Bed Load Particles Research Articles

Upscaling Sediment‐Flux‐Dependent Fluvial Bedrock Incision to Long Timescales

AbstractFluvial bedrock incision is driven by the impact of moving bedload particles. Mechanistic, sediment‐flux‐dependent incision models have been proposed, but the stream power incision model (SPIM) is frequently used to model landscape evolution over large spatial and temporal scales. This disconnect between the mechanistic understanding of fluvial bedrock incision on the process scale, and the way it is modeled on long time scales presents one of the current challenges in quantitative geomorphology. Here, a mechanistic model of fluvial bedrock incision that is rooted in current process understanding is explicitly upscaled to long time scales by integrating over the distribution of discharge. The model predicts a channel long profile form equivalent to the one yielded by the SPIM, but explicitly resolves the effects of channel width, cross‐sectional shape, bedrock erodibility, and discharge variability. The channel long profile chiefly depends on the mechanics of bedload transport, rather than bedrock incision. In addition to the imposed boundary conditions specifying the upstream supply of water and sediment, and the incision rate, the model includes four free parameters, describing the at‐a‐station hydraulic geometry of channel width, the dependence of bedload transport capacity on channel width, the threshold discharge of bedload motion, and reach‐scale cover dynamics. For certain parameter combinations, no solutions exist. However, by adjusting the free parameters, one or several solutions can usually be found. The controls on and the feedbacks between the free parameters have so far been little studied, but may exert important controls on bedrock channel morphology and dynamics.

Numerical Simulations of Meanders Migrating Laterally as They Incise Into Bedrock

AbstractThe problem of meandering in mixed bedrock‐alluvial rivers is more challenging than that of purely alluvial streams, in that alluvial, bed incisional and bank incisional morphodynamics must be accounted for. Here we present a numerical formulation that addresses heretofore unanswered questions. Bed incision is based on abrasion due to saltating grains. The model satisfies mass conservation of alluvium over a partially covered bedrock surface. Bank incision is treated in terms of a measure of the incipient collision of bedload particles with the bank. It is assumed that land accretes along the inside of point bars when the water depth falls below a specified shallow value. All but one of the runs are performed with repetitive two‐step hydrographs. Runs starting from a low‐amplitude sine‐generated curve indicate that sinuosities at least as high as 2.5 can be achieved. The rate of increase of sinuosity declines in time, but does not vanish. For the same hydrograph, increasing the initial thickness of alluvium on the bed causes the rate of vertical bedrock incision to decline, and bend sinuosity to increase at a faster rate. At a sufficiently high thickness, the channel migrates laterally without bed lowering. Bend shape can change dramatically with increasing alluvial thickness, with high thickness favoring more regular bend trains. For the same initial alluvial thickness, increasing the peak flow of the hydrograph causes the vertical incision rate and the rate of sinuosity growth to increase. The model thus captures a wide range of behavior associated with bedrock meandering.

A velocity-variation-based formulation for bedload particle hops in rivers

Abstract

The influence of channel morphology on bedload path lengths: Insights from a survival process model

SummaryTracer studies are a commonly used tool to develop and test Einstein‐type stochastic bedload transport models. The movements of these tracers are controlled by many factors including grain characteristics, hydrologic forcing, and channel morphology. Although the influence of these sediment storage zones related to morphological features (e.g., bars, pools, riffles) have long been observed to “trap” bedload particles in transport, this influence has not been adequately quantified. In this paper we explore the influence of channel morphology on particle travel distances through the development of a Bayesian survival process model. This model simulates particle path length distributions using a location‐specific “trapping probability” parameter (pi), which is estimated using the starting and ending locations of bedload tracers. We test this model using a field tracer study from Halfmoon Creek, Colorado. We find that (1) the model is able to adequately recreate the observed multi‐modal path length distributions, (2) particles tend to accumulate in trapping zones, especially during large floods, and (3) particles entrained near a trapping zone will travel a shorter distance than one that is further away. Particle starting positions can affect path lengths by as much as a factor of two, which we confirm by modelling “starting‐location‐specific” path length probability distributions. This study highlights the importance of considering both tracer locations and channel topography in examinations of field tracer studies. © 2020 John Wiley & Sons, Ltd.

