are modified by a coarsening upward subsequence in the upstream bar region where spiral flow is developing from the bend upstream. 相似文献
5.
The effects of aquatic macrophytes on flow and turbulence were studied in a tightly curving meander bend. Three field measurement campaigns were carried out within a one year period to capture effects of seasonal changes in macrophyte cover. They comprised three‐dimensional velocity measurements and mappings of vegetation cover and bathymetry. Flow accelerates and converges over the run into an axial pool in a jet‐like flow pattern bordered by outer and inner bank flow separation zones. The jet and widening of the cross‐section anticipate helical flow until the second half of the bend, where an asymmetric pool developed. Submerged vegetation at the riffles preserves the jet at much lower discharges during the summer period by concentrating high momentum fluid near the surface. Plants locally modify the velocity and stress patterns, reduce bed shear stresses, create zones of fine sediment accumulation and reinforce the bed and banks with roots and rhizomes. Plant patches colonising the banks and the point bar confine secondary flow cells laterally and affect shape and magnitude of the transverse flow profiles near their edges. The morphology of the bend was very stable over the observation period and neither bank erosion nor pool scouring occurred. However, fine sediments accumulate within vegetation patches and in the recirculation zones while the remaining open areas tend to erode slightly. With the decay of macrophytes in winter, sediment accumulations are mobilised again and the bathymetry levels, supporting cyclic models of morphologic change in vegetated bends. In the second part of the paper, semi‐empirical models for the three predominant flow types were tested and discussed; velocity and stress models of vegetated mixing layers and plane turbulent jets, and Rozovskii's model for the transverse flow in bends. Copyright © 2012 John Wiley & Sons, Ltd. 相似文献
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Koen Blanckaert Maarten G. Kleinhans Stuart J. McLelland Wim S. J. Uijttewaal Brendan J. Murphy Anja van de Kruijs Daniel R. Parsons Qiuwen Chen 《地球表面变化过程与地形》2013,38(7):696-716
There is a paucity of data and insight in the mechanisms of, and controls on flow separation and recirculation at natural sharply‐curved river bends. Herein we report on successful laboratory experiments that elucidate flow structure in one constant‐width bend and a second bend with an outer‐bank widening. The experiments were performed with both a flat immobile gravel bed and mobile sand bed with dominant bedload sediment transport. In the constant‐width bend with immobile bed, a zone of mainly horizontal flow separation (vertical rotational axis) formed at the inner bank that did not contain detectable flow recirculation, and an outer‐bank cell of secondary flow with streamwise oriented rotational axis. Surprisingly, the bend with widening at the outer bank and immobile bed did not lead to a transverse expansion of the flow. Rather, flow in the outer‐bank widening weakly recirculated around a vertical axis and hardly interacted with the inner part of the bend, which behaved as a constant‐width bend. In the mobile bed experiment, downstream of the bend apex a pronounced depositional bar developed at the inside of the bend and pronounced scour occurred at the outside. Moreover the deformed bed promoted flow separation over the bar, including return currents. In the constant‐width bend, the topographic steering impeded the generation of an outer‐bank cell of secondary flow. In the bend with outer‐bank widening, the topographic steering induced an outward expansion of the flow, whereby the major part of the discharge was conveyed in the central part of the widening section. Flow in the outer‐bank widening was highly three dimensional and included return currents near the bottom. In conclusion, the experiments elucidated three distinct processes of flow separation common in sharp bends: flow separation at the inner bank, an outer‐bank cell of secondary flow, and flow separation and recirculation in an outer‐bank widening. Copyright © 2012 John Wiley & Sons, Ltd. 相似文献
8.
