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1.
In the Erlenbach stream, a pre‐alpine steep channel in Switzerland, sediment transport has been monitored for more than 25 years. Near the confluence with the main valley river, stream flow is monitored and sediment is collected in a retention basin with a capacity of about 2000 m3. The basin is surveyed at regular intervals and after large flood events. In addition, sediment transport has been continuously monitored with piezoelectric bedload impact and geophone sensors since 1986. In 2008–2009, the measuring system in the Erlenbach stream was enhanced by installing an automatic system to obtain bedload samples. Movable metal baskets are mounted on a rail at the downstream wall of the large check dam above the retention basin, and they can be moved automatically into the flow to take bedload transport samples. The wire mesh of the baskets has a spacing of 10 mm to sample all sediment particles coarser than this size (which is about the limiting grain size detected by the geophones). The upgraded measuring system permits to obtain bedload samples over short sampling periods and to measure the grain size distribution of the transported material and its variation over time and with discharge. The analysis of calibration relationships for the geophone measuring system confirms findings from very similar measurements which were performed until 1999 with piezoelectric bedload impact sensors; there is a linear relationship between impulse counts and bedload mass passing over the sensors. Findings from flume experiments are used to discuss the most important factors which affect the calibration of the geophone signal. The bedload transport rates as measured by the moving baskets are among the highest measured in natural streams, with values of the order of several kilograms per meter per second. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   

2.
Field data are essential in evaluating the adequacy of predictive equations for sediment transport. Each dataset based on the sediment transport rates and other relevant information gives an increased understanding and improved quantification of different factors influencing the sediment transport regime in the specific environment. Data collected for 33 sites on 31 mountain streams and rivers in Central Idaho have enabled the analysis of sediment transport characteristics in streams and rivers with different geological, topographic, morphological, hydrological, hydraulic, and sedimentological characteristics. All of these streams and rivers have armored, poorly sorted bed material with the median particle size of surface layer coarser than the subsurface layer. The fact that the largest particles in the bedload samples did not exceed the median particle size of the bed surface material indicates that the armor layer is stable for the observed flow discharges (generally bankfull or less, and in some cases two times higher than bankfull discharge). The bedload transport is size‐selective. The transport rates are generally low, since sediment supply is less than the ability of flow to move the sediment for one range of flow discharges, or, the hydraulic ability of the stream is insufficient for entrainment of the coarse bed material. Detailed analyses of bedload transport rates, bedload and bed material characteristics were performed for each site. The obtained results and conclusions are used to identify different influences on bedload transport rates in analyzed gravel‐bed rivers. Copyright © 2008 John Wiley & Sons, Ltd.  相似文献   

3.
A comprehensive monitoring programme focusing on bedload transport behaviour was conducted at a large gravel‐bed river. Innovative monitoring strategies were developed during five years of preconstruction observations accompanying a restoration project. A bedload basket sampler was used to perform 55 cross‐sectional measurements, which cover the entire water discharge spectrum from a 200‐year flood event in 2013 to a rare low flow event. The monitoring activities provide essential knowledge regarding bedload transport processes in large rivers. We have identified the initiation of motion under low flow conditions and a decrease in the rate of bedload discharge with increasing water discharge around bankfull conditions. Bedload flux strongly increases again during high flood events when the entire inundation area is flooded. No bedload hysteresis was observed. The effective discharge for bedload transport was determined to be near mean flow conditions, which is therefore at a lower flow discharge than expected. A numerical sediment transport model was able to reproduce the measured sediment transport patterns. The unique dataset enables the characterisation of bedload transport patterns in a large and regulated gravel‐bed river, evaluation of modern river engineering measures on the Danube, and, as a pilot project has recently been under construction, is able to address ongoing river bed incision, unsatisfactory ecological conditions for the adjacent national park and insufficient water depths for inland navigation. Copyright © 2017 John Wiley & Sons, Ltd.  相似文献   