An Image-Based Uninterrupted Lagrangian Tracking Method for Measurement of Sediment Motion at Grain Scale

The imaging technique provides an efficient non-intrusive way for studying local sediment transport with a low rate in open-channel flows. It aims to track all sediment trajectories above the background consists of similar particles (i.e., top-view images of the channel). For this area of interest, currently used imaging methods can be summarized as a two-step framework for identifying and matching active bed-load particles. While effective against a simple and clear background, the two-step approach fails to yield accurate and uninterrupted Lagrangian paths for the sediment particles against complex image background consists of similar particles. This study presents a three-step approach to improve the accuracy of the tracking method. The first two steps of this approach based on image subtraction, centroid exaction and Kalman filter entail improvements to those of the classic methods. The third step based on the nearest neighbor algorithm and spline interpolation manage to identify broken chains and connect them to reconstruct uninterrupted Lagrangian paths of the sediment particles. The verification against simulated images and experimental data shows that the improved three-step approach yields more accurate estimation of bed-load kinematics than the classic two-step method.

Bed-Load Collision Sound Filtering through Separation of Pipe Hydrophone Frequency Bands

Bed-load discharge of a river can be monitored by indirectly measuring the acoustic pulses generated when the bed load collides with a steel pipe installed on the riverbed (i.e., pipe hydrophone measurement). However, existing methods used for filtering pulses from acoustic signals reflect a combination of bed-load collision frequency bands, thereby limiting characterization capabilities. This study proposes an improved filtering method that separates and efficiently examines frequency bands that are highly correlated with bed-load collision characteristics. Herein, an experimental hydraulic model and bed-load collision sound-measurement system were constructed, and bed-load collision experiments were repeatedly performed for collecting acoustic data using a pipe hydrophone. Fast Fourier Transform analysis was performed on data to select the specific frequency bands and pressures reflecting the bed-load particle size. Furthermore, a bandpass method to examine bed-load collision sounds is also presented herein. Results indicate that in comparison with existing filtering methods, the proposed bandpass method yields higher detection rates under bed-load conditions of low flow rate and small particle size, thereby demonstrating its enhanced effectiveness.

A Mechanistic Model for Lateral Erosion of Bedrock Channel Banks by Bedload Particle Impacts

AbstractBedrock incision plays a key role in determining the pace of landscape evolution. Much is known about how bedrock rivers incise vertically, but less is known about lateral erosion. Lateral erosion is widely thought to occur when the bed is alluviated, which prevents vertical erosion and deflects the downstream transport of bedload particles into channel walls. Here we develop a model for lateral erosion by bedload particle impacts. The lateral erosion rate is the product of the volume eroded per particle impact and the impact rate. The volume eroded per particle impact is modeled by tracking the motion of bedload particles from collision with roughness elements to impacts on the wall. The impact rate on the wall is calculated from deflection rates on roughness elements. The numerical model further incorporates the coevolution of wall morphology, shear stress, and erosion rate. The model predicts the undercut wall shape observed in physical experiments. The nondimensional lateral erosion rate is used to explore how lateral erosion varies under different relative sediment supply (ratio of supply to transport capacity) and transport stage conditions. Maximum lateral erosion rates occur at high relative sediment supply rates (~0.7) and moderate transport stages (~10). The competition between lateral and vertical erosion is investigated by coupling the saltation‐abrasion vertical erosion model with our lateral erosion model. The results suggest that vertical erosion dominates under near 75% of supply and transport stage conditions but is outpaced by lateral erosion near the threshold for full bed coverage.

