Uniform Sediment Research Articles

Coevolution of bed surface patchiness and channel morphology: 2. Numerical experiments

In gravel bed rivers, bed topography and the bed surface grain size distribution evolve simultaneously, but it is not clear how feedbacks between topography and grain sorting affect channel morphology. In this, the second of a pair of papers examining interactions between bed topography and bed surface sorting in gravel bed rivers, we use a two‐dimensional morphodynamic model to perform numerical experiments designed to explore the coevolution of both free and forced bars and bed surface patches. Model runs were carried out on a computational grid simulating a 200 m long, 2.75 m wide, straight, rectangular channel, with an initially flat bed at a slope of 0.0137. Over five numerical experiments, we varied (a) whether an obstruction was present, (b) whether the sediment was a gravel mixture or a single size, and (c) whether the bed surface grain size feeds back on the hydraulic roughness field. Experiments with channel obstructions developed a train of alternate bars that became stationary and were connected to the obstruction. Freely migrating alternate bars formed in the experiments without channel obstructions. Simulations incorporating roughness feedbacks between the bed surface and flow field produced flatter, broader, and longer bars than simulations using constant roughness or uniform sediment. Our findings suggest that patches are not simply a by‐product of bed topography, but they interact with the evolving bed and influence morphologic evolution.

Variability of residual fluxes of suspended sediment in a multiple tidal-inlet system: the Dutch Wadden Sea

In multiple tidal-inlet systems such as the Dutch Wadden Sea, the exchange of sediments between the coastal lagoon and the adjacent sea is controlled by the combined effect of the tides, wind-driven flows, and density-driven flows. We investigate the variability of residual (tidally averaged) fluxes of suspended sediment with the three-dimensional numerical model GETM in relation to forcing mechanisms and model parameters. Sediment transport is modeled with three sediment classes. A modified Partheniades-Krone formulation describes the erosion and deposition fluxes from a single-layer sediment pool. The model is initialized with a uniform sediment pool for each class and the spin up period amounts to six months. Simulations span 1 year. Comparisons with observations show that model results are fairly realistic. Residual fluxes of water and suspended sediment are episodic in nature and vary strongly throughout the year, mainly due to wind variability. The net balance between import and export of material is very sensitive to model parameters. Residual fluxes are sensitive to the geographical orientation and location of the inlets, and the effect of driving mechanisms on the residual fluxes and concentrations can be organized hierarchically, with wind forcing having the largest effect on concentration levels and variability.

Morphology of the Kosi megafan channels

Abstract. We report on a new set of measurements on the morphology of braided and meandering threads of the Kosi megafan, North Bihar, India. All threads develop on a uniform sandy sediment and under a similar climate. The data set is composed of the width, depth, water discharge and grain size of 51 threads. Downstream slopes and sinuosity are also available. Using this data set, we show that braided and meandering threads share common hydraulic geometries. We then use the threshold theory to explain why the aspect ratio of threads is almost naturally detrended, and rescale the data according to this theory. As expected, the rescaled dimensions of braided and meandering threads are weakly correlated to water discharge. We propose that the large dispersion observed, which is common to meandering and braided threads, is the signature of sediment transport, vegetation or cohesion effects.

Reduction of Local Scour Around A Bridge Pier Using Triple Rectangular Plates

The performance of vertical triple plates as a new countermeasure in control of local scour around a cylindrical model pier is studied. Two ones of the plates are attached to the side wall one of the pier at one pier diameter distance, extending toward upstream distance and the third one is located in the middle, attached to the pier nose. All the three plates are parallel to flow direction. Experiments are conducted for five different depths of flow and two different lengths of the lateral plates, namely 8 and 14 cm. all the runs are performed under the clear water flow over uniform sediment. The results showed a maximum efficiency of 76% in scour reduction for 8 cm long side plates and 85% for 14 cm long ones.it is also found that the proposed setup act simultaneously as both the bed armoring and flow altering countermeasure.

