Suspended Load Transport Research Articles

Downstream fining processes in sandy lowland rivers

Downstream fining of river bed sediments is well known for gravel-bed rivers, but also occurs in sand-bed rivers. The objective of this study was to determine the effect of selective transport processes, dune sorting, overbank deposition, channel migration and river bifurcations on downstream fining in sandy lowland rivers subject to relative sea level rise, tidal water level variations and increasing river width. A numerical simulation model was built using the Delft3D software. Selective suspended-load transport appeared to be dominant over selective bed-load transport. Dune sorting and channel migration increase fining rates, but overbank deposition has an opposite effect. River bifurcations affect the fining rate because of their effect on the sediment supply to bifurcating lowland rivers. Downstream fining was found to develop within decades, but it took few centuries before the longitudinal bed profile and the fining rate attained dynamic equilibrium. A comparison of the modelled fining rates with those observed in the Rhine River shows reasonable agreement.

Analytical Solution of Collisional Sheet Flows

This work focuses on the sediment transport that is dominated by collisional exchange of momentum between particles and can be identified as the transition regime between bed-load and suspended-load transport. Idealizing the particles as inelastic frictional spheres and accounting for the role of the interstitial fluid on particle collisions, three regions characterize such flows: a diffuse collisional layer neighboring the free surface, in which a simple trapezium rule is employed to solve a boundary-value problem based on the kinetic theory; a dense, algebraic layer, in which there is an algebraic balance between production and dissipation of particle fluctuation energy, the concentration is approximately constant, and correlated motion between the particles exists; and a macroviscous layer, close to the erodible bed, in which the collisions are inelastic and the fluid viscous force dominates the momentum exchange. Using boundary conditions of no-slip and yielding at the erodible bed and vanishing of th...

PHYSICAL MODEL EXPERIMENTS ON RECLAMATION PROJECT OF FANGCHENG PORT

Tidal flat reclamation works is an effective way to relieve the contradiction between supply and demand of land in the coastal region. The reclamation project in the bay will decrease tidal current velocities, change tidal currents field, then slowdown the water exchange in the bay ,which will influence water quality in the bay and increase sediment deposit at navigation channels. To study impacts of hydrodynamic environmental and sediment movement due to reclamation projects in Fangcheng Port, the physical model for tide-current and sediment was constructed in NHRI. The horizontal scale and vertical scale of the Fangcheng Port model is 1/660 and 1/100 respectively. The siltation in the bay and channels mainly caused by suspended load transport under the action of tidal currents and waves.The ability of water exchange in the bay is discussed by simulating sewage diffusion characteristics before and after reclamation projects in the bay. The physical model test results are shown the total tidal volume is decreased by 20% after the reclamation plan, of which 17.5% reduction in the East Bay, 2.5% in West Bay. The tidal currents field in the channel are discussed. The sedimentation rate increases 3-10cm/a in navigation channels and basins of Fangcheng Port.

Suspended sediment effects on hydraulic jump characteristics

In the present study, the effect of suspended load transport on the characteristics of a hydraulic jump in a rectangular channel was investigated experimentally. Sediment concentration and initial Froude numbers in the range 0·42–16·15% and 2·5–8, respectively, were considered. Two grain sizes, 0·15 and 0·03 mm, were used in the experiments. Hydraulic jump characteristics including velocity and concentration distributions, length of jump, the sequent depth ratio and the energy dissipation were studied for both the smooth and rough channel beds. The results showed that the sequent depth ratio and length of the jump in the presence of suspended sediment were smaller than those due to clear water flow. It was also found that the reduction of sequent depth ratio and the jump length were more in rough beds.

Multiple time scales of fluvial processes — theory and applications

Fluvial processes comprise water flow, sediment transport and bed evolution, which normally feature distinct time scales. The time scales of sediment transport and bed deformation relative to the flow essentially measure how fast sediment transport adapts to capacity region in line with local flow scenario and the bed deforms in comparison with the flow, which literally dictates if a capacity based and/or decoupled model is justified. This paper synthesizes the recently developed multiscale theory for sediment-laden flows over erodible bed, with bed load and suspended load transport, respectively. It is unravelled that bed load transport can adapt to capacity sufficiently rapidly even under highly unsteady flows and thus a capacity model is mostly applicable, whereas a non-capacity model is critical for suspended sediment because of the lower rate of adaptation to capacity. Physically coupled modelling is critical for fluvial processes characterized by rapid bed variation. Applications are outlined on very active bed load sediment transported by flash floods and landslide dam break floods.