The impact of wastewater flow on the channel bed morphology was evaluated in four ephemeral streams in Israel and the Palestinian Territories: Nahal Og, Nahal Kidron, Nahal Qeult and Nahal Hebron. Channel changes before, during and after the halting of wastewater flow were monitored. The wastewater flow causes a shift from a dry ephemeral channel with intermittent floods to a continuous flow pattern similar to that of humid areas. Within a few months, nutrient‐rich wastewater flow leads to rapid development of vegetation along channel and bars. The colonization of part of the active channel by vegetation increases flow resistance as well as bank and bed stability, and limits sediment availability from bars and other sediment stores along the channels. In some cases the established vegetation covers the entire channel width and halts the transport of bed material along the channel. During low and medium size flood events, bars remain stable and the vegetation intact. Extreme events destroy the vegetation and activate the bars. The wastewater flow results in the development of new small bars, which are usually destroyed by flood flows. Due to the vegetation establishment, the active channel width decreases by up to 700 per cent. The deposition of fine sediment and organic material changed the sediment texture within the stable bar surface and the whole bed surface texture in Nahal Hebron. The recovery of Nahal Og after the halting of the wastewater flow was relatively fast; within two flood seasons the channel almost returned to pre‐wastewater characteristics. The results of the study could be used to indicate what would happen if wastewater flows were introduced along natural desert streams. Also, the results could be used to predict the consequences of vegetation removal as a result of human intervention within the active channel of humid streams. Copyright © 2001 John Wiley & Sons, Ltd. 相似文献
9.
The important role of vegetation in adding cohesion and stabilizing streambanks has been widely recognized in several aspects of fluvial geomorphology, including stream restoration and studies of long‐term channel change. Changes in planform between braided, meandering, and anabranching forms have been attributed to the impacts of vegetation on hydraulic roughness and bank stability. However, these studies focus either on flume studies where analog vegetation is used, or case studies featuring one species, which is commonly invasive. We present functional differences of bank‐stabilizing root characteristics and added cohesion, with vegetation categorized as woody and non‐woody and by the vegetation groups of trees, shrubs, graminoids, and forbs. We analyzed root morphology and tensile strength of 14 species common to riparian areas in the southern Rocky Mountains, in field sites along streambanks in the montane and subalpine zones of the Colorado Front Range. Using the vegetation root component (RipRoot) of a physically‐based bank stability model (BSTEM), we estimated the added cohesion for various sediment textures with the addition of each of the 14 species. Significant differences exist between woody and non‐woody vegetation and between the four vegetation categories with respect to the coefficient of the root tensile strength curve, lateral root extent, and maximum root diameter. Woody vegetation (trees and shrubs) have higher values of all three parameters than non‐woody species. Tree roots add significantly more cohesion to streambanks than forb roots. Additionally, rhizomes may play an important role in determining the reach‐scale effects of roots on bank stabilization. Differences in root characteristics and added cohesion among vegetation categories have several important implications, including determining the likelihood of planform change, developing guidelines for the use of bank‐stabilizing vegetation, and linking the effect of vegetation to geomorphic structure that can benefit ecosystem functioning. Copyright © 2014 John Wiley & Sons, Ltd. 相似文献
10.
J. A. Milne 《地球表面变化过程与地形》1982,7(3):227-240
Seventy-eight riffle to riffle and 80 bend spacings along eleven coarse-bedload, low sinuosity stream channels in upland Britain have been surveyed. Frequency distributions of these spacings are notably right-skewed. The most common repeating distances between riffles and bend inflections are between 4 and 6 channel widths although spacings up to 20 widths are also present. Riffle and pool locations around bends at different stages of planform development indicate that change is largely through increased sinuosity between two consecutive riffles of an original straight reach. Observed straight segments exhibit alternating riffles and pools evenly spaced at 4-6 widths, and most bends have similarly spaced riffles at their inflections in plan, with the intermediate pool at their apex. However angular deflections between axial lines joining inflections indicate existing sequences of bends did not develop from a single straight reach. Bends which are significantly longer than 4-6 widths are of low sinuosity and represent variability in naturally irregular planforms rather than arcs in advanced stages of meander growth. Adjusted bed topography around such long bends takes two forms. Either a single riffle-pool cycle is present with one or both bed forms being longer than average, or a 4-6 widths spacing is maintained by more than one riffle-pool sequence. Locally, the cross-section characteristics of riffles and pools are also influenced by planform location. 相似文献
11.
