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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.
Alternate bars were formed by sediment transport in a flume with Froude-modelled flow and relative roughness characteristic of gravel-boulder channels with steep slopes. The flume (0.3 m wide × 7.5 m long) was filled with a sand-gravel mixture, which was also fed into the top of the flume at a constant rate under constant discharge. Channel slope was set at 0.03. Initially, coarse particles accumulated on incipient bar heads near one side of the flume and diverted flow and bedload transport across the flume toward a pool scoured against the opposite flume wall downstream. Sorting in the pool directed coarse particles onto the next bar head downstream. Alternate sequences of pools and coarse bar heads were thereby linked down the entire flume by interactions of sediment sorting, flow, and channel morphology. During episodes of bar construction, unsorted bedload invaded interior bar surfaces and was deposited. Persistent deposition of coarse particles on bar heads prevented downstream migration of bars by inhibiting bar-head erosion and bedload transport over bars. Likely factors leading to bar-head stabilization in modelled gravel-bed channels are coarse mixed-size sediment, steep channel gradients, and shallow depths.  相似文献   

3.
Understanding bedload transport fluctuations in rivers is crucial for complementing the existing knowledge on sediment transport theory. In this contribution, we use a natural-scale laboratory flume to analyse bedload transport fluctuations in non-uniform sand under normal flow conditions. Based on the significance of downward seepage, we incorporate the seepage effect on bedload transport over a non-uniform sand bed channel. The weight of the dry material was measured, and the volumetric transport rate per unit width (bedload transport rate) was estimated. An important observation is that the bedload transport rate initially rapidly increases with time and reaches a maximum value. Based on experimental data, we propose an empirical expression to estimate temporal bedload transport. In addition, an empirical model for bedload transport is proposed by incorporating downward seepage among other variables. The performance of several existing bedload transport formulae was also taken into account by the experimental datasets.  相似文献   

4.
We report on bedload transport observations using piezoelectric bedload impact sensors (PBIS), an indirect method of estimating the volume of bedload transport of coarse sediment. The PBIS device registers vibrations produced by bedload (particle diameter >～20 mm) and records the signal as a sum of the number of impulses per time. Sediment transport at the Erlenbach stream has been continuously monitored with a PBIS array starting in 1986. The sensor array spans the width of an entire cross‐section and is mounted flush with the surface of a check dam immediately upstream of a sediment retention basin. We compare PBIS data with long‐term sedimentation records obtained from repeated surveys of material stored in the sediment retention basin, with artificial sediment input under controlled conditions in the field, and also with laboratory experiments. The rate of bedload transport is proportional to the number of impacts on the sensor per unit time. The reliability of the calibration relationship increases with the length of the observation period, e.g. for higher numbers of impacts and larger bedload volumes. Sediment volumes for individual flood events estimated with the PBIS method are in agreement with volumes estimated using an independent empirical method based on the effective runoff volume of water, the peak water discharge, and the critical discharge for the onset of sediment transport. Copyright © 2007 John Wiley & Sons, Ltd.  相似文献   

5.
Sediment transport during flood events often reveals hysteretic patterns because flow discharge can peak before (counterclockwise hysteresis) or after (clockwise hysteresis) the peak of bedload. Hysteresis in sediment transport has been used in the literature to infer the degree of sediment availability. Counterclockwise and clockwise hysteresis have been in fact interpreted as limited and unlimited sediment supply conditions, respectively. Hysteresis has been mainly explored for the case of suspended sediment transport, but it was rarely reported for bedload transport in mountain streams. This work focuses on the temporal variability of bedload transport in an alpine catchment (Saldur basin, 18.6 km2, Italian Alps) where bedload transport was monitored by means of an acoustic pipe sensor which detects the acoustic vibrations induced by particles hitting a 0.5m‐long steel pipe. Runoff dynamics are dominated by snowmelt in late spring/early summer, mostly by glacier melt in late summer/early autumn, and by a combination of the snow and glacier melt in mid‐summer. The results indicate that hysteretic patterns during daily discharge fluctuations are predominantly clockwise during the snowmelt period, likely due to the ready availability of unpacked sediments within the channel or through bank erosion in the lower part of the basin. On the contrary, counterclockwise hysteresis tend to be more frequent during late glacier melting period, possibly due to the time lag needed for sediment provided by the glacial and peri‐glacial area to be transported to the monitoring section. However, intense rainfall events occurring during the glacier melt period generated predominantly clockwise hysteresis, thus indicating the activation of different sediment sources. These results indicate that runoff generation processes play a crucial role on sediment supply and temporal availability in mountain streams. Copyright © 2014 John Wiley & Sons, Ltd.  相似文献   

