Investigation of hydraulics transport time scales within the Arvand River estuary,Iran

Transport time scales are key parameters for understanding the hydrodynamic and biochemical processes within estuaries. In this study, the flushing and residence times within the Arvand River estuary have been estimated using a two‐dimensional hydrodynamic model called CE‐QUAL‐W2. The model has been calibrated and verified by two different sets of field data and using the k‐ε vertical eddy diffusivity scheme. Flushing time has been estimated using different methods such as the tidal prism and fraction of freshwater methods. Moreover, residence times have been investigated using pulse residence time, estuarine residence time and remnant function approaches. The results have shown that different methods yield different time scales, and freshwater inflow has the greatest impact upon estimation of residence time, whereas tidal circulation hardly contributes to residence time at all. It has also been shown that the neap‐spring circulation and start phase of simulations have negligible effects on the Arvand's time scales. The investigation of bathymetry showed that two sills of the estuary tend to significantly increase residence time. Understanding the applicability of these time scales and their estimation approaches helps us to evaluate the water quality management of estuaries. Copyright © 2013 John Wiley & Sons, Ltd.     

Reconciled bedload sediment transport rates in ephemeral and perennial rivers

It 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.     

Back‐calculation of bedload transport in steep channels with a numerical model

In August 2005 severe flood events occurred in the Alps. A sediment routing model for steep torrent channel networks called SETRAC has been applied to six well‐documented case study streams with substantial sediment transport in Austria and Switzerland. For these streams information on the sediment budget along the main channel is available. Flood hydrographs were reconstructed based on precipitation data and stream gauges in neighbouring catchments. Different scenarios are modelled and discussed regarding sediment availability and the effect of armouring and macro‐roughness on sediment transport calculations. The simulation results show the importance of considering increased flow resistance for small relative flow depth when modelling bedload transport during high‐intensity flood events in torrents and mountain rivers. Without any correction of increased flow resistance using a reduced energy slope, the predicted bedload volumes are about a factor of 10 higher on average than the observed values. Simulation results were also used for a back‐calculation of macro‐roughness effects from bedload transport data, and compared with an independent estimate of flow resistance partitioning based on flow resistance data. Copyright © 2010 John Wiley & Sons, Ltd.     

Insights into bedload transport processes of a large regulated gravel‐bed river

A comprehensive monitoring programme focusing on bedload transport behaviour was conducted at a large gravel‐bed river. Innovative monitoring strategies were developed during five years of preconstruction observations accompanying a restoration project. A bedload basket sampler was used to perform 55 cross‐sectional measurements, which cover the entire water discharge spectrum from a 200‐year flood event in 2013 to a rare low flow event. The monitoring activities provide essential knowledge regarding bedload transport processes in large rivers. We have identified the initiation of motion under low flow conditions and a decrease in the rate of bedload discharge with increasing water discharge around bankfull conditions. Bedload flux strongly increases again during high flood events when the entire inundation area is flooded. No bedload hysteresis was observed. The effective discharge for bedload transport was determined to be near mean flow conditions, which is therefore at a lower flow discharge than expected. A numerical sediment transport model was able to reproduce the measured sediment transport patterns. The unique dataset enables the characterisation of bedload transport patterns in a large and regulated gravel‐bed river, evaluation of modern river engineering measures on the Danube, and, as a pilot project has recently been under construction, is able to address ongoing river bed incision, unsatisfactory ecological conditions for the adjacent national park and insufficient water depths for inland navigation. Copyright © 2017 John Wiley & Sons, Ltd.     

Experimental investigation of bedload transport processes under unsteady flow conditions

Hydraulic engineering is usually based on theoretical analysis and/or numerical modelling simulation. As the dynamic behaviour of sediment movement under unsteady flow is still unclear, and field measurement is comparatively difficult during a large flood, prior investigations through flume experiments are required. A series of flume experiments, conducted using different inflow hydrographs without sediment supply from upstream, was carried out to investigate the sediment transport process under unsteady flow conditions. A series of triangular hydrographs were performed in the experiments. The results indicate that a temporal lag was found between the flow hydrograph peak and the sediment hydrograph peak because large size sand dunes lasted for a short period in the falling limb of the flow hydrograph. The temporal lag was found to be about equal to 6–15% of the flow hydrograph duration. Owing to the temporal lag, the total bedload yield in the rising period was less than that in the falling period. Furthermore, the measured total bedload yield in the unsteady flow experiments was larger than the predicted value, which was estimated by using the results obtained from the equivalent steady flow experiment. The peak bedload transport rate for unsteady flow conditions was also larger than the predicted value. The ratios of the measured to the predicted quantities mentioned above were found to be constant values for different shapes of hydrographs. It is, therefore, expected that the analytical results of sediment transport from equivalent steady flow can be a good reference for sediment transport under unsteady flow conditions. Copyright © 2004 John Wiley & Sons, Ltd.     

