Gravel Transport Research Articles

Quantifying the role of bed surface topography in controlling sediment stability in water‐worked gravel deposits

This study uses measured bed topography scans and fractional bed load transport rate data obtained from previous laboratory studies of gravel transport to examine the influence of the local grain‐scale topography on grain entrainment and fractional transport rates. A simple steady state fluid model is developed to impose fluid forces onto the exposed areas of the bed surface. Image analysis is used to identify the surface grains and simple physically based algorithms are used to include the effects of sheltering. Comparisons of modeled and experimental fractional transport rates show that remote sheltering has a very strong influence on grain entrainment. With the effects of remote sheltering included, the model was found to be able to predict fractional transport rates well for a range of bed topographies and feed conditions. Further studies using a wider range of bed shear stresses would allow the effects of turbulence and bed re‐arrangement to be included in the model.

Sequence stacking patterns in the Western Canada foredeep: influence of tectonics, sediment loading and eustasy on deposition of the Upper Cretaceous Kaskapau and Cardium Formations

AbstractThe Kaskapau and Cardium Formations span Late Cenomanian to Early Coniacian time and were deposited on a low‐gradient foredeep ramp. The studied portion of the Kaskapau Formation spans ca 3·5 Myr and forms a mudstone‐dominated wedge thinning from 700 to <50 m from SW to NE over ca 300 km. In contrast, the Cardium Formation spans about 2·1 Myr, is about 100 m thick, sandstone‐rich and broadly tabular. The Kaskapau and Cardium Formations are divided, respectively, into 28 and nine allomembers, each bounded by marine flooding surfaces. Kaskapau allomembers 1 to 7 show about 200 km of offlap from the forebulge, accompanied by progradation of thin sandstones from the eroded forebulge crest. In contrast, Kaskapau allomembers 8 to 28 and Cardium allomembers C1 to C9 show overall onlap onto the forebulge of about 350 km, and contain no forebulge‐derived sandstones. This broad pattern is interpreted as recording a latest Cenomanian pulse of tectonic loading which led to shoreline back‐step in the proximal foredeep and coeval uplift of the forebulge, leading to erosion. The advance of the sediment wedge after Kaskapau allomember 7 is attributed primarily to the isostatic effect of a distributed sediment load; the advance of the orogenic wedge had a subordinate effect on subsidence of the forebulge. For Kaskapau allomembers 1 to 6, isopachs trend north to south, suggesting a load directly to the west; allomembers 7 to 28 show an abrupt rotation of isopachs to NW–SE, suggesting that the load shifted several hundred kilometres to the south. This re‐orientation might be related to a change from an approximately orthogonal to a dextral transpressive stress regime.Within the longer‐term offlap–onlap cycle recorded by the Kaskapau and Cardium Formations, individual allomembers are grouped into packages reflecting higher‐frequency onlap–offlap cycles, each spanning ca 0·5 to 0·7 Myr. Offlap from the forebulge tends to be accompanied by more pronounced transgression in the foredeep, whereas onlap onto the forebulge is accompanied by progradation of tongues of shoreface sandstone. This relationship suggests that changes in deformation rate in the orogenic wedge modulated proximal subsidence rate, enhancing or suppressing shoreline progradation, and also causing subtle uplift or subsidence of the forebulge region. Wedge‐shaped allomembers in the Kaskapau Formation contain shoreface sandstone and conglomerate that prograded, respectively, <40 and <25 km from the preserved basin margin; progradation of coarse clastics was limited by rapid flexural subsidence. Tabular allomembers of the Cardium Formation imply a low flexural subsidence rate and contain sandy and conglomeratic shoreface deposits that prograded up to ca 180 km from the preserved basin margin. This relationship suggests that low rates of flexural subsidence promoted steeper alluvial gradients, more vigorous gravel transport and more extensive shoreface progradation. Overall, observed stratal geometry and facies distribution is explained readily in terms of current elastic flexural models. Most shoreface sandstones in the proximal foredeep show evidence of forced regression. Eustasy provides the most plausible explanation for relative sea‐level rise–fall cycles on the 125 kyr allomember timescale. Geometric relationships suggest eustatic oscillations of about 10 m. Forced regressive shoreface development was suppressed during Kaskapau allomembers 1 to 10 when the rate of flexural subsidence was at its highest.

