Equilibrium Sediment Transport Research Articles

Study on Vertical Distribution of Sediment Concentration in the Tidal Estuary

The vertical distribution of sediment concentration in estuaries is the core issue of suspended sediment transport. At present, there are many researches on the vertical distribution of sediment concentration in rivers, and a lot of results have been achieved, but that in estuaries is seldom studied. The Yellow River Estuary (YRE) and the Qiantang River Estuary (QRE) are typical tidal estuaries with strong erosion/deposition and high concentration in China. Under the action of alternating tidal flow, the suspended sediment transport shows significant time-varying and non-equilibrium transport characteristics. The vertical distribution of sediment concentration is quite different from that of ordinary rivers. This paper analyzes the vertical distribution of sediment concentration under the action of tidal currents based on the theory of sediment movement and diffusion. Compared with equilibrium sediment transport, the vertical distribution of sediment concentration only differs by one vertical distribution coefficient. Based on the analysis of the measured data, it is found that the vertical distribution of sediment concentration in the YRE and QRE is mostly exponential. Firstly, the multivariate relationship between the vertical distribution coefficient and the sediment saturation and suspension index is established by means of the measured water and sediment data of the QRE. The correlation coefficient <I>R</I><sup>2</sup> was above 0.64, which has a good correlation. Finally, the established relationship was validated using measured water and sediment data from the YRE, and the results were basically consistent, indicating that the established formula for vertical distribution of sediment concentration is suitable for general tidal estuaries, and has a certain reference value for the simulation of sediment transport in the YRE and QRE.

Effect of equilibrium and non-equilibrium sediment transport flows on the shear velocity in an open channel.

Equilibrium and non-equilibrium sediment transport flow in natural rivers and artificial channels can result in degradation or aggradation of the river bed, which will affect many hydraulic parameters, one of which is shear velocity. This study evaluates the influence of sediment transport on shear velocity in open channels. In this study, laboratory experiments were carried out using a sediment-recirculating flume, and measurements of the velocity and Reynolds shear stress distributions were measured using a Sontek-acoustic Doppler velocimeter (ADV). This experimental study measured 120 velocity distributions, and Reynolds stress profiles, consisting of 12 different sets of measurements involving flow with sediment transport and movable sand bed. The results showed an average shear velocity, u*, difference between sediment feeding and without sediment feeding by 9.41%. For the integration constant Br value, the average difference between sediment feeding and without sediment feeding was 5.58%.

Evaluating Manning's Roughness Coefficient for Flows with Equilibrium and Non-equilibrium Sediment Transport

One of the challenges in using Manning’s equation lies in accurately determining Manning’s roughness coefficient, especially due to sediment transport. The condition of sediment transport within a specific section of a river can be in equilibrium, meaning that the amount of sediment entering and exiting the segment is balanced or it can be in non-equilibrium, where there is an imbalance between the sediment entering and leaving the segment. Experiments were carried out in the laboratory to simulate both transport conditions, one involving sediment feeding (equilibrium condition) and the other without sediment feeding (non-equilibrium condition). A total of 180 mean velocity profiles were measured using an acoustic doppler velocimeter (ADV) in fixed and movable bed flows. The present experiment was combined with theoretical approaches to evaluate Manning’s coefficient in open channels under sediment transport-flow conditions and reach conclusive results. The shape of the velocity profile, u/U, for flows over a movable bed with sediment feeding is “slender” compared to those without sediment feeding. The presence of sediment transport affects the shape of the velocity profile, especially in the inner region, influencing Manning's roughness coefficient. This coefficient can be determined from two-point velocity measurements at z/H = 0.1 and 0.2 for fixed and movable bed flows with and without sediment feeding, respectively. On average, sediment feeding increases the coefficient by 19% compared to the non-sediment feeding case. KEYWORDS: Manning’s roughness coefficient, Velocity profile, Equilibrium and non-equilibrium, Sediment transport, Open-channel flow

Debris flow weakens the ecological role of river microhabitat heterogeneity in mountainous regions