6.
It is important to evaluate bedload discharge and temporal changes of the bed surface, and bed deformation can be estimated during floods if the bedload discharge is properly evaluated in an arbitrary cross‐section. With the exception of grain size and its distribution within the bedload, bedload discharge has been measured using both direct and indirect methods. Bedload slot is a direct method but cannot be used to measure bedload during a flood because of volume limitations. Indirect methods require correlation between the signals and sediment volume measured using another method. In the present study, a small, automatically recording bedload sensor with an iron plate and a pair of load cells is developed in order to evaluate not only large particles but also sand particles as bedload. Bedload mass is calculated by integrating with respect to both the velocity of sediment particles and the averaged particle weight as measured by a pair of load cells, and, as an example, the velocity is estimated by the cross‐correlation function of weights measured by load cells. The applicability of the proposed sensor is discussed based on the results of flume tests in the laboratory (2014) and the observation flume of the Hodaka Sedimentation Observatory of Kyoto University in Japan (2015). The system was installed in the observation flume in November of 2012, and flume data were obtained using natural sediment particles. In particular, it was difficult to estimate the velocity of averaged bedload particles, and it was better to apply a cross‐correlation function in the laboratory tests. However, it appears that the previous estimation can estimate these velocities in the observation flume using a connecting tube and submerged load‐cell systems. Copyright © 2017 John Wiley & Sons, Ltd.  相似文献   

7.
Anthropogenic climate change is expected to change the discharge and sediment transport regime of river systems. Because rivers adjust their channels to accommodate their typical inputs of water and sediment, changes in these variables can potentially alter river morphology. In this study, a hierarchical modeling approach was developed and applied to examine potential changes in reach‐averaged bedload transport and spatial patterns of erosion and deposition for three snowmelt‐dominated gravel‐bed rivers in the interior Pacific Northwest. The modeling hierarchy was based on discharge and suspended‐sediment load from a basin‐scale hydrologic model driven by a range of downscaled climate‐change scenarios. In the field, channel morphology and sediment grain‐size data for all three rivers were collected. Changes in reach‐averaged bedload transport were estimated using the Bedload Assessment of Gravel‐bedded Streams (BAGS) software, and the Cellular Automaton Evolutionary Slope and River (CAESAR) model was used to simulate the spatial pattern of erosion and deposition within each reach to infer potential changes in channel geometry and planform. The duration of critical discharge was found to control bedload transport. Changes in channel geometry were simulated for the two higher‐energy river reaches, but no significant morphological changes were found for a lower‐energy reach with steep, cohesive banks. Changes in sediment transport and river morphology resulting from climate change could affect the management of river systems for human and ecological uses. Copyright © 2015 John Wiley & Sons, Ltd.  相似文献   

8.
To describe the exchange of water and sediment through the Venice Lagoon inlets a 3-D hydrodynamic and sediment transport model has been developed and applied to a domain comprising Venice Lagoon and a part of the Adriatic Sea. The model has been validated for both current velocities and suspended particle concentration against direct observations and from observations empirically derived fluxes from upward-looking acoustic Doppler current profiler probes installed inside each inlet. The model provides estimates of the suspended sediment transport in the lower 3 m of the water column that is not detected by acoustic Doppler current profiler sensors. The bedload model prediction has been validated against measured sand transport rates collected by sand traps deployed in the Lido and Chioggia inlets. Results indicate that, in the Lido inlet, 87% of the total load is in suspension, while the rest moves as bedload.  相似文献   

9.
In August 2005 severe flood events occurred in the Alps. A sediment routing model for steep torrent channel networks called SETRAC has been applied to six well‐documented case study streams with substantial sediment transport in Austria and Switzerland. For these streams information on the sediment budget along the main channel is available. Flood hydrographs were reconstructed based on precipitation data and stream gauges in neighbouring catchments. Different scenarios are modelled and discussed regarding sediment availability and the effect of armouring and macro‐roughness on sediment transport calculations. The simulation results show the importance of considering increased flow resistance for small relative flow depth when modelling bedload transport during high‐intensity flood events in torrents and mountain rivers. Without any correction of increased flow resistance using a reduced energy slope, the predicted bedload volumes are about a factor of 10 higher on average than the observed values. Simulation results were also used for a back‐calculation of macro‐roughness effects from bedload transport data, and compared with an independent estimate of flow resistance partitioning based on flow resistance data. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   

