Bedload Transport Measurements Research Articles

Path length and sediment transport estimation from DEMs of difference: a signal processing approach

Abstract. The difficulties of measuring bedload transport in gravel-bed rivers have given rise to the morphological method wherein sediment transport can be inferred from changes in riverbed elevation and estimates of the distance traveled by sediment: its path length. Because current methods for estimating path length are time- and labor-intensive, we present a method to estimate a characteristic path length from repeat digital elevation models (DEMs of difference, i.e., DoDs). We propose an automated method to extract the spacing between erosional and depositional sites on the DoD by the application of variational mode decomposition (VMD), a signal processing method, to quantify the spacing as a proxy for path length. We developed this method using flume experiments where bed topography and sediment flux were measured and then applied it to published field data with physical path length measured from tracer measurements. Our sediment transport estimates were not significantly different than the measured sediment flux at lower discharges in the lab. However, we observed an underestimation of sediment flux at the higher discharges in the flume study. We interpret this as a limit of the method in confined settings, where sediment transport becomes decoupled from morphological changes. We also explore how the time between survey acquisitions, the morphological active width relative to the channel width, and DoD thresholding techniques affect the proposed method and the potential issues they pose for the morphological method in general.

Anatomy of an Alpine Bedload Transport Event: A Watershed‐Scale Seismic‐Network Perspective

AbstractThe way Alpine rivers mobilize, convey and store coarse material during high‐magnitude events is poorly understood, notably because it is difficult to obtain measurements of bedload transport at the watershed scale. Seismic sensor data, evaluated with appropriate seismic physical models, can provide that missing link by yielding time‐varying estimates of bedload transport albeit with non‐negligible uncertainty. Low cost and ease of installation allow for networks of sensors to be deployed, providing continuous, watershed‐scale insights into bedload transport dynamics. Here, we deploy a network of 24 seismic sensors to estimate coarse material fluxes in a 13.4 km2 Alpine watershed during a high‐magnitude transport event. First, we benchmark the seismic inversion routine with an independent time‐series of bedload transport obtained with a calibrated acoustic system. Then, we apply the procedure to the other seismic sensors across the watershed. Propagation velocities derived from cross‐correlation analysis between spatially consecutive bedload transport time‐series were too high with respect to typical bedload transport velocity suggesting that a faster‐moving water wave (re‐)mobilizes local coarse material. Spatially distributed estimates of bedload transport reveal a relative inefficiency of Alpine watersheds in evacuating coarse material, even during a relatively infrequent high‐magnitude bedload transport event. Significant inputs estimated for some tributaries were rapidly attenuated as the main river crossed less hydraulically efficient reaches. Only a small proportion of the total amount of material mobilized in the watershed was exported at the outlet. Multiple periods of competent flows are likely necessary to evacuate coarse material mobilized throughout the watershed during individual bedload transport events.

Meta-analysis of a large bedload transport rate dataset

The estimation of bedload transport, a type of coarse fluvial sediment motion, is challenging because of higher-order, complex, and time-variable environmental processes. Still, models for calculating bedload transport mostly rely on semi-empirical formulae derived from time-averaged laboratory flume or field measurements and involve cross-section averaging of hydro-environmental parameters. Additionally, many models assume infinite, constant availability of sediment, which is unrealistic in natural environments. While most published models yield excellent results on the dataset they were trained on, they may be significantly inaccurate when applied to other cases. Also, more recent formulae relying on considerable amounts of data still lead to high calculation uncertainty. While the importance of short-term dynamics, for example, driven by turbulence, for bedload transport is well known, this study investigates the relevance of time-variable processes at monthly to seasonal scales. Our analysis of a large dataset and remote sensing data shows evidence for seasonal variability in measurements of bedload transport rates and challenges the validity of Gaussian statistics for interpolating bedload transport models. Measured bedload transport rates were smaller when snowmelt was taking place and when no glacier was present in the catchment, and were best represented by extreme value distributions. By controlling for biases from measurement devices, geomorphic patterns, and the presence of an upstream dam, we show that steady, constant at-a-station hydraulics only explain a limited share of the variance in the measurements. Ultimately, a model for estimating bedload transport rates should consider the temporal variability of processes across all scales, from turbulence to climate.

