Bedload Transport Rates Research Articles

Bedload Sediment Transport in Connection with the Geomorphological Transition of Gravel-Bed Streams in the Moravskoslezské Beskydy Mountains

This paper summarizes the results of bedload transport research in the Morávka, Mohelnice and Ostravice basins, conducted using the BAGS (Bedload Assessment for Gravel-bed Streams) spreadsheet-based program and unit stream power empirical formulas. The presented results only show the potential rate of bedload transport, because sediment inputs and barriers were not included in the model. Trends in the potential bedload transport rate represent a good instrument for identifying the range of disconnected sediment transport along the longitudinal profile. Modelling on cross-sections with a high rate of fluvial erosion shows an increase in potential bedload transport and larger diameters of the largest possible transported particles, calculated using unit stream power empirical formulas. The results may be applied to improve local watershed management.

Patterns of Morphometric Parameters in a Large Bedform Field: Development and Application of a Tool for Automated Bedform Morphometry

Patterns of Morphometric Parameters in a Large Bedform Field: Development and Application of a Tool for Automated Bedform Morphometry

Two Stochastic Fraction Bedload Transport Rate Models for Nonuniform Sediment

Two Stochastic Fraction Bedload Transport Rate Models for Nonuniform Sediment

Analysis of Bed Load Equations and River Bed Level Variations Using Basin-Scale Process-Based Modelling Approach

Bed load transport is a key process in maintaining the dynamically stable channel geometry for restoring the form and function of river ecosystems. Bed load consists of relatively large sediment particles that are moved along the streambed by rolling, sliding or saltation. Currently, various empirical correlations are used to estimate bed load transport rates since no single procedure, whether theoretical or empirical, has yet to be universally accepted as completely satisfactory in this aspect. Bed load particles are primarily sourced from river bed materials or banks. The amount of bed load and its spatial distribution contributes significantly to river bed level changes. Hillslope sediment contribution, mostly available to the river in the form of suspended load, also plays an important role in river bed level changes. This study aims to analyse different bed load equations and the resultant computations of river bed level variations using a process-based sediment dynamic model. Analyses have revealed that different bed load equations were mainly deduced from the concept of relating bed shear stresses to their critical values which are highly factored by the slope gradient, water discharge and particle sizes. In this study, river bed level variations are calculated by estimating total surplus or deficit sediment loads (suspended loads and bed loads) in a channel section. This paper describes the application of different widely used bed load equations, and evaluation of their various parameters and relative performances for a case study area (Abukuma River Basin, Japan) using a basin-scale process-based modelling approach. Relative performances of river bed level simulations obtained by using different bed load equations are also presented. This paper elaborates on the modelling approaches for river bed load and bed level simulations. Although verifications were not done due to unavailability of field data for bed load, qualitative evaluations were conducted vis-a-vis field data on flow and suspended sediment loads as well as the bed loads presented in different past studies.

Evaluation of bedload transport predictions using flow resistance equations to account for macro‐roughness in steep mountain streams

Steep mountain streams typically feature macro‐roughness elements like boulders, step‐pool sequences, and a varying channel width. Flow resistance because of such roughness elements appears to be an important control on bedload transport rates. Many commonly used bedload transport equations overestimate the transport in steep streams by orders of magnitude. Few approaches take into account the typical macro‐roughness elements, and systematic tests of these models with field observations are lacking. In the present study several approaches were considered that allow calculating the contribution of macro‐roughness elements to flow resistance. These approaches were combined with bedload transport equations and the predictions were compared to field measurements of discharge, transported bedload volumes, and channel characteristics in 13 Swiss mountain streams. The streams have channel slopes ranging from 2% to 19%, and catchment areas of 0.5 to 170 km2. For six streams there were time series of sediment yields, mostly measured annually, and for the other seven streams sediment volume estimates were available for large flood events in 2000 and 2005. All tested equation combinations achieved an improvement in bedload prediction compared to a reference equation that was uncorrected for macro‐roughness. The prediction accuracy mainly depended on the size and density of the macro‐roughness and on flow conditions. The best performance overall was achieved by an empirical approach accounting for macro‐roughness, on the basis of an independent data set of flow resistance measurements.