Experimental and numerical investigations on degradation of channel bed of cohesive sediment mixtures

Experimental observations on a degradation profile of channel beds consisting of cohesive sediment mixtures are presented. The sediment used for the experimentation consisted of fine gravel mixed with clay in varying proportions (10%–50% by weight) and fine gravel and fine sand in equal proportions mixed with varying proportions of clay (10%–50% by weight). The clay percentage in bed material and unconfined compressive strength of channel bed were found to be the main factors controlling the depth of degradation of channel bed of cohesive sediment mixtures, as degradation depth reduces considerably with an increase in clay percentage and unconfined compressive strength of sediment bed. An algorithm is also proposed to compute transient bed and water surface profiles in a channel bed made up of such cohesive sediment mixtures. For this purpose the predictor‐corrector‐based, essentially nonoscillatory finite difference numerical scheme is utilized for the solution of governing equations through the use of appropriate initial and boundary conditions. The recently proposed relationships by the authors for bed and suspended load transport of cohesive sediment mixtures were used as the auxiliary equations in the proposed algorithm. The observations on degradation of channel beds consisting of such cohesive sediment mixtures have not been reported so far in the literature to the best of our knowledge.

Numerical simulation on seasonal transport variations and mechanisms of suspended sediment discharged from the Yellow River to the Bohai Sea

Based on sediment and discharge flux data for the Yellow River, realistic forcing fields and bathymetry of the Bohai Sea, a suspended sediment transport module is driven by a wave-current coupled model to research seasonal variations and mechanisms of suspended load transport to the Bohai Sea. It could be concluded that surface sediment concentration indicates a distinct spatial distribution characteristic that varies seasonally in the Bohai Sea. Sediment concentration is rather high near the Yellow River estuary, seasonal variations of which are controlled by quantity of sediment from the Yellow River, suspended sediment concentration reaches its maximum during summer and fall. Furthermore, sediment concentration decreases rapidly in other seas far from the Yellow River estuary and maintains a very low level in the center of the Bohai Sea, and is dominated by seasonal variations of climatology wind field in the Bohai Sea. Only a small amount of sediments imported from the Yellow River are delivered northwestward to the southern coast of the Bohai Bay. Majority of sediments are transported southeastward to the Laizhou Bay, where sediments are continuously delivered into the center of the Bohai Sea in a northeastward direction, and part of them are transported eastward alongshore through the Bohai Strait. 69% of sediments from the Yellow River are deposited near the river delta, 31% conveyed seaward, within which, 4% exported to the northern Yellow Sea through the Bohai Strait. Wind wave is the most essential contributor to seasonal variations of sediment concentration in the Bohai Sea, and the contribution of tidal currents is also significant in shallow waters when wind speed is low.

A model for the simulation of coupled flow‐bed form evolution in turbulent flows

We develop a three‐dimensional numerical model to simulate bed form dynamics in a turbulent boundary layer. In the numerical model, hydrodynamics is solved in a moving generalized boundary‐fitted curvilinear coordinate system, such that the domain boundary exactly follows complex time‐dependent bed form geometry. The resolved turbulent features are computed via large‐eddy simulation, while the subgrid scale turbulent motions are modeled with a dynamic mixed model. A second‐order accurate arbitrary Lagrangian‐Eulerian method is used to guarantee conservation of sediment mass, while the grid moves arbitrarily due to the motion of the bed. Transport of suspended load is modeled using the Eulerian approach with a pickup function as the bottom boundary condition for sediment entrainment at the bed. Transport of bed load and suspended load are combined in a mass balance equation for the bed, which evolves due to the spatiotemporally varying bed stress induced by the turbulent flow field above the bed and gravity (gravity‐induced avalanche flow). Motion of the bed in turn affects the flow field in a coupled hydrodynamic moving bed simulation, in which bed features evolve due to resolved details of the turbulent flow. We compare different bed elevation models and demonstrate the capability of the present model through simulation of sand ripple formation and evolution induced by turbulence in an oscillatory flow. A resolution study demonstrates the need for fine grid resolution to resolve a bulk of the near‐wall turbulence, which is essential for bed form initiation.