Grain shear stress and bed-load transport in open channel flow with emergent vegetation

The prediction of bedload transport capacity in riverbed with aquatic vegetation is a pressing need for river restoration. Laboratory experiments were conducted to investigate the weak bed-load transport rates with emergent vegetation for different energy slopes and vegetation densities. Such rate was observed to increase as the energy slope increased and the vegetation density decreased. The bed shear stress calculated by subtracting the vegetation drag from the total flow driving force was unable to predict the measured bed-load transport rate. On the other side, the estimation of the grain shear stress using the Einstein partitioning method considering the form drag provided a reasonable prediction. On the basis of these results, an explicit formulation for estimating grain shear stress was developed to predict the bed-load transport rate in a vegetated channel by considering the superimposition of forces. In addition, a simple relationship between grain shear stress and total shear stress was established by the boundary layer model. It was inferred that bed-load transport rate decreases with the increase of vegetation blocking factor while the total shear stress remains constant. Comparatively, the proposed models were more accurate than literature models, improving the understanding of the bed-load transport in a vegetated flow.

Comparison of calibration characteristics of different acoustic impact systems for measuring bedload transport in mountain streams

Abstract. The Swiss plate geophone (SPG) system has been installed and tested in more than 20 steep gravel-bed streams and rivers, and related studies generally resulted in rather robust calibration relations between signal impulse counts and transported bedload mass. Here, we compare this system with three alternative surrogate measuring systems. A variant of the SPG system uses the same frame (housing) set-up but with an accelerometer instead of a geophone sensor to measure the vibrations of the plate (GP-Acc, for geophone plate accelerometer). The miniplate accelerometer (MPA) system has a smaller dimension of the impact plate and is embedded in more elastomer material than the SPG system. The Japanese pipe microphone (JPM) is a 1 m long version of the system that has been installed in many streams in Japan. To compare the performance of the four systems, we used calibration measurements with direct bedload samples from three field sites and an outdoor flume facility with controlled sediment feed. At our field sites, the systems with an accelerometer and a microphone showed partly large temporal variations in the background noise level, which may have impaired the calibration measurements obtained during certain time periods. Excluding these periods, the SPG, GP-Acc, and JPM all resulted in robust calibration relations, whereas the calibration of the MPA system showed a poorer performance at all sites.

Experiments on the Sediment Transport Along Pool‐Riffle Unit

AbstractThis study examines channel dynamics and bed load transport through a riffle and pool sequence forced by downstream channel width variations within an experimental flume. The experiment consisted of four runs across, which we compare and contrast local and spatially averaged bed surface texture and topography, sediment transport rates, and sediment mobility at five locations across a pool‐riffle pair. Sediment transport was measured using mini Helley‐Smith samplers and particle tracers seeded in the monitored riffle and pool. In this study, “local” sediment transport rates were highly variable across the five monitoring locations. The lowest sediment transport rate was recorded at the riffle tail whereas the highest rates were measured at the riffle head and the pool center. The texture of the bed surface and transported load do not explain measured bed load transport trends and depending on how the measurements are aggregated differing interpretations are supported. In general, the bed texture in the pool was finer than the texture in the riffle, however, specific grain‐size percentile classes derived from pooled population analysis suggests little to no difference between pool and riffle texture. The combined results highlight the importance of acknowledging and applying analysis techniques to better understand the inherent variability of bed load transport within channel reaches where morphology differs, such as pools and riffles.

