Coarse Surface Layer Research Articles

Vertical reworking of sediment by the cased caddisfly Glossosomatidae (Agapetus fuscipes) increases sand exposure and availability in armoured gravel-bed rivers

Landscapes and ecosystems are the result of two-way interactions between hydro-geomorphic and biological processes. Many animals, particularly those that build structures or transport sediment, are important biogeomorphic agents. Glossosomatidae caddisfly larvae (Insecta, Trichoptera) are globally widespread and abundant inhabitants of gravel-bed rivers. Glossosomatidae build mobile cases from sand that they transport over the river bed. However, there is limited understanding on how Glossosomatidae bioconstructions may influence sand distribution in rivers or how their zoogeomorphic behaviours are influenced by hydraulics or characteristics of the river bed. First, we conducted surveys to quantify the magnitude of sand incorporated into Glossosomatidae (Agapetus fuscipes) cases within a UK river. Second, we studied A. fuscipes movement behaviour and quantified the direction and magnitude of sediment reworking, in a flume, under differing flow velocity and gravel size treatments. We found that 99 % of A. fuscipes larvae transported sediment vertically upwards. This resulted in an average conveyance per larvae of 0.06 g sand upwards by 25 mm (maximum of 50 mm). In gravel beds with a coarse surface layer, this resulted in displacement of sand from sheltered interstices onto the surface of exposed gravel particles. In the flume, this behaviour was maintained even at high flows, sufficient to entrain empty cases from these locations. Whilst the mass of sediment moved by individual larvae is small, dense populations of Glossosomatidae larvae may have important consequences for the vertical distribution of sand in rivers. At our field site, A. fuscipes case density averaged 2192 cases m−2, equivalent to 1.4 t km−1. This finding is important because in gravel-bed rivers frequented by Glossosomatidae larvae, sediment transport is typically limited by the availability of entrainable fine grain sediment at the surface. We discuss the implications of this sediment movement for river bed sedimentary structure, the transport of sand and gravel, and the possible role of Glossosomatidae larvae as ecosystem engineers.

Bed-material entrainment in a mountain river affected by hydropeaking

Hydropeaking, by artificially generated flow peaks, influences hydro-sedimentary dynamics on rivers and, consequently, affects bed material entrainment and transport. This study examines the onset of motion of sediment particles in four sections of a Pyrenean gravel-to-cobble bed river exposed to frequent hydropeaking (once per day, on average). Five criteria of particle entrainment have been used to assess the prediction of the initiation of grain motion at-a-section scale. Theoretical entrainment conditions were validated using real observations of mobility by means of tracers. It was found that the maximum flow discharged by the hydropower plant mostly affects the furthest downstream section, located almost 17 km downstream, in which the finer fractions of the bed are entrained. The mobile grain sizes include up to coarse gravels (≈ 30 mm). Differences in sediment supply (imposed by tributaries), the value of the bed slope and the structure of the coarse surface layer decisively control the downstream variability of incipient particle motion between sections. Results from a 17 km study segment indicated that hydropeaking generate partial transport, that is, a partially size-selective transport that occurs downstream from the hydropower plant and winnows the sand and small gravel further downstream, increasing armouring and depleting fine sediments.

Rainfall-runoff properties of tephra: Simulated effects of grain-size and antecedent rainfall