Field Application and Validation of a Seismic Bedload Transport Model

AbstractBedload transport drives morphological changes in gravel‐bed streams and sediment transfer in catchments. The large impact forces associated with bedload motion and its highly dynamic spatiotemporal nature make it difficult to monitor bedload transport in the field. In this study, we revise a physically‐based model of bedload‐induced seismic ground motion proposed by Tsai et al. (2012) and apply it to invert bedload flux from seismic measurements alongside an Alpine stream. First, we constrain the seismic response of a braided river reach with a simple active experiment using a series of large‐rock impacts. This allows the characterization of surface wave propagation and attenuation with distance from the impact source. Second, we distinguish bedload‐generated ground vibrations from those caused by turbulent flow using frequency‐based scaling relationships between seismic power and discharge. Finally, absolute bedload transport rates are quantified from seismic measurements using inverse modeling based on a simplified formulation of bedload particle motion. The results are verified with a large data set of bedload samples (between 0.01 and 1 kg/m/s), demonstrating that seismic measurements can provide an indirect measure for bedload flux with an uncertainty of less than one order of magnitude. Larger deviations may be due to the contribution of turbulent flow effects and remaining uncertainties in particle impact mechanics (elasticity and velocity). When constraining these uncertainties, seismic monitoring may provide an accurate and continuous means to investigate bedload dynamics in gravel‐bed streams.

Numerical investigation on the incipient motion of non-spherical sediment particles in bedload regime of open channel flows

Theoretical analysis and nowadays advanced numerical simulations for bedload particle transport widely share a popular assumption: All sediments are spherical and can be simplified as spheres, which apparently differs from reality. In order to explore the shape effects of sediment particles on their transport and to find out the possible consequence originated from the spherical assumption, a particle resolved computational model is established by combining the large eddy simulation for fluid and the discrete element method for the solid particles using an immersed boundary (IB) technique. Using this model, simulations are conducted for spherical and non-spherical bedload particles in both laminar and turbulent open channel flows. Simulation results show that shape of sediment particles has direct influence on the incipient motion in both laminar and turbulent flows. In laminar flows, the widely adopted spherical particles tend to exhibit over 5 times higher traveling speed compared with the non-spherical particles, and over 4 times longer time lag during incipient motion. In turbulent flows, the disk-shaped non-spherical particle yields up to 60% higher travel speed, 50% larger saltation height and 126% longer saltation length compared with spherical particles. The travel modes of non-spherical sediment particles during incipient motion are also analyzed.

Generalization of Hop Distance‐Time Scaling and Particle Velocity Distributions via a Two‐Regime Formalism of Bedload Particle Motions

AbstractTo date, there is no consensus on the probability distribution of particle velocities during bedload transport, with some studies suggesting an exponential‐like distribution while others a Gaussian‐like distribution. Yet, the form of this distribution is key for the determination of sediment flux and the dispersion characteristics of tracers in rivers. Combining theoretical analysis of the Fokker‐Planck equation for particle motions, numerical simulations of the corresponding Langevin equation, and measurements of motion in high‐speed imagery from particle‐tracking experiments, we examine the statistics of bedload particle trajectories, revealing a two‐regime distance‐time (L‐Tp) scaling for the particle hops (measured from start to stop). We show that particles of short hop distances scale as L ~ giving rise to the Weibull‐like front of the hop distance distribution, while particles of long hop distances transition to a different scaling regime of L ~ Tp leading to the exponential‐like tail of the hop distance distribution. By demonstrating that the predominance of mostly long hop particles results in a Gaussian‐like velocity distribution, while a mixture of both short and long hop distance particles leads to an exponential‐like velocity distribution, we argue that the form of the probability distribution of particle velocities can depend on the physical environment within which particle transport occurs, explaining and unifying disparate views on particle velocity statistics reported in the literature.