Mountain rivers provide habitat or refuges and create migration corridors for diverse aquatic and riparian organisms. River microhabitat heterogeneity (RMH), which plays a key role in ecological restoration, is sensitive to external disturbances in mountain rivers. However, the effects of RMH, induced by hydro-geomorphological processes, on local macroinvertebrates have not been quantitatively studied. To explore the ecological significance of RMH, we proposed a new RMH index (RMHI) for quantitative evaluation of RMH and selected five debris flow-dominated mountain rivers (DMR) and five equilibrium sediment transport mountain rivers (EMR) as contrasting examples based on the richness of sediment supply. We found that RMH supported macroinvertebrate α-diversity and functional richness in both DMR and EMR, but debris flow weakened the ecological role of RMH in DMR. The proposed RMHI should be ≥ 8.0 to maintain the ecological health of mountain rivers. Besides, the macroinvertebrate communities were mainly driven by species turnover in EMR, while species turnover and nestedness were balanced in DMR. And macroinvertebrate community shift from an R-strategy to a K-strategy due to deposition and erosion. According to the research results, we put forward the following suggestions. At the watershed scale, ecological conservation of mountain rivers requires a regional approach focusing on multiple sites in EMR. Priority should be given to protect the river with high macroinvertebrate species richness and close attention needs to be paid to multiple sites of DMR. At the river scale, we can protect the biodiversity of mountain rivers by maintain RMHI above its threshold.

Effects of sediment supply on scour at submerged grade control structures

Grade control structures (GCSs) are common river training structures constructed to prevent riverbed degradation. Existing scour predictors for GCSs are based on flume experiments with either no sediment supply or equilibrium sediment transport conditions, the quantitative impacts of sediment supply on the scour at submerged GCSs are still unclear. The present study reports a series of flume experiments to investigate the effects of sediment supply on scour at submerged GCSs. The experimental results show that, compared with the equilibrium sediment transport condition, a decrease of upstream sediment supply rate can reduce the upstream scour depth (down to zero), increase the downstream scour depth (maximumly by 2.3 times) and change the scour process at a submerged GCS. If the sediment supply rate is close to the sediment transport capacity of the flow, the downstream scour depth first increases rapidly and then oscillates obviously around an equilibrium value (i.e. quasi-live-bed scour). If the sediment supply rate is close to zero, the downstream scour depth oscillates initially, then increases monotonically with time towards an equilibrium value (i.e. quasi-clear-water scour). Based on the experimental data and theoretical analysis, the quantitative impacts of sediment supply rate on the equilibrium scour depth at submerged GCSs are evaluated; and calculation methods considering the sediment supply are proposed for estimating the equilibrium scour depth at submerged GCSs.

Flow-driven soil erosion processes in a calcareous semiarid soil: Rill length and flow rate impacts

Understanding the dynamic mechanisms of rill erosion is vital for accurately modeling of soil erosion. In this research, rill erosion was experimentally studied using different rill lengths. A flume, 11 m long and 0.2 m wide adjusted to 3% slope, was used for constructing rectangular rills of 0.05 m width and six different lengths (1 to 12 m). The experiments were carried out on a clayey calcareous soil from a semiarid region under two flow rates of QL = 0.025 and QH = 0.045 L s−1. Temporal sediment concentrations (C) were measured at the rill outlet for 40 min and the sediment size distribution (PSD) was recorded twice during each test. The common decreasing trend of C was affected by both flow rate and rill length. The formation of head-cuts along the bed of long rills was an important erosion process affecting C and PSD. The compiled data were used to determine the exponential models' parameters of both C and soil detachment rate based on the steady, unsteady, and event-average values. While, the equilibrium sediment transport occurred by increasing rill length beyond 7 m under QL, the tested rills under QH were not sufficiently long for the flow to reach its maximum transport capacity at the gentle slope used and for the calcareous soil tested. Based on the sediment PSD, the selective transport of coarse particles was evidenced. Selectivity transport was affected by rill length reached its maximum at the rill length of 7 m.The findings provides better understanding of rill erosion processes for calcareous soils on gentle slopes in semiarid regions. Further research is needed for better understanding of the rill erosion processes on short gentle slopes to provide experimentally supports for the existing theoretical concepts of rill erosion models.