Rain-triggered lahars (RTLs) are a significant and often persistent secondary volcanic hazard at many volcanoes around the world. Rainfall on unconsolidated volcaniclastic material is the primary initiation mechanism of RTLs: the resultant flows have the potential for large runout distances (>100km) and present a substantial hazard to downstream infrastructure and communities. RTLs are frequently anticipated in the aftermath of eruptions, but the pattern, timing and scale of lahars varies on an eruption-by-eruption and even catchment-by-catchment basis. This variability is driven by a set of local factors including the grain size distribution, thickness, stratigraphy and spatial distribution of source material in addition to topography, vegetation coverage and rainfall conditions. These factors are often qualitatively discussed in RTL studies based on post-eruption lahar observations or instrumental detections. Conversely, this study aims to move towards a quantitative assessment of RTL hazard in order to facilitate RTL predictions and forecasts based on constrained rainfall, grain size distribution and isopach data. Calibrated simulated rainfall and laboratory-constructed tephra beds are used within a repeatable experimental set-up to isolate the effects of individual parameters and to examine runoff and infiltration processes from analogous RTL source conditions.Laboratory experiments show that increased antecedent rainfall and finer-grained surface tephra individually increase runoff rates and decrease runoff lag times, while a combination of these factors produces a compound effect. These impacts are driven by increased residual moisture content and decreased permeability due to surface sealing, and have previously been inferred from downstream observations of lahars but not identified at source. Water and sediment transport mechanisms differ based on surface grain size distribution: a fine-grained surface layer displayed airborne remobilisation, accretionary pellet formation, rapid surface sealing and infiltration-excess overland flow generation whilst a coarse surface layer demonstrated exclusively rainsplash-driven particle detachment throughout the rainfall simulations. This experimental protocol has the potential to quantitatively examine the effects of a variety of individual parameters in RTL initiation under controlled conditions.

Procesos fluviales en lechos con materiales gruesos

Fluvíal processes ingravel-bed rívers. The bedload transport processes ingravel-bed rívers are not very known. The two most frequent features that characterize these rívers are the presence of a scour layer and the pavement. The coarse surface layer causes a great resistance to flow in gravel-bed rívers. So, flow resítance equations for computing average velocity can be applíed.

Reexposure and advection of 14C‐depleted organic carbon from old deposits at the upper continental slope

Outcrops of old strata at the shelf edge resulting from erosive gravity‐driven flows have been globally described on continental margins. The reexposure of old strata allows for the reintroduction of aged organic carbon (OC), sequestered in marine sediments for thousands of years, into the modern carbon cycle. This pool of reworked material represents an additional source of 14C‐depleted organic carbon supplied to the ocean, in parallel with the weathering of fossil organic carbon delivered by rivers from land. To understand the dynamics and implications of this reexposure at the shelf edge, a biogeochemical study was carried out in the Gulf of Lions (Mediterranean Sea) where erosive processes, driven by shelf dense water cascading, are currently shaping the seafloor at the canyon heads. Mooring lines equipped with sediment traps and current meters were deployed during the cascading season in the southwestern canyon heads, whereas sediment cores were collected along the sediment dispersal system from the prodelta regions down to the canyon heads. Evidence from grain‐size, X‐radiographs and 210Pb activity indicate the presence in the upper slope of a shelly‐coarse surface stratum overlying a consolidated deposit. This erosive discontinuity was interpreted as being a result of dense water cascading that is able to generate sufficient shear stress at the canyon heads to mobilize the coarse surface layer, eroding the basal strata. As a result, a pool of aged organic carbon (Δ14C = −944.5 ± 24.7‰; mean age 23,650 ± 3,321 ybp) outcrops at the modern seafloor and is reexposed to the contemporary carbon cycle. This basal deposit was found to have relatively high terrigenous organic carbon (lignin = 1.48 ± 0.14 mg/100 mg OC), suggesting that this material was deposited during the last low sea‐level stand. A few sediment trap samples showed anomalously depleted radiocarbon concentrations (Δ14C = −704.4 ± 62.5‰) relative to inner shelf (Δ14C = −293.4 ± 134.0‰), mid‐shelf (Δ14C = −366.6 ± 51.1‰), and outer shelf (Δ14C = −384 ± 47.8‰) surface sediments. Therefore, although the major source of particulate material during the cascading season is resuspended shelf deposits, there is evidence that this aged pool of organic carbon can be eroded and laterally advected downslope.