The use of continuous sediment‐transport measurements to improve sand‐load estimates in a large sand‐bedded river: The lower Chippewa River, Wisconsin

AbstractAccurately determining sediment loads is necessary for managing river environments but is difficult because multiple processes can lead to large discharge‐independent changes in sediment transport. Thus, estimations of sediment load using discharge–sediment rating curves fit to sparse or historical sediment‐transport measurements can be inaccurate, necessitating alternative approaches to reduce uncertainty. Continuous sediment‐transport measurements reduce uncertainty because they can be used to detect discharge‐independent changes in transport and are therefore unaffected by hysteresis.We used largely continuous approaches to measure sand transport in the lower Chippewa River, a large sand‐supplying tributary to the Mississippi River. We used side‐looking acoustic‐Doppler profilers to continuously measure suspended‐sand concentration, and bedform‐tracking techniques to episodically measure bedload transport. Bedload transport was then continuously estimated using a discharge‐dependent ratio of bedload to suspended‐sand transport. This approach allowed determination of sand loads that were not estimated based only on water discharge. Our continuous suspended‐sand measurements show that hysteresis between discharge and suspended‐sand concentration occurs during most floods. Quasi‐continuous bed‐elevation measurements using a scour monitor show that lags between discharge and dune geometric adjustment is also common, causing hysteresis between discharge and bedload transport during floods. Furthermore, comparisons of our measurements with historical sediment‐transport measurements indicate large discharge‐independent declines in both suspended‐sand and bedload transport since the 1980s. These findings indicate that sand transport is a non‐stationary function of water discharge over timescales ranging from within individual floods to decades. Consequently, although our continuous‐measurement approach yields only a ~20–30% improvement over rating‐curve estimates of sand load over multi‐year periods, our approach yields up to a factor‐of‐five improvement in sand‐load estimates over the shorter, within a flood timescales, over which the largest discharge‐independent changes in sand transport occur.

A Physical Model for Acoustic Noise Generated by Bedload Transport in Rivers

AbstractMonitoring bedload transport is of interest for studying river morphology evolution and hydraulic structures stability (e.g., dams andbridges). Bedload self‐generated noise measured by hydrophones has been experimentally correlated to bedload flux in several studies. However, the lack of theoretical background linking the recorded acoustic noise to bedload and river characteristics has prevented a good understanding of the experimental results. Here, we develop a model of the acoustic noise generated by bedload transport in rivers. The model provides an estimation of the acoustic power generated by impacts of bedload particles with riverbed particles. In this model, we account for bedload kinematics (e.g., impact velocities, saltation length) and the environment in which acoustic wave is propagated (e.g., acoustic wave attenuation). Sensitivity analysis shows that the noise generated by bedload transport depends on the grain size distribution and river characteristics such as slope, water level, and propagation effects. We tested the model on a field data set comprising acoustic and direct bedload measurements from different rivers. The acoustic powers predicted by the model are consistent with field measurements for some rivers while questionable for other ones. The analysis has shown a great sensitivity of the model to bedload kinematics (in particular the way the grain velocity is computed) and riverbed grain size distribution. The model provides a first basis that can serve as a framework in future work concerned with acoustic measurements of bedload transport. However, the model is still imperfect and it is limited by today's knowledge of the physics of bedload transport.

Bedload transport measurements on the Maros river, Hungary

Sediment transport is a vital component in hydrological and fluvial geomorphological studies, however, the temporal and spatial changes in sediment fluxes, and the efficiency of bedload samplers are rarely analysed, as bedload measurements are quite difficult. The aim of the present study is to measure the bedload transport of the sand-bedded Maros River (Hungary) at low stages using the Helley-Smith bedload sampler. In order to understand the variability in the bedload transport, the water stage and discharge across the channel section were also measured. The variability of the bedload was caused by an active in-channel bar and translational sediment pulses. The created bedload transport rating curve could be applied just below 300 m3/s water discharge thus further measurements are needed to evaluate the bedload transport of higher discharges.