Sediment size effects on rip channel dynamics

Field observations indicate that both wave conditions and sediment characteristics exert the controlling effects on the beach states and determine the development of rip channel system. Although the role of wave conditions on the rip channel dynamics has been extensively studied, little attention has been paid to the effects of sediment characteristics. In this paper, we use a nonlinear morphodynamic model, XBeach, to investigate the impact of grain size on the formation and nonlinear evolution of rip channels under the condition of uniform sediments. We find that the grain size can affect a number of aspects of rip channel dynamics. With increasing grain size, both longshore migration speed and growth rate decrease significantly, while the response and saturation times increase. On the other hand, the influence of grain size on the geometrical properties of the rip channel seems to be much less pronounced, as the wavelength of the channels hardly changes and the three-dimensionality features only vary slightly. The “global analysis” method is applied to investigate the mechanism underlying the variations of growth rate and migration speed for varying sediment size. The calculations reveal that these variations are mainly caused by the amount of sea bed sediments being stirred up which is clearly grain-size-dependent. Quantitative relationships between predicted rip channel characteristics, sediment grain size and the hydrodynamic conditions are tentatively established, which is amendable to improvement by extending the analysis to other or more general situations.

Mass conservation in computational morphodynamics: uniform sediment and infinite availability

SummaryComputational morphodynamics in finite volume methods are based on the evaluation of the rate of bed level change in the vertices on the deforming bed. With the use of finite volume methods on collocated (unstructured) grids, the rate of bed level change needs to be interpolated from the mesh faces to the vertices. First, this work reviews two methods based on a vectorial shape of the bed evolution equation (no scalar contributions from storage, erosion and deposition) in terms of their mass conserving properties. Second, a method that allows for scalar contributions in the bed evolution equation (the Exner equation) is proposed for general, unstructured meshes, and an analytical derivation for the simple one‐dimensional problem on a non‐equidistantly discretised grid is considered. The solution is compared with the general two‐dimensional formulation. The two‐dimensional formulation leads to the formulation of a geometric sand sliding routine on unstructured grids. The newly proposed interpolation method and the sand sliding routine are tested, and mass conservation of the sediment is considered with special emphasis on the effect of the solution accuracy for the suspended sediment transport. Discussions on other interpolation methods and their mass conserving properties are given with a special focus of the distance weighted interpolation method directly available and easily applied in OpenFOAM. Furthermore, effects from horizontal displacements of the vertices, explicit filtering of the evolving bed and morphological acceleration on global mass conservation, are discussed. Copyright © 2015 John Wiley & Sons, Ltd.

A conceptual model of pore-space blockage in mixed sediments using a new numerical approach, with implications for sediment bed stabilization

In mixed sediment beds, erosion resistance can change relative to that of beds composed of a uniform sediment because of varying textural and/or other grain-size parameters, with effects on pore water flow that are difficult to quantify by means of analogue techniques. To overcome this difficulty, a three-dimensional numerical model was developed using a finite difference method (FDM) flow model coupled with a distinct element method (DEM) particle model. The main aim was to investigate, at a high spatial resolution, the physical processes occurring during the initiation of motion of single grains at the sediment–water interface and in the shallow subsurface of simplified sediment beds under different flow velocities. Increasing proportions of very fine sand (D50=0.08 mm) were mixed into a coarse sand matrix (D50=0.6 mm) to simulate mixed sediment beds, starting with a pure coarse sand bed in experiment 1 (0 wt% fines), and proceeding through experiment 2 (6.5 wt% fines), experiment 3 (10.5 wt% fines), and experiment 4 (28.7 wt% fines). All mixed beds were tested for their erosion behavior at predefined flow velocities varying in the range of U 1-5=10–30 cm/s. The experiments show that, with increasing fine content, the smaller particles increasingly fill the spaces between the larger particles. As a consequence, pore water inflow into the sediment is increasingly blocked, i.e., there is a decrease in pore water flow velocity and, hence, in the flow momentum available to entrain particles. These findings are portrayed in a new conceptual model of enhanced sediment bed stabilization.