Physical basis for quasi‐universal relations describing bankfull hydraulic geometry of single‐thread gravel bed rivers

We examine relations for hydraulic geometry of alluvial, single‐thread gravel bed rivers with definable bankfull geometries. Four baseline data sets determine relations for bankfull geometry, i.e., bankfull depth, bankfull width, and down‐channel slope as functions of bankfull discharge and bed surface median sediment size. These relations show a considerable degree of universality. This universality applies not only within the four sets used to determine the forms but also to three independent data sets as well. We study the physical basis for this universality in terms of four relations, the coefficients and exponents of which can be back calculated from the data: (1) a Manning‐Strickler‐type relation for channel resistance, (2) a channel‐forming relation expressed in terms of the ratio of bankfull Shields number to critical Shields number, (3) a relation for critical Shields number as a function of dimensionless discharge, and (4) a “gravel yield” relation specifying the (estimated) gravel transport rate at bankfull flow as a function of bankfull discharge and gravel size. We use these underlying relations to explore why the dimensionless bankfull relations are only quasi‐universal and to quantify the degree to which deviation from universality can be expected. The analysis presented here represents an alternative to extremal formulations to predict hydraulic geometry.

The Unified Gravel‐Sand (TUGS) Model: Simulating Sediment Transport and Gravel/Sand Grain Size Distributions in Gravel‐Bedded Rivers

This paper presents The Unified Gravel‐Sand (TUGS) model that simulates the transport, erosion, and deposition of both gravel and sand. TUGS model employs the surface‐based bed load equation of Wilcock and Crowe (2003) and links grain size distributions in the bed load, surface layer, and subsurface with the gravel transfer function of Hoey and Ferguson (1994) and Toro‐Escobar et al. (1996), a hypothetical sand transfer function, and hypothetical functions for sand entrainment/infiltration from/into the subsurface. The model is capable of exploring the dynamics of grain size distributions, including the fractions of sand in sediment deposits and on the channel bed surface, and is potentially useful in exploring gravel‐sand transitions and reservoir sedimentation processes. Simulation of three sets of large‐scale flume experiments indicates that the model, with minor adjustment to the Wilcock‐Crowe equation, excellently reproduced bed profile and grain size distributions of the sediment deposits, including the fractions of sand within the deposits. Simulation of a flushing flow experiment indicated that the sand entrainment function is potentially capable of simulating the short‐term processes such as flushing flow events.

Channel adjustments, bedload transport and sediment sources in a gravel‐bed river, Brenta River, Italy

AbstractThis paper deals with the Brenta River (the Alps, Italy), a gravel‐bed river strongly affected by human impact. The aims of this work are (a) to reconstruct the channel adjustments that have occurred during the last decades and (b) to analyse the present condition of the river in terms of bedload transport and sediment sources. The study reach, which is 23 km long, was investigated using historical maps, aerial photographs, 12 monumented cross‐sections, laser altimetry data and field survey. Bedload transport rates were estimated by a morphological approach, which allows the construction of a sediment budget.Dramatic channel adjustments have occurred in response to an alteration of sediment fluxes caused by different human interventions (gravel mining, dams, torrent‐control works). Major channel adjustments, specifically incision (up to 8–9 m) and narrowing, took place between the 1950s and the 1980s. On the other hand, widening has become the most important process, and incision has continued only in the upper part of the study reach in more recent times (about the last 20–25 years).The estimate of bedload transport rates for the period 1984–1997 was carried out starting from the erosion and deposition volumes of the single sub‐reaches. Such an estimate has shown that (a) a high spatial variability of gravel transport rates exists (from 0 up to 73 200–85 400 m3/year) and (b) most of the bedload is supplied by local sources. Bank erosion is the main source of sediments for bedload transport and its contribution can be estimated as from 9 up to 20 times higher than the upstream contribution. Finally, from a practical point of view, results from this study should be taken into account in addressing maintenance, sediment management and restoration along a dynamic and unstable river such as the Brenta. Copyright © 2007 John Wiley & Sons, Ltd.