Mobilization of coarse surface layers in gravel‐bedded rivers by finer gravel bed load

Additions of sand to gravel beds greatly increase the mobility and flux of gravel. However, it is not known how additions of finer gravel to coarser gravel beds will affect the mobility of bed material. Here we examine the effect of fine gravel pulses on gravel bed material transport and near‐bed flow dynamics in a series of flume experiments. Bed material refers exclusively to sediment in the channel prior to the pulse introduction. The observations indicate that fine sediment pulses tend to migrate downstream in low‐amplitude waves. As the waves pass over the gravel bed, the interstitial pockets in the bed material surface fill and coarse gravel particles are entrained. This increases bed material transport rates and causes a distinct shift from a selective mobility transport regime where particles coarser than the bed material median (8 mm) make up <30% of the load to an equal mobility transport regime where bed materials coarser than 8 mm and finer than 8 mm are transported in equal proportions. The only possible source for this coarser bed load material is the sediment bed, suggesting that portions of the coarse surface layer are being mobilized. Observations of near‐bed velocity and turbulence suggest that fine gravel pulses cause fluid acceleration in the near‐bed region associated with a reduction in the level of turbulence produced at the sediment boundary. This accelerated fluid at the bed increases drag exerted on coarse surface layer particles, promoting their mobilization. Our findings suggest that, in general, finer bed sediment (not just sand) can mobilize coarser sediment and that expressions for the influence of sand on bed mobility need to be generalized on the basis of grain ratios.

Strong effect of coarse surface layer on moisture within gravel bars: Results from an outdoor experiment

We propose that the coarse surface layer found on most gravel bars acts as mulch, decreasing evaporation from the soil surface and maintaining higher moisture levels in the underlying fine matrix, as compared with the case of sand bed streams. Because of the importance that such process would have for vegetation establishment, we conducted an outdoor evaporation experiment on soil columns containing diverse mixtures of fines and gravel, covered with 0, 4, and 8 cm thick gravel layers, as well as columns with homogeneous sand. The columns were saturated, drained, and then exposed to the atmosphere for 58 days. They were weighed and their soil moisture was computed on eight different dates. Both the coarse layer thickness and the texture of the underlying fine matrix, as well as their interaction, had significant effects on soil moisture. The presence of 4 and 8 cm thick gravel layers on the surface increased moisture up to 3.7 and 5.3 times, respectively, as compared with homogeneous sand. Effects of the fine matrix texture were significant only when the fines were exposed to the atmosphere at the top of the columns, but not when a coarse layer was present. Our results show that the presence of a clean gravel layer maintains high moisture levels within the underlying fines, for periods of weeks without water input. We suggest that this rock mulching effect could be fundamental for successful plant colonization of bare gravel bars, so that it should be incorporated into any vegetation establishment model developed for gravel bed rivers.

The influence of lee sediment behind large bed elements on bedload transport rates in supply-limited channels

A coarse surface layer can help to limit bedload transport rates in channels with cobble and gravel beds. In these systems, periodic boulder-sized clasts often exist with small deposits of fine material in the lee of these large bed elements. A combined field and flume study was conducted to investigate the potential impact of lee deposits with distinctly finer sediment-sizes behind boulders on bedload transport rates. Detailed sediment characterizations were performed on surface, subsurface, and lee sediments in two coarse-bedded Connecticut channels. Bedload measurements also were conducted in a series of flows that approached the bankfull level in these two systems to determine transport rates and the size distribution of bedload material. A 6-m long, 0.5-m wide flume was used to model these systems with fine sediment passing over a fixed bed of sediment particles with uniform-sized, large bed elements. Sediment distributions of the lee deposits in the two Connecticut channels indicate that lee deposits may be produced from winnowing of sediments from the surface layer. Lee deposits also exhibit sediment distributions similar to bedload sediment distributions from low to near-bankfull flow in one of the two channels. Bedload sediments in the second channel were finer than lee deposits, presumably from selective entrainment of fines. Flume experiments demonstrate that bedload transport rates are lower for periods of steady flow relative to periods that include either an increase or decrease in discharge. The results show that lee sediments establish a metastable deposit behind each obstruction for a given discharge. Either increases or decreases in discharge disrupt this temporary stability and increase sediment delivery to the main flow. The study suggests that the influence of the rate of change in discharge may be as important as the absolute magnitude of discharge on sediment transport rates at moderate and low discharges in sediment-limited systems with large bed elements.