Combining multi-physical measurements to quantify bedload transport and morphodynamics interactions in an Alpine braiding river reach

Understanding the interactions between bedload transport and morpholdynamics in braided streams has important applications in river management and restoration. Direct field measurements addressing this question are however scarce as they are often challenging to perform. Here, we report an extensive two-month field campaign in an Alpine braided reach (La Séveraisse river, French Alps) that experienced predictable daily peak discharge (48 events observed) generating significant bedload transport and morphological changes during the melting season. We monitored these processes using a wide range of direct and indirect techniques (bedload sampling, continuous seismic measurements, pebbles tracking, topographic surveys, remote sensing using ground control cameras and drone flights). Doing so, surrogate measurements allowed to extend temporally discrete manual bedload sampling, and to extend spatially local riverbed cross section measurements. These measurements provide unique complementary constraints on the targeted physics, at various spatial and temporal scales which enabled us to draw robust conclusions. Data showed a progressive decrease in bedload transport for a given flow rate along the two months period. Simultaneously, river morphology in the braided sections changed from an incised to a more distributed configuration which led to a decrease of local maxima in dimensionless shear stresses in the braided reach for similar flow conditions. This control of bedload transport by maximum local shear stresses was in line with tracked pebble surveys indicating that coarse bedload particles were mostly transported in the main active channel. At the reach scale, this transport was found to be more efficient in laterally confined sections than in braided ones which has important implications in terms of bedload estimation in alternative confined and braided (unconfined) rivers. Finally, this study highlight the interest to combine a large variety of traditional and innovative measurements techniques to better understand complex sediment transport processes in the field.

Particle image velocimetry validation for quantifying bedload movement

A Particle Image Velocimetry (PIV) technique was validated for predicting movements of bedload particles. Synthetic images, similar to those from testing a physical model, were created with different image particle densities and particle displacements. A set of surface particles were digitally moved by rotating and translating them on top of a layer of stationary particles to replicate the mobile-bed layer in the physical model. The images were processed using PIV software and performance of the software to measure these movements was evaluated. The effect of image particle density, particle displacement, and interrogation window size on the PIV software performance was investigated. Study results showed that all investigated variables had a significant effect on the predicted velocities. Guidelines are provided for application of PIV to bedload transport based on study results. Optimum bedload velocity predictions were found with the combination of highest particle density and highest particle displacements.

Transient Anomalous Diffusion and Advective Slowdown of Bedload Tracers by Particle Burial and Exhumation

AbstractThe process of burial and exhumation of bedload particles within a certain depth of the riverbed leads to vertical exchange of particles, which significantly affects the characteristics of streamwise bedload transport. In this paper, we revisit the classic active layer formulation and extend it by incorporating the burial and exhumation through conceptualizing the fluctuations of bed surface as the relative vertical movement of buried tracer particles in the substrate layer (i.e., we change the static reference system to the fluctuating riverbed surface). We theoretically demonstrate, for the first time, the emergence of the transient anomalous diffusion (both superdiffusion and subdiffusion) and power‐law advective slowdown at the intermediate timescales, which are induced by the nonequilibrium transport as characterized by the inhomogeneous vertical mixing of tracers due to particle burial and exhumation. Neglecting the ballistic regime at extremely short times, at small and large timescales, the transport regimes show normal diffusion. This result further implies that for the most typical fluvial riverbed with finite vertical exchange depth (i.e. nonaggrading or nondegrading bed), the subdiffusion of bedload tracers for large timescale transport may still be transient, which will eventually converge to the normal diffusion as time increases. Comparing the obtained analytical solutions with available numerical results as well as field observations, we show that the proposed formulation can capture well anomalous diffusion and the power‐law slowdown of the advective velocity of bedload tracers at intermediate timescales, and more importantly the transition from anomalous to normal diffusion at large timescales.

