Uniform Sediment Research Articles

Grain Size‐Specific Engelund‐Hansen Type Relation for Bed Material Load in Sand‐Bed Rivers, With Application to the Mississippi River

AbstractMany sand‐bed rivers worldwide have been experiencing significant reductions in sediment load over the past several decades. This is one of the causes of river delta drowning worldwide. This problem, however, has not been studied in detail in the context of sediment grain sorting. Considering the good performance of the original Engelund‐Hansen relation (OEH) for uniform sediment, and the fact that bulk grain size‐specific relations for bed material load that allow for sorting are relatively rare in the case of sand‐bed rivers, a grain size‐specific Engelund‐Hansen type relation (SEH) is proposed in this study based on data from a large flume. We embed both the OEH and the SEH in a one‐dimensional river morphodynamic model to simulate the morphodynamic evolution of the middle Mississippi River in response to the upstream cutoff of sediment supply. Simulation results using a single characteristic grain size show that bed material load delivered to the delta reduces only gradually in response to the cutoff of sediment supply, in agreement with previous studies. However, implementing the full grain size distribution of sediment leads to a much faster reduction of bed material load delivered to the delta, because the bed surface coarsens in response to grain sorting. This armoring inhibits the bed degradation that would replenish sediment load along the channel. The bed material load of finer sediment declines more rapidly than that of coarser sediment. The results of this study have practical implications for river delta restoration.

Graded bed load transport in sediment supply limited channels under unsteady flow hydrographs

Natural river channels that are sediment supply limited due to upstream water management practices and/or flow regulation are often subject to in-channel bed degradation and bed surface restructuring during natural flood hydrographs or controlled flow releases from upstream dams. The heterogeneity of sediment sizes present in these channels means that different size classes are transported more actively during different parts of the hydrograph. A series of laboratory experiments is conducted to investigate the in-channel response of a graded sediment bed to a range of design flow hydrographs when no sediment is supplied at the upstream boundary. The results show varying temporal lag and bed load transport hysteresis for different transported fractions, defined by fine, medium and coarse size classes in the graded sediment mixture. The coarse size class typically exhibits clockwise hysteresis, indicating more active transport during the rising hydrograph limb, whereas the fine size class demonstrates either no/mixed or counterclockwise hysteresis, as it becomes more active during the falling limb. On this separate limb basis, predictions of fractional bed load transport rates are improved by calculating unique reference threshold shear stresses for each size class at the initiation and cessation of fractional grain motions on the rising and falling limbs, respectively. Corresponding temporal variations in the bed load median grain size vary depending on the hydrograph total water work and unsteadiness, with peak values generally attained during the rising limb and overall bed load fining observed during the falling limb. Analysis of the three size classes also indicates that the medium-coarse and fine fractions are transported in larger relative proportions during smaller magnitude, more flashy hydrographs and larger magnitude, flatter hydrographs, respectively. The resulting armouring of the post-hydrograph bed surface layer is most significant at the upstream end of the channel and decreases exponentially in the downstream direction. Two empirical models, based on combined hydrograph and bed sediment descriptors, are also shown to predict reasonably well the overall bed load yields generated under different flow hydrographs from the satisfactory collapse of the current experimental data and previous datasets for both uniform and graded bed sediments. These bed load yields provide an empirical means to describe the extent of upstream bed armouring and the downstream fining of the bed surface layer after the passage of individual hydrographs.

Experiment on fundamental behaviours of wavelength and height of vegetated alternate bar in river

The study on the alternate bar has been implemented by many researchers previously. However, the detailed analysis of the formation of the vegetated alternate bar is still not sufficient, especially on the bar wavelength, height, and width. Therefore, this present study provides an analysis of the experimental approach on the vegetated alternate bar formations. The focus was on the characteristics of wavelength, height, and width of the alternate bar. The objective of the study is to investigate the relationship of these parameters for three different conditions of alternate bar formations of non-vegetated, full and half patches of vegetation alternate bar. The flume of 10 m long and 1 m wide with erodible bed and fixed banks were used for the experiments with uniform bed sediment of size 0.8 mm. The wavelength and height of both full and half vegetated alternate bars increased compared to non-vegetated thus stabilized the formation of the alternate bar in the channels. The vegetation also reduced the value of the shield parameter to produce high deposition on the bar. The width of the bar increased at the head and body of the vegetated bar however it was decreased in the area of bar edge. Therefore, it could say that vegetation gives positive impacts on the alternate bar formation characteristics especially on its formation and stability in the rivers.