4.
A reliable characterization of bedload transport is required to gauge the engineering and theoretical issues related to the dynamics of sediments transport in rivers. However, while significant advances have been made in the development of monitoring techniques, robust quantitative predictive relationships have proven difficult to derive. In this article, we develop a dedicated signal processing technique aimed at improving the usage of impact plate measurements for material transport characterization. Our set‐up consists of a piezoelectric hydrophone mounted on the bottom side of a stainless steel plate, thus acting as a ‘sediment vibration sensor’. While the classical analysis with such systems is usually limited to rather simple procedures, such as impact counting, a large amount of useful information is contained in the actual waveform of the impact signal, which conveys the force and the contact time that the bedload imposes on the plate. An advanced signal processing technique called ‘first arrival atomic decomposition’ is used to improve the characterization of bedload transport by analysing the amplitude and frequency attributes of each single impact. This new processing approach proves to be well suited for bedload transport monitoring using plate systems and allows us to establish a relationship between the median grain size (D50) and the impact signal properties. This link is first observed and validated with controlled flume experiments and then applied to continuous impact records in a small gravel‐bed river during a flood event. The estimated D50 offers a novel possibility to observe the time‐varying grain size distribution of bedload transport. Copyright © 2014 John Wiley & Sons, Ltd.  相似文献   

5.
A reliable estimation of sediment transport in gravel‐bed streams is important for various practical engineering and biological studies (e.g., channel stability design, bed degradation/aggradation, restoration of spawning habitat). In the present work, we report original laboratory experiments investigating the transport of gravel particles at low bed shear stresses. The laboratory tests were conducted under unsteady flow conditions inducing low bed shear stresses, with detailed monitoring of the bed topography using a laser scanner. Effects of bed surface arrangements were documented by testing loose and packed bed configurations. Effects of fine sediments were examined by testing beds with sand, artificial fine sand or cohesive silt infiltrated in the gravel matrix. Analysis of the experimental data revealed that the transport of gravel particles depends upon the bed arrangement, the bed material properties (e.g., size and shape, consolidation index, permeability) and the concentration of fine sediments within the surface layer of moving grains. This concentration is directly related to the distribution of fine particles within the gravel matrix (i.e., bottom‐up infiltration or bridging) and their transport mode (i.e., bedload or suspended load). Compared to loose beds, the mobility of gravel is reduced for packed beds and for beds clogged from the bottom up with cohesive fine sediments; in both cases, the bed shear stress for gravel entrainment increases by about 12%. On the other hand, the mobility of gravel increases significantly (bed shear stress for particle motion decreasing up to 40%) for beds clogged at the surface by non‐cohesive sand particles. Copyright © 2017 John Wiley & Sons, Ltd.  相似文献   

6.
The morphological active width, defined as the lateral extent of bed material displacement over time, is a fundamental parameter in multi‐threaded gravel‐bed rivers, linking complex channel dynamics to bedload transport. Here, results are presented from five constant discharge experiments, and three event hydrographs, covering a range of flow strengths and channel configurations for which morphological change, bedload transport rates, and stream power were measured in a physical model. Changes in channel morphology were determined via differencing of photogrammetrically‐derived digital elevation models (DEMs) of the model surface generated at regular intervals over the course of ~115 h of experimental runs. Independent measures of total bedload output were made using downstream sediment baskets. Results indicate that the morphological active width increases with total and dimensionless stream power and is strongly and positively correlated with bulk change (total volume of bed material displaced over time) and active braiding intensity (ABI). Although there is considerable scatter due to the inherent variability in braided river morphodynamics, the active width is positively correlated with independent measurements of bedload transport rate. Active width, bulk change, and bedload transport rates were all negligible below a dimensionless stream power threshold value of ~ 0.09, above which all increase with flow strength. Therefore, the active width could be used as a general predictor of bulk change and bedload transport rates, which in turn could be approximated from total and dimensionless stream power or ABI in gravel‐bed braided rivers. Furthermore, results highlight the importance of the active width, rather than the morphological active depth, in predicting volumes of change and bedload transport rates. The results contribute to the larger goals of better understanding braided river morphodynamics, creating large high‐resolution datasets of channel change for model calibration and validation, and developing morphological methods for predicting bedload transport rates in braiding river systems. Copyright © 2018 John Wiley & Sons, Ltd.  相似文献   