Effects of vegetation on channel morphodynamics: results and insights from laboratory experiments 总被引：1，自引：0，他引：1
A series of laboratory experiments demonstrates that riparian vegetation can cause a braided channel to self‐organize to, and maintain, a dynamic, single‐thread channel. The initial condition for the experiments was steady‐state braiding in non‐cohesive sand under uniform discharge. From here, an experiment consisted of repeated cycles alternating a short duration high flow with a long duration low flow, and uniform dispersal of alfalfa seeds over the bed at the end of each high flow. Plants established on freshly deposited bars and areas of braidplain that were unoccupied during low flow. The presence of the plants had the effect of progressively focusing the high flow so that a single dominant channel developed. The single‐thread channel self‐adjusted to carry the high flow. Vegetation also slowed the rate of bank erosion. Matching of deposition along the point bar with erosion along the outer bend enabled the channel to develop sinuosity and migrate laterally while suppressing channel splitting and the creation of new channel width. The experimental channels spontaneously reproduced many of the mechanisms by which natural meandering channels migrate and maintain a single dominant channel, in particular bend growth and channel cutoff. In contrast with the braided system, where channel switching is a nearly continuous process, vegetation maintained a coherent channel until wholesale diversion of flow via cutoff and/or avulsion occurred, by which point the previous channel tended to be highly unfavorable for flow. Thus vegetation discouraged the coexistence of multiple channels. Varying discharge was key to allowing expression of feedbacks between the plants and the flow and promoting the transition from braiding to a single‐thread channel that was then dynamically maintained. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
12.
《国际泥沙研究》2016,(4):360-367
Studies regarding the influence of emergent vegetation on sediment transport are scarce and have mainly focused on flume conditions. To fill this gap and also meet the international need, we aimed to evaluate the influence of emergent vegetation (Echinodorus macrophyllus) on sediment transport of Capibaribe River, Brazil. Bedload and suspended sediment measurements were carried out using the US BLH 84 and US DH 48 samplers, respectively. Measurements of stem diameter, stem spacing and plant density were performed in conjunction with flow and sediment field measurements. Based on our results, 0.45 m s ? 1 was the threshold of mean flow velocity supported by E. macrophyllus under field conditions. This value can be helpful for other rivers with gravel-bed river to armoured layer ratio (AR ? D50-surface)/D50-subsurface ? 12.50) – natural conditions observed in Capibaribe River – or where the vegetation can provide positive effects, such as increase the bed stability, assist water restoration/rehabilitation and decrease water turbidity. Our results can hopefully be used in engineering practice and ecosystem management. In general, both the drag coefficient and drag force varied inversely and directly with the mean flow velocity and vegetation density, respectively. The vegetation resistance force was inversely proportional to the bedload transport owing to the resistance caused by emergent vegetation. This finding was supported by the clear decoupling between nonvegetated and emergent vegetated conditions indicated by cluster analysis. The study results provided a reasonable understanding of the interaction between emergent vegetation, water flow and sediment transport in the Capibaribe River.&2016 International Research and Training Centre on Erosion and Sedimentation/the World Association 相似文献
13.