10.
A 1:50 scale hydraulic model was designed, based on Froude number similarity and using hydrological and sediment data from a small braided gravel-bed river (the North Branch of the Ashburton River, Canterbury, New Zealand). Eighteen experiments were conducted; seven using steady flows, and eleven using unsteady flows. The experiments were carried out in a 20 m × 3 m tilting flume equipped with a continuous sediment feed and an automated data acquisition and control system. In all experiments water at 30°C was used to reduce viscosity-related scale effects. Analyses of the experimental data revealed that bedload transport rates in braided channels are highly variable, with relative variability being inversely related to mean bedload transport rate. Variability was also found to be cyclic with short-term variations being caused by the migration of bedforms. Bedload transport was found to be more efficient under steady flow than under unsteady flow, and it was postulated that this is caused by a tendency for channel form to evolve towards a condition which maximizes bedload transport for the occurring flow. Average bedload transport rate was found to vary with channel form, although insufficient measurements were made to define a relationship.  相似文献   

11.
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   

12.
A system has been installed to automatically monitor rainfall, streamflow, bedload discharge and suspended sediment concentration in the arid to hyper‐arid setting of Nahal Rahaf, Southern Judean Desert in Israel. The Rahaf gauging station is located in a relatively steep, wide channel with an unsteady bed driven by flash floods. It is an attempt to deploy modern automatic equipment for continuous sediment transport monitoring in harsh, arid fluvial environments. Unit bedload discharges are the highest recorded hitherto, suggesting they may represent an upper end member in the worldwide climate–bedload discharge relationship. Suspended sediment concentration is much higher than is typical of perennial fluvial humid environments. There is high correlation between suspended sediment concentration and water discharge on an event scale, with diverse intra‐event relations. The sediment yield of individual events is large, but the small number of floods limits the mean annual sediment yield to low values in this arid environment. This also has environmental implications, as large‐scale quarrying requires a long period of self‐restoration in such an arid fluvial setting. Copyright © 2005 John Wiley & Sons, Ltd.  相似文献   

13.
Hydraulic engineering is usually based on theoretical analysis and/or numerical modelling simulation. As the dynamic behaviour of sediment movement under unsteady flow is still unclear, and field measurement is comparatively difficult during a large flood, prior investigations through flume experiments are required. A series of flume experiments, conducted using different inflow hydrographs without sediment supply from upstream, was carried out to investigate the sediment transport process under unsteady flow conditions. A series of triangular hydrographs were performed in the experiments. The results indicate that a temporal lag was found between the flow hydrograph peak and the sediment hydrograph peak because large size sand dunes lasted for a short period in the falling limb of the flow hydrograph. The temporal lag was found to be about equal to 6–15% of the flow hydrograph duration. Owing to the temporal lag, the total bedload yield in the rising period was less than that in the falling period. Furthermore, the measured total bedload yield in the unsteady flow experiments was larger than the predicted value, which was estimated by using the results obtained from the equivalent steady flow experiment. The peak bedload transport rate for unsteady flow conditions was also larger than the predicted value. The ratios of the measured to the predicted quantities mentioned above were found to be constant values for different shapes of hydrographs. It is, therefore, expected that the analytical results of sediment transport from equivalent steady flow can be a good reference for sediment transport under unsteady flow conditions. Copyright © 2004 John Wiley & Sons, Ltd.  相似文献   