Land use changes, torrent control works and sediment mining: effects on channel morphology and sediment flux, case study of the Reno River (Northern Italy)

AbstractAs with most Italian rivers, the Reno River has a long history of human modification, related also to morphological changes of the lower Po River since Roman times, but in the last decades, significant land use changes in the headwaters, dam construction, torrent control works and extensive bed material mining have caused important channel morphology and sediment budget changes. In this paper, two main types of channel adjustment, riverbed incision and channel narrowing, are analysed. Riverbed degradation is discussed by comparing four different longitudinal profiles surveyed in 1928, 1951, 1970 and 1998 in the 120 km long reach upstream of the outlet. The analysis of channel narrowing is carried out by comparing a number of cross‐sections surveyed in different years across the same downstream reach. Field sediment transport measurements of seven major floods that occurred between 2003 and 2006 are compared with the bedload transport rates predicted by the most renowned equations. The current low bedload yield is discussed in terms of sediment supply limited conditions due to land use changes, erosion‐control works and extensive and out of control bed material mining that have affected the Reno during the last decades. Copyright © 2011 John Wiley & Sons, Ltd.

A seismic signature of river bedload transport during storm events

A seismic signature of river bedload transport during storm events

Probability distributions for bed form–dominated bed load transport: The Hamamori distribution revisited

The Hamamori distribution is frequently used to describe the variability of bed load transport rates in bed form–dominated environments. However, the original derivation contains several incorrect steps. Here a corrected and generalized derivation is presented. It is shown that Hamamori's original distribution function arises if the underlying assumptions are adapted slightly. Two formalisms are presented to obtain probability distributions of instantaneous bed load transport rates from growth functions of ripples and from the distribution of ripple heights. Six distribution functions, among them the exponential distribution, the Hamamori distribution, and the gamma distribution, are compared to two laboratory data sets, one for gravel and one for sand transport, and to field data from a sand‐bed river. For the gravel bed laboratory measurements, the Hamamori distribution gives a good description of the data for intermediate transport rates, while the gamma distribution performs best overall. For the sand‐bed laboratory data, the exponential distribution shows the best performance. A distribution function derived from Rayleigh‐distributed ripple heights performs best for the field data.

1D finite volume model of unsteady flow over mobile bed

Summary A one dimensional (1D) finite volume method (FVM) model was developed for simulating unsteady flow, such as dam break flow, and flood routing over mobile alluvium. The governing equation is the modified 1D shallow water equation and the Exner equation that take both bed load and suspended load transport into account. The non-equilibrium sediment transport algorithm was adopted in the model, and the van Rijn method was employed to calculate the bed-load transport rate and the concentration of suspended sediment at the reference level. Flux terms in the governing equations were discretised using the upwind flux scheme, Harten et al. (1983) (HLL) and HLLC schemes, Roe’s scheme and the Weighted Average Flux (WAF) schemes with the Double Minmod and Minmod flux limiters. The model was tested under a fixed bed condition to evaluate the performance of several different numerical schemes and then applied to an experimental case of dam break flow over a mobile bed and a flood event in the Rillito River, Tucson, Arizona. For dam break flow over movable bed, all tested schemes were proved to be capable of reasonably simulating water surface profiles, but failed to accurately capture the hydraulic jump. The WAF schemes produced slight spurious oscillations at the water surface and bed profiles and over-estimated the scour depth. When applying the model to the Rillito River, the simulated results generally agreed well with the field measurements of flow discharges and bed elevation changes. Modeling results of bed elevation changes were sensitive to the suspended load recovery coefficient and the bed load adaptation length, which require further theoretical and experimental investigations.

Geometric bed roughness and the bed state storm cycle

[1] Temporal variation in the geometric roughness of the mobile sandy seabed during wave forcing events is investigated as a function of bed state. The bed states include irregular ripples, cross ripples, linear transition ripples, and flat bed, each appearing repeatedly within a different range of wave energies, as part of the bed state storm cycle. Ripple wavelengths determined from ensemble-averaged roughness spectra indicate that irregular and cross ripples are suborbital, whereas linear transition ripples are anorbital. The observed ripple steepnesses indicate that irregular and linear transition ripples fall slightly below Nielsen's (1981) field data relation, whereas cross ripple steepnesses are anomalously low in comparison. Time series of geometric roughness are coherent across spatial frequency. Abrupt changes in geometric roughness occurred on timescales of ∼3 h on average during both the onset and the waning stages of storm events: i.e., for both decreasing and increasing roughness, respectively. Predicted response times, based on ripple volume and bed load transport rate, are in agreement with the observations when the best fit form of the Meyer-Peter and Muller (1948)-type bed load relation obtained by Ribberink (1998) is used. In contrast, the more standard form of this relation and the associated parameters yield predicted response times much shorter than those observed.