Testing bedload transport formulae using morphologic transport estimates and field data: lower Fraser River,British Columbia

Morphologic transport estimates available for a 65‐km stretch of Fraser River over the period 1952–1999 provide a unique opportunity to evaluate the performance of bedload transport formulae for a large river over decadal time scales. Formulae tested in this paper include the original and rational versions of the Bagnold formula, the Meyer‐Peter and Muller formula and a stream power correlation. The generalized approach adopted herein does not account for spatial variability in flow, bed structure and channel morphology. However, river managers and engineers, as well as those studying rivers within the context of long‐term landscape change, may find this approach satisfactory as it has minimal data requirements and provides a level of process specification that may be commensurable with longer time scales. Hydraulic geometry equations for width and depth are defined using morphologic maps based on aerial photography and bathymetric survey data. Comparison of transport predictions with bedload transport measurements completed at Mission indicates that the original Bagnold formula most closely approximates the main trends in the field data. Sensitivity analyses are conducted to evaluate the impact of inaccuracies in input variables width, depth, slope and grain size on transport predictions. The formulae differ in their sensitivity to input variables and between reaches. Average annual bedload transport predictions for the four formulae show that they vary between each other as well as from the morphologic transport estimates. The original Bagnold and Meyer‐Peter and Muller formulae provide the best transport predictions, although the former underestimates while the latter overestimates transport rates. Based on our findings, an error margin of up to an order of magnitude can be expected when adopting generalized approaches for the prediction of bedload transport. Copyright © 2005 John Wiley & Sons, Ltd.     

Continuous measurement of sediment transport in the Erlenbach stream using piezoelectric bedload impact sensors

We report on bedload transport observations using piezoelectric bedload impact sensors (PBIS), an indirect method of estimating the volume of bedload transport of coarse sediment. The PBIS device registers vibrations produced by bedload (particle diameter >～20 mm) and records the signal as a sum of the number of impulses per time. Sediment transport at the Erlenbach stream has been continuously monitored with a PBIS array starting in 1986. The sensor array spans the width of an entire cross‐section and is mounted flush with the surface of a check dam immediately upstream of a sediment retention basin. We compare PBIS data with long‐term sedimentation records obtained from repeated surveys of material stored in the sediment retention basin, with artificial sediment input under controlled conditions in the field, and also with laboratory experiments. The rate of bedload transport is proportional to the number of impacts on the sensor per unit time. The reliability of the calibration relationship increases with the length of the observation period, e.g. for higher numbers of impacts and larger bedload volumes. Sediment volumes for individual flood events estimated with the PBIS method are in agreement with volumes estimated using an independent empirical method based on the effective runoff volume of water, the peak water discharge, and the critical discharge for the onset of sediment transport. Copyright © 2007 John Wiley & Sons, Ltd.     

The causes of bedload pulses in a gravel channel: the implications of bedload grain‐size distributions

Unsteady bedload transport was measured in two c. 5 m wide anabranches of a gravel‐bed braided stream draining the Haut Glacier d'Arolla, Switzerland, during the 1998 and 1999 melt seasons. Bedload was directly sampled using 152 mm square Helley–Smith type samplers deployed from a portable measuring bridge, and independent transport rate estimates for the coarser size fractions were obtained from the dispersion of magnetically tagged tracer pebbles. Bedload transport time series show pulsing behaviour under both marginal (1998) and partial (1999) transport regimes. There are generally weak correlations between transport rates and shear stresses determined from velocity data recorded at the measuring bridge. Characteristic parameters of the bedload grain‐size distributions (D₅₀, D₈₄) are weakly correlated with transport rates. Analysis of full bedload grain‐size distributions reveals greater structure, with a tendency for transport to become less size selective at higher transport rates. The bedload time series show autoregressive behaviour but are dif?cult to distinguish by this method. State–space plots, and associated measures of time‐series separation, reveal the structure of the time series more clearly. The measured pulses have distinctly different time‐series characteristics from those modelled using a one‐dimensional sediment routing model in which bed shear stress and grain size are varied randomly. These results suggest a mechanism of pulse generation based on irregular low‐amplitude bedforms, that may be generated in‐channel or may represent the advection of material supplied by bank erosion events. Copyright © 2003 John Wiley & Sons, Ltd.     