14.
Only comparatively few experimental studies have been carried out to investigate the performance of the HEC-6 river morphological model. The model was developed by the Hydrologic Engineering Center of the US Army Corps of Engineers. In this study, experiments were carried out in a 20 m long concrete flume 0.6 m wide with varying rectangular cross-sections. The channel bed is paved with uniform sand of D50 = 0.9 mm and D90 = 1.2 mm within the test reach of 12 m. Two types of experiments were carried out with sediment transport, one under steady uniform flow and another under steady non-uniform flow conditions. Nine steady uniform flow experiments were carried out to compare the measured equilibrium relationship of flow and sediment transport rate with two bedload formulae, namely, Du Boys and Meyer–Peter and Muller, and with three total load formulae, namely, Toffaleti, Laursen and Yang. It was found that even though the sediment transport consists of a certain portion of bedload, the total load formulae give satisfactory results and better agreement than the two bedload formulae. Five steady non-uniform flow experiments were carried out under various conditions of varying bed profile and channel width and also with sediment addition and withdrawal. The measured transient water surface and bed profiles are compared with the computed results from the HEC-6 model. It was found that the Toffaleti and Yang total load formulae used in the HEC-6 model give the most satisfactory prediction of actual bed profiles under various conditions of non-uniform flow and sediment transport. The effects of Manning's n, variations of sediment inflow, various sediment transport formulae, sediment grain size and the model numerical parameters, i.e. distance interval Δx and numerical weighting factor, on the computed water surface and bed profiles were determined. It was found that the selection of the sediment transport formulae has the most significant effect on the computed results. It can be concluded that the HEC-6 model can predict satisfactorily a long-term average pattern of local scour and deposition along a channel with either a small abrupt change in geometry or gradually varying cross-sections. However, the accuracy of the model prediction is reduced in the regions where highly non-uniform flow occurs.  相似文献   

15.
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   

16.
Non-uniform sediment deposited in a confined, steep mountain channel can alter the bed surface composition. This study evaluates the contribution of geometric and resistance parameters to bed sta-bilization and the reduction in sediment transport. Flume experiments were done under various hydraulic conditions with non-uniform bed material and no sediment supply from upstream. Results indicate that flume channels respond in a sequence of coarsening and with the formation of bedform-roughness features such as rapids, cascades, and steps. A bedform development coefficient is introduced and is shown to increase (i.e. vertical sinuosity develops) in response to increasing shear stress during the organization process. The bedform development coefficient also is positively correlated with the critical Shields number and Manning's roughness coefficient, suggesting the evolution of flow resistance with increasing bedform development. The sediment transport rate decreases with increasing bed shear stress and bedform development, further illustrating the effect of bed stabilization. An empirical sedi-ment transport model for an equilibrium condition is proposed that uses the bedform development coefficient, relative particle submergence (i.e. the ratio of mean water depth and maximum sediment diameter), modified bed slope, and discharge. The model suggests bedform development can play a primary role in reducing sediment transport (increasing bed stabilization). The model is an extension of Lane's (1955) relation specifically adapted for mountain streams. These results explain the significance of bedform development in heightening flow resistance, stabilizing the bed, and reducing sediment transport in coarse, steep channels.  相似文献   

17.
Recent advances in fluvial seismology have provided solid observational and theoretical evidence that near-river seismic ground motion may be used to monitor and quantify coarse sediment transport. However, inversions of sediment transport rates from seismic observations have not been fully tested against independent measurements, and thus have unknown but potentially large uncertainties. In the present study, we provide the first robust test of existing theory by conducting dedicated sediment transport experiments in a flume laboratory under fully turbulent and rough flow conditions. We monitor grain-scale physics with the use of ‘smart rocks’ that consist of accelerometers embedded into manufactured rocks, and we quantitatively link bedload mechanics and seismic observations under various prescribed flow and sediment transport conditions. From our grain-scale observations, we find that bedload grain hop times are widely distributed, with impacts being on average much more frequent than predicted by existing saltation models. Impact velocities are observed to be a linear function of average downstream cobble velocities, and both velocities show a bed-slope dependency that is not represented in existing saltation models. Incorporating these effects in an improved bedload-induced seismic noise model allows sediment flux to be inverted from seismic noise within a factor of two uncertainty. This result holds over nearly two orders of magnitude of prescribed sediment fluxes with different sediment sizes and channel-bed slopes, and particle–particle collisions observed at the highest investigated rates are found to have negligible effect on the generated seismic power. These results support the applicability of the seismic-inversion framework to mountain rivers, although further experiments remain to be conducted at sediment transport near transport capacity. © 2018 John Wiley & Sons, Ltd.  相似文献   