On How Episodic Sediment Supply Influences the Evolution of Channel Morphology, Bedload Transport and Channel Stability in an Experimental Step‐Pool Channel

AbstractWe present results from flume experiments in which an 8% steep channel with longitudinal width variations and step‐pool morphology was subjected to sediment feed pulses of different magnitude and frequency under constant water discharge. The channel response to these pulses included (a) large bedload transport rates, (b) bed aggradation, (c) fining of the bed surface, and (d) continuous formation and collapse of steps. In between pulses, the bed surface coarsened, and bedload transport rates dropped by several orders of magnitude. Steps continuously formed and collapsed during and shortly after the pulses, but their stability increased when the sediment feed was turned off. High pulse magnitude enhanced step formation, while low pulse frequency (i.e., long interpulse period) enhanced step stability. We back‐calculated the threshold for motion based on measured bedload transport rates and bed shear stress. Changes in the threshold for motion were much larger than changes in bed surface slope. By accounting for energy dissipation through the effective slope based on flow resistance partitioning, a better prediction was obtained. The threshold for motion decreased following sediment pulses then increased immediately after and fluctuated until the next sediment pulse. Our results indicate that longitudinal width variations and episodic sediment supply are primary controls on the evolution of step‐pool channels. Sediment feed magnitude affects mostly morphological changes, while sediment feed frequency controls channel stability.

Effect of Sediment Supply on Cyclic Fluctuations of the Disequilibrium Ratio and Threshold Transport Discharge, Inferred From Bedload Transport Measurements Over 27 Years at the Swiss Erlenbach Stream

AbstractBedload transport in natural channels typically shows large fluctuations even for constant hydraulic forcing. It has been shown that the threshold shear stress for initiation of motion depends, for example, on the bed morphology and on channel slope and that it may be influenced by flood history effects. This study is based on a unique data set of continuous field observations of bedload transport acquired during 27 years with impact plate measurements and involving more than 500 sediment‐transporting flood events. A disequilibrium ratio is defined which represents the observed bedload mass per event divided by the bedload mass calculated via a bedload transport equation integrated over the time of a flood event. The disequilibrium ratio varies in a cyclic behavior around equilibrium conditions defined by its median value. The mean duration of a cycle depends on upstream sediment supply or availability on the bed, and longer cycles are associated with a larger active layer on the bed. A memory effect is evident regarding the temporal evolution of both disequilibrium ratio and bed state as reflected by threshold transport conditions, determined from the observations as the critical discharge at the start and at the end of a bedload‐transporting event. The disequilibrium ratio and the threshold transport discharge are inversely correlated with each other, likely providing a feedback mechanism governing the fluctuations around an equilibrium state. Accounting for the memory effect, that is, for the bed state after the previous event, improves the prediction of bedload transport for the following event.

Disturbances in coarse bedload transport in a high-mountain stream channel system (Western Tatras, Poland)

Bedload transport measurements in the formerly glaciated Chochołowski catchment located in the Western Tatras in Poland were performed in the period 1975 to 2018. Measurement of bedload displacement allowed to determine the role of bedload transport disturbances in both system connectivity and sediment transfer. Bedload transport occurred as many as triggered up to several times a year. The longest distances were observed during rain-on-snow events: between 12 m in headwaters and over 100 m in the fluvial valley. Bedload became activated along the entire length of the channel system (10.5 km) every 2 to 5 years when the stream discharge exceeded 10 m3 s−1 (with an average of 1.25 m3 s−1). In such situations, bedload dynamics increased downstream, and stream power was sufficient to overcome local barriers (i.e. boulder and log steps). Downstream increases in bedload dynamics may become disturbed and inverted by sudden snowmelt and locally heavy rainfall. These types of events caused the dynamics of bedload transport to be 190% to 320% greater in the upper part of the studied catchment and to decline in the downstream direction over a distance of 7 km to yield an attenuation effect. The same pattern was observed in local tributaries where the dynamics of bedload transport were 200% higher in the upper part and attenuation was observed along a distance of 1.5 km in the downstream direction. All events of this type cause seasonal disturbances in bedload transport. However, the greatest effect on bedload dynamics was produced by natural deforestation. A 16% decrease in forest cover causes a fluvial system disequilibrium manifested in an intensification of hydro-geomorphologic processes and formation of new landforms.