Opposite hysteresis of sand and gravel transport upstream and downstream of a bifurcation during a flood in the River Rhine, the Netherlands

AbstractAt river bifurcations water and sediment is divided among the downstream branches. Prediction of the sediment transport rate and division thereof at bifurcations is of utmost importance for understanding the evolution of the bifurcates for short-term management purposes and for long-term fluvial plain development. However, measured sediment transports in rivers rarely show a uniquely determined relation with hydrodynamic parameters. Commonly a hysteresis is observed of transport rate as a function of discharge or shear stress which cannot be explained with the standard sediment transport predictor approach. The aim of this paper is to investigate the causes of hysteresis at a bifurcation of the lower Rhine river, a meandering river with stable banks, large dunes during flood, and poorly sorted bed sediment. The hydrodynamics and bed sediment transport were measured in detail during a discharge wave with a recurrence interval larger than 10 years. Surprisingly, the hysteresis in bedload against discharge was in the opposite direction upstream and downstream of the bifurcation. The upstream clockwise hysteresis is caused by the lagging development of dunes during the flood. The counter-clockwise hysteresis downstream of the bifurcation is caused by a combination of processes in addition to dune lagging, namely 1) formation of a scour zone upstream of the bifurcation, causing a migrating fine sediment wave, and 2) vertical bed sorting of the bed sediment by dunes with avalanching lee-sides, together leading to surface-sediment fining and increased transport during and after the flood. These findings lead to challenges for future morphological models, particularly for bifurcations, which will have to deal with varying discharge, sediment sorting in the channel bed, lagging dunes and related hydraulic roughness.

Modelling gravel transport and morphology for the Fraser River Gravel Reach, British Columbia

A two-dimensional (2D) numerical hydrodynamic-morphological model is developed to investigate gravel transport and channel morphology in a large wandering gravel-bed river, the Fraser River Gravel Reach, in British Columbia, Canada. The model takes into count multi-fraction bedload transport, including the effects of surface coarsening, hiding and protrusion. Model outputs together with river discharge statistics were analyzed, producing distributed sediment budget and well-defined, localised zones of aggradation and degradation along the gravel reach. Long-term channel response to gravel extraction from aggrading zones as a flood hazard mitigation measure was also investigated numerically to assess the effectiveness of such an extraction. The total computed sediment budget agrees well with results based on field measurements of gravel transport available to us. This study points to the importance of a number of factors to bedload predictions: the gravel-to-sand ratio, the adequacy of resolving the wandering planform, and the distinction between bed shear stress driving bedload transport and bed resistance on the flow. These are in addition to the physical processes governing the flow field and gravel mobilization. The methodology presented in this paper can provide a scientific basis for gravel management including monitoring and extraction in order to maintain adequate flood protection and navigation, while preserving the ecosystem.

Selective bed-load transport in Las Vegas Wash, a gravel-bed stream

Summary Bed-load measurements collected at the Las Vegas Wash, a gravel-bed stream near Las Vegas, Nevada, were used to study selective transport of sand and gravel in uni-modal or weakly bi-modal river sediment. Measurements showed that size selectivity in a sediment mixture decreases as shear stress increases. Transport of variously sized sediment particles approaches equal mobility as the transported bed load is composed approximately of the same size particles as surface-bed material. Consequently, a hiding function was derived to account for the increase or reduction in reference shear stress for an individual size class in a sediment mixture as compared with that in an uniformly sized sediment. An empirical equation for determining fractional bed-load transport rate was then formulated by correlating the dimensionless, fractional bed-load transport rate with the dimensionless bed-shear stress. This equation indicated that the hiding function depends not only on the size of individual size class but also on the flow depth used to quantify the magnitude of shear stress. The present study contributes to the body of knowledge used in predicting selective transport of sediment mixtures in gravel-bed streams.

