Bed Load Transport Relations Research Articles

The role of bed-penetrating Kelvin–Helmholtz vortices on local and instantaneous bedload sediment transport

We present results from open-channel, Euler–Lagrange (EL) simulations of turbulent flow over an erodible particle bed at a shear Reynolds number of$Re_{\tau }=180$. Upon space and time averaging, our simulations correctly reproduce the Wong & Parker (ASCE J. Hydraul. Engng, vol. 132, issue 11, 2006, pp. 1159–1168) bedload transport relation (WP). However, local and instantaneous sediment flux shows orders of magnitude scatter around the WP prediction. Visualization of the vortical structures using swirling strength shows the existence of bed-penetrating Kelvin–Helmholtz (KH) vortex packets, which coupled with particle inertia are primarily responsible for the large scatter. The results also show that Euler–Euler (EE) simulations, where the individual sediment grains are not distinguished, are still able to reliably capture the turbulent shear stress variation, however, they do not capture the wide distribution of sediment flux indicative of saltating transport. The KH vortices induce non-zero streamwise and bed-normal velocities at the upper surface of the bed, which must be considered in EE simulations.

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.

Passive Acoustic Monitoring of Bed Load for Fluvial Applications

Passive Acoustic Monitoring of Bed Load for Fluvial Applications

Applicability of bed load transport models for mixed‐size sediments in steep streams considering macro‐roughness

AbstractIn steep mountain streams, macro‐roughness elements typically increase both flow energy dissipation and the threshold of motion compared to lower‐gradient channels, reducing the part of the flow energy available for bed load transport. Bed load transport models typically take account of these effects either by reducing the acting bed shear stress or by increasing the critical parameters for particle entrainment. Here we evaluate bed load transport models for mixed‐size sediments and models based on a median grain size using a large field data set of fractional bed load transport rates. We derive reference shear stresses and bed load transport relations based on both the total boundary shear stress and a reduced (or “effective”) shear stress that accounts for flow resistance due to macro‐roughness. When reference shear stresses are derived from the total boundary shear stress, they are closely related to channel slope, but when they are derived from the effective shear stress, they are almost invariant with channel slope. The performance of bed load transport models is generally comparable when using the total shear stress and a channel slope‐related reference shear stress, or when using the effective shear stress and a constant reference shear stress. However, dimensionless bed load transport relations are significantly steeper for the total stress approach, whereas they are similar to the commonly used fractional Wilcock and Crowe (WC) transport model for the effective stress approach. This similarity in the relations allows the WC model, developed for lower‐gradient streams, to be used in combination with an effective shear stress approach, in steep mountain streams.

A Comparative Analysis of Lajeunesse Model with other Used Bed Load Models - Effects on River Morphological Changes

Among the phenomena that greatly influence the river morphology is the sediment transport, especially the bed load mode causing a significant changes in the river morphology. Indeed, the choice of a model or a methodology that can better quantify sediment transport, remains always poorly understood. In this context, a new approach to studying the morphological evolution of rivers is proposed by Charru in 2004, it is based on a model for the erosion and deposition of the particles under laminar flow. In 2006, Charru proposes an extension of this model to turbulent flow. In more advanced research, Lajeunesse in 2010, realizes an experimental study to support the erosion deposition model of Charru, and proposes a new formula to calculate the bed load transport rate. The current research focuses on the effects of bed load transport on the morphological changes in rivers. In the first part, a comparative analysis of empirical laws of bed load transport with experimental data was conducted, in order to test and validate the new bed load model proposed by Lajeunesse, then to check the grain size effect on the sediment transport capacity. In the second part, we are interested in the study of the morphological evolution in rivers. It was performed through numerical modeling using TELEMAC 2D coupled with SISYPHE. The aim is to understand and analyze the morphological changes in the channel bottom. The analysis of the results presented in this paper showed that through the calculated score, most formulas give satisfactory results. In particular at the grain scale, the new bed load transport relation of Lajeunesse, provides an excellent fit to the experimental data. Finally, we were interested in the study of the morphological changes in the channel bottom, it appers clear that the bed load transport has large impacts on river morphology.

Predicting bed load transport of sand and gravel on Goodwin Creek

Bed load transport rates are difficult to predict in channels with bed material composed of sand and gravel mixtures. The transport of bed load was measured on Goodwin Creek, and in a laboratory flume channel with a similar bed material size distribution. The range of bed load transport rates measured in the laboratory channel were similar to those measured in the channel of Goodwin Creek; however, the shear stresses calculated on Goodwin Creek were three times greater than in the laboratory channel for similar bed load transport rates. Much of this difference in shear stress was removed by applying the drag partitioning technique of Einstein, although significant differences between the two sets of data remain. Predictions of bed load transport rates using three previously published transport relations were good for most flows for the laboratory data. For the Goodwin Creek bed load data, predicted transport rates were close to measured ones for low flows but diverged by an order of magnitude or more for high shear stresses. Improved methods of shear stress partitioning are needed to improve the performance of bed load transport relations on streams of this type.