7.
The monitoring of bedload flux under flash flood conditions has been successfully achieved since 1992 using slot samplers in the semiarid Nahal Eshtemoa. In the present study, a surrogate bedload monitoring technique - the Japanese plate microphone - has been deployed and calibrated against data from the slot samplers. Since a slot sampler has a sensitivity threshold that becomes especially important when transport rates are low, different averaging periods should be considered for high and low fluxes. In order to overcome the deficiencies of time-based aggregation used hitherto, we have developed a new method involving mass aggregation and commensurably variable intervals, thereby enabling a more accurate analysis and optimizing the bedload sampler's capabilities. The data derived with this new method has then been utilized to calibrate the Japanese plate microphone. The Eshtemoa is an ephemeral gravel bed channel with a high proportion of fine gravel (< 0.02 m); for these conditions, acoustic sensors have not been calibrated as yet. Two multiple linear regression models incorporating the effect of median bedload grain size on pulse rate have been established to predict bedload flux and cumulative transported bedload mass. The coefficients in these models are statistically significant. Good predictions are obtained for bedload flux (adj. r2 = 0.83) and for cumulative bedload mass (adj. r2 = 0.98) during flood recession. Overall, the multiple linear regression models, used in conjunction with the mass aggregation method of estimating bedload flux, suggest that field calibration of acoustic devices is feasible under these conditions for ca. 90% of the duration of bedload transport. © 2020 John Wiley & Sons, Ltd.  相似文献   

8.
We monitor bedload transport and water discharge at six stations in two forested headwater streams of the Columbia Mountains, Canada. The nested monitoring network is designed to examine the effects of channel bed texture, and the influence of alluvial (i.e. step pools and riffle pools) and semialluvial morphologies (i.e. boulder cascades and forced step pools) on bedload entrainment and transport. Results indicate that dynamics of bedload entrainment are influenced by differences in flow resistance attributable to morphology. Scaled fractional analysis shows that in reaches with high form resistance most bedload transport occurs in partial mobility fashion relative to the available bed material, while calibers finer than 16 mm attain full mobility during bankfull flows. Equal mobility transport for a wider range of grain sizes is achieved in reaches exhibiting reduced form resistance. Our findings confirm that the Shields value for mobilization of the median surface grain size depends on channel gradient and relative submergence; however, we also find that these relations vary considerably for cobble and gravel bed channels due to proportionality between dimensionless shear stress and grain size. Exponents of bedload rating curves across sites correlate most with the D90s of the mobile bed, however, where grain effects are controlled (i.e. along individual streams), differences in form resistance across morphologies exert a primary control on bedload transport dynamics. Application of empirical formulae developed for use in steep alpine channels present variable success in predicting transport rates in forested snowmelt streams. Formulae that explicitly account for reductions in mobile bed area and high morphological resistance associated with woody debris provide the best approximation to observed empirical data. Copyright © 2014 John Wiley & Sons, Ltd.  相似文献   

9.
Suspended load transport can strongly impact ecosystems, dam filling and water resources. However, contrary to bedload, the use of physically based predicting equations is very challenging because of the complexity of interactions between suspended load and the river system. Through the analysis of extensive data sets, we investigated extent to which one or several river bed or flow parameters could be used as a proxy for quantifying suspended fluxes in gravel bed rivers. For this purpose, we gathered in the literature nearly 2400 instantaneous field measurements collected in 56 gravel bed rivers. Among all standard dimensionless parameters tested, the strongest correlation was observed between the suspended sediment concentration and the dimensionless bedload rate. An empirical relation between these two parameters was calibrated. Used with a reach average bedload transport formula, the approach allowed to successfully reproduce suspended fluxes measured during major flood events in seven gravel bed alpine rivers, morphodynamically active and distant from hillslope sources. These results are discussed in light of the complexity of the processes potentially influencing suspended load in a mountainous context. The approach proposed in this paper will never replace direct field measurements, which can be considered the only confident method to assess sediment fluxes in alpine streams; however, it can increment existing panel tools that help river managers to estimate even rough but not unrealistic suspended fluxes when measurements are totally absent. © 2019 John Wiley & Sons, Ltd.  相似文献   

10.
In bedload transport modelling, it is usually presumed that transported material is fed by the bed itself. This may not be true in some mountain streams where the bed can be very coarse and immobile for the majority of common floods, whereas a finer material, supplied by bed‐external sources, is efficiently transported during floods, with marginal morphological activities. This transport mode was introduced in an earlier paper as ‘travelling bedload’. It could be considered an extension of the washload concept of suspension, applied to bedload transport in high‐energy, heavily armoured streams. Since this fine material is poorly represented in the bed surface, standard surface‐based approaches are likely to strongly underestimate the true transport in such streams. This paper proposes a simple method to account for travelling bedload in bedload transport estimations. The method is tested on published datasets and on a typical Alpine stream, the Roize (Voreppe, France). The results, particularly on active streams that experience greater transport than expected from the grain sizes of their bed material, reinforce the necessity of accounting for the ‘travelling bedload concept’ in bedload computation. The method relevance is discussed regarding varying flood magnitudes, geomorphic responses and eventual anthropic origin of the ‘travelling bedload’ phenomena. To conclude, this paper considers how to compute bedload transport for a wide range of situations, ranging from sediment‐starved cases to the general mobile bed alluvial case, including the intermediate situation of external source supply on armoured bed. Copyright © 2017 John Wiley & Sons, Ltd.  相似文献   