Non-universality of von Kármán's κ in fluvial streams

A distinguished universal aspect of wall-bounded turbulent flows is Theodore von Kármán's universal logarithmic law (log-law) of the wall, describing the time-averaged streamwise velocity variation with distance from the wall. The log-law has a universal von Kármán constant κ governing the velocity gradient. There remain a number of cases of the non-universality of κ in fluvial streams. In particular, it behaves as a variable in flows with low submergence, or if there is bed- and suspended-load transport. This research focuses on the non-universality of κ raising various issues and inviting future research directions.

Produção e transporte da carga suspensa fluvial: teoria e método para rios de médio porte

The analysis of the suspended load of a river is very important for understanding the basin dynamics and environmental condition. The objective of this study is to comment various methods of analysis for fluvial suspended load. The flow, beginning along the valley slope increases in volume and erosive strength, enters in channel and can deposits in floodplain or be transferred to an upper order channel. In its path suspended load in fundamental not only on alluvial plain morphology but on ecological functioning of river system. Basin land use or hydrological alterations directly affect the quality and quantity of suspended load. Under this perspective this paper presents the fundamentals of suspended load formation and transport processes and the more common methods in their analysis.

Experimental and numerical investigation of local scour around a submerged vertical circular cylinder in steady currents

Local scour around a submerged vertical circular cylinder in steady currents was studied both experimentally and numerically. The physical experiments were conducted for two different cylinder diameters with a range of cylinder height-to-diameter ratios. Transient scour depth at the stagnation point (upstream edge) of the cylinder was measured using the so-called conductivity scour probes. Three-dimensional (3D) seabed topography around each model cylinder was measured using a laser profiler. The effect of the height-to-diameter ratio on the scour depth was investigated. The experimental results show that the scour depth at the stagnation point is independent on cylinder height-to-diameter ratio when the later is smaller than 2. The increase rate of equilibrium scour depth with cylinder height increases with an increase in Shields parameter. A three-dimensional finite element numerical model is developed for simulating local scour around submerged vertical cylinders. Steady flow around the cylinder is simulated by solving the Reynolds-Averaged Navier–Stokes (RANS) equations with a k– ω turbulence closure. Both suspended load and bed load sediment transport rates were included in the model. Efforts are made to reduce the computational time associated with three-dimensional morphological modelling including the use of wall-function to avoid resolving the near-wall boundary layers, large morphological time step and parallel computing techniques. Scour around a submerged vertical circular cylinder founded on the seabed is simulated and the numerical results are validated against the test data. The scour mechanisms around the submerged cylinder are investigated using the numerical model.

Suspended Wash Load Transport of Nonuniform Sediments

The results of an experimental study on transport of suspended wash load through a coarse-bed stream are presented. The experiments were conducted under different concentrations of fine suspended sediment (wash load of uniform size, 0.064 mm diameter) and with three different coarse-bed sediments: two having uniform sizes and one with nonuniform size distribution. For any equilibrium concentration of wash load in suspension, a definite proportion of the wash material was observed to be present within the bed material. No difference is found in this regard between wash load and suspended load transport. Therefore, the relationship, as stated by Samaga et al., for the parameter representing sheltering—exposure and interference effects in the suspended load transport of nonuniform sediments was applied in a modified form by using the present data and the data collected from the literature.

Sediment Transport on Roberts Bank: A Sandy Intertidal Flat on the Fraser River Delta

Abstract Waves and currents were monitored over a range of incident forcing between March 23 and April 24, 2003, on Roberts Bank, the sandy intertidal portion of the Fraser River Delta. A comparison of the dimensionless, current-induced skin friction with the critical skin friction for the initiation of sediment motion suggests that the currents are only capable of entraining sediment briefly with the ebbing tide or when enhanced by the wind. Since these wind-generated currents are associated with storm waves, which typically exceed the critical skin friction, they have a disproportionately large impact on the direction of the sediment transport. An energetics-based model, driven by locally measured near-bottom currents, is used to characterize the rate and direction of bedload and suspended load transport. The largest transport rates were predicted in response to storm waves and were initially directed onshore with weak oscillatory transport and alongshore by wind-generated currents that turned offshore ...