Flow Characteristics and Bed Morphology in a Compound Channel between Two Single Channels

In mountainous areas, a river can widen from a single channel to a compound channel under the influence of geological conditions or human impacts, bringing about challenges in terms of flood control and channel regulation. This paper reports the results of tests conducted in a 26 m long flume with a uniform sediment bed (grain size = 0.5 mm), investigating the flow characteristics and bed morphology in a compound channel between two single channels. The stage‒discharge relationship in the compound channel and the longitudinal and cross-sectional bed profile in the compound channel between two single channels are presented and analyzed. The experimental results indicate that the flow characteristics and bed morphology in a compound channel between two single channels are significantly different from those in a normal compound channel. Based on the experimental data and observations, the mechanisms of flow and sediment transport in the compound channel between two single channels are illuminated.

Experimental investigation of scour around circular arrangement pile groups

Scour prediction is essential for the design of offshore foundation structures. In this study, we experimentally investigate the clear-water scour around pile groups of the circular arrangement, with uniform bed sediment and steady current. Five pile-group arrangements and two flow velocities are considered. We illustrate the scouring process around each arrangement and derive the equilibrium scour depth. Based on our experimental results as well as previously published data, we check the accuracy of three widely-used empirical methods in predicting equilibrium scour depth of pile groups. It is found that, based on the effective diameter calculated for a rectangular arrangement, the predictions of all the selected methods lead to significant overestimation. Here, we propose a new effective diameter to represent the characteristic geometry of a circular arrangement. Essentially the projected width of a pile group is calculated by the diameter of a monopile that has the same cross-sectional area of the pile group. Using the new effective diameter, the predictions from the selected empirical methods are remarkably improved.

Experimental Investigation of Scour Prediction Methods for Offshore Tripod and Hexapod Foundations

Scour prediction is essential for the design of offshore foundations. Several methods have been proposed to predict the equilibrium scour depth for monopiles. By introducing an effective diameter, such methods could also be applied to predicting scour depth for pile groups. Yet, there are still difficulties in estimating the equilibrium scour depth of foundations in complex shapes, such as the tripod foundation. This study investigates the clear-water scour around the tripod and hexapod foundations through laboratory experiments, with uniform bed sediment and steady current. Here, the authors propose an approach to calculate the effective diameter for the tripod and hexapod models, which is similarly as for the pile groups. Three widely-used methods in predicting equilibrium scour depth have been evaluated, and the best method is recommended.

Corrigendum: Hydrology Research 50 (6), 1645–1664: Difference in the bed load transport of graded and uniform sediments during floods: An experimental investigation, Khabat Khosravi, Amir H. N. Chegini, Andrew D. Binns, Prasad Daggupati and Luca Mao, https://doi.org/10.2166/nh.2019.078

Corrigendum: <i>Hydrology Research</i> 50 (6), 1645–1664: Difference in the bed load transport of graded and uniform sediments during floods: An experimental investigation, Khabat Khosravi, Amir H. N. Chegini, Andrew D. Binns, Prasad Daggupati and Luca Mao, https://doi.org/10.2166/nh.2019.078

Experimental Investigation on Using Chloride/Hydroxide Aerosol to Control Spontaneous Combustion of Lignite in Underground Coal Mines

To provide a sustained and reliable approach for suppressing spontaneous combustion hazard in underground goafs, chloride/hydroxide aerosols were used for the first time to inhibit spontaneous combustion of lignite. A supersonic wave generator was used to generate aerosols for continuously treating Yunnan lignite of a particle size of <1 mm. A uniform sediment of aerosols was formed when chloride and hydroxide aerosols (AlCl3–Na2SiO3, MgCl2–Na2SiO3, CaCl2–Na2SiO3, and NaHCO3–AlCl3) simultaneously flow through the coal surface. According to the oxidation experiments and microscopic tests, the controlling effect of cold aerosol was 66.28–81.28%. After treatment for 20–60 min, the produced precipitation accounted for 1.0–7.5% (by mass) of the original specimen. The TG/DSC test showed that the temperature of exothermic peak was increased by about 100 °C, and the release of heat was restrained. Infrared spectra test illustrated that the influence of cold aerosol on the organic functional groups in coal was mainly embodied in −OH, −CH2–, C–O, −COOH, etc. The superfine sediments produced by the reaction between carbonate and silicate aerosol adhere to the coal surface and continuously inhibit the oxygen absorption of coal, which offers potential as a clean, sustainable technique that can be used to prevent development of spontaneous fire.