Equilibrium sediment transport by dilute turbidity currents: Comparison of competence‐based and capacity‐based models

AbstractEquilibrium sediment transport is the condition of zero net entrainment and deposition by sediment‐transporting flow (i.e. grade or regime). Here criteria for equilibrium sediment transport, or those used as proxies for equilibrium (for example, onset of erosion, onset of particle setting or suppression of turbulence) for dilute, suspended‐load‐dominated, turbidity currents, are tested against laboratory and natural data. The examined criteria are restricted to those describing flow over a bed of loose particulate material involving non‐cohesive sediment. Models include both monodisperse and polydisperse formulations that represent sediment non‐uniformity by using a single characteristic grain size or discretization of the grain‐size distribution, respectively. Analysis shows that a polydisperse‐type flux‐balance model, that equates erosional and depositional fluxes and where erosion is related to the power used to lift sediment mass from the bed (the ‘Flow‐Power Flux‐Balance’ model) provides predictions most consistent with observational data. Other equilibrium models tested, monodisperse or polydisperse, fail to predict realistic bed slopes and/or flow durations for concentrations, velocities and depths within limits for natural flows. Results of the Flow‐Power Flux‐Balance model are used to quantify sediment transport fields, equilibrium Shields numbers and slopes for turbidity currents of variable flow and particle properties.

Assessing an overland flow resistance approach under equilibrium sediment transport conditions

In this study, for the first time, a theoretically deduced flow resistance equation was tested for an overland flow under equilibrium sediment transport conditions using available experimental data by Liu et al. for five Chinese soils. Initially the relationship among the velocity profile parameter Γ, the channel slope, the flow Reynolds number, the Froude number and the sediment concentration was calibrated using 90 measurements of the available database (Loessial, Cinnamon and Black soil) and tested by other 60 measurements (Red and Purple soil). The results proved that the Darcy–Weisbach friction factor can be accurately estimated by the proposed theoretical approach, with errors that are always less than or equal to ± 5%. Then, using mean values of the exponents of Froude number, slope, sediment concentration and Reynolds number (Eq. 7), the calibration was carried out for each soil type to attribute the soil effect on the velocity profile parameter and flow resistance to the scale factor a. This analysis demonstrated that the a coefficient increases with the transportability of soil particles, which is associated to a decrease of both the flow energy required for sediment transport and the Darcy–Weisbach friction factor.

Analysis of equilibrium sediment transport in the middle and lower reaches of the Yangtze River after the impoundment of the Three Gorges Reservoir

Analysis of equilibrium sediment transport in the middle and lower reaches of the Yangtze River after the impoundment of the Three Gorges Reservoir

Equilibrium sediment transport, grade and discharge for suspended-load-dominated flows on Earth, Mars and Titan

Recent surface observations have emphasised the importance of bedload transport by rivers and streams on Mars and Titan. Previous “hydraulic” analysis, however, has also shown that transport as suspended load should be possible, if not more prevalent due to lower critical shear stresses, under reduced gravity conditions compared with Earth; where the dominant mode of sediment flux by rivers is as suspended load. A new suspended-load equilibrium condition (zero net erosion and deposition) constraint is advanced, using a flux-balance model applicable to transport (capacity) limited alluvial channels. The theory unifies Gilbert's relation for slope and flow capacity model with the Shields criterion for incipient motion. Shields-stress analysis shows that sediment transport thresholds on Earth, Mars and Titan are dynamically similar. Thus, despite large differences in dimensional shear stresses, variations in the critical slopes and flow depths required for threshold and equilibrium conditions are comparatively modest. Under reduced gravity conditions, there exists a slightly higher potential for sediment transport as suspended load; except for systems involving high particle-fluid density ratios, as may apply to the transport of organic particles on Titan. Critically we show that graded suspended-load channels should develop gentler slopes under reduced gravity, and not steeper slopes as previously inferred based on generalized hydraulic geometry relationships of terrestrial river and submarine channels. We further argue that the use of submarine channels as analogues for channels formed under reduced gravity is not physically justified. Finally, compared with bedload channels, suspended-load channels should also have markedly higher discharges, with implications for mechanistic-based discharge predictions and precipitation models.