Using Historical Data to Examine the Accuracy of Sand Transport Field Measurements in Two Nearshore Marine Settings

Amos, C.L.; Kassem, H.; Townend, I.; Umgiesser, G.; Madricardo, F.; Zaggia, L.; Manfe, G.; Lorenzetti, G., and Gomez, E., 2020. Using historical data to examine the accuracy of sand transport field measurements in two nearshore marine settings. Journal of Coastal Research, 36(5), 1013–1028. Coconut Creek (Florida), ISSN 0749-0208.A re-analysis of historical data from two field campaigns was undertaken to examine the accuracy of measurements of bed load (Qb) and total transport (Qtot) of sand in (1) a wave-dominant shoreface off western Newfoundland, Canada, and (2) a tide-dominant inlet of Venice Lagoon, Italy. Video tapes recorded within Sea Carousel (a benthic annular flume) deployed off Newfoundland were used to determine the transport of medium to coarse sand under controlled unidirectional flow conditions. These results were compared with Helley-Smith sand trap measurements of bed load of fine to medium sand in a tidal inlet of Venice Lagoon, Italy. Ripple migration rates in Sea Carousel were similar to those measured in rivers and shallow marine settings at similar flows. Accuracy of sand transport rate (derived from ripple motion) was assessed by comparison to fundamental methods presented in the literature. Some of the scatter in correlations with earlier methods was removed by using a nondimensional form of total sand transport and correlating it to excess stream power (i.e. above a traction threshold). Better correlations were found between immersed (bed load) transport rate and excess stream power by applying a published adjustment to the observations for flow depth and grain diameter. Total immersed (normalized) sand transport () in Sea Carousel correlated with excess stream power in a fashion similar to results reported in the literature: = 0.288(ω – ωcr)1.65 kg m–1 s–1, where the immersed total sand transport is normalized with respect to flow depth and grain diameter. The sand trap data also followed this fit in part (2006 data only) but demonstrated greater scatter. The data herein thus fell in line with those reported in the literature from a wide variety of flume and field settings and for a wide variety of grain sizes. It is concluded that annular benthic flumes offer a reasonable and reliable method of assessing sand transport under controlled conditions of flow. The results from Sea Carousel and the Helley-Smith traps appear to follow the same relationships and so appear compatible. However, benthic sand traps show a higher degree of scatter, perhaps due to the uncertainties in how they sit on the seabed, and due to the arbitrary conditions of flow to which they are subjected when deployed.

Bedload Transport Monitoring in Alpine Rivers: Variability in Swiss Plate Geophone Response.

Acoustic sensors are increasingly used to measure bedload transport in Alpine streams, notably the Swiss plate geophone (SPG) system. An impact experiment using artificial weights is developed in this paper to assess the variability in individual plate response and to evaluate the extent to which calibration coefficients can be transferred from calibrated plates to non-calibrated plates at a given measuring site and/or to other measuring sites. Results of the experiment over 43 plates at four measuring sites have notably shown (a) that the maximum amplitude (V) recorded by individual plates tends to evolve as a power law function of the impact energy (J), with an exponent slightly larger than 1, for all the plates at all measuring sites; (b) that there is a substantial propagation of energy across plates that should be taken into account for a better understanding of the signal response; (c) that the response of individual plates is in most cases consistent, which suggests that calibration coefficients are comparable within and in between measuring sites, but site-specific and plate-specific variabilities in signal response have to be considered for a detailed comparison.