Numerical modelling of hydrodynamics and sediment transport processes during storm events in a non-perennial river

Numerical modelling of hydrodynamics and sediment transport processes during storm events in a non-perennial riverThis article presents the numerical modelling of hydrodynamics and sediment transport processes in the seasonal Pathri Rao River, which flows in the Northern part of India. Modelling is made by the coupled application of the hydrodynamic model called TELEMAC-2D and the morphodynamic model called SediMorph. The hydrodynamic model results are validated with the observed data and it has been found that the present model provides reasonable predictions. Likewise, the validation of the morphodynamic model is also presented. For this purpose, the suspended and bed load transport modules are validated separately for four runoff events observed during the investigations. The validation of the former is approached by comparing the depth-averaged suspended transport concentrations against experimental measurements made at the make-shift gauging station and subsequent comparison against experimental measurements. On the other hand, due to non-availability of the observed values on bed load sediment transport, the bed load sediment transport and bed evolution numerical results could not be validated quantitatively and was approached only on qualitative basis. In general, both validations present an acceptable agreement with measurements and scientific facts. Further, this study demonstrates that the coupled TELEMAC-2D/SediMorph system could be used with confidence for practical applications in the Pathri Rao River.

The partitioning of the total sediment load of a river into suspended load and bedload: a review of empirical data

The partitioning of the total sediment load of a river into suspended load and bedload is an important problem in fluvial geomorphology, sedimentation engineering and sedimentology. Bedload transport rates are notoriously hard to measure and at many sites only suspended load data are available. Often the bedload fraction is estimated with rule-of-thumb methods such as Maddock’s Table, which are inadequately field-tested. Here the partitioning of sediment load for the Pitzbach is discussed, an Austrian mountain stream for which high temporal resolution data on both bedload and suspended load are available. The available data show large scatter on all scales. The fraction of the total load transported in suspension may vary between zero and one at the Pitzbach, while its average decreases with rising discharge (i.e. bedload transport is more important during floods). Existing data on short- and long-term partitioning is reviewed and an empirical equation to estimate bedload transport rates from measured suspended load transport rates is suggested. The partitioning averaged over a flood can vary strongly from event to event. Similar variations may occur in the year-to-year averages. Using published simultaneous short-term field measurements of bedload and suspended load transport rates, Maddock’s table is reviewed and updated. Long-term average partitioning could be a function of the catchment geology, the fraction of the catchment covered by glaciers and the extent of forest, but the available data are insufficient to draw final conclusions. At a given drainage area, scatter is large, but the data show a minimal fraction of sediment transported in suspended load, which increases with increasing drainage area and with decreasing rock strength for gravel bed rivers, whereby in large catchments the bedload fraction is insignificant at ~1%. For sand bed rivers, the bedload fraction may be substantial (30-50%) even for large catchments. However, available data are scarce and of varying quality. Long-term partitioning varies widely among catchments and the available data are currently not sufficient to effectively discriminate control parameters.

Numerical simulation of tide-induced transport of heterogeneous sediments in the English Channel

Abstract The three-dimensional numerical model COHERENS (COupled Hydrodynamical-Ecological model for REgioNal and Shelf seas) has been adapted to compute the rates of transport as bedload and suspended load of heterogeneous bottom sediments induced by the dominant M2 tide in the English Channel. A pre-processing of an extensive surficial sediments dataset has been performed to determine the seabed composition (grain-size distribution or presence of rocks) at the computational grid nodes. Maximum bedload and suspended load transport rates over the tidal cycle as well as the contributions of the 10 different sedimentary classes to the mean transports are computed. Highest sediment transport rates occur in fine sediments areas located in the surroundings of high shear stresses areas. Medium sand ( d 4 = 350 μ m ) is found to be predominant in bedload, while suspension load implies mainly silts ( d 1 = 25 μ m ) in the inner shoreface and both fine and medium sands ( d 3 = 150 μ m , d 4 = 350 μ m ) in the outer shoreface. The offshore residual bedload transport pathways are orientated westerly in the western part of the Channel and easterly in the eastern part defining a “parting” zone which runs from the Isle of Wight to the Cotentin Peninsula. An offshore “bedload convergence” occurs in the southwest of the Dover Strait; a narrow transport pathway bypassing it along the French coastline. These features reproduce those predicted by Grochowski et al. (1993a) and provide higher resolution features like inshore headland-induced gyres, particularly along the English coastline. The new predicted general pattern of residual suspended load transport is very similar to the bedload pattern. Differences arise in the central “divergence” zone which exhibits a “Y” shape with two branches ending on both sides of the Isle of Wight, in the Baie de Seine characterized by a central “convergence” and along the English coastline studded with many headland-induced recirculations.