Impacts of subsurface hydrologic conditions on rill sediment transport capacity

Sediment transport capacity (Tc) is the equilibrium sediment transport value under steady-state flow and channel conditions. Given the difficulties in controlling hydrologic parameters close to steady-state in the field, most previous studies measured transport capacity in the laboratory on either non-erodible beds or erodible beds without drainage. However, infiltration under drainage conditions and exfiltration under seepage conditions cannot be ignored in the real world, and their impacts on soil strength and water discharge may affect transport capacity. The objective of this study was to estimate the effects of subsurface hydrologic conditions on rill sediment transport capacity, and the observations are discussed under both detachment-limited and transport-limited conditions. Series of experiments were carried out with a relatively uniform sand using three water discharges on a 4.56% slope flume with four rills from 0.5 to 3.0 m long under four subsurface hydrologic conditions varying from free drainage to 10 cm seepage head. The determination of equilibrium sediment transport capacity was based on the spatial change of sediment transport as slope lengths increased and the elevation change of the erodible surface in rills. Results from 107 runs indicated that there was only one equilibrium sediment transport value for a given surface and subsurface hydrologic condition, given similar observations were obtained under detachment-limited and transport-limited conditions. For the studied sand, observed critical shear stress decreased around 20% from the drainage to the saturation condition, and decreased slightly from the saturation to the seepage condition. Measured transport capacity increased 13% to 15% from drainage to seepage conditions for the studied water discharges, which was the result of decreased soil strength and increased water discharge. The differences in transport capacities between drainage and saturation increased as water discharge increased, but the differences between saturation and seepage were relatively stable for all studied water discharges.

The roles of flood magnitude and duration in controlling channel width and complexity on the Green River in Canyonlands, Utah, USA

Predictions of river channel adjustment to changes in streamflow regime based on relations between mean channel characteristics and mean flood magnitude can be useful to evaluate average channel response. However, because these relations assume equilibrium sediment transport, their applicability to cases where streamflow and sediment transport are decoupled may be limited. These general relations also lack the specificity that is required to connect specific characteristics of the streamflow and sediment regime with the dynamics of channel morphological change that create channel complexity, which is often of ecological interest. We integrate historical records of channel change, observations of scour and fill during a snowmelt flood, measurements of sediment transport, and predictions from a two-dimensional streamflow model to describe how annual peak flow magnitude and peak-flow duration interact with the upstream sediment supply to control channel form for a 15-km study reach on the regulated Green River in Canyonlands National Park, Utah. Two major decadal-scale episodes of channel narrowing have occurred within the study area. For each of these episodes, the reduction in average channel width was consistent with the change predicted by hydraulic geometry relations as a function of average flood magnitude. However, channel narrowing occurred during periods of exceptionally low annual floods. The most recent episode of channel narrowing occurred between 1988 and 2009, during low-flow cycles when the 5-yr mean peak flow was less than 60% of the long-term (1959–2016) mean peak flow. These findings, together with findings from previous studies, demonstrate that decreases in peak-flow magnitude caused by streamflow regulation, climate change, or a combination of those factors have driven episodes of channel narrowing on the Green River. Observations of streamflow, sediment-transport, and morphologic change coupled with predictions from a two-dimensional streamflow model indicate that peak flow magnitudes of at least 75% of the long-term mean peak flow are required to transport bed-material sand in suspension in all regions of the multi-thread channel and that the ~2-month duration of the snowmelt flood played an important role in creating conditions necessary to maintain channel conveyance. These results indicate that detailed characterizations of channel response such as these are needed to predict how river channels will respond to changes in streamflow regime that affect annual peak flow magnitude and duration.

One Flood Is Not Enough: Pool‐Riffle Self‐Maintenance Under Time‐Varying Flows and Nonequilibrium Multifractional Sediment Transport

AbstractThe interaction of sediment supply and hydrographs can affect, on a mesoscale, geomorphic features like pools and riffles, which are fundamental units of many gravel bed rivers. In the past decades, different hypotheses have been developed to characterize the hydrodynamics of pool‐riffle sequences; however, most of the previous studies considered equilibrium or near‐equilibrium sediment transport conditions. Here we investigate the stability of pools and riffles during a sequence of different hydrographs representative of a natural flow regime, without satisfying the equilibrium sediment transport condition. In the current study, the effects of bed geometry, sediment sorting and hydrograph duration, are explained and quantified. The results show that under nonequilibrium conditions, the reversal episodes are not always competent enough for complete self‐maintenance during a single flood. However, width variations and grain sorting effects prevented the pools to be completely filled up with the upstream sediment supply. Hydrograph duration had a significant role in the riffle bed geometry. Even though a single flood (irrespective of the magnitude) was not competent enough to restore the pool‐riffle feature, a sequence of floods progressively improved conditions for self‐maintenance. These findings can bring more insight into flow management strategies, in terms of the importance of multiple sequential floods for restoring rivers with high sediment supply.