Effect of Coarse Surface Layer on Bed-Load Transport

Existing bed-load transport formulas may overestimate the transport rate in mountain rivers by two orders of magnitude or more. Recently published field data sets provide an opportunity to take a fresh look at the bed-load transport relationship and it is hypothesized that the overestimate is due to a failure to account for the effect of a coarse surface layer of bed material inhibiting the release of fine subsurface material. Bed-load transport is determined as gs=aρ(q−qc) where q=water discharge per unit width; qc=critical value for initiation of bed material movement; ρ=water density; and a=coefficient. The gs∕q relationship is typically piecewise linear, characterized by two transport phases with, respectively, low and high rates of change. Twenty-one flume and 25 field data sets were used to quantify the relationship for Phase 2. The flume data confirm the dependence of a on S1.5, where S=channel slope, in agreement with earlier studies. The field data additionally show that a varies inversely with the degree of bed armoring, given by the ratio of surface to subsurface bed material size. The finding is consistent with the hypothesis and suggests the need to account for the bed material supply limitation in the bed-load transport formula. However, the available data are not entirely sufficient to rule out an alternative dependency, or codependency, on flow resistance. The critical conditions for initiation of Phase 2 transport are also quantified as a function of bed material size and channel slope. The resulting set of equations allows a more accurate estimation of Phase 2 bed-load transport rates. However, the equations are empirical and should be restricted for use within the range of conditions used in their development, to determine mean rather than instantaneous transport rates and to determine bulk transport rates, not transport by size fraction.

Persistence of the surface texture of a gravel‐bed river during a large flood

AbstractWe present herein clear field evidence for the persistence of a coarse surface layer in a gravel‐bed river during flows capable of transporting all grain sizes present on the channel bed. Detailed field measurements of channel topography and bed surface grain size were made in a gravel‐bed reach of the Colorado River prior to a flood in 2003. Runoff produced during the 2003 snowmelt was far above average, resulting in a sustained period of high flow with a peak discharge of 27 m3/s (170% of normal peak flow); all available grain sizes within the study reach were mobilized in this period of time. During the 2003 peak flow, the river avulsed immediately upstream of the study reach, thereby abandoning approximately one half kilometer of the former channel. The abandonment was rapid (probably within a few hours), leaving the bed texture essentially frozen in place at the peak of the flood. All locations sampled prior to the flood were resampled following the stream abandonment. In response to the high flow, the surface median grain size (D50s) coarsened slightly in the outer part of the bend while remaining nearly constant along the inner part of the bend, resulting in an overall increase from 18 to 21 mm for the study reach. Thus, the coarse bed surface texture persisted despite shear stresses throughout the bend that were well above the critical entrainment value. This may be explained because the response of the bed texture to increases in flow strength depends primarily upon the continued availability of the various grain size percentiles in the supply, which in this case was essentially unlimited for all sizes present in the channel. Copyright © 2007 John Wiley & Sons, Ltd.

Development of texture contrast soils by a combination of bioturbation and translocation

Soils and weathering profiles in a wide variety of parent materials and environmental settings exhibit coarse-over-fine vertical textural contrasts. Where these cannot be attributed to inherited texture contrasts or erosion–deposition, the most common explanations are based on translocation (eluviation–illuviation) which removes clays from surface layers and deposits them in the subsoil; or bioturbation, where preferentially fine material is delivered to the surface by organisms, from whence erosional winnowing creates a coarse surface layer. In some soils of the lower coastal plain of North Carolina, U.S.A., neither explanation is sufficient to explain the observed texture contrasts. A heuristic model based on a combination of translocation of fine material from surface to subsoil, and bioturbation-driven delivery and recycling of material to the surface can explain the observed vertical textural contrasts. The key elements in the model are coastal plain sediments which include some fine material; eluviation–illuviation by percolating water; delivery of additional fine and mixed grain size material to surface by bioturbation, making it available for translocation; concentration of fine material originally scattered throughout the parent material in a B horizon; and maintenance of vertical moisture fluxes by bioturbation. The model is supported by morphological evidence of the key mechanisms, argillic horizons that are finer than both the surface layers and underlying parent material, evidence that argillic horizon formation is not limited by the rate of clay synthesis, and the absence of texture contrasts in nearby soils formed from dune sands which lack fines.