Influence of bank vegetation and gravel bed on velocity and Reynolds stress distributions 总被引：2，自引：1，他引：1
This paper presents the results of a laboratory flume experimental study on the interaction of bank vegetation and gravel bed on the flow velocity （primarily on the location of the maximum velocity, Umax） and the Reynolds stress distributions. The results reveal that the dip of the maximum velocity below the water surface is up to 35% of flow depth and the difference between Umax and the velocity at the water surface is considerable in the presence of vegetation on the walls. The zone of the log-law varies from y/h=2 up to 15 percent of flow depth and it does not depend on distance from the wall. Deviation of the velocity profile in the outer layer over a gravel bed with vegetation cover on the walls is much larger than the case of flow over a gravel bed without vegetation cover on the walls. The presence of vegetation on the walls changes uniform flow to non-uniform flow. This fact can be explained by considering the nonlinear Reynolds stress distribution and location of maximum velocity in each profile at different distances across the flume. The Reynolds stress distributions at the distance 0.02 m from the wall have negative values and away from the wall, they change the sign taking positive values with specific convex form with apex in higher location. Average of von Karman constant κ for this study is equal to 0.16. Based on to=0.16, the methods of Clauser and the Reynolds stress are compatible for determination of shear velocity. 相似文献
14.
Experimental observations in a tilting flume having a bed covered with rice plants (Oryza sativa) are used to analyse the flow characteristics of flexible emergent vegetation with downward seepage. The flow velocity for no-seepage and with seepage is reduced by, on average, 52% and 33%, respectively, as the flow reaches the downstream end with vegetation. Higher Reynolds stress occurs at the start of the vegetation zone; hence, bed material transport occurs in this region. The results indicate that the bed is no longer the primary source of turbulence generation in vegetated flow; rather it is dominated by turbulence generated by the vegetation stems. The local effect of the presence of vegetation causes variations in the hydrodynamic characteristics along the vegetated portion of the channel, which leads to erosion and deposition in the vegetation zone. The experiments show that vegetation can provide considerable stability to channels by reducing channel erosion even with downward seepage. 相似文献
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This paper presents an approach to modeling the depth-averaged velocity and bed shear stress in compound channels with emergent and submerged vegetation. The depth-averaged equation of vegetated compound channel flow is given by considering the drag force and the blockage effect of vegetation, based on the Shiono and Knight method (1991) [40]. The analytical solution to the transverse variation of depth-averaged velocity is presented, including the effects of bed friction, lateral momentum transfer, secondary flows and drag force due to vegetation. The model is then applied to compound channels with completely vegetated floodplains and with one-line vegetation along the floodplain edge. The modeled results agree well with the available experimental data, indicating that the proposed model is capable of accurately predicting the lateral distributions of depth-averaged velocity and bed shear stress in vegetated compound channels with secondary flows. The secondary flow parameter and dimensionless eddy viscosity are also discussed and analyzed. The study shows that the sign of the secondary flow parameter is determined by the rotational direction of secondary current cells and its value is dependent on the flow depth. In the application of the model, ignoring the secondary flow leads to a large computational error, especially in the non-vegetated main channel. 相似文献
16.
Influence of junction angle on three‐dimensional flow structure and bed morphology at confluent meander bends during different hydrological conditions 下载免费PDF全文
下载免费PDF全文 Recent field and modeling investigations have examined the fluvial dynamics of confluent meander bends where a straight tributary channel enters a meandering river at the apex of a bend with a 90° junction angle. Past work on confluences with asymmetrical and symmetrical planforms has shown that the angle of tributary entry has a strong influence on mutual deflection of confluent flows and the spatial extent of confluence hydrodynamic and morphodynamic features. This paper examines three‐dimensional flow structure and bed morphology for incoming flows with high and low momentum‐flux ratios at two large, natural confluent meander bends that have different tributary entry angles. At the high‐angle (90°) confluent meander bend, mutual deflection of converging flows abruptly turns fluid from the lateral tributary into the downstream channel and flow in the main river is deflected away from the outer bank of the bend by a bar that extends downstream of the junction corner along the inner bank of the tributary. Two counter‐rotating helical cells inherited from upstream flow curvature flank the mixing interface, which overlies a central pool. A large influx of sediment to the confluence from a meander cutoff immediately upstream has produced substantial morphologic change during large, tributary‐dominant discharge events, resulting in displacement of the pool inward and substantial erosion of the point bar in the main channel. In contrast, flow deflection is less pronounced at the low‐angle (36°) confluent meander bend, where the converging flows are nearly parallel to one another upon entering the confluence. A large helical cell imparted from upstream flow curvature in the main river occupies most of the downstream channel for prevailing low momentum‐flux ratio conditions and a weak counter‐rotating cell forms during infrequent tributary‐dominant flow events. Bed morphology remains relatively stable and does not exhibit extensive scour that often occurs at confluences with concordant beds. Copyright © 2014 John Wiley & Sons, Ltd. 相似文献
17.