Continuous time random walk model for non-uniform bed-load transport with heavy-tailed hop distances and waiting times

Bed-load transport along widely graded river-beds typically exhibits anomalous dynamics, whose efficient characterization may require parsimonious stochastic models with pre-defined statistics involving the waiting time and hop distance distributions for sediment particles. This study employs a continuous time random walk (CTRW) model to characterize bed-load particle motions on a widely graded gravel-bed with cluster microforms built in our lab. Flume experiments guide the selection of the Mittag-Leffler (M-L) function as the waiting time distribution function, and the Lévy α-stable density for the hop distance distribution function in the CTRW model. Monte Carlo simulations show that the resulting CTRW model can well capture the observed flume experimental data (with either a continuous or an instantaneous source) with coexisting super- and sub-dispersion behaviors in the bed-load transport process. Analyses further discover the dual impact of clusters on the dynamics of fine sediment particles. Some particles are more likely to be blocked or trapped by clusters, while others have a high probability to be accelerated by the flow accelerating belt between the clusters. Therefore, with proper statistical distributions and relevant parameters for sediment waiting times and hop distances, the CTRW model may efficiently capture the complex dynamics in sediment transport.

Acoustic wave propagation in rivers: an experimental study

Abstract. This research has been conducted to develop the use of passive acoustic monitoring (PAM) in rivers, a surrogate method for bedload monitoring. PAM consists in measuring the underwater noise naturally generated by bedload particles when impacting the river bed. Monitored bedload acoustic signals depend on bedload characteristics (e.g., grain size distribution, fluxes) but are also affected by the environment in which the acoustic waves are propagated. This study focuses on the determination of propagation effects in rivers. An experimental approach has been conducted in several streams to estimate acoustic propagation laws in field conditions. It is found that acoustic waves are differently propagated according to their frequency. As reported in other studies, acoustic waves are affected by the existence of a cutoff frequency in the kilohertz region. This cutoff frequency is inversely proportional to the water depth: larger water depth enables a better propagation of the acoustic waves at low frequency. Above the cutoff frequency, attenuation coefficients are found to increase linearly with frequency. The power of bedload sounds is more attenuated at higher frequencies than at low frequencies, which means that, above the cutoff frequency, sounds of big particles are better propagated than sounds of small particles. Finally, it is observed that attenuation coefficients are variable within 2 orders of magnitude from one river to another. Attenuation coefficients are compared to several characteristics of the river (e.g., bed slope, surface grain size). It is found that acoustic waves are better propagated in rivers characterized by smaller bed slopes. Bed roughness and the presence of air bubbles in the water column are suspected to constrain the attenuation of acoustic wave in rivers.

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.

Spatial and temporal patterns of sediment storage over 45 years in Carnation Creek, BC, a previously glaciated mountain catchment

AbstractThe movement of sediment through mountain river networks remains difficult to predict, as processes beyond streamflow and particle size are responsible for the entrainment and transport of bedload sediment. In deglaciated catchments, additional complexity arises from glacial impacts on landscape organization. Research to date indicates that the quantity of sediment stored in the channel is an important component of sediment transport in systems which alternate between supply and transport limited states, but limited long‐term field data exist which can capture storage‐transfer dynamics over a timescale encompassing episodic supply typical of mountain streams. We use a 45‐year dataset with annual and decadal‐scale data on sediment storage, channel morphology, and wood loading to investigate the spatial and temporal organization of storage in Carnation Creek, a previously glaciated 11 km2 catchment on Vancouver Island, British Columbia.Sediment is supplied episodically to the channel, including additions from debris flows in the early 1980s just upstream of the studied channel region. Analyzing the spatial and temporal organization of sediment storage along 3.0 km of channel mainstem reveals a characteristic storage wavelength similar to the annual bedload particle travel distance. Over time, two scales of variation in storage are observed: small‐scale fluctuation of 3–10 years corresponding to local erosional and depositional processes, and larger scale response over 25–35 years related to supply of sediment from hillslopes. Complex relationships between storage and sediment transfer (i.e., annual change in storage) are identified, with decadal‐scale hysteresis present in storage‐transfer relations in sites influenced by hillslope sediment and logjams. We propose a conceptual model linking landscape organization to temporal variability in storage and to storage–export cycles. Collectively, our results reaffirm the importance of storage to sediment transport and channel morphology, and highlight the complexity of storage–transport interactions. © 2019 John Wiley & Sons, Ltd.