Estimating Mean Bedload Transport Rates and Their Uncertainty

Measuring bedload transport rates usually involves measuring the flux of sediment or collecting sediment during a certain interval of time Δt. Because bedload transport rates exhibit significant non‐Gaussian fluctuations, their time‐averaged rates depend a great deal on Δt. We begin by exploring this issue theoretically within the framework of Markov processes. We define the bedload transport rate either as the particle flux through a control surface or as a quantity related to the number of moving particles and their velocities in a control volume. These quantities are double averaged; that is, we calculate their ensemble and time averages. Both definitions lead to the same expression for the double‐averaged mean rate and to the same scaling for the variance's dependence on the length of the sampling duration Δt. These findings lead us to propose a protocol for measuring double‐averaged transport rates. We apply this protocol to an experiment we ran in a narrow flume using steady‐state conditions (constant water discharge and sediment feed rates), in which the time variations in the particle flux, the number of moving particles, and their velocities were measured using high‐speed cameras. The data agree well with the previously defined theoretical relationships. Lastly, we apply our experimental protocol to other flow conditions (a long laboratory flume and a gravel‐bed river) to show its potential across various contexts.

How urban stormwater regimes drive geomorphic degradation of receiving streams

For streams draining urban catchments, sediment transport capacity is the key driver of physical impacts including bed sediment removal and channel incision. The main unanswered question is the relative role of flow alteration compared to sediment supply in influencing sediment transport capacity. With this objective, we computed sand and gravel bed sediment transport capacity using the Wilcock and Kenworthy two-fraction bedload transport relation for nine streams in catchments covering a gradient of urbanisation. Computations were done for typical natural bed surface material, based on conditions in the least urban study streams. We compared transport capacity distributions and cumulative transport capacity over one-year between the streams. Transport capacity was up to three orders of magnitude higher in urban streams than in forested-catchment streams. This was driven overwhelmingly by the urbanisation-induced alterations to the flow regime, with only minor feedback from channel form changes. Transport capacity was two to three orders of magnitude greater than measured bedload transport in all but the least urban streams. This excess bedload transport capacity mobilises and removes bed sediment, produces channel incision and enlargement and reduces channel complexity. Rebalancing transport capacity with sediment supply therefore requires significant flow mitigation towards pre-urban conditions. Other responses, which may theoretically help to regain this balance – channel widening, grade control, increasing roughness, sediment augmentation – are either inappropriate or only feasible following flow mitigation measures.

The Dynamics of Suspended Sediment in a Typical Alpine Alluvial River Reach: Insight From a Seasonal Survey

AbstractThe transport of suspended sediment is associated with important social, economic, and environmental issues. It is still unclear, however, how suspended sediments eroded on hillslopes are transferred downstream through the river system. In this study, we aimed to investigate this process by applying a sediment budget approach to a typical 3.5‐km‐long Alpine braided reach. Using high‐frequency suspended load measurements combined with Monte Carlo simulations for uncertainty propagation, we observed a significant buffering behavior of the reach studied. Thirty‐two of the 48 events observed during the 2‐month campaign showed significant differences between upstream and downstream mass transported as suspension, despite the reach studied was short compared to the upstream drainage area (130 km2). These differences at the event scale varied widely within an envelope comprised between a net erosion equivalent to 74% of upstream suspended mass and a net deposition equivalent to 71%. Budgets were found to be controlled at a nearly instantaneous time scale by the liquid discharges and the suspended sediment concentrations in an opposite way: for low upstream concentrations, net erosion increased when the discharges increased, while above a certain concentration, net deposition increased when the concentrations increased. Moreover, coarse particles mobility in the reach (characterized via bedload transport measurements) appeared to have a strong influence on the availability of suspended particles as both quantities evolved concomitantly through time. These observations have important implications for our understanding and modeling of the transfer of suspended particles in gravel bedded streams.