The mechanism of energy balance in an open-channel flow with submerged vegetation was investigated. The energy borrowed from the local flow, energy spending caused by vegetation drag and flow resistance, and energy transition along the water depth were calculated on the basis of the computational results of velocity and Reynolds stress. Further analysis showed that the energy spending in a cross-section was a maximum around the top of the vegetation, and its value decreased progressively until reaching zero at the flume bed or water surface. The energy borrowed from the local flow in the vegetated region could not provide for spending; therefore, surplus borrowed energy in the non-vegetated region was transmitted to the vegetated region. In addition, the total energy transition in the cross-section was zero; therefore, the total energy borrowed from the flow balanced the energy loss in the whole cross-section. At the same time, we found that there were three effects of vegetation on the flow: turbulence restriction due to vegetation, turbulence source due to vegetation and energy transference due to vegetation, where the second effect was the strongest one. 相似文献
18.
Experimental results of the mean flow field and turbulence characteristics for flow in a model channel bend with a mobile sand bed are presented. Acoustic Doppler velocimeters (ADVs) were used to measure the three components of instantaneous velocities at multiple cross sections in a 135° channel bend for two separate experiments at different stages of clear water scour conditions. With measurements at multiple cross sections through the bend it was possible to map the changes in both the spatial distribution of the mean velocity field and the three Reynolds shear stresses. Turbulent stresses are known to contribute to sediment transport and the three‐dimensionality inherent to flow in open channel bends presents a useful case for determining specific relations between three‐dimensional turbulence and sediment entrainment and transport. These measurements will also provide the necessary data for validating numerical simulations of turbulent flow and sediment transport. The results show that the magnitude and distribution of three‐dimensional Reynolds stresses increase through the bend, with streamwise‐cross stream and cross stream‐vertical components exceeding the maximum principal Reynolds stress through the bend. The most intriguing observation is that near‐bed maximum positive streamwise‐cross stream Reynolds stress coincides with the leading edge of the outer bank scour hole (or thalweg), while maximum cross stream‐vertical Reynolds stress (in combination with high negative streamwise‐cross stream Reynolds stress near the bend apex) coincides with the leading edge of the inner bank bar. Maximum Reynolds stress and average turbulent kinetic energy appear to be greater and more localized over the scour hole before final equilibrium scour is reached. This suggests that the turbulent energy in the flow is higher while the channel bed is developing, and both lower turbulent energy and a broader distribution of turbulent stresses near the bed are required for cessation of particle mobilization and transport. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
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20.
The movement of bedload in subcritical flow produces additional roughness as compared to flow in a rigid bed. The magnitude of this bed load roughness is proportional to the thickness of the sediment layer moving along the bed, the particle size and the sediment concentration. In a supercritical flow, however, further resistance is expected due to the momentum absorption by the high flow velocity. In this study the effect of sediment movement on the flow resistance in supercritical flow was experimentally investigated. The experiments included flows over smooth and rough beds carrying sediment of mean diameters D50=2.80, 5.42 and 7.06 mm in a rigid rectangular channel. The results show that the sediment transport may increase the friction factor by up to 90% and 60% in smooth and rough beds, respectively. Bedload extracts its momentum from the flow, which causes a reduction of near bed flow velocity and steeper velocity gradient near the bed resulting in an increase in shear velocity as well as in roughness height. The increase in friction factor is directly related to bedload concentration and particle size. 相似文献