Bed load sediment transport in ephemeral and perennial gravel bed streams

Perennial gravel streams are usually dominated by a bed material stratified in terms of grain size, with a coarse surface layer overlying a finer subsurface (Figure 1a). However, field observations from some ephemeral gravel streams in the Middle East and other locations exhibit the opposite stratification (Figure 1b).Recent studies have found that under the same range of flow strengths, such an ephemeral stream has a much higher transport efficiency of bed load sediment (the movement of particles by rolling and sliding on the bed, or by making short hops of a few grain diameters in distance), which suggests a completely different behavior from perennial streams [e.g., Laronne and Reid, 1993; Reid and Laronne, 1995].

Transport processes of uniform and mixed sands in oscillatory sheet flow

New experiments were carried out in the Large Oscillating Water Tunnel of WL|Delft Hydraulics (scale 1:1) using asymmetric 2nd-order Stokes waves. The main aim was to gain a better understanding of size-selective sediment transport processes under oscillatory plane-bed/sheet-flow conditions. The new data show that for uniform sand sizes between 0.2 < D < 1.0 mm, measured net transport rates are hardly affected by the grain size and are proportional to the third-order velocity moment. However for finer grains ( D = 0.13 mm) net sand transport rates change from the ‘onshore’ direction into the ‘offshore’ direction in the high velocity range. A new measuring technique for sediment concentrations, based on the measurement of electro-resistance (see [McLean, S.R., Ribberink, J.S., Dohmen-Janssen, C.M. and Hassan, W.N.M., 2001. Sediment transport measurements within the sheet flow layer under waves and currents. J. Waterw., Port, Coast., Ocean Eng., ISSN 0733-950X]), was developed further for the improved measurement of sediment dynamics inside the sheet-flow layer. This technique enabled the measurements of particle velocities during the complete wave cycle. It is observed that for long period waves ( T = 12.0 s), time-dependent concentrations inside the sheet-flow layer are nearly in phase with the time-dependent flow velocities. As the wave period decreases, the sediment entrainment from the bed as well as the deposition process back to the bed lags behind the wave motion more and more. The new data show that size-gradation has almost no effect on the net total transport rates, provided the grain sizes of the sand mixture are in the range of 0.2 < D < 1.0 mm. However, if very fine grains ( D = 0.13 mm) are present in the mixture, net total transport rates of graded sand are generally reduced in comparison with uniform sand with the same D 50. The transport rates of individual size fractions of a mixture are strongly influenced by the presence of other fractions in a mixture. Fine particles in sand mixtures are relatively less transported than in that uniform sand case, while the opposite occurs for coarse fractions in a mixture. The relative contribution of the coarse grains to the net total transport is therefore larger than would be expected based on their volume proportion in the original sand mixture. This partial transport behaviour is opposite to what is generally observed in uni-directional (e.g. river) flows. This is caused by vertical sorting of grain sizes in the upper bed layer and in the sheet flow and suspension layers. Kinematic sorting is believed to be responsible for the development of a coarse surface layer on top of a relatively fine sub-layer, providing in this way a relatively large flow exposure for the coarser sizes. Furthermore fine grains are suspended more easily than coarse grains to higher elevations in the flow where they are subject to increasing phase-lag effects (settling lags). The latter also leads to reduced net transport rates of these finer sizes.