Downstream effects of impounding a natural lake: the Snake River downstream from Jackson Lake Dam, Wyoming, USA

ABSTRACTA sediment mass balance constructed for a 16‐km reach of the Snake River downstream from Jackson Lake Dam (JLD) indicates that river regulation has reduced the magnitude of sediment mass balance deficit that would naturally exist in the absence of the dam. The sediment budget was constructed from calibrated bed load transport relations, which were used to model sediment flux into and through the study reach. Calibration of the transport relations was based on bed load transport data collected over a wide range of flows on the Snake River and its two major tributaries within the study area in 2006 and 2007. Comparison of actual flows with unregulated flows for the period since 1957 shows that operations of JLD have reduced annual peak flows and increased late summer flows. Painted tracer stones placed at five locations during the 2005 spring flood demonstrate that despite the reduction in flood magnitudes, common floods are capable of mobilizing the bed material. The sediment mass balance demonstrates that more sediment exits the study reach than is being supplied by tributaries. However, the volume of sediment exported using estimated unregulated hydrology indicates that the magnitude of the deficit would be greater in the absence of JLD. Calculations suggest that the Snake River was not in equilibrium before construction of JLD, but was naturally in sediment deficit. The conclusion that impoundment lessened a natural sediment deficit condition rather than causing sediment surplus could not have been predicted in the absence of sediment transport data, and highlights the value of transport data and calculation of sediment mass balance in informing dam operations. Copyright © 2011 John Wiley & Sons, Ltd.

River morphodynamics with creation/consumption of grain size stratigraphy 1: laboratory experiments

Rivers with poorly-sorted bed sediment create their own stratigraphy as they deposit sediment. Prediction of the subsequent river degradation into its own deposit requires knowledge of the spatial structure of the grain size variation of the deposit. The ultimate goal of the present work is the development, testing and verification against experimental data of a numerical model of morphodynamics that can store stratigraphy into memory as it is created by aggradation, and can subsequently consume this stratigraphy if and when the river later degrades into the deposit. Such a morphodynamic model is tested using a surface-based bedload transport relation known to be applicable to the experiments considered here. Part 1 of a two-part paper addressing this issue describes the laboratory experiments and uses the experimental results performed at mobile-bed equilibrium to evaluate a bedload transport relation. In the companion paper, this bedload transport relation is installed into a morphodynamic model that specifically includes the creation/consumption of stratigraphy. The model is then tested against the experimental data.

Bedload transport and bed resistance associated with density and turbidity currents

Turbidity currents in the ocean are driven by suspended sediment. Yet results from surveys of the modern sea floor and turbidite outcrops indicate that they are capable of transporting as bedload and depositing particles as coarse as cobble sizes. While bedload cannot drive turbidity currents, it can strongly influence the nature of the deposits they emplace. This paper reports on the first set of experiments which focus on bedload transport of granular material by density underflows. These underflows include saline density flows, hybrid saline/turbidity currents and a pure turbidity current. The use of dissolved salt is a surrogate for suspended mud which is so fine that it does not settle out readily. Thus, all the currents can be considered to be model turbidity currents. The data cover four bed conditions: plane bed, dunes, upstream-migrating antidunes and downstream-migrating antidunes. The bedload transport relation obtained from the data is very similar to those obtained for open-channel flows and, in fact, is fitted well by an existing relation determined for open-channel flows. In the case of dunes and downstream-migrating antidunes, for which flow separation on the lee sides was observed, form drag falls in a range that is similar to that due to dunes in sand-bed rivers. This form drag can be removed from the total bed shear stress using an existing relation developed for rivers. Once this form drag is subtracted, the bedload data for these cases collapse to follow the same relation as for plane beds and upstream-migrating antidunes, for which no flow separation was observed. A relation for flow resistance developed for open-channel flows agrees well with the data when adapted to density underflows. Comparison of the data with a regime diagram for field-scale sand-bed rivers at bankfull flow and field-scale measurements of turbidity currents at Monterey Submarine Canyon, together with Shields number and densimetric Froude number similarity analyses, provide strong evidence that the experimental relations apply at field scale as well.