Crayfish and fish as bioturbators of streambed sediments: Assessing joint effects of species with different mechanistic abilities

Many studies illustrate that bioturbating animal species individually affect aquatic sediments through diverse mechanistic abilities, whereas assessments of joint effects of such species on sediments are relatively rare. Such joint effects have implications for real systems, in which different bioturbators coexist, but are difficult to predict for two reasons. First, they can be additive (being the sum of the individual effects of each species) or they can be positive or negative interactive (being greater or smaller than the sum of the individual effects). Second, if interactive, they can depend on biotic interactions that affect the bioturbating activities of the species and/or they can depend on physical interactions among bioturbator-induced sediment modifications. Using experimental streams, we assessed such joint effects on gravel–sand sediments for flow and sediment conditions preferred by barbel ( Barbus barbus) but also used by gudgeon ( Gobio gobio) and, in a second experiment, for flow and sediment conditions preferred by both male crayfish ( Orconectes limosus) and gudgeon. These species have different mechanistic abilities to affect gravel and/or sand in stream beds. In each experiment, we measured (i) the transport of gravel and sand at baseflow (during 12 experimental days); (ii) four sediment surface characteristics (after 12 d); and (iii) the critical shear stress ( τ c) causing incipient gravel and sand motion during experimental floods (after 12 d). Gudgeon contributed differently to the joint effects in the two experiments, which related to its individual weight, prevailing baseflow shear stress, sediment particle weight, and sediment mixture (availability of surface sand). Overall, the species pairs had predominantly negative interactive joint effects on the sediment variables assessed by us. Both a literature survey and observations during the experiments provided no evidence for direct biotic interactions between barbel and gudgeon or crayfish and gudgeon, so one would reasonably associate their negative interactive effects on the sediments with physical interactions among bioturbator-induced sediment surface modifications. Individually, each species reduced the percentage of sand in the surface layer and the surface algal cover to relatively low values so that the species pairs could not accomplish much greater joint effects on these variables, explaining their negative interactive effects on them. As algal cover particularly affected the τ c for gravel and sand, the negative interactive effects of the animals on this surface variable chained toward the τ c for the sediments, on which the species pairs also had negative interactive effects. Such chained negative interactive effects on sediment variables are seemingly a general pattern of joint bioturbator effects on aquatic sediments, i.e., the many so far described single-species effects should be smaller than their sum if the species coexists in nature.

Simulating Bed-Load Transport in a Complex Gravel-Bed River

An existing two-dimensional mobile-bed hydrodynamic model has been modified to simulate bed-load transport in a complex gravel-bed river. We investigated the sensitivity of predicted bed load to control parameters, and compared model predictions of flow depth, shear stress, and gravel transport with field measurements made from the river. The predictions are based on concurrent field data of flow discharge, water level, and sediment for model input. The model takes into account multiple-fraction transport rates, and continuously updates the river bed and surface grain-size distribution. The model predictions are in reasonable agreement with field measurements.

One‐dimensional modeling of bed evolution in a gravel bed river subject to a cycled flood hydrograph

How does a mountain river adjust to accommodate repeated flood hydrographs? Do flood hydrographs cause major cycles of aggradation and degradation of the river bed? Here flume experiments are used to explore this problem. The response of a gravel bed river to repeated floods is modeled in the simplest possible way. The gravel is well sorted, the flume is operated in sediment feed mode, and the gravel feed rate is held constant. The flow discharge, on the other hand, is specified in terms of the repetition of the same hydrograph until mobile bed equilibrium (averaged over the hydrograph) is achieved. The results of the experiments demonstrate a remarkable trade‐off. In a short inlet "boundary layer" (transition region) the bed elevation and bed slope fluctuate cyclically with the changing flow discharge, while the gravel transport rate remains nearly equal to the constant feed rate. Downstream of this short reach, however, the bed elevation and bed slope do not fluctuate in response to the hydrograph; all the fluctuation is transferred to the gravel transport rate. These results are verified in terms of one‐dimensional analytical and numerical modeling. This modeling shows that the trade‐off is inevitable as long as the morphologic response time of the reach in question is sufficiently long compared to the duration of a single hydrograph. The implication is that gravel bed rivers tend to adjust to hydrographs so as to minimize the response of the bed and maximize the response of the bed load transport rate to fluctuating flow discharge.