Predicting fine sediment dynamics along a pool-riffle mountain channel

Fine sediment dynamics in mountain rivers are of concern because of implications for aquatic habitat, channel stability, and downstream sediment yields. Many mountain river systems have episodic fine sediment transport because of infrequent, point-source sediment inputs from landslides; basin instability triggered by land uses such as logging; or infrequent mobilization of the coarse surface layer in channels. Dam removal, which is now more likely along mountain rivers, may also provide a substantial fine sediment input to downstream channel reaches. Fine sediment storage in the interstices of spawning gravels and within pools along mountain rivers is of particular interest because of impacts to aquatic organisms. In this study we focus on sediment dynamics within pools of the North Fork Poudre River in Colorado as an example of the processes controlling fine sediment deposition, storage, and transport within laterally constricted pools. The 1996 release of ∼7000 m 3 of silt-to gravel-sized sediment from a reservoir on the North Fork provided an opportunity to develop a field data set of fine sediment dynamics and to test the predictions of three different one- or two-dimensional sediment transport and hydraulic models against the field observations. The models were calibrated against quantitative measurements of pool scour and fill. One-dimensional HEC-6 results indicate that robust simulations yield the greatest agreement between predicted and measured pool bed elevation change. Model calibration on two pools and validation on one pool indicate that at least 58% of observed bed changes after the sediment release were predicted by HEC-6. Modeling accuracy using quasi-two-dimensional GSTARS 2.0 was considerably more variable, and no pool-wide trends were obtained. The two-dimensional model RMA2 substantially improved the representation of eddy pool hydraulics within a compound pool of the North Fork. Results from the hydraulic modeling, coupled with bed load and total load computations, delineate areas of scour and deposition which are consistent with observations in the field. A conceptual model of sediment delivery and storage for laterally confined pools suggests that persistent deposition of fine sediment within eddies distal from the sediment source may result from sediment releases. The original loss of channel capacity facilitated additional deposition within eddies as sediment within upstream proximal pools became mobilized. At high discharges, the development of a strong shear zone prevents degradation of sediment deposits within the eddy. Central portions of these proximal pools may clear according to existing models, whereas deposition within recirculating zones may be long-term. Water managers could use these models to estimate minimum pool volume for overwinter habitat and residence time of pool sediment.

Defining phases of bedload transport using piecewise regression

AbstractDifferences in the transport rate and size of bedload exist for varying levels of flow in coarse‐grained channels. For gravel‐bed rivers, at least two phases of bedload transport, with notably differing qualities, have been described in the literature. Phase I consists primarily of sand and small gravel moving at relatively low rates over a stable channel surface. Transport rates during Phase II are considerably greater than Phase I and more coarse grains are moved, including material from both the channel surface and subsurface. Transition from Phase I to Phase II indicates initiation and transport of grains comprising the coarse surface layer common in steep mountain channels. While the existence of different phases of transport is generally acknowledged, the threshold between them is often poorly defined. We present the results of the application of a piecewise regression analysis to data on bedload transport collected at 12 gravel‐bed channels in Colorado and Wyoming, USA. The piecewise regression recognizes the existence of different linear relationships over different ranges of discharge. The inflection, where the fitted functions intersect, is interpreted as the point of transition from Phase I to Phase II transport; this is termed breakpoint. A comparison of grain sizes moved during the two phases shows that coarse gravel is rarely trapped in the samplers during Phase I transport, indicating negligible movement of grains in this size range. Gravel larger than about D16 of the channel surface is more consistently trapped during Phase II transport. The persistence of coarse gravel in bedload samples provides good evidence that conditions suitable for coarse grain transport have been reached, even though the size of the sediment approaches the size limits of the sampler (76 mm in all cases). A relative breakpoint (Rbr) was defined by the ratio between the discharge at the breakpoint and the 1·5‐year flow (a surrogate for bankfull discharge) expressed as a percentage. The median value of Rbr was about 80 percent, suggesting that Phase II begins at about 80 percent of the bankfull discharge, though the observed values of Rbr ranged from about 60 to 100 percent. Variation in this value appears to be independent of drainage area, median grain size, sorting of bed materials, and channel gradient, at least for the range of parameters measured in 12 gravel‐bed channels. Published in 2002 by John Wiley &amp; Sons, Ltd.