Alluvial Morphodynamics of Bedrock Reaches Transporting Mixed‐Size Sand. Laboratory Experiments

AbstractResearch on the morphodynamics of bedrock rivers has primarily focused on bedrock incision, and little is known about the alluvial morphodynamics of rivers with exposed bedrock surfaces. More specifically, there is a lack of information on the morphodynamics of low slope bedrock reaches due to the recent recognition of such systems. Here, we present the results of laboratory experiments specifically designed to gain novel insight into flow resistances, flow hydrodynamics, and sediment transport processes in equilibrium partially exposed bedrock reaches transporting nonuniform sand as bed material in low slope areas. The experiments show that (1) downstream of a stable alluvial‐bedrock transition flow depth decreases in the streamwise direction, (2) bedform amplitude may decrease in the streamwise direction, and (3) stable patterns of downstream fining may form. Given the bedrock geometry, the water surface elevation at downstream boundary and the characteristics of the bedform regime in an alluvial channel subject to the same flow rate and sediment supply at equilibrium control bedform characteristics and sediment sorting patterns in the bedrock reach. When this distance is significantly smaller than the alluvial equilibrium flow depth or when the alluvial equilibrium bedform regime is close to the dune‐antidune transition, bedforms in the bedrock reach are closer to the dune‐antidune transition than at alluvial equilibrium with a consequent reduction in bedform amplitude. If the distance between the water level at the downstream boundary and the bedrock surface is close to the alluvial equilibrium flow depth and the alluvial equilibrium bedforms are well in the dune regime, a stable pattern of downstream fining can be expected. The comparisons between experimental and modeled sediment transport rates and equilibrium grain size distributions of the sediment further show that surface‐based bedload transport models derived for alluvial systems reasonably predict equilibrium sediment transport rates and bed surface size distributions in bedrock reaches if the presence of exposed bedrock is accounted for in terms of alluvial cover fraction.

Morphological Computation of Dune Evolution with Equilibrium and Non-Equilibrium Sediment Transport Models

Morphological Computation of Dune Evolution with Equilibrium and Non-Equilibrium Sediment Transport Models

Morphological Computation of Dune Evolution with Equilibrium and Non-Equilibrium Sediment Transport Models

Morphological Computation of Dune Evolution with Equilibrium and Non-Equilibrium Sediment Transport Models

Two-Dimensional Numerical Simulation Study on Bed-Load Transport in the Fluctuating Backwater Area: A Case-Study Reservoir in China

Numerical modeling of sedimentation and erosion in reservoirs is an active field of reservoir research. However, simulation of the bed-load transport phenomena has rarely been applied to other water bodies, in particular, the fluctuating backwater area. This is because the complex morphological processes interacting between hydrodynamics and sediment transport are generally challenging to accurately predict. Most researchers assert that the shape of a river channel is mainly determined by the upstream water and sediment, and the physical boundary conditions of the river channel, rather than random events. In this study, the refinement and application of a two-dimensional shallow-water and bed-load transport model to the fluctuating backwater area is described. The model employs the finite volume method of the Godunov scheme and equilibrium sediment transport equations. The model was verified using experimental data produced by a scaled physical model, and the results indicated that the numerical model is believable. The numerical model was then applied to actual reservoir operations, including reservoir storage, reservoir drawdown, and the continuous flood process, to predict the morphology of reservoir sedimentation and sediment transport rates, and the changes in bed level in the fluctuating backwater area. It was found that the location and morphology of sedimentation affected by the downstream water level result in random evolution of the river bed, and bed-load sedimentation is moved from upstream to downstream as the slope of the longitudinal section of the river bed is reduced. Moreover, the research shows that the river channel sedimentation morphology is changed by the change water level of the downstream reach, causing the dislocation of the beach and channel and random events that will affect the river, which is of certain reference value for waterway regulation.