An equation for bed-load transport capacities in gravel-bed rivers

Summary Detailed hydraulic and sedimentary information is needed to accurately predict bed-load transport rates in natural gravel-bed rivers. Yet, being able to estimate maximum transport rates from simple measurements would greatly benefit various sediment-related river management practices. To this end, a new concept of bed-load transport capacity for heterogeneous grains in gravel-bed rivers was introduced as the maximum possible transport rate a gravel-bed river can have for a given value of dimensionless shear stress, calculated using the median size of bed-load grains. Flows that can transport bed load at capacity may be identified by the criterion that the median size of bed-load grains must be greater than or equal to that of the bed substrate. Then, a single coefficient, power equation was developed to predict such capacities using bed-load capacity data covering both low flows with an armor layer and high flows without it. The good performance of this empirical equation was confirmed by comparing its predictability with that of Mayer Peter and Muller’s and Bagnold’s bed-load equations. Using an independent data compiled from six gravel-bed rivers in Idaho, not only was the empirical equation validated but also the criterion for identifying the condition under which bed load is transported at capacity was tested. In practice, the empirical equation can be used to estimate the maximum possible bed-load transport rates during high flow events, which is useful for various sediment-related river managements.

Experimental and Numerical Investigation of Bed-Load Transport under Unsteady Flows

The dynamic behavior of bed-load sediment transport under unsteady flow conditions is experimentally and numerically investigated. A series of experiments are conducted in a rectangular flume (18 m in length, 0.80 m in width) with various triangular and trapezoidal shaped hydrographs. The flume bed of 8 cm in height consists of scraped uniform small gravel of D50=4.8 mm. Analysis of the experimental results showed that bed-load transport rates followed the temporal variation of the triangular and trapezoidal hydrographs with a time lag on the average of 11 and 30 s, respectively. The experimental data were also qualitatively investigated employing the unsteady-flow parameter and total flow work index. The analysis results revealed that total yield increased exponentially with the total flow work. An original expression which is based on the net acceleration concept was proposed for the unsteadiness parameter. Analysis of the results then revealed that the total yield increased exponentially with the incr...

MODELING BIOGEOMORPHOLOGICAL INTERACTIONS IN UNDERWATER NOURISHMENTS

To prevent sandy coastlines from further erosion, nourishments are executed in which sand is usually put underwater at the foreshore. Waves and currents transport the sand on the beach and in this way stabilize the coastal profile. Little is known about the interactions of these so-called shoreface nourishments with the benthic populations inhabiting the coastal strip. Benthos is affected by the nourishments, but benthic populations could in turn affect the morphological evolution of the nourished coast. Monitoring has shown that the benthic community will mainly recovery after ca. 1 year. However, the impact of benthos on the sediment dynamics and hydrodynamics is unknown. In this paper we focus on tube building worms, which have a large abundance in the foreshore, live in patches of several square meters in diameter and protrude several centimeters from the sediment in the water column. Tube building worms are included in a numerical modeling tool (Delft3D), by explicitly accounting for the influence of cylindrical structures on drag and turbulence by an extra source term of friction force in the momentum equation and an extra source term of Total Kinetic Energy (TKE) and turbulent energy dissipation in the k-ε equations respectively. The model is validated against field and flume experiments and it shows a significant influence on flow velocities near the bed, bed shear stress and bed-load transport rates. Moreover, model results reveal that tube building worms are able to stabilize nourishments and slow down the migration of the outer breaker bar. Present model explorations indicate that future research should focus on the measurement of the patchy distribution of bio-engineers in the foreshore and their impact on the sediment dynamics and hydrodynamics. Such knowledge will enable process based modeling of the spatial and temporal variation in biological activity on the morphological development of the coastal profile and also it will lead to validation of the proposed model with field measurements.