Bed load transport on the shoreface by currents and waves

Tide-driven bed load transport is an important portion of the net annual sediment transport rate in many shoreface and shelf environments. However, bed load transport under waves cannot be measured in the field and bed load transport by currents without waves is barely measurable, even in spring tidal conditions. There is, consequently, a strong lack of field data and validated models. The present field site was on the shoreface and inner shelf at 2 to 8.5 km offshore the central Dutch coast (far outside the surfzone), where tidal currents flow parallel to the coast. Bed load transports were carefully measured with a calibrated sampler in spring tidal conditions without waves at a water depth of 13–18 m with fine and medium sands. The near-bed flow was measured over nearly a year and used for integration to annual transport rates. An empirical bed load model was derived, which predicts bed load transports that are a factor of > 5 smaller than predicted by existing models. However, they agree with laboratory data of sand and gravel transport in currents near incipient motion. The damped transport rates may have been caused by cohesion of sediment or turbulence damping due to mud or biological activity. The annual bed load transport rate was calculated using a probability density function (pdf) derived from the near-bed current and orbital velocity data which represented the current and wave climate well when compared to 30 years of data from a nearby wave station. The effect of wave stirring was included in the transport calculations. The net bed load transport rate is a few m 2/year. This is much less than predicted in an earlier model study, which is partly due to different bed load models but also due to the difference in velocity pdf. The annual transport rate is very sensitive to the probability of the largest current velocities.

Discussion of “Measurement of Coarse Gravel and Cobble Transport Using Portable Bedload Traps” by Kristin Bunte, Steven R. Abt, John P. Potyondy, and Sandra E. Ryan

Discussion of “Measurement of Coarse Gravel and Cobble Transport Using Portable Bedload Traps” by Kristin Bunte, Steven R. Abt, John P. Potyondy, and Sandra E. Ryan

Closure to “Measurement of Coarse Gravel and Cobble Transport Using Portable Bedload Traps” by Kristin Bunte, Steven R. Abt, John P. Potyondy, and Sandra E. Ryan

Closure to “Measurement of Coarse Gravel and Cobble Transport Using Portable Bedload Traps” by Kristin Bunte, Steven R. Abt, John P. Potyondy, and Sandra E. Ryan

Assessing potential abiotic and biotic complications of crayfish-induced gravel transport in experimental streams

Biogeomorphology adds the element “biological dynamics” (of populations or communities) to chemical and physical geomorphic factors and thus complicates the framework of geomorphic processes. Such biological complications of the animal-induced transport of solids in streams should be particularly important in crayfish, as crayfish affect this transport through their overall activity and intraspecific aggression levels, which could be modified by shelter availability or the establishment of dominance hierarchies among individuals not knowing each other. Using experimental streams, we tested these hypotheses by measuring how shelter availability or residential crayfish group invasion by unknown individuals affected the impact of the crayfish Orconectes limosus on the (i) transport of gravel at baseflow (during 12 experimental days); (ii) sediment surface characteristics (after 12 days); and (iii) critical shear stress causing incipient gravel motion during simulated floods (after 12 days). The two potentially important factors shelter availability or residential group invasion negligibly affected the crayfish impact on gravel sediments, suggesting that habitat unfamiliarity (a third potentially important factor affecting crayfish activity) should increase the crayfish-induced sediment transport. Because habitat unfamiliarity is associated with sporadic long-distance migrations of a few crayfish individuals, this third factor should play a minor role in real streams, where crayfish biomass should be a key factor in relations with crayfish effects on sediments. Therefore, we combined the results of this study with those of previous crayfish experiments to assess how crayfish biomass could serve in modelling the gravel transport. Crayfish biomass explained 47% of the variability in the baseflow gravel transport and, in combination with the coefficient of variation of the bed elevation and algal cover, 72% of the variability in the critical gravel shear stress. These results encourage more research on the topic, as an increasing number of eliminations of abiotic and biotic factors that could complicate the animal-induced sediment transport in streams would facilitate the use of biological variables (e.g., bioturbator biomass) in future modelling of the transport of solids.