Evidence for Self‐Similar Bedload Transport on Andean Alluvial Fans, Iglesia Basin, South Central Argentina

AbstractSelf‐similar downstream grain‐size fining trends in fluvial deposits are being increasingly used to simplify equilibrium sediment transport dynamics in numerical models. Their ability to collapse time‐averaged behavior of a depositional system into a simple mass balance framework makes them ideal for exploring the sensitivity of sediment routing systems to their climatic and tectonic boundary conditions. This is important if we want to better understand the sensitivity of landscapes to environmental change over timescales >102 years. However, the extent to which self‐similarity is detectable in the deposits of natural rivers is not fully constrained. In transport‐limited rivers, stored sediment can be remobilized or “recycled” and this behavior has been highlighted as a mechanism by which externally forced grain‐size fining trends are distorted. Here we evaluate evidence of self‐similarity in surface gravel‐size distributions on three geomorphically diverse alluvial fans in the Iglesia basin, south Central Argentine Andes. We find that size distributions are self‐similar, deviating from that condition only when significant variability occurs in the coarse tails of the distributions. Our analysis indicates a strong correlation between the degree of sediment recycling and the proportion of coarse clasts present on the bed surface. However, by fitting a relative mobility transfer function, we demonstrate that size‐selectivity alone can explain the bulk size distributions observed. This strengthens the application of self‐similar grain size fining models to solving problems of mass balance in a range of geomorphic settings, with an aim for reconstructing environmental boundary conditions from stratigraphy.

Sediment carbon fate in phreatic karst (Part 2): Numerical model development and application

The authors develop a numerical model to elucidate time-distributed processes controlling sediment carbon fate in phreatic karst. Sediment carbon processes simulated in the new numerical model include in-conduit erosion and deposition, sediment carbon transport, surficial fine grained laminae evolution, carbon pool mixing, microbial oxidation, and the understudied process of sediment carbon exchange during equilibrium transport. The authors perform a model evaluation procedure that includes generalized likelihood uncertainty estimation to quantify uncertainty of the model results. Modeling results suggest that phreatic karst conduits sustain sediment transport activity long after surface storm events cease. The sustained sediment transport has the potential to shift the baseflow sediment yield of the phreatic karst to be on par with stormflow sediment yield. The sustained activity is suggested to promote the exchange of sediment carbon between the water column and subsurface karst deposits during equilibrium sediment transport conditions. In turn, the sediment carbon exchange impacts the mixing of new and old carbon pools and the flux of carbon from phreatic karst. Integrated numerical model results from this study support the concept that phreatic karst act as a biologically active conveyor of sediment carbon that temporarily stores sediment, turns over carbon at higher rates than surface streams, and recharges degraded carbon back to the fluvial system. The numerical modeling method adopted in this paper shows the efficacy of coupling carbon isotope fingerprinting with water quality modeling to study sediment carbon in phreatic karst.

Sediment grain size variation along a cross-shore profile – representative d50

The coastal zones management not only relies on a profound knowledge of the shoreline but also on a good assessment of morphodynamics, sediments transport and coastal interventions required. Many coastal studies, such as beach slope estimates, equilibrium beach profile definition or sediment transport analysis, depend on the sediment grain size at the site. This paper aims to gain a better understanding of grain size distribution along the cross-shore profile and variation of d50, allowing to define a representative d50 to characterize the beaches. On the study site chosen, Barra beach, Aveiro, an extensive field campaign was performed, from October 2010 to May 2011. In a weekly basis, samples from 5 points along a cross-shore profile were collected. During this period, data related to wave climate was collected from the Portuguese Hydrographic Institute in temporal series of 10 min. Tidal projections from Hydrographic Institute were also identified for the period of the field campaigns. The sediment grain size distributions showed that, although the mass-median-diameter does not usually pass the 1 mm, d50 presented a great variation during winter, in the intertidal zone. On the other side, the first and last points of the cross-shore profile, located far from the intertidal zone, presented the smaller d50 and variation through time. The significant wave height presented a mean value of 2.16 m. The most energetic wave climates happened in November, January and February. During those months, an increase of d50 in the wave breaking zone was noticed. Generally, the expected behavior of the cross-shore sediment grain size distribution is in line with the data collected in this paper and it was verified that the grain size is coarser on the intertidal zone and after storm periods. Also, despite the complexity of defining a characteristic d50 due to its great temporal and spatial variation, sensitivity analysis on the data obtained helped the identification of the upper foreshore limit, at high tide, as the better location for a sediment sample collection representative of a cross-shore profile.