Coarse bedload transport in a mountain river

Coarse bedload transport dynamics are investigated utilizing hydrodynamic and sediment transport data obtained in an extensively instrumented study reach located in Squaw Creek, Montana, USA. During 1991 and 1992, a number of discrete bedload transport events associated with the daily rise and fall in stream discharge were investigated. Data show that initiation of sediment transport was accompanied by a reduction in bed roughness and by changes in bulk hydraulic parameters. For larger discharges, coarser fractions of the bed material mobilized, and bedload transport rates and average hydraulic parameters stabilized. As discharge reduced, mobile coarse particles became less frequent and deposited fine particles were removed, resulting in an increase in bed roughness. These observations are attributed to the downstream translation of bar sediments during the passage of a hydrograph. Bedload pulses were aperiodic but spatially variable. Flow turbulence and velocity profile data obtained during low flows allowed comparison between average bed shear stress and apparent bed roughness estimates obtained using different approaches. © 1998 John Wiley & Sons, Ltd.     

Temporal variations in bedload transport rates associated with the migration of bedforms

Temporal variations in bedload transport rates that occur at a variety of timescales, even under steady flow conditions, are accepted as an inherent component of the bedload transport process. Rarely, however, has the cause of such variations been explained clearly. We consider three data sets, obtained from laboratory experiments, that refer to measurements of bedload transport made with continuously recording bedload traps. Each data set is characterized by a predominant low-frequency oscillation, on which additional higher-frequency oscillations generally are superimposed. The period of these oscillations, as isolated through the use of spectral analysts, ranged between 0·47 and 168 minutes, and was associated unequivocally with the migration of bedforms such as ripples, dunes, and bars. The extent to which such oscillatory behaviour may be recognized in a data set depends on the duration of sampling and the length of the sampling time, with respect to the period of a given bedform. Several theoretical probability distribution functions have been developed to describe the frequency distributions of (relative) bedload transport rates that are associated with the migration of bedforms (Einstein, 1937b; Hamamori, 1962; Carey and Hubbell, 1986). These distribution functions were derived without reference to a sampling interval. We present a modification of Hamamori's (1962) probability distribution function, generated by Monte Carlo simulation, which permits one to specify the sampling interval, in relation to the length of a bedform. Comparisons between the simulated and observed frequency distributions, that were undertaken on the basis of the data described herein, are good (significant at the 90 per cent confidence level). Finally, the implications that temporal variability, which is associated with the migration of bedforms, have for the accurate determination of bedload transport rates are considered.     

Differential bedload transport rates in a gravel-bed stream: A grain-size distribution approach

Two methods that were recently proposed for calculating fractional bedload transport rates in gravel-bed streams are examined closely. Both of them employ the Oak Creek bedload data. The Diplas (PD) method is guided by dimensional analysis and, therefore, can be used to predict bedload transport in different gravel rivers. The only requirement for using this method is the knowledge of the subsurface material size distribution of the stream of interest. The expression for the fractional bedload transport obtained by the Shih and Komar (SK) method for Oak Creek cannot be used for other streams. Its use for a given stream requires information that is rarely available. For the Oak Creek case both methods demonstrate similar predictive ability.     

Defining phases of bedload transport using piecewise regression

Differences 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 (R_br) 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 R_br was about 80 percent, suggesting that Phase II begins at about 80 percent of the bankfull discharge, though the observed values of R_br 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 & Sons, Ltd.     