A laboratory investigation of bed-load transport of gravel sediments under dam break flow

Dam break flows and resulting river bed erosion can have disastrous impacts on human safety, infrastructure, and environmental quality. However, there is a lack of research on the mobility of non-uniform sediment mixtures resulting from dam break flows and how these differ from uniform sized sediment. In this paper, laboratory flume experiments revealed that coarse and fine fractions in non-uniform sediment had a higher and a lower bed-load parameter, respectively, than uniform sediments of the same size. Thus, the finer fractions were more stable and the coarser fractions were more erodible in a non-uniform bed compared to a uniform-grained bed. These differences can be explained by the hiding and protrusion of these fractions, respectively. By investigating changes in mobility of the mixed-size fractions with reservoir water levels, the results revealed that at low water levels, when the coarser fractions were only just mobile, the bed-load parameter of the finer fractions was higher than the coarser fractions. The opposite was observed at a higher water level, when a significant proportion of the coarsest fractions was mobilized. The higher protrusion of these grains had an important effect on their mobility relative to the finer grains. The transported sediment on these mixed-sized beds was coarser than the initial bed sediment, and became coarser with an increase in reservoir water level.

Sound Velocity Estimation of Seabed Sediment Based on Parametric Array Sonar

Backscattered sound waves of seabed sediments are important information carriers in seafloor detection and acoustic characteristic parameters inversion. Most of the existing methods for estimating geoacoustic parameters are based on multiangle seabed backscattered signal processing and suitable for flat seafloor conditions with uniform sediment thickness. This usually deviates from the real field conditions and affects the accuracy of parameter estimation. In this paper, the sound ray propagation theory is studied and analysed under the condition of sloping seabed and uneven sediment thickness. Based on the phased parameter array sonar system, a method of acoustic parameters estimation of the sediment under inclined seabed conditions is proposed. The simulation results show that the new method shows good adaptability to different inclination angles of the seabed and solves the problem of accuracy of acoustic parameter estimation of the inclined seabed sediments. The model will greatly reduce the seafloor topography requirements in the sediment acoustic parameter inversion, such as velocity, layer thickness, and acoustic impedance.

Vertical concentration profile of nonuniform sediment

A new formula for the concentration profile of nonuniform sediment is derived using the diffusion differential equation with equilibrium bed concentration. The interaction coefficient for nonuniform sediment particles as a function of both relative diameter and geometric standard deviation of nonuniform sediment is taken into account in the settling velocity. The diffusion coefficient is obtained from the logarithmic velocity profile. This new formula possesses several advantages as follows: (1) avoids theoretical defect of the Rouse formula, which states that the sediment concentration is infinite at the bottom and zero at the water surface. (2) suitable for an arbitrary fraction of nonuniform sediment, and (3) easy to apply with a simple form. The formula yields less difference among concentration profiles for various particles than that for uniform sediment and reveals the law for fractional concentration distribution of nonuniform sediment. The calculated concentration agrees well with the measured data from the Yangtze River.

Experimental study on uniform and mixed bed-load sediment transport under unsteady flow

Experimental study on uniform and mixed bed-load sediment transport under unsteady flow

Experimental Analysis of Velocity Distribution in a Coarse-Grained Debris Flow: A Modified Bagnold’s Equation

Today, Bagnold’s theory is still applied to gravity-driven flows under the assumption of uniform sediment concentration. This study presents findings of flume experiments conducted to investigate the velocity and concentration distributions within the debris body by using high-resolution images. The analysis has shown that the concentration and mobility of grains vary along the depth. A linear law to interpret the grains concentration distribution, starting from the knowledge of the packing concentration and of the surface concentration, Cs, has been identified. By considering such a law, modified expressions of the Bagnold’s number and the velocity in stony-type debris flows are also presented. By using these expressions, three regimes of motion have been identified along the depth, and the velocity profile within the debris body is determined as a function of the parameter Cs. It has been verified that the velocity profiles estimated by using the modified equation compare well (mean square error less than 0.1) with the literature’s measured profiles when Cs is correctly measured or estimated. Results of cutting tests, conducted for a sample of the used material, have also allowed us to verify that Cs could be determined as a function of the static friction angle of the material.

Scour at River Bend: A Parametric Study

Scour at river bend is a complex process involving various parameters. Experiments were conducted in two laboratory flumes having 90° and 180° bends to study the influence of different parameters on scour at bends. Uniform non-cohesive sediments having median diameters varying from 0.28 to 1.18 mm were used as bed materials under different flow conditions. Three different cross-sectional shapes of the bed, namely flat, trapezoidal and parabolic, were used to study the effect of shape on scour. From the experiments, it is found that, at 90° bend, maximum scour and deposition occurred on concave and convex bank, respectively, between 50° and 60° azimuthal sections, whereas for 180° bend, the location was between 130° and 140° sections. Magnitude of scour was maximum for parabolic bed on the concave bank under flooded condition and deposition was maximum for flat bed on convex side. The deposition pattern on trapezoidal and parabolic bed was also different than that of flat bed. Again, for the same shear velocity ratio, the bigger size bed sediment produced larger scour depth but, for the same bed particle size, scour was more for greater approach shear velocity ratio. Further, measured three-dimensional time averaged velocity components before and after the scour on the 180° bend having initial parabolic bed shows the dominance of a spiral flow at the first half of the bend, and cross-flow from concave to convex at lower layer of the second half of the bend. The combined effect of vortex, cross-flow and accelerated flow is responsible for the erosion on the concave bank and deposition on the convex bank.