11.
The pipe microphone has been shown to be an effective means for monitoring bedload transport in mountain streams. It is commonly installed perpendicular to the flow direction on a stable river bed, such as that of a check dam. Acoustic pulses caused by bedload collisions with the pipe are detected by a microphone. However, bedload particles saltating over the pipe remain undetected. To overcome this disadvantage, we installed a horizontal as well as a vertical pipe microphone in the Ashi‐arai‐dani supercritical channel located in the Hodaka mountain range, Japan. The vertical pipe was installed on the wall of the channel and the horizontal pipe was installed on the channel bed. The acoustic response of the horizontal pipe is expected to be larger than that of the vertical pipe, because the bedload concentration decreases with increasing height above the bed. However, at high amplifications, the peak pulse value from the vertical pipe is higher than that from the horizontal pipe. We explain this observation as follows: under high bedload discharge conditions, the pulses of the horizontal pipe are saturated but those of the vertical pipe are not. We proposed a ratio (Rhv) between the pulses detected by these sensors, and applied this ratio for calibrating the contemporaneous pulses detected by a microphone located immediately upstream of a bedload slot sampler. Indeed the Rhv‐corrected pulses correlated well with the bedload discharge calculated from the sampler, supporting our explanation. We conclude that bedload monitoring using concomitant vertical and horizontal pipe microphones can be used to calibrate centrally located pipe microphones when the bedload concentration is approximately homogeneous laterally across the width of the channel cross‐section, and thereby represent bedload discharges more accurately than with only a single pipe microphone. Copyright © 2017 John Wiley & Sons, Ltd.  相似文献   

12.
River incision is fundamental in shaping the Earth's surface. In mountainous regions with steep river beds, fluvial bedrock erosion by bedload transport is an important mechanism forming channels. However, there are only a few complete field datasets that can be used to improve process understanding and evaluate erosion models, especially at the process scale. To provide a simultaneous dataset of hydraulics, bedload transport and bedrock erosion at high temporal and spatial resolution, a new measuring device has been installed in the Erlenbach, a gauged stream in the Swiss Pre‐Alps. In this stream, bedload transport rates can be calculated from surveying deposits and from geophone plate sensors and bedload transport samples can be taken directly by an automated moving basket system. To measure bedrock erosion rates simultaneously, two natural stone slabs were mounted flush with the channel bed in a steel frame hosting various measurement devices. Force sensors below the slabs record normal stress and shear stress. At‐a‐point erosion rates on the slab surfaces are continuously measured at sub‐millimetre precision at three locations on each slab. In addition, the slab topography is monitored following erosive flood events. In this article (i) the ‘erosion scale’ device is described, (ii) data resolution and data quality is assessed by means of tests and event data, and (iii) the first transport event is discussed. The erosion scales are confirmed to provide data at high spatio‐temporal resolution for process analysis. The preliminary data show evidence for the tools effect in bedrock erosion. The bedrock slabs can be exchanged to obtain measurements for catchments with different lithologies for comparison. Copyright © 2014 John Wiley & Sons, Ltd.  相似文献   

13.
Bedload, the transport of sediment remaining in contact with the stream bed, has mainly been studied from the perspective of the correlation between fluid driving forces and the responding sediment flux. Yet grain–grain interactions are important and bedload should also be considered as a granular phenomenon. We review progress made recently in the study of granular flows, especially on segregation and rheology, that better illuminates the nature of bedload. Granular flows may exhibit gas‐like or fluid‐like flow, or quasi‐solid deformation. All three conditions might be duplicated in bedload. Understanding of intense bedload transport occurring continuously in a layer several grains deep – typical of sand beds – might greatly benefit from results in granular physics, as illustrated by grain‐inspired bedload results. However, processes restricted to the surface of the bed, when particles move intermittently and the bed becomes structured, while characteristic in gravel‐bed channels, are not well addressed in granular physics. Mutual study of these phenomena may benefit both physics and fluvial geomorphology. We intend, therefore, to contribute to an enhanced dialogue between granular physics and bedload science communities. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   