Effects of the alien Pacific oyster (Crassostrea gigas) on subtidal macrozoobenthos communities

Since 2006, the Pacific oyster Crassostrea gigas has been a permanent resident on the west cost of Sweden. Because C. gigas is nonindigenous in Scandinavia, it may modify ecosystems and affect the resident biota. Individuals of C. gigas often settle in large aggregations, and the physical structure of the resulting reef provides attachment points and refuges for many secondary species. However, C. gigas also has the potential to change the macrofaunal community structure of the associated sediment, for example by stabilization or enrichment of the sediment. Here, we assess the macrozoobenthos community of sediments within C. gigas reefs and contrast the results with the comparable community within beds of the native blue mussel (Mytilus edulis) and with that of uniform bare sediment. We show that the communities within oyster reefs and mussel beds contained more species and had a higher abundance of organisms compared with the bare sediment. In addition, we show significant differences between the communities within oyster reefs and mussel beds and consistently a larger total abundance in the former.

Characterization of flow turbulence in mobile boundary channels

An experimental investigation on measurement of 3-D instantaneous velocity profiles in a mobile boundary channel with uniform and non-uniform sediments are reported. The estimated velocity fluctuations from measured velocity profiles along the center line of the flume are used to quantify the nature of bursting events, and turbulence characteristics on uniform (finer and coarser) and non-uniform sediments. The analysis presented in the paper indicated that sediment characteristics play very significant roles for existence of aforesaid characteristics (turbulence characteristics and bursting events) near the channel beds and fluid flow regions in the main flow. Further investigations on aforesaid theme are recommended, while taking the measurements on velocity profiles along the depths across the channel width including the center line depth of the channel.

Effect of Accelerating on Sediment Incipient Motion over Dunes under Shallow Water Condition

Due to the complexity of bed form morphology and dune movement, this paper selected a series of 2-D asymmetric dunes in order to address the development of a flow region associated with velocity profile characteristics and sediment incipient motion under shallow water condition. Experiments were conducted over four asymmetric dunes of one crest length (4 m), two crest height (0.3m,0.4m) and two uniform sediments with mean grain sizes D=0.5 and 0.7 mm at the TianJin Research Institute For Water Transport Engineering, China. By using Acoustic Doppler Velocimeter (ADV), the instantaneous velocities were measured at four cross sections giving a total of 16 measured profiles. The variations of velocity profile along the flow affected by the wavy bottom have been studied. The results show that the effect of flow acceleration plays an important role in governing incipient motion over dunes.

Study on changes in bed characteristics and friction factor in the presence of wash load in suspension

Results of an experimental study on the effects of different concentrations of wash load on the size of bed features and resistance to flow in a laboratory flume are presented. The experiments were carried out under different hydraulic conditions in a 30 m long, 0.204 m wide and 0.5 m deep tilting flume under clear water condition and in the presence of different concentration of wash load in the flow. The bed material used consisted of uniform sediment of size 0.96 mm. Analysis of the data indicates that the characteristics of the bed features change and friction factor increases in the presence of different concentration of wash load in the flow. The reasons for changes in the characteristics of the bed features and increase in friction factor in the presence of wash load are identified and a relationship for predicting friction factor in the presence of wash load has been established.

Mechanics of Pipeline Scour Propagation in the Spanwise Direction

AbstractThis paper presents the results of an experimental study on the mechanics of pipeline scour propagation in the spanwise direction. The experiments were conducted with uniform sediment under a unidirectional steady current in clear-water conditions. The study aims to provide a better understanding of how the differential pressure between the upstream and the downstream sides of the pipeline and local bed shear stress at the span shoulder affect pipeline scour propagation in the spanwise direction. The measured data show that a high differential pressure significantly enhances pipeline scour hole development in the spanwise direction. Its presence increases the propagation velocity of the scour hole and differentiates the rapid from the slack phase of development identified in previous studies. It may be inferred from the relationship of the propagation velocity on pressure gradient that the two factors, namely, the differential pressure and the local bed shear stress, vary proportionally with the s...