PREDICTION OF NET BEDLOAD TRANSPORT RATES OBTAINED IN OSCILLATING WATER TUNNELS AND APPLICABILITY TO REAL SURF ZONE WAVES

Experimental studies of sediment transport rates due to nearshore waves are often conducted in oscillating water tunnels (OWTs). In an OWT, the oscillatory motion produced by the piston propagates almost instantaneously along the entire tunnel. Consequently, unlike the wave motion in the sea or in a wave flume, flow in an OWT is uniform along the tunnel, and second-order wave propagation effects (such as Longuet-Higgins's streaming) are absent. The effect of these hydrodynamic differences between OWT and sea waves on sediment transport rates has generally been neglected. In this paper we present a simple, practical formulation to evaluate bed shear stresses and bedload transport rates due to asymmetric and skewed waves plus a current in an OWT, based on fitting the exact results of a rigorous, analytical model of the OWT wave-current boundary layer. By then accounting for real wave effects we find that wave propagation significantly affects the predicted period-averaged net sediment transport rates. Such real wave effects can therefore not be neglected when comparing nearshore transport models with OWT data.

MECHANISM OF FLOW RESISTANCE IN ALLUVIAL STREAMS WITH SAND WAVES AND PRESICTION OF SEDIMENT TRANSPORT SYSTEM

Prediction methods of flow resistance and bed-load transport rate in alluvial streams with sand waves under lower-regime are proposed in this study. At first, the skin friction equation is derived by integrating the bed shear stress distribution equation on a sand wave surface. We propose the equation of hydraulic resistance based on the equations of skin friction and form drag next. The hydraulic resistance equation is verified with estimations of mean depth of flow over sand waves. The reach averaged bed-load transport rate can be calculated reasonably by the Meyer Peter & Müller’s equation using the proposed skin friction equation with the estimated mean flow depth. The proposed method in this study can predict flow depth, geometrical scale of sand waves, and bed-load transport rate by giving water discharge, bed gradient and grain diameter. The proposed method also clarifies that the ratio of a form drag to a skin friction for sand waves is different in a uniform sediment bed and a non-uniform sediment bed.

Bed scouring downstream of hydraulic structures under steady flow conditions: Experimental analysis of space and time scales and implications for mathematical modeling

Abstract Sediment transport and bed deformation in alluvial rivers are space and time dependent. The knowledge of the space and time scales can be helpful for the definition of predictive procedures of an alluvial reach response to changing boundary conditions. In this work, attention is paid to scouring process occurring downstream of the rigid basement of a hydraulic structure. The analysis is focused on transient bed profiles which are determined, under steady flow conditions, by a decrease of the upstream sediment transport rate. In particular, the paper is aimed at improving understanding of the space and time scales required by the alluvial system to reach the equilibrium conditions. For this purpose, movable-bed runs are carried out in a straight laboratory channel, for two values of the water discharge. During each run, the longitudinal bed profiles are measured at different times. Based on experimental data, spatial scale factors of the bed scouring and of the bed load transport to reach the equilibrium conditions are defined. Particularly, it is found that the scour length varies especially during the first evolution phase and then it oscillates around of a mean value that is about 40% of the channel reach interested by the phenomenon under consideration; an expression is found for the adaptation coefficient which allows the estimation of the spatial delay of the bed load transport rate. Using both the collected data and literature data an expression defining the time scale of scouring is deducted. According to this expression the non-dimensional time scale is related to Shields parameter through an exponential law. Then, by the help of a 1-D numerical model previously developed by the writer, the implications for choosing the spatial and temporal resolutions, on which numerical results depend significantly, are shortly discussed. The results show that the time and space scales, which represent physical constraints of the spatial and temporal resolutions, need to be properly defined in order to simulate the scouring process evolution.

Functional relationships between vegetation, channel morphology, and flow efficiency in an alluvial (anabranching) river