REGREENING OF THE NORTHERN ETHIOPIAN MOUNTAINS: EFFECTS ON FLOODING AND ON WATER BALANCE


 
 
 The hydro-geomorphology of mountain catchments is mainly determined by vegetation cover. This study was carried out to analyse the impact of vegetation cover dynamics on flooding and water balance in 11 steep (0.27-0.65 m m-1) catchments of the western Rift Valley escarpment of Northern Ethiopia, an area that experienced severe deforestation and degradation until the first half of the 1980s and considerable reforestation thereafter. Land cover change analysis was carried out using aerial photos (1936,1965 and 1986) and Google Earth imaging (2005 and 2014). Peak discharge heights of 332 events and the median diameter of the 10 coarsest bedload particles (Max10) moved in each event in three rainy seasons (2012-2014) were monitored. The result indicates a strong reduction in flooding (R2 = 0.85, P<0.01) and bedload sediment supply (R2 = 0.58, P<0.05) with increasing vegetation cover. Overall, this study demonstrates that in reforesting steep tropical mountain catchments, magnitude of flooding, water balance and bedload movement is strongly determined by vegetation cover dynamics.
 KEY WORDS: HYDRO-GEOMORPHOLOGY, REFORESTATION, CREST STAGE, PEAK DISCHARGE, BEDLOAD
 
 

Experimental Censorship of Bed Load Particle Motions and Bias Correction of the Associated Frequency Distributions

Knowledge of the statistical distributions of particle hop properties (distances, travel, and rest times) enables a deeper understanding of bed load sediment transport. However, the measurement of particle hops is prone to censorship: Since many hops cross the boundaries of a spatial‐temporal observation window, one knows that they exist but does not know how long they are. An option is to build particle hop samples considering only the hops that are completely observed and excluding (censoring) those observed only partially. Such a choice, however, biases the frequency distributions of the hop properties. Moreover, censorship acts in both space and time, and a hop censored in time will also not contribute to a sample of hop lengths, and vice versa. Time censorship similarly applies to particle rest times. This paper presents a theoretical formulation of censorship that leads to nonparametric bias corrections recovering estimates of values of the underlying distributions of hop distance, travel time, and rest time up to sampling window dimensions. We illustrate the occurrence and consequences of experimental censorship, and the benefit of applying the bias corrections, for both synthetic and laboratory samples of particle hops. The corrections reasonably recover the relative proportions of frequency distributions represented by the data up to the sampling dimensions and improve the estimates of the first two moments of particle hop properties. Recommendations are given regarding how the size of an observation window may be chosen to reduce the bias to below some prescribed value, if the forms of the underlying distributions are known.

Diffusive Regimes of the Motion of Bed Load Particles in Open Channel Flows at Low Transport Stages

AbstractThe stochasticity of fluid and sediment parameters has been identified as a source of diffusion, particularly anomalous diffusion at different temporal and spatial scales of bed load particle trajectories. Data from two sets of flume experiments are presented, one data set has gravel particle trajectories tracked over a limited area and was used in identifying the influence of different shear stress conditions on diffusive processes. A new experiment was performed using spherical particles moving as bed load in an annular flume in order to address concerns about censorship effects caused by the size of the detection window. An annular flume allowed collection of practically uncensored particle trajectories over longer time period than has been previously possible in the laboratory. Three diffusive regimes were observed at distinct stages of particle motion: (i) ballistic regime at the local range; (ii) Fickian diffusion at the intermediate range; (iii) subdiffusion at the global range. Characteristic time scales separate the regimes and correlate with the mean traveling and resting times of particles. Fickian diffusion in the intermediate range is first recognized as a result of the balance between intermittent weak transport and near‐bed turbulence, as first predicted by Nikora et al. (2002, https://doi.org/10.1029/2001WR000513). In the global range, extreme values were observed in the distribution of particle resting times, suggesting that two types of distributions (related to surface motion and vertical mixing) were responsible for the subdiffusion at longer time scales. Diffusion was found to be anisotropic at all stages of particle motion.