Clear-water local scour at wide piers in shallow-water flow

Laboratory data for local scour depth regarding the size of wide piers are presented. Clear water scour tests were performed for various pier widths (0.06, 0.076, 0.102, 0.14 and 0.165 m), two types of pier shapes (circular and rectangular) and two types of uniform cohesionless bed sediment (d50 = 0.23 and d50 = 0.80 mm). New data are presented and used to demonstrate the effects of pier width, pier shape and sediment size on scour depth. The influence of equilibrium time (te) on scouring processes is also discussed. Equilibrium scour depths were found to decrease with increasing values of b/d50. The temporal development of equilibrium local scour depth with new laboratory data is demonstrated for flow intensity V/Vc = 0.95. On the other hand, the results of scour mechanism have shown a significant relationship between normalized volume of scoured and deposited with pier width, b. The experimental data obtained in this study and data available from the literature for wide piers are used to evaluate predictions of existing methods.

Flume Experiments to Constrain Bedload Adaptation Length

Spatially variable channel geometry in natural rivers produces nonuniform flow and spatial gradients in the shear stress field. The travel distance required for the flow to acquire the capacity bedload concentration and attain a new equilibrium bedload transport rate upon encountering a region of higher or lower shear stress is defined as the bedload adaptation length (Lb). Estimates of Lb are used by some numerical morphodynamic models to account for nonequilibrium bedload transport in the computation of local transport rates. However, current methods for estimating this parameter are uncertain and often crude. The authors therefore conducted experiments designed to measure Lb for a uniform sediment mixture in a laboratory flume. Instantaneous bedload transport rates were determined by counting passing sediment particles on digital imagery collected at variable distances downstream from a zero-transport boundary in a small flume. The flume was operated at three bed slopes in order to assess Lb over a range of hydraulic conditions. Bedload adaptation length was found to be about 30±8 particle diameters at a relatively low excess dimensionless shear stress (θ−θc=0.018, where θ is the dimensionless shear stress and θc=0.0436 is the critical dimensionless shear stress) and about 100±30 particle diameters at a moderate level of excess dimensionless shear stress (θ−θc=0.032). The experiments failed to resolve Lb at higher shear stresses. These results support physically based models that cast Lb as an increasing function of excess shear stress. They also suggest that Lb may be small relative to the resolution of the numerical mesh used in many modeling applications. In such cases, model performance may be insensitive to the choice of any arbitrary small value of Lb.

Live-bed local scour around vertical-wall abutments

Local scour experiments were performed with four different uniform cohesionless sediment diameters d50 = 0.26, 0.42, 1.06, and 1.92 mm and five vertical-wall abutments with projected lengths l = 0.04, 0.06, 0.08, 0.10, and 0.12 m perpendicular to the flow. These tests were conducted at different flow velocity (V) varying from 0.71 to 5 times of the critical velocity (Vc). The plotting of scour depths dse vs. V/Vc shows that the scour depths increase up to the threshold value of flow (V/Vc = 1) and after that it reduces and again increases to attain the live-bed maximum in the flow range V/Vc = 3–5. It is observed that the scour depth increases with the increase of sediment sizes and abutment lengths. Further, the scour depth decreases with the increases of nonuniformity under all flow conditions. A design equation is proposed based on the present experimental data and existing live-bed scour data. The measured values of scour depth are compared with the calculated values using proposed equation and also with three different live-bed local scour equations for the conditions of the tests.

Overtopping dike-breach: effect of grain size distribution

ABSTRACTA series of plane dike overtopping laboratory tests is reported, in which the effect of sediment mixture on the breach process is investigated. Three different sediment mixtures were tested, in addition to the reference dike consisting of uniform non-cohesive sediment. No sealing elements or surface protection were applied. The adopted constant reservoir inflow led to a falling reservoir scenario. The experiments demonstrate that the grain size distribution has a small effect on the overall breach process within the given test range, as the hydraulic conditions during the dike breach are much above the sediment entrainment condition. No increased erosion resistance was observed for the sediment mixtures. Therefore, the mean sediment diameter adequately describes the non-cohesive material characteristics and general dike-breach features in laboratory scale can be investigated using uniform material. This finding is significant both for laboratory experimentation and for prototype dike construction.