Water and sediment flux interactions are examined in Magela Creek, an alluvial (anabranching) sand bed river in the northern Australian tropics. Dense riparian vegetation stabilizes the channels and floodplains thereby preventing erosional instability at flow depths up to 6.2 times bankfull and discharges up to 15 times bankfull. Narrow anabranching channels characterize >92% of the alluvial reach and transport bed load more efficiently than short reaches of wide single‐channels, yet overall 29 ± 12% of the bed load is sequestered and the average vertical accretion rate is 0.41 ± 0.17 mm yr−1 along the 12 km study reach. The most effective discharge for transporting sediment (40–45 m3 s−1) is consistent at all 5 stations (10 channels) examined and is equivalent to the channel‐forming discharge. It has an average recurrence interval of 1.01 years, occurs for an exceptionally long portion (13–15%) of the annual flow duration, and averages a remarkable 2.1 times bankfull. The high flow efficiency (i.e., bed load transport rate to stream power ratio) of the anabranches is facilitated by low width/depth channels with banks reinforced by vegetation. Colonnades of bank top trees confine high‐velocity flows overbed (i.e., over the channel bed) at stages well above bankfull. At even larger overbank flows, momentum exchange between the channels and forested floodplains restrains overbed velocities, in some cases causing them to decline, thereby limiting erosion. Magela Creek exhibits a complicated set of planform, cross‐sectional and vegetative adjustments that boost overbed velocities and enhance bed load yield in multiple channels while restraining velocities and erosion at the largest discharges.

Sand Transport offshore the Saemangeum Dike, Midwest Coast of Korea

Abstract Geological investigations conducted over a vast seabed offshore the giant Saemangeum Dike on the west coast of Korea indicate that the surface area of fine to very fine sands has rapidly expanded southward in recent years (2002–2008). To unravel the processes responsible for this phenomenon, two field campaigns of hydrodynamic measurements were carried out at the same station with an instrumented tripod during May 2007 and January 2008. The measurements show the definite occurrence of wind-generated residual currents that were more distinctive and consistent during winter compared with the relatively weak, inconsistent analogues of the late-spring season. The simple algebraic calculations for bedload transport based on a cubic relationship between the bedload transport rate and steady currents suggest that the southward movements of surface sands result from interplay of wintertime residual currents and macrotidal currents. In addition, numerical model experiments illuminate that the dike constru...

Understanding the influence of slope on the threshold of coarse grain motion: Revisiting critical stream power

This paper investigates the slope-dependent variation in critical mean bed shear stress for coarse grain motion, and evaluates stream power per unit bed area as an alternative threshold parameter. Explanations for observed slope dependency and existing approaches for predicting the critical stream power per unit bed area are reviewed. An analysis of secondary bed-load transport datasets is used to examine the strength of associations between stream power per unit bed area, mean bed shear stress and mean velocity, with bed-load transport rate. Data from an original flume study are combined with secondary data from similar flume experiments to investigate the effect of slope on both critical stream power per unit bed area and critical mean bed shear stress. Results suggest that stream power per unit bed area is most closely correlated with bed-load transport rate, and also that critical stream power per unit bed area is less variable with slope than critical mean bed shear stress. Alternative solutions to approximating critical stream power are explored. These include: (1) modifying existing expressions for critical stream power to account for higher critical mean bed shear stresses at higher slopes, and (2) applying a constant dimensionless critical stream power criterion.

Reconciled bedload sediment transport rates in ephemeral and perennial rivers

AbstractIt has been thought for some time that bedload sediment transport rates may differ markedly in ephemeral and perennial rivers and, supporting this thought, there has been observation of very high rates of bedload transport by flash floods in the ephemeral river Nahal Yatir. However, until now, there has been no quantitative model resolving the observation, nor a theory capable of explaining why bedload transport rates by unsteady flash floods can be reasonably well described by bedload transport capacity formulae initially derived for steady flows. Here a time scale analysis of bedload transport is presented as pertaining to Nahal Yatir, which demonstrates that bedload transport can adapt sufficiently rapidly to capacity determined exclusively by local flow regime, and accordingly the transport capacity formulations developed for steady flows can be applied even under unsteady flows such as flash floods. Complementing the time scale analysis, a series of computational exercises using a coupled shallow water hydrodynamic model are shown to adequately resolve the observation of the very high rates of bedload transport by flash floods in Nahal Yatir. While bedload transport rates in ephemeral and perennial rivers differ remarkably when evaluated against a pure flow parameter such as specific stream power, they are essentially reconciled if assessed with a physically sensible parameter incorporating not only the flow regime but also the sediment particle size. The present finding underpins the practice of fluvial geomorphologists relating measured bedload transport to local flow and sediment characteristics only, irrespective of whether the flow is unsteady or steady. Copyright © 2010 John Wiley & Sons, Ltd.