First results and interpretation of energy-flux measurements over Alpine permafrost

AbstractThe interaction of energy-exchange processes between the atmosphere and the Earth surface determines the surface temperature regime. It is of fundamental importance to the question whether frozen ground exists at a given site and how rapidly it may decay in response to a climatic perturbation. To further our understanding of these processes, measurements concerning near-surface energy-exchange processes were initiated in January 1997 on creeping permafrost at a high mountain site, Murtèl-Corvatsch, upper Engadin, Swiss Alps. Data on all important energy-balance fluxes were collected. In this paper, we present ground-temperature and energy-balance measurements from Murtèl-Corvatsch for a 2 year period, 1997–99. We will examine the relative importance of the energy-balance components and discuss special problems relating to the coarse surface layer. The results indicate a non-zero energy budget, with a positive deviation of up to 78 W m 4 in winter and a negative deviation of up to –130 W nT2 in summer. We propose that this overall imbalance of the energy-exchange fluxes, as well as the significant difference between mean annual surface and ground temperatures/can be explained by unmeasured advective energy fluxes that occur within the layer of large boulder blocks at the top of the permafrost.

Patterns and processes of sediment sorting in gravel-bed rivers

Sedimentological studies of coarse-grained alluvial rivers reveal patterns of bed material sorting at a variety of spatial scales ranging from downstream fining over the length of the long profile to the vertical segregation of a coarse surface layer at the scale of individual particles. This article reviews the mechanisms that sort bed material by size during sediment entrainment, transport and deposition and discusses some of the inter-relationships that exist between patterns and processes of sediment sorting at different spatial and temporal scales. At initiation of motion, sorting can arise from the preferential entrainment of the finer fractions from the heterogeneous bed sediments. Bedload grain-size distributions are modified during transport as different size fractions are routed along different transport pathways under the influence of nonuniform bed topography and associated flow patterns, and during deposition as the variable pocket geometry of the rough bed surface and turbulence intensity of the flow control the size of the particles that deposit. The review highlights the poor understanding of the many feedback linkages that exist between patterns and processes of sediment sorting at different scales and the need for a greater awareness of the spatial and temporal bounds of these linkages.

Experiments on Downstream Fining of Gravel: I. Narrow-Channel Runs

We present the results of three laboratory experiments in which longitudinally sorted deposits were formed by feeding poorly sorted sediment at the upstream end of a narrow, 45-m-long channel. The input sediment had a median size of 6 mm and included significant amounts of material up to 64 mm and down to 0.2 mm. Water discharge was constant at 49 L/s and sediment discharge varied from 0.048 to 0.19 kg/s. Downstream fining was produced in all three runs; the variation in sediment-feed rate had relatively little effect on the fining profiles. In all three runs, the formation of a longitudinally sorted deposit was mediated by the formation of a coarse surface layer. The surface layer remained at the top of the deposit during aggradation by continually reforming itself at the deposit surface. The coarse surface layer fined by approximately a factor of 2 in D90 and D50 consistently in all three experiments. The deposit (subsurface) fined less, with D90 fining more strongly than D50. The short channel length a...

Armor-Limited Clear-Water Contraction Scour at Bridges

A simplified model of streambed armoring is used to extend Laursen's well-known equation to include the limiting effects of a coarse surface layer when calculating clear-water contraction scour at bridges. An empirical relation for selective entrainment of gravel from naturally sorted riverbed material is used to estimate the smallest nontransportable particle in the armor layer. If the smallest nontransportable particle approaches the largest-sized particles in the bed-material mixture, evidence indicates that the armor layer will be unstable and particles of all sizes will be nearly equally mobile. However, if the armor layer is stable, clear-water contraction-scour depth estimates might be significantly less than for nonarmored conditions. To calculate armor-limited contraction-scour depths, two equations need to be solved simultaneously, one for the depth of the so-called active layer provided by the armor layer model, and one for depth of clear-water scour in a long constriction. The simple computati...

Numerical Method on Sediment Sorting and Bed Variation in Meander Channels

It is often found that a coarse surface layer exists in the outer-bank region, and a finer one in the inner-bank region in natural stream bends. This phenomenongenerally results from the sortingof graded bed load. A method for the calculation of bed load discharge is proposed, in which the effects of bed slope and flowdirection on sediment transport have been taken into accounts. Furthermore, a two-dimensional numerical simulation modelis developed, which is useful for predicting of the spatial and temporal variations of grain size distribution as well as of the bed configuration. Discussions are made on the sorting process and the effects of grain size distribution on the bed topography.