Sediment Erosion and Transport Experiments in Laboratory using Artificial Rainfall Simulator

Catchments soil erosion, one of the most serious problems in the mountainous environment of the world, consists of a complex phenomenon involving the detachment of individual soil particles from the soil mass and their transport, storage and overland flow of rainfall, and infiltration. Sediment size distribution during erosion processes appear to depend on many factors such as rainfall characteristics, vegetation cover, hydraulic flow, soil properties and slope. This study involved laboratory flume experiments carried out under simulated rainfall in a 3.0 m long × 0.8 m wide × 0.7 m deep flume, set at 17° slope. Five experimental cases, consisting of twelve experiments using three different sediments with two different rainfall conditions, are reported. The experiments consisted of detailed observations of particle size distribution of the out-flow sediment. Sediment water mixture out-flow hydrograph and sediment mass out-flow rate over time, moisture profiles at different points within the soil domain, and seepage outflow were also reported. Moisture profiles, seepage outflow, and movement of overland flow were clearly found to be controlled by water retention function and hydraulic function of the soil. The difference of grain size distribution of original soil bed and the out-flow sediment was found to be insignificant in the cases of uniform sediment used experiments. However, in the cases of non-uniform sediment used experiments the outflow sediment was found to be coarser than the original soil domain. The results indicated that the sediment transport mechanism is the combination of particle segregation, suspension/saltation and rolling along the travel distance.

Scour and flow around submerged structures

The results of an experimental investigation on scour of non-cohesive uniform and non-uniform sediment beds and flow around fully submerged structures over a fixed plane bed are discussed. In the study, the submergence factor, Froude number and cylinder Reynolds number for all the experiments were kept in the ranges of 0·25–0·75, 0·23–0·70 and 13 500–58 650 respectively. The free surface profile over the submerged structures consists of distinct upper and lower layers; the lower layer is characterised by continuous mixing with the generated downstream wake whereas the characteristics of the upper layer depend on the submergence factor and the Froude number. Using observed scour depths at different times, the time variation of scour depth is scaled by an exponential law. The non-dimensional time scale decreases following a power-law relationship with an increase in densimetric Froude number. It is evident that there is a considerable reduction in the non-dimensional scour depth with an increase in the submergence ratio, mainly due to the decrease in the intensity of the horseshoe vortex. It is also evident that the normalised equilibrium scour depth decreases with an increase in the ratio of cylinder diameter to sediment size. A significant reduction of 40% of the scour depth in uniform sediment was observed in experiments with non-uniform sediments. The three-dimensional flow fields around the submerged structure over a fixed plane bed were measured with an acoustic Doppler velocimeter. The vertical velocity distributions indicate deceleration of the approaching flow, acceleration around the submerged body and the approach to the original state of open-channel flow downstream of the submerged structure.

Tracer dispersion in bedload transport

Abstract. Bedload particles entrained by rivers tends to disperse as they move downstream. In this paper, we use the erosion-deposition model of Charru et al. (2004) to describe the velocity and the spreading of a plume of tracer particles. We restrict our analysis to steady-state transport above a flat bed of uniform sediment. The transport of tracer particles is then controlled by downstream advection and particle exchange with the immobile bed. After a transitional regime dominated by initial conditions, the evolution of a plume of markers tends asymptotically towards classical advection-diffusion: its average position grows linearly with time, whereas it spreads like the square root of time.

Numerical modelling of sediment bed aggradation in open rectangular drainage channels

A numerical modelling investigation of bed aggradation in open rectangular drainage channels is presented in this paper. A semi-coupled modelling approach for uniform sediments based on 1D-shallow water De Saint Venant - Exner equations was adopted to describe the temporal evolution of the bed in the deposition and transport phenomena associated with aggradation process. Six well-established sediment transport formulas were used to evaluate the sediment discharge over the aggrading bed. Simulations enabled the comparison of results obtained with the selected formulas against a comprehensive set of experimental measurements reported in the literature. Results have shown the adopted model to describe successfully the evolution of sediment bed and water level profiles during the aggradation experiments, with minor differences depending on the used sediment transport relationship.