14.
Differences in the transport rate and size of bedload exist for varying levels of flow in coarse‐grained channels. For gravel‐bed rivers, at least two phases of bedload transport, with notably differing qualities, have been described in the literature. Phase I consists primarily of sand and small gravel moving at relatively low rates over a stable channel surface. Transport rates during Phase II are considerably greater than Phase I and more coarse grains are moved, including material from both the channel surface and subsurface. Transition from Phase I to Phase II indicates initiation and transport of grains comprising the coarse surface layer common in steep mountain channels. While the existence of different phases of transport is generally acknowledged, the threshold between them is often poorly defined. We present the results of the application of a piecewise regression analysis to data on bedload transport collected at 12 gravel‐bed channels in Colorado and Wyoming, USA. The piecewise regression recognizes the existence of different linear relationships over different ranges of discharge. The inflection, where the fitted functions intersect, is interpreted as the point of transition from Phase I to Phase II transport; this is termed breakpoint. A comparison of grain sizes moved during the two phases shows that coarse gravel is rarely trapped in the samplers during Phase I transport, indicating negligible movement of grains in this size range. Gravel larger than about D16 of the channel surface is more consistently trapped during Phase II transport. The persistence of coarse gravel in bedload samples provides good evidence that conditions suitable for coarse grain transport have been reached, even though the size of the sediment approaches the size limits of the sampler (76 mm in all cases). A relative breakpoint (Rbr) was defined by the ratio between the discharge at the breakpoint and the 1·5‐year flow (a surrogate for bankfull discharge) expressed as a percentage. The median value of Rbr was about 80 percent, suggesting that Phase II begins at about 80 percent of the bankfull discharge, though the observed values of Rbr ranged from about 60 to 100 percent. Variation in this value appears to be independent of drainage area, median grain size, sorting of bed materials, and channel gradient, at least for the range of parameters measured in 12 gravel‐bed channels. Published in 2002 by John Wiley & Sons, Ltd.  相似文献   

15.
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16.
The question: ‘how does a streambed change over a minor flood?’ does not have a clear answer due to lack of measurement methods during high flows. We investigate bedload transport and disentrainment during a 1.5‐year flood by linking field measurements using fiber optic distributed temperature sensing (DTS) cable with sediment transport theory and an existing explicit analytical solution to predict depth of sediment deposition from amplitude and phase changes of the diurnal near‐bed pore‐water temperature. The method facilitates the study of gravel transport by using near‐bed temperature time series to estimate rates of sediment deposition continuously over the duration of a high flow event coinciding with bar formation. The observations indicate that all gravel and cobble particles present were transported along the riffle at a relatively low Shields Number for the median particle size, and were re‐deposited on the lee side of the bar at rates that varied over time during a constant flow. Approximately 1–6% of the bed was predicted to be mobile during the 1.5‐year flood, indicating that large inactive regions of the bed, particularly between riffles, persist between years despite field observations of narrow zones of local transport and bar growth on the order ~3–5 times the median particle size. In contrast, during a seven‐year flood approximately 8–55% of the bed was predicted to become mobile, indicating that the continuous along‐stream mobility required to mobilize coarse gravel through long pools and downstream to the next riffle is infrequent. Copyright © 2017 John Wiley & Sons, Ltd.  相似文献   

17.
Bedload transport is known to be a highly fluctuating temporal phenomenon, even under constant (mean) flow conditions, as a consequence of stochasticity, bedform migration, grain sorting, hysteresis, or sediment supply limitation. Because bedload transport formulas usually refer to a single mean transport value for a given flow condition, one can expect that prediction accuracy (when compared to measurements) will depend on the amplitude and duration of fluctuations, which in turn depend on the time scale used for observations. This paper aims to identify how the time scale considered can affect bedload prediction. This was done by testing 16 common bedload transport formulas with four data sets corresponding to different measurement period durations: (i) highly fluctuating (quasi‐)instantaneous field measurements; (ii) volumes accumulated at the event scale on two small alpine gravel‐bed rivers, potentially affected by seasonal fluctuations; (iii) volumes accumulated at the interannual scale in a meandering gravel bed river, thought to be weakly subject to fluctuations; (iv) time‐integrated flume measurements with nearly uniform sediments. The tests confirmed that the longer the measurement period, the better the precision of the formula's prediction interval. They also demonstrate several consequential limitations. Most threshold formulas are no longer valid when the flow condition is below two times the threshold condition for the largest elements' motion on the bed surface (considering D84). In such conditions, equations either predict zero transport, or largely overestimate the real transport, especially when D84 is high. There is a need for new sediment data collected with highly reliable techniques such as recording slot bedload samplers to further investigate this topic. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   