Variation in the reference Shields stress for bed load transport in gravel‐bed streams and rivers

The present study examines variations in the reference shear stress for bed load transport (τr) using coupled measurements of flow and bed load transport in 45 gravel‐bed streams and rivers. The study streams encompass a wide range in bank‐full discharge (1–2600 m3/s), average channel gradient (0.0003–0.05), and median surface grain size (0.027–0.21 m). A bed load transport relation was formed for each site by plotting individual values of the dimensionless transport rate W* versus the reach‐average dimensionless shear stress τ*. The reference dimensionless shear stress τ*r was then estimated by selecting the value of τ* corresponding to a reference transport rate of W* = 0.002. The results indicate that the discharge corresponding to τ*r averages 67% of the bank‐full discharge, with the variation independent of reach‐scale morphologic and sediment properties. However, values of τ*r increase systematically with average channel gradient, ranging from 0.025–0.035 at sites with slopes of 0.001–0.006 to values greater than 0.10 at sites with slopes greater than 0.02. A corresponding relation for the bank‐full dimensionless shear stress τ*bf, formulated with data from 159 sites in North America and England, mirrors the relation between τ*r and channel gradient, suggesting that the bank‐full channel geometry of gravel‐ and cobble‐bedded streams is adjusted to a relatively constant excess shear stress, τ*bf − τ*r, across a wide range of slopes.

Bed load transport in an obstruction-formed pool in a forest, gravelbed stream

This paper examines channel dynamics and bed load transport relations through an obstruction-forced pool in a forest, gravel-bed stream by comparing flow conditions, sediment mobility, and bed morphology among transects at the pool head, centre, and tail. Variable sediment supply from within and outside of the channel led to a complex pattern of scour and fill hysteresis. Despite the large flood magnitude, large portions of the bed did not scour. Scour was observed at three distinct locations: two of these were adjacent to large woody debris (LWD), and the third was along the flow path deflected by a major LWD obstruction. Bed material texture showed little change in size distribution of either surface or subsurface material, suggesting lack of disruption of the pre-flood bed. Fractions larger than the median size of the bed surface material were rarely mobile. Sediment rating relations were similar, although temporal variation within and among stations was relatively high. Relations between bed load size distribution and discharge were complex, showing coarsening with increasing discharge followed by fining as more sand was mobilized at high flow. Lack of local scour in the pool combined with bed load fining and net fill by relatively fine material implied that the dominant sources of mobile sediment were upstream storage sites and local bank collapse. Patterns of flow, channel dynamics, and sediment mobility were strongly affected by a LWD flow obstruction in the pool centre that created turbulent effects, thereby enhancing entrainment and transport in a manner similar to scour at bridge piers.

Bedload Transport in Gravel-Bed Streams with Unimodal Sediment

Bedload transport in many gravel-bed streams becomes highly complicated because of the nonuniformity of the grain size and the vertical stratification of channel bed material. A new relation for computing bedload transport rates in gravel-bed streams is proposed here. In an effort to account for the variation of the makeup of the surface material within a wide range of Shields stresses, the relation employs a two-parameter approach, one related to the material in the pavement and the other to that in the subpavement layers. The mode is used to represent the grain sizes of each layer. Available bedload transport data from gravel-bed streams with unimodal sediment are used to test the accuracy of the relation. A comparison with other bedload transport relations is also considered.

Comparison of bed load transport in torrents and gravel bed streams

Data are compiled on bed load transport and discharge in gravel bed rivers and torrents with bed slopes up to 0.17. The transport characteristics of 19 streams are compared with simple bed load transport equations and with measurements from flume experiments. In simple form the bed load transport is a function of an “effective” discharge times a bed slope factor. The efficiency of the streams in transporting bed load is defined as the deviation from the transport function. It varies over several orders of magnitude, particularly for smaller and steeper streams, whereas flume experiments and larger streams are in better agreement with the simple bed load transport relation. The large variation and the strong decrease in efficiency in smaller streams appears to be associated with substantial form losses for relative flow depths (defined by h/d90) smaller than ∼4–6.

Prediction of Live-Bed Scour at Bridge Abutments

A semi-empirical analysis on the time-averaged equilibrium live-bed scour at an abutment, placed perpendicular to the flow direction, is presented. The approach is based on the flow and sediment mass continuity equations, scour geometry, flow resistance, and a generalized bed-load transport relation in an alluvial channel. The proposed equation can be reduced to the equilibrium clear-water scour equation proposed earlier by the first writer (Lim). A total of 111 live-bed scour data from various sources were analyzed and compared with the developed equation. The agreement between the computed and measured data is acceptable.

Surface-based bedload transport relation for gravel rivers

Bedload transport in gravel-bed rivers is accomplished by means of the mobilization of grains exposed on the bed surface. This mobilization is due to the action of fluid forces on the exposed grains. Substrate particles can participate in the bedload only to the extent that local or global scour results in their exposure on the surface. It follows that a calculation of the bedload transport rate of mixtures should be based on the availability of each size range in the surface layer. Herein an existing empirical substrate-based bedload relation for gravel mixtures, developed solely with reference to field data, is transformed into a surface-based relation.