18.
Bastos  A.  Collins  M.  Kenyon  N. 《Ocean Dynamics》2003,53(3):309-321
Numerical simulations of tidal flow and sand transport around a coastal headland (Portland Bill, southern UK) were undertaken to investigate patterns of sand transport during the development of tidally induced transient eddies. Results obtained from a 2-D finite-element hydrodynamic model (TELEMAC-2D) were combined with a sediment transport model (SEDTRANS), to simulate the sand transport processes around the headland. Simulation of the tidal flow around Portland Bill has shown the formation and evolution of tidally induced transient eddies, around the headland. During the evolution of these transient eddies, no current-induced bedload (transport) eddy is formed for either side of the headland. Net bedload sand transport direction, around a coastal headland, is the result of instantaneous gradients in bedload transport rates, during flood and ebb flows, rather than the average (residual) flow. Thus, the use of residual (water) circulation to describe patterns of sediment movement as bedload is not an appropriatedapproach. In the case study presented here, the distinct characteristics of the coastal and seabed morphology around the Isle of Portland (i.e. headland shape and the bathymetry) indicate that these parameters can be influencing tidal (flow) and sediment dispersion around the headland. Such an interpretation has broader implications and applications to headland-associated sandbanks elsewhere.Responsible Editor: Hans Burchard  相似文献   

19.
《国际泥沙研究》2021,36(6):723-735
This numerical modeling study (i) assesses the influence of the sediment erosion process on the sediment dynamics and subsequent morphological changes of a mixed-sediment environment, the macrotidal Seine estuary, when non-cohesive particles are dominant within bed mixtures (non-cohesive regime), and (ii) investigates respective contributions of bedload and suspended load in these dynamics. A three dimensional (3D) process-based morphodynamic model was set up and run under realistic forcings (including tide, waves, wind, and river discharge) during a 1-year period. Applying erosion homogeneously to bed sediment in the non-cohesive regime, i.e., average erosion parameters in the erosion law (especially the erodibility parameter, E0), leads to higher resuspension of fine sediment due to the presence of coarser fractions within mixtures, compared to the case of an independent treatment of erosion for each sediment class. This results in more pronounced horizontal sediment flux (two-fold increase for sand, +30% for mud) and erosion/deposition patterns (up to a two-fold increase in erosion over shoals, generally associated with some coarsening of bed sediment). Compared to observed bathymetric changes, more relevant erosion/deposition patterns are derived from the model when independent resuspension fluxes are considered in the non-cohesive regime. These results suggest that this kind of approach may be more relevant when local grain-size distributions become heterogeneous and multimodal for non-cohesive particles. Bedload transport appears to be a non-dominant but significant contributor to the sediment dynamics of the Seine Estuary mouth. The residual bedload flux represents, on average, between 17 and 38% of the suspended sand flux, its contribution generally increasing when bed sediment becomes coarser (can become dominant at specific locations). The average orientation of residual fluxes and erosion/deposition patterns caused by bedload generally follow those resulting from suspended sediment dynamics. Sediment mass budgets cumulated over the simulated year reveal a relative contribution of bedload to total mass budgets around 25% over large erosion areas of shoals, which can even become higher in sedimentation zones. However, bedload-induced dynamics can locally differ from the dynamics related to suspended load, resulting in specific residual transport, erosion/deposition patterns, and changes in seabed nature.  相似文献   

20.
Using the 160‐m‐long flume at Tsukuba University we undertook an experiment to provide a first estimate of the virtual velocity of sand in the size range 0.5–2.0 mm. For the flow velocity used in our experiment this sediment‐size range would conventionally be regarded as suspended sediment. The virtual velocity was found to be 37–41% of the flow velocity. Paradoxically, virtual velocity decreases as particle size decreases. Such a lower virtual velocity of finer sediment is not inconceivable. First, trapping of the sediment appears to be a function of bed roughness, and there is a probable relationship between bed roughness and trapping efficiency for particles of different sizes. Second, finer particles are more likely to find sheltered positions on a rough bed and thus experience lower mobility, relative to the more exposed coarser grains, as observed for bedload transport. Third, the virtual velocity of particles undergoing bedload transport has been found, in some instances, to be lower for finer clasts. We combine our data with previous studies of virtual velocity of bedload to develop, for the first time, a hypothesis for a holistic analysis of sediment movement in rivers. Copyright © 2017 John Wiley & Sons, Ltd.  相似文献   

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