Experimental Study on Uniform and Mixed Bed-Load Sediment Transport under Unsteady Flow

The scouring and deposition of sediment caused by unsteady flows (e.g., storm waves and floods) produces many secondary disasters. The resultant bed-load movement exhibits different transport laws compared with that by steady flow. In this study, the flume experiments were performed to study the bed-load movement under unsteady flow with different velocity skewness. The movement of uniform and non-uniform non-cohesive sediment under unsteady flow as well as the influence of the steady and unsteady flow on sediment transport rate are compared. Additionally, the non-uniform sediment transport formula of fine-to-coarse particle diameter ratio was investigated. The results showed that the sediment transport rate between uniform and non-uniform sand under the same median diameter is different. The non-uniform sediment transport rate is 1.27-, 3.19-, and 0.68-times as large as that in uniform sediment under d50 = 0.664, 1.333, and 2.639 mm under unsteady flow, respectively. For non-uniform sand, the transport rate of non-uniform sand with a larger adjacent particle size ratio (δ = 0.29) was 1.31-times greater than that of the non-uniform sand with a smaller adjacent particle size ratio (δ = 0.50). Moreover, theoretical deduction was carried out and the incipient sediment motion was analyzed from the force mechanism. A new unsteadiness parameter based on the acceleration concept was proposed. The relationship between the travel distance and velocity skewness of sediment particles was set up. The experimental results and theoretical analysis showed that sediment under unsteady flow were easier to start and transport than those under steady flow in the same flow effect. The travel distance of sediment particles was longer under unsteady flow than that under steady flow.

Semi-analytical solution for one-dimensional unsteady sediment transport model in open channel with concentration-dependent settling velocity

This paper aims to solve the one-dimensional unsteady suspended sediment transport equation in open channels through a semi-analytical approach. The presence of a large amount of particles in flow change the settling velocity of a particle, and this phenomenon, commonly known as hindered settling effect, must be taken into account to deal with high concentrated flows. Inclusion of this mechanism makes the governing equation nonlinear, and together with this nonlinear governing equation, a generalized bottom boundary condition is taken in terms of deposition velocity and equilibrium bottom concentration. An explicit series solution is presented using the method of lines based homotopy analysis method, and the convergence of the series solution is gained through a convergence control parameter. The solution is validated by comparing it with the existing solution as well as a numerical one. Apart from that, the solution has also been validated under a steady-state condition with available experimental data. Results are interpreted both graphically and physically. It is found that the hindered settling effect is dominant in the main flow region only, for sediment free inlet for all types of turbulent diffusion coefficients. On the other hand, in the case of uniform sediment concentration at the inlet, hindered settling affects the concentration in the top portion of the channel too for linear and parabolic profiles of turbulent diffusion coefficients.

Source to sink reconstruction of a Holocene Fjord‐infill: Depositional patterns, suspended sediment yields, wind‐induced circulation patterns and trapping efficiency for Lake Strynevatnet, inner Nordfjord, Norway

AbstractThis paper reconstructs the sedimentation volumes and patterns, suspended sediment yields, wind‐induced circulation patterns and sediment trapping efficiency of Lake Strynevatnet, western Norway as an integrated source to sink system. The lake became deglaciated ca 11 ky cal bp, with glacio‐isostatic uplift isolating the basin from the nearby fjord (Nordfjord) ca 9.2 ky cal bp. Based on geophysical data collected in 2010, the upper 15–20 m of Holocene sediment accumulation in the lake was mapped. A sediment body in the centre of the lake indicates a depositional mechanism dominated by suspension sedimentation. The source of this sediment is associated with the adjacent glaciated catchments westward of the lake. Three seismic units were identified based on seismic facies generating an evolutionary model utilizing three depositional units (U1, U2 and U3), in which unit U2 represents the Storegga tsunami event. Unit U3 is further divided into three subunits; U3a, U3b and U3c based on their spatial continuity and subtle downlapping and onlapping relationships. The degree to which wind conditions could have affected the lake depositional patterns were studied utilizing an open‐source coupled hydrodynamic and sediment transport model. The results show that fluvial discharge alone is incapable of generating a circulation pattern in the lake currents. Suspended sediment concentrations in the lake are highest for strong winds. Modelled sediment accumulation on the lake floor shows that mild or absent winds lead to a proximal to distal sediment thickness trend, while strong winds result in uniform sediment thickness. Based on this it is argued that the thickness trends of seismic subunits U3a‐c are related to a variable palaeowind climate. As such, seismic data of lake infills, in combination with numerical modelling, may provide valuable palaeoclimatic information on wind patterns.