Bedload transport measurements in the Gilgel Abay River, Lake Tana Basin, Ethiopia

Little is known about the contribution of bedload flux to the total sediment load exported from tropical sand-bed rivers. Yet, predicting bedload transport rates contributes to better river management, more appropriate river engineering works design, accurate knowledge of sediment delivery rates, which greatly benefits sediment-related water resources management. Therefore, bedload, suspended load and runoff discharge were measured at the lower Gilgel Abay River in the Blue Nile catchment of Ethiopia. Catchment area is 3887 km2, average stream discharge in the rainy season 315 (±164) m3 s−1, and the stream surface gradient at the sampling station 0.0007 m m−1. A standard Helley-Smith (U.S. BL-84) cable-suspended sampler was used to take 315 samples to measure bedload fluxes for the 2017 rainy season. The bedload predominantly comprises well to moderately well sorted, medium-sized sand (250–500 µm) with a symmetric and leptokurtic distribution. The mean daily observed bedload discharge varies from 0.7 to 427.5 Mg d−1 with an average rate of 175.1 Mg d−1. Field observations also show that the bedload dynamics become larger near the river banks and decrease towards the middle of the river section due to the presence of a central bridge pier. The average suspended sediment load ranges from 247 to 220,028 Mg d−1 and dominates the total sediment yield (>94.6%). The bedload accounts for only 0.1 to 5.4% of the total sediment load. The area-specific bedload and suspended load are also 0.6 and 22 Mg ha−1 yr−1 respectively. The elongated planform growth of the Gilgel Abay River delta in Lake Tana is also in line with the predominance of very fine, suspended load in the sediment transport of this river, in addition to limited alongshore drifting given the low waves of Tana lake. Among the tested bedload prediction criteria, the stream-power based equations of Martin and modified Bagnold performed better. Yet, the bedload prediction performance of simple power regression equations based on runoff discharge and excess unit stream power parameters provided the best agreement with the observed bedload data (Adj. R2 0.82–0.87). Finally, this study provides plausible estimates of bedload fluxes that are within predicted ranges reported for sand-bed rivers in Africa and other tropical regions.

Sand particle velocities over a subaqueous dune slope using high‐frequency image capturing

AbstractThis work presents measurements and analysis of sand particle velocities over a subaqueous dune with median sand diameter of 0.85 mm. Time‐lapse images of the mobile bed and an automated particle image velocimetry (PIV)‐based cross‐correlation method are used to obtain mean velocity of sand particles. This technique is shown to be consistent with measurements obtained with manual tracing. The measurements indicate an increase in mean particle velocity over a dune slope. Three regions are distinguished over the dune slope: (1) region of fluctuating particle velocity, (2) region of increasing particle velocity, and (3) region of maximum particle velocity. The observations are aligned with experimental and numerical modelling studies, indicating fluctuations in flow velocity over a dune stoss slope. We furthermore show that the standard deviation of the mean particle velocity is affected by the slope location and decreases from the lower slope towards the upper slope. The particle velocity variability is discussed in the context of general onset and cessation of sediment transport, the effect of the reattachment zone, sweep‐transport events, and the existence of superimposed bedforms. With this work we bridge the gap between measurements of bedload transport at the particle‐scale and at the bedform‐scale. © 2019 John Wiley & Sons, Ltd.

Evaluation of bed load equations using field measured bed load and bed material load

ABSTRACT Bed load transport can be determined by direct measurement of bed load being transported by a stream. In India, measurement of bed load is rarely done. In the present study, bed load has been measured by unequal width incremental method using United States Geological Survey (USGS) bed load sampler (Modified-Helley–Smith bed load sampler) and bed material has been measured using scoop type sampler for the Ver stream in Mandvi region of Surat, Gujarat, India. From the field measurements and analysis, it is seen that the Ver stream has dominance of bed load transport with a Rouse number higher than 2.5 for all the selected cross sections. The measured average bed load transport rate is found to be 7.77 t/day. The measured bed load transport rate has been tested using two prominent bed load models. In 1950 Brown proposed the reach averaged approach which was augmented and supported by Recking in 2013. Both Brown's and Recking's reach averaged approaches have been used for computing bed load of Ver stream using particle size of bed load and bed material. Bed load computed using Recking's approach matches well with the observed bed load while Brown's model over-predicted bed load transport rate using bed load particle size. Overall, Recking's reach-averaged approach performs well using bed load particle size and bed material particle size.