Transport of gravel and cobble on a mixed-sediment inner bank shoreline of a large inlet, Grays Harbor, Washington

Gravel and cobble transport measurements were obtained by particle tracing experiments from a mixed sand, gravel and cobble beach at the head of a crenulate-shaped shoreline, Half Moon Bay in Grays Harbor, Washington. The direct measurements and results provide insight regarding the differential transport of gravel and cobble sized material on mixed-sediment beaches. The results are of relevance to the development of both predictive formulae for coarse-grained sediment transport and guidance for practical gravel and cobble beach design. Net alongshore transport of gravel and cobble is generally several times greater than the net cross-shore transport at Half Moon Bay. Larger and smaller particles both move preferentially alongshore; however, smaller particles tend to move across-shore more than larger particles. Particle transport distance during a tidal cycle increases with particle size (or mass) up to a point, beyond which the particle transport rate begins to decrease with increasing size. The direct relationship between transport rate and particle size may reflect the selective entrainment and the rejection (or overpassing) of larger particles, which are more exposed to fluid forces on the beach surface than smaller particles, which are sheltered within the matrix of larger particles, as well as the higher susceptibility to burial of smaller particles. The decrease in transport rate for the largest sizes may reflect the limited competency of the fluid forces to transport larger and heavier particles under the observed conditions. The direct transport measurements are consistent with the overall particle size and shape distributions observed on this crenulate-shaped inner bank beach whereby the larger and flatter particles have tended to outrun the smaller and more spherical particles in the downdrift direction.

Coarse sediment transport in mountain streams in Colorado and Wyoming, USA

AbstractSince the early 1990s, US Forest Service researchers have made thousands of bedload measurements in steep, coarse‐grained channels in Colorado and Wyoming, USA. In this paper we use data from 19 of those sites to characterize patterns and rates of coarse sediment transport for a range of channel types and sizes, including step–pool, plane‐bed, pool–riffle, and near‐braided channels. This effort builds upon previous work where we applied a piecewise regression model to (1) relate flow to rates of bedload transport and (2) define phases of transport in coarse‐grained channels. Earlier, the model was tested using bedload data from eight sites on the Fraser Experimental Forest near Fraser, Colorado. The analysis showed good application to those data and to data from four supplementary channels to which the procedure was applied. The earlier results were, however, derived from data collected at sites that, for the most part, have quite similar geology and runoff regimes. In this paper we evaluate further the application of piecewise regression to data from channels with a wider range of geomorphic conditions. The results corroborate with those from the earlier work in that there is a relatively narrow range of discharges at which a substantial change in the nature of bedload transport occurs. The transition from primarily low rates of sand transport (phase I) to higher rates of sand and coarse gravel transport (phase II) occurs, on average, at about 80 per cent of the bankfull (1·5‐year return interval) discharge. A comparison of grain sizes moved during the two phases showed that coarse gravel is rarely trapped in the samplers during phase I transport. Moreover, the movement and capture of the D16 to D25 grain size of the bed surface seems to correspond with the onset of phase II transport, particularly in systems with largely static channel surfaces. However, while there were many similarities in observed patterns of bedload transport at the 19 studied sites, each had its own ‘bedload signal’ in that the rate and size of materials transported largely reflected the nature of flow and sediment particular to that system. Published in 2005 by John Wiley & Sons, Ltd.

Deep-water leveed-channel complexes of the Cerro Toro Formation, Upper Cretaceous, southern Chile