Interference between pier models and its effects on scour depth

Excessive scouring of sediment around bridge piers plays a major role causing failure of waterway structures. To prevent scouring around bridge piers, it is necessary to determine the maximum scour depth. In the past, researchers have conducted extensive laboratory work around flow structures with the help of scaled models. But most of the work has been carried out around an isolated pier and the work around a group of piers is relatively much lesser. To fill up the gaps in information, an experimental study was conducted in a recirculating water flume 15 m long, 0.4 m wide and 0.6 m high containing uniform sediment of mean size, d50 = 0.30 mm. Experiments on cylindrical pier models of size 42 mm were run at a velocity equal to 0.89 times the critical velocity, uc. Three different arrangements i.e. Tandem, Side by Side and Staggered have been used to analyze the effect of spacing and orientation on scour. From the initial observations it was concluded that, in the case of Tandem arrangement when the spacing between the piers was 16 times the diameter of the pier and more, both the piers showed independent behavior. But if the spacing between the bridge piers is 0D, then both the piers behave like a single pier and the scour depth was found to be 41% greater than that of a single pier. However, when the clear spacing was greater than 1.5 times for side by side and 2.5 times for staggered arrangement, the piers behaved independently.

Petrography and Geochemistry of the Upper Triassic Sandstones from the Western Ordos Basin, NW China: Provenance and Tectonic Implications

Petrographic and geochemical characteristics of the Upper Triassic sandstones in the western Ordos Basin were studied to provide insight into weathering characteristics, provenance, and tectonic implications. Petrographic features show that the sandstones are characterized by low‐medium compositional maturity and textural maturity. The CIA and CIW values reveal weak and moderate weathering history in the source area. The geochemical characteristics together with palaeocurrent data show that the northwestern sediments were mainly derived from the Alxa Block with a typical recycled nature, while the provenance of the western and southwestern sediments were mainly from the Qinling‐Qilian Orogenic Belt. The tectonic setting discrimination diagrams signify that the parent rocks of sandstones in the western and southern Ordos Basin were mainly developed from continental island arc, which is closely related to the evolution of the Qinling‐Qilian Orogenic Belt. However, the sandstones in the northwestern Ordos Basin show complex features, which may be resulted from a typical recycling process. Overall evidence from petrography, geochemistry and sedimentology, together with previous researches suggest the Kongtongshan and Helanshan areas were the southwestern and northwestern boundary of the Ordos Basin, respectively, and there was no clear boundary between the Hexi Corridor Belt and Ordos Basin, where a large, uniform sediment dispersal system developed during the Late Triassic.

Uniform and graded bed-load sediment transport in a degrading channel with non-equilibrium conditions

Bed-load transport plays a critical role in river morphological change and has an important impact on river ecology. Although there is good understanding of the role of the variation of river bed grain size on transport dynamics in equilibrium conditions, much less is understood for non-equilibrium conditions when the channel is either aggrading or degrading. In particular, the relative role of different grain sizes in the promotion and hindering of the transport of coarse and fine fractions in a degrading channel has yet to be investigated. The current study attempts to provide new understanding through a series of flume experiments done using uniform and graded sediment particles. The experiments revealed coarser grain-size fractions for a poorly-sorted sediment, relative to uniform-sized sediment, reduced the transport of finer grains and finer fractions enhanced the transport of coarse grains. This hindering-promotion effect, caused by relative hiding and exposure of finer and coarse fractions, increased with bed slope and decreased with relative submergence. In particular, as relative submergence increased, the graded fractions tended towards behaving more like their uniform-sized counterparts. Also, the bed-load parameter of the graded fractions increased more with a rise in bed slope than observed for the uniform-sized counterparts. These results revealed, for degrading channel conditions, such as downstream of a dam, bed-load equations developed for uniform bed sediment are inappropriate for use in natural river systems, particularly in mountain streams. Furthermore, changes in river bed composition due to activities that enhance the input of hill-slope sediment, such as fire, logging, and agricultural development, are likely to cause significant changes in river morphology.