Influence of cohesion on suspended load transport of non-uniform sediments

Experimental results are presented on suspended load transport rates of clay eroded from a channel bed made up of two types of cohesive sediment mixtures. The first mixture was prepared using fine gravel and clay with the clay proportion by weight varying from 10 to 50%. In the second mixture, equal proportions of fine gravel and fine sand were mixed with clay in proportions of 10 to 50%. Based on dimensional considerations a computational procedure was developed to estimate the rate of suspended load transport of the clay fraction eroded from the cohesive sediment mixtures. In the absence of cohesion, the proposed computational method transforms to a method for computation of suspended load transport rate of the cohesionless sediment mixtures. Studies on suspended load transport rates of cohesive sediment mixtures containing gravel as presented here were not reported so far in the literature to the best of our knowledge.

Suspended load in flows on erodible bed

Steady state suspended-load of sediment transported in flow over erodible beds usually is treated by the advection-diffusion approach, though in recent years, it is being treated as a two-phase flow phenomenon incorporating kinetics of sediment particles. Among the advection-diffusion approaches, Rouse's (1937) equation is the well-known, although a number of researchers in later periods have attempted to improve it by modifying the mixing length concept taking into account other aspects. In this paper, the advection-diffusion approach and associated logarithmic law of flow velocity are revisited. It is concluded from the logarithmic law that the Reynolds shear stress is a linear function of height above the bed, which reduces to bed shear stress in the case of a long horizontal channel. As a consequence, it is shown that the volumetric concentration of sediment is best approximated by the sum of two power laws of height above the bed. An equation is derived for the suspended-load transport rate in terms of elementary functions.

3D Numerical Simulation for Water Flows and Sediment Deposition in Dam Areas of the Three Gorges Project

This paper presents a three-dimensional (3D) mathematical model for suspended load transport in turbulent flows. Based on the stochastic theory of turbulent flow proposed by Dou, numerical schemes of Reynolds stresses for anisotropic turbulent flows are obtained. Instead of a logarithmic law, a specific wall function is used to describe the velocity profile close to wall boundaries. The equations for two-dimensional suspended load motion and sorting of bed material have been improved for a 3D case. Numerical results are in good agreement with the measured data of the Gezhouba Project. The present method has been employed to simulate sediment erosion and deposition in the vicinity of the Three Gorges Dam. The size distribution of the deposits and bed material, and flow and sediment concentration at different times and elevations, are predicted. The results agree well with the observations in physical experiments. Thus, a new method is established for 3D simulation of sediment motion in the vicinity of dams.

“Chevrons” are not mega-tsunami deposits—A sedimentologic assessment

Since the introduction of the term “chevron” for large v- or u-shaped bed forms in Egypt and the Bahamas, others have adopted the term to describe large-scale coastal bed forms in Australia, Madagascar, and elsewhere. These authors interpret “chevron” bed forms as deposits of mega-tsunamis resulting from Holocene oceanic asteroid impacts. We reason that chevron-type bed forms are common and are present far enough from the coast to preclude tsunami genesis. Moreover, we argue that “chevrons” are not mega-tsunami deposits by modeling tsunami behavior and evaluating sediment-transport conditions under which such features formed. We model the southern Madagascar case, with an impact source in the Indian Ocean, and show that a modeled wave approach is inconsistent with “chevron” orientation. We then evaluate sediment-transport conditions under which these “chevron” bed forms could persist, i.e., bed-load transport. In our analysis, no conditions specified generate pure bed-load transport, and most result in pure suspended-load transport.