The Cerro Toro Formation in the Torres del Paine National Park, southern Chile, contains a series of deep-water channel complexes deposited in an elongate Andean foreland basin during the Late Cretaceous. This stratigraphic interval represents an essentially continuous depositional record of migrating, leveed-channel complexes. Collectively, the channel-fill units in the study area form a belt approximately 5 km (3 mi) wide and several hundred meters thick. Within the study area, four sets of channel complexes are identified. This paper focuses on the best exposed of these channel-complex sets (channel-complex set 3). The channels are filled by bedded conglomerate and amalgamated sandstones interpreted to represent the deposits of high-concentration turbidity currents and debris flows. Large-scale cross-beds in some of the conglomerates indicate significant bed-load transport of gravel- and cobble-forming bars in the channels. Channel axis to margin facies changes between clast-supported conglomerate and either (1) thick-bedded sandstone or (2) matrix-supported conglomerate are observed. Channel-fill facies lie on erosional surfaces that cut into adjacent interchannel facies. Beds thin and onlap these surfaces toward the channel margins. Shale or siltstone drapes of the channel cuts are uncommon and laterally discontinuous. Bed continuity between channel and adjacent, interchannel facies is not observed. The interchannel strata are interpreted to represent levee successions that bound the channels. Stratigraphy in the levee units is defined to include (1) basal, sandy lobe deposits comprised of medium- to thick-bedded turbidites and (2) overbank facies consisting primarily of packages of fining- and thinning-upward, fine-grained, thin-bedded turbidites. This vertical succession is transitional. Distal levee facies include mudstones with thin-bedded, laterally continuous sandstones. Proximal levee facies include mudstones with both thin- and thick-bedded sandstones; however, the thick-bedded sandstones have lower lateral continuity. The proximal levee facies have a higher sandstone percentage than the distal levee, but also have greater depositional and postdepositional complexity, with sand-filled crevasses, erosional truncation, and slumped beds. Field observations suggest that these leveed channels formed in stages that are represented by depositional and/or erosional events. In chronological order, these are (1) an initial stage of relatively unconfined, sand-rich deposition; (2) aggradation of a mud-rich, confining levee system resulting from overbank deposition as turbidity flows bypass the area; (3) erosion as the channel becomes entrenched or as the channel migrates; and (4) filling of the channel-margin relief by onlap of channel-fill sediments. These stages appear to have repeated several times during the formation of a series of channel complexes. In these ways, the Cerro Toro Formation appears analogous to leveed-channel systems observed in late Pleistocene submarine fans and subsurface examples.

Tradeoffs associated with sediment‐maintenance flushing flows: a simulation approach to exploring non‐inferior options

AbstractSediment‐maintenance flushing flows designed to mimic the action of natural floods in removing the accumulated fine sediments from the channel and loosening the gravel bed have been increasingly proposed as an effective alternative in dam management and a required component of riverine restoration programmes. However, reservoir releases are generally associated with financial and environmental costs, thus it is highly desirable to specify flushing flows as accurately as possible. In this paper we present a simulation approach to evaluating flushing flows and exploring the tradeoffs associated with non‐inferior flushing options. A two‐fraction sediment routing model is used to simulate the gravel‐sand bed response to flushing flows. The results reveal that the sand cleansing effect propagates from upstream to downstream and from surface to subsurface. Under a steady gravel supply from upstream, an equilibrium state of gravel transport and bed degradation is eventually reached in the simulation reach. The flushing flow and sediment transport system investigated in this study involves a transient state variable (bed sand content), a decision variable (flushing flow discharge), a flushing goal (ultimate bed sand content), and three outcomes to be minimized (flushing duration, released water volume, and total gravel loss). A series of numerical simulations are carried out with a range of flows and pre‐flushing bed sediment conditions. The results reveal that the flushing efficiency is higher for the larger flow. However, for flows greater than ∼100 m3/s the flushing duration is less sensitive to the flow discharge, thus the system may be simplified as a bi‐objective one. The gravel loss and water volume are two conflicting outcomes within the non‐inferior flow region. Under a worse bed sediment condition, the feasible flushing options are constrained in a narrower range and also associated with higher costs. The tradeoffs between the conflicting outcomes are quantitatively displayed with the transformed feasible solutions in the objective space. We provide here a general and practical approach permitting a quantitative evaluation of the different flushing options that is appropriate to the level of data typically available. Copyright © 2004 John Wiley & Sons, Ltd.

The morphological destruction and subsequent restoration programmes of large rivers in Europe

This paper presents a synopsis of the rivers Rhine and Danube in Europe, connected by the Rhine-Main-Danube waterway. Their morphological destruction and subsequent ecological impacts are documented, and the ongoing and future restoration programmes are discussed. While the River Rhine has improved significantly in water quality and still is channelized to a large extent (despite considerable restoration), the River Danube features more near natural stretches and flood plains while still being largely polluted in the middle and lower parts. In both catchments, the goals of research/management are unlimited fish migration (e. g., salmon in the Rhine, and sturgeon in the Danube), some gravel and sediment transport, the conservation/restoration of flood plains as hot spots of biodiversity and flood retention areas, and the sustainable development of the river basin. While the River Rhine authorities (including five countries) are advanced in the political process of river basin management, the River Danube authorities (including 16 countries) still must find a harmonized cooperation.