Patterns of bedload entrainment and transport in forested headwater streams of the Columbia Mountains,Canada

We monitor bedload transport and water discharge at six stations in two forested headwater streams of the Columbia Mountains, Canada. The nested monitoring network is designed to examine the effects of channel bed texture, and the influence of alluvial (i.e. step pools and riffle pools) and semialluvial morphologies (i.e. boulder cascades and forced step pools) on bedload entrainment and transport. Results indicate that dynamics of bedload entrainment are influenced by differences in flow resistance attributable to morphology. Scaled fractional analysis shows that in reaches with high form resistance most bedload transport occurs in partial mobility fashion relative to the available bed material, while calibers finer than 16 mm attain full mobility during bankfull flows. Equal mobility transport for a wider range of grain sizes is achieved in reaches exhibiting reduced form resistance. Our findings confirm that the Shields value for mobilization of the median surface grain size depends on channel gradient and relative submergence; however, we also find that these relations vary considerably for cobble and gravel bed channels due to proportionality between dimensionless shear stress and grain size. Exponents of bedload rating curves across sites correlate most with the D_90s of the mobile bed, however, where grain effects are controlled (i.e. along individual streams), differences in form resistance across morphologies exert a primary control on bedload transport dynamics. Application of empirical formulae developed for use in steep alpine channels present variable success in predicting transport rates in forested snowmelt streams. Formulae that explicitly account for reductions in mobile bed area and high morphological resistance associated with woody debris provide the best approximation to observed empirical data. Copyright © 2014 John Wiley & Sons, Ltd.     

Climate change impacts on bedload transport in alpine drainage basins with hydropower exploitation

Sediment transport is known to have a significant impact on hydropower infrastructures and changes in sediment transport rates are important for sediment management measures and hydroelectricity production. In this study, we present how climate change may affect bedload transport in 66 high alpine catchments used by hydropower companies in the Valais, Switzerland. Future sediment yield is estimated with a runoff‐based sediment transport model for the two future 30 year time periods 2021–2050 and 2070–2099. The analysis is integrated into a modelling chain in which error‐corrected and downscaled climate scenarios generated in the framework of the ENSEMBLES project are coupled to the hydrological model PREVAH, glacier retreat and bedload transport. To calibrate the sediment transport model, we used the observed sediment volumes in water intakes and reservoirs if such data were available. The results obtained show on average a decrease of sediment yield due to the reduced runoff generation during summer, especially for the scenario period 2070–2099. A shift of the seasonal sediment transport regime with a current maximum during July and August to earlier months in the year is predicted. Projections of future sediment yield rely on the accuracy of the individual modeling chain elements. The different sources of uncertainty are discussed qualitatively. Copyright © 2015 John Wiley & Sons, Ltd.     

Analysis of bedload transport characteristics of Idaho streams and rivers

Field data are essential in evaluating the adequacy of predictive equations for sediment transport. Each dataset based on the sediment transport rates and other relevant information gives an increased understanding and improved quantification of different factors influencing the sediment transport regime in the specific environment. Data collected for 33 sites on 31 mountain streams and rivers in Central Idaho have enabled the analysis of sediment transport characteristics in streams and rivers with different geological, topographic, morphological, hydrological, hydraulic, and sedimentological characteristics. All of these streams and rivers have armored, poorly sorted bed material with the median particle size of surface layer coarser than the subsurface layer. The fact that the largest particles in the bedload samples did not exceed the median particle size of the bed surface material indicates that the armor layer is stable for the observed flow discharges (generally bankfull or less, and in some cases two times higher than bankfull discharge). The bedload transport is size‐selective. The transport rates are generally low, since sediment supply is less than the ability of flow to move the sediment for one range of flow discharges, or, the hydraulic ability of the stream is insufficient for entrainment of the coarse bed material. Detailed analyses of bedload transport rates, bedload and bed material characteristics were performed for each site. The obtained results and conclusions are used to identify different influences on bedload transport rates in analyzed gravel‐bed rivers. Copyright © 2008 John Wiley & Sons, Ltd.     

Measurement of the spatial distribution of fluvial bedload transport velocity in both sand and gravel

Maps are presented of the spatial distribution of two‐dimensional bedload transport velocity vectors. Bedload velocity data were collected using the bottom tracking feature of an acoustic Doppler current pro?ler (aDcp) in both a gravel‐bed reach and a sand‐bed reach of Fraser River, British Columbia. Block‐averaged bedload velocity vectors, and bedload velocity vectors interpolated onto a uniform grid, revealed coherent patterns in the bedload velocity distribution. Concurrent Helley‐Smith bedload sampling in the sand‐bed reach corroborated the trends observed in the bedload velocity map. Contemporaneous 2D vector maps of near‐bed water velocity (velocity in bins centered between 25 cm and 50 cm from the bottom) and depth‐averaged water velocity were also generated from the aDcp data. Using a vector correlation coef?cient, which is independent of the choice of coordinate system, the bedload velocity distribution was signi?cantly correlated to the near‐bed and depth‐averaged water velocity distributions. The bedload velocity distribution also compared favorably with variations in depth and estimates of the spatial distribution of shear stress. Published in 2004 by John Wiley & Sons, Ltd.     