18.
We report bedload data and acoustic impulse measurements due to particle impact from the Pitzbach in Austria. Impulse counts can be viewed as a measure of the energy delivered to the bed by moving particles. Impulse counts show a large scatter even for the same discharge and bedload supply. This scatter is due to varying grain size distribution, grain shape, mode of transport of the sediment particles and spatial and temporal distribution of the sediment load. The mean impulse count at given hydraulic conditions may increase or decrease with increasing sediment supply, suggesting that both tools and cover effects are active on the channel bed. Dependent on the local balance between sediment supply and transport capacity, either effect may be dominant at different locations along the cross‐section at the same time. Furthermore, the same bed location may respond to increasing sediment supply as tools‐dominated at some discharges and cover‐dominated at other discharges. Our observations may have implications for modelling of bedrock erosion in landscape evolution models and of bedrock channel morphology. Erosion models that do not incorporate both tools and cover effects are not sufficient to describe observations. Furthermore, a local erosion law cannot in general be used to describe erosion averaged over the channel cross‐section. The changing balance between sediment supply and transport capacity with increasing discharge highlights that a single representative discharge is not sufficient to capture the full erosion dynamics. Copyright © 2008 John Wiley & Sons, Ltd.  相似文献   

19.
Monitoring sediment transport in morphologically complex and labile channels remains a difficult task, even at the laboratory scale. To address this challenge, a fully automated imagery technique for continuously mapping the spatial and temporal variability of bedload transport is proposed. This method uses differentiated time‐lapse imagery taken from a fixed camera to detect bed variations induced by grain displacement. The technique is not based on tracking the individual particles; rather, it evaluates macroscopic colour changes within a region that contains several grains, which depend on the occurrence and intensity of the bedload transport. Image‐derived data were compared with the sediment flux measured during four flume experiments, and produced good correspondence. The method provides continuous tracking of the location of the transporting channels, and enables estimation of local variations in the magnitude of the bedload flux. Moreover, the spatial extent of the monitoring area offers an unprecedented opportunity to aggregate spatially dense and continuous data at the reach scale, as needed to properly capture the full range of variability of morphologically complex and rapidly evolving gravel‐bed rivers. Despite being limited to laboratory‐scale physical experiments, the method provides useful data to investigate fundamental morphodynamic processes such as bar migration, bank erosion, anabranches opening/closure, and the associate spatial and temporal scales. Further, the data obtained have the potential to enhance numerical model calibration and improve our understanding of the complex dynamics of real‐world settings. Copyright © 2017 John Wiley & Sons, Ltd.  相似文献   

20.
There is growing acknowledgement of the interaction between animals and the river bed on which they live and the implications of biological activity for geomorphic processes. It has been observed that signal crayfish (Pacifastacus leniusculus) disturb gravel substrates, potentially promoting sediment transport and impacting ecological communities. However, the mechanisms involved and the extent of their impact remain poorly understood, especially in relation to other processes that affect grain mobility in gravel‐bed rivers. A series of flume experiments, using loose and water‐worked gravel beds of narrowly graded grain sizes that were exposed to 6 h of crayfish activity under low‐velocity flows, showed a substantial increase in the number of grains entrained by subsequent higher‐velocity flows when compared with control runs in which crayfish were never introduced. Crayfish alter the topography of their substrate by constructing pits and mounds, which affect grain protrusion. When walking and foraging, they also alter gravel fabric by reorienting and changing the friction angle of surface grains. In water‐worked surfaces, this fabric rearrangement is shown to lead to a statistically significant, partial reversal of the structuring that had been achieved by antecedent flow. For these previously water‐worked surfaces, the increase in entrainment arising from disturbance by crayfish was statistically significant, with grain transport nearly twice as great. This suggests that signal crayfish, an increasingly widespread invasive species in temperate latitudes beyond their native NW North America, have the potential to enhance coarse‐grained bedload flux by altering the surface structure of gravel river beds and reducing the stability of surface grains. This study illustrates further the importance of acknowledging the impact of mobile organisms in conditioning the river bed when assessing sediment entrainment mechanics in the context of predicting bedload flux. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

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