Dependence of sediment sorting on bedload transport phase in a river meander

It is widely recognized nowadays that there are at least two different phases of bedload sediment transport in gravel‐bed rivers. However, the transition between these phases is still poorly or subjectively defined, especially at bends in rivers, where cross‐stream sediment transport can strongly influence changes in the texture of the transported sediment. In this paper, we use piecewise models to identify objectively, at two points in the cross‐section of a river bend, the discharge at which the transition between bedload transport phases occurs. Piecewise models were applied to a new bedload data set collected during a wide range of discharges while analysing the associated changes in sediment texture. Results allowed the identification of two well‐differentiated phases of sediment transport (phase I and phase II), with a breakpoint located around bankfull discharge. Associated with each phase there was a change in bedload texture. In phase I there was non‐dominance in the transport of fine or coarse fractions at a particular sampling point; but in phase II bedload texture was strongly linked to the position of the sampling point across the channel. In this phase, fine particles tended to be transported to the inner bank, while coarse sizes were transferred throughout the middle parts of the channel. Moreover, bedload texture at the inner sampling point became bimodal while the transport of pebble‐sized particles was increasing in the central parts of the river channel. It is suggested that this general pattern may be related both to secondary currents, which transfer finer particles from the outer to the inner bank, and to the progressive dismantling of the riverbed surface layer. Copyright © 2018 John Wiley & Sons, Ltd.     

A demonstration of the importance of bedload transport for fluvial bedrock erosion and knickpoint propagation

We provide field evidence for the role of bedload in driving fluvial incision and knickpoint propagation. Using aerial photographs, field surveys, and hydrological data, we constrain the incision history of a bedrock gorge 1200 m long and up to 20 m deep cut by Da'an River in western Taiwan. This reach of the river experienced 10 m of uplift during the 1999 Chi‐Chi earthquake. For five years following the earthquake, bedload was prevented from entering the uplift zone, the knickpoint was static and little incision took place. Bedload transport across the uplift zone resumed in 2004, initiating extremely rapid incision, with 620 m of knickpoint propagation and up to 20 m of downcutting by 2008. This change highlights the relative inefficiency of suspended sediment and the dominant role of bedload as a tool for fluvial erosion and knickpoint propagation. Once bedload tools became available, knickpoint propagation was influenced by geological structure, lithology, and drainage organization. In particular, a change in dip of the sandstone beds at the site caused a decrease of knickpoint propagation velocity. Copyright © 2012 John Wiley & Sons, Ltd.     

Investigating the transport dynamics and the properties of bedload material with a hydro‐acoustic measuring system

This article deals with the following two questions. Are acoustic measurements in running waters appropriate for a highly resolved investigation of the bedload transport? Which characterizations of the bedload regarding mass and shape are possible via the acoustic signals? The signals were recorded by means of data recorders (Tascam Inc. DAP1 Portable Data Recorder) and hydrophones (International Transducer Corp. ITC‐4001 A). The ITC‐4001 is a shallow water omnidirectional transducer containing a flexural disc transducer utilizing Channelite‐5400 ceramics mounted in a rugged corrosion‐resistant housing. These hydrophones were screwed onto the bottom side of stainless steel plates, serving as a contact surface for the bedload in motion above them. After more than 100 series of tests in the laboratory, which indicated the basic relations between the dimension, shape and weight of the bedload and the resulting signal, field tests of the measuring system were conducted. By artificially produced flood waves in the small brooks Riverisbach, Olewiger Bach and by a winter flood wave in the River Moselle, it is possible to elaborate similar structures of the signal course of the bedload movement. The highest transport rates can be observed at the beginning of the increasing limbs and behind the peaks of the waves. At the beginning of the waves, the increasing transport power of the water and the loose material can be considered as the cause for this result. The high stream velocity behind the wave peaks explains the increase in the bedload transport so that material from the channel beds is unfastened and will be mobilized. The characterization of the bedload regarding the shape and mass is still limited regarding the field measurements and could be solved only for homogeneous grain sizes and single stones under laboratory conditions. Copyright © 2007 John Wiley & Sons, Ltd.