Sediment Continuity Research Articles

Numerical modeling of flow and bed evolution in meandering submarine channels

We present a numerical model of flow and bed morphology in meandering submarine channels. The conservation equations of fluid mass and momentum, suspended sediment concentration, and turbulent kinetic energy are expressed in depth‐averaged form and an intrinsic coordinate system. These equations, along with the Exner equation of bed sediment continuity, are solved using a predictor‐corrector explicit finite difference scheme. The numerical model is validated against laboratory experiments conducted in a straight flume. The model is generalized to any sinuous planform geometry, but this study focuses on sine‐generated sinuous channels. A series of numerical experiments have been performed with various inflow conditions and channel shapes to characterize the nature of in‐channel flow, deposition, and erosion, as well to investigate the tendency for meander migration. The model predicts an overall decrease in depth‐averaged current velocity and suspended sediment concentration in the down‐channel direction. Variations of current thickness, streamwise velocity, concentration, and bed level across the channel have been found to increase with an increase in channel curvature. Model runs made with erosional currents display significant change in bed level in the lateral direction because of erosion near the out side and deposition near the inside of a bend. The lateral bed slope generated by the flow of a depositional current is found to be relatively small. Model results are compared with the sedimentation pattern of a seismically interpreted buried submarine channel.

Experimental and numerical analysis of the scouring effects of flushing waves on sediment deposits

In the management of urban drainage networks, great interest has been recently addressed to the removal of sediment deposits from sewer collectors. In particular, sewer cleansing can be carried out by flushing devices able to discharge a great volume of water during a short period of time. In this paper an experimental and numerical investigation on the scouring effects of flushing waves on sewer sediment deposits is presented. Experiments were carried out on a laboratory channel adopting a simple flushing device and considering different flushing conditions. Accurate measurements on the evolution of mobile-bed deposits and on sediment transported during the flushing operations were carried out. Subsequently, a numerical model was specifically developed for the analysis of the flushing wave propagation and for the description of the sediment scouring on mobile-bed channels. The model is based on the semi-coupled solution of the complete De Saint Venant equations for the water flow and of the Exner equation for the sediment continuity. Numerical results were compared with the experimental measurements in order to derive indications on the scouring processes consequent to flushing operations.

Three‐dimensional flow modelling and sediment transport in the River Klarälven

AbstractA three‐dimensional flow model that uses the RNG k‐ε turbulence model and a non‐equilibrium wall function was applied to the River Klarälven in the southwest part of Sweden. The objectives were to study the nature of the flow in the river bifurcation and to investigate the short‐term sediment transport patterns in the river. The effectiveness of three‐dimensional flow models depends upon: (1) how well the river geometry and it surface roughness are modelled; and (2) the choice of the closure model. Improvements were obtained by modelling the river in two parts: the entire river reach, and a selected part. Composite Manning coefficients were used to account for roughness properties. The method requires a calibration process that ensures the water surface profiles match the field data. The k‐ε model under‐predicted both the extent of flow separation zones and the number of secondary flow regions having a spiral motion, in comparison with the RNG k‐ε model. The 3‐D model could predict with good accuracy both the general and secondary flow fields in the river. The results agreed well with the 3‐D velocity measurements using an acoustic Doppler current profiler. A conceptual model was developed that accounts for the development of secondary flows in a river bifurcation having two bends. The main flow feature in the river cross‐sections was the existence of multiple counter‐rotating spiral motions. The number of spiral motions increased as the river bends were approached. The river bends also caused vorticity intensification and increased the vertical velocities. The application of the 3‐D flow model was extended by solving the sediment continuity equation. The sediment transport patterns were related to the secondary flow fields in the river. The sediment transport patterns at the river bifurcations are characterized by the growth of a sandbank. Copyright © 2004 John Wiley & Sons, Ltd.

Vertical sorting and the morphodynamics of bed form–dominated rivers: A modeling framework

Uncertainties and errors in predictions by morphodynamic models of rivers with nonuniform sediment are usually attributed to shortcomings pertaining to the submodel of sediment transport. This mistakenly neglects shortcomings in the submodel of sediment continuity, which describes the vertical sorting and the bed surface composition, whereas the latter is one of the main input parameters for calculating sediment transport and thus morphological changes. Hirano [1971, 1972] was the first to develop a sediment continuity model for nonuniform sediment. Since this commonly used Hirano active layer model and its variants suffer from a number of shortcomings, the authors have developed a new type of sediment continuity model that describes the bed composition and vertical sorting fluxes without distinguishing discrete bed layers. This continuum sorting model is aimed at conditions dominated by bed forms and bed load transport. It is based on (1) the Parker‐Paola‐Leclair framework for sediment continuity, (2) the Einstein step length formulation, (3) a newly developed lee sorting function, and (4) a newly developed method to account for the variability in bed form trough elevations. The resulting model is deterministic in the computation of the vertical sorting profile and is probabilistic in terms of the riverbed surface due to the presence of dunes.

Phase‐shifts in shear stress as an explanation for the maintenance of pool–riffle sequences

AbstractThe stability of the pool–riffle sequence is one of the most fundamental features of alluvial streams. For several decades, the process of velocity, or shear stress, reversal has been proposed as an explanation for an increase in the amplitude of pool–riffle sequence bars during high flows, offsetting gradual scour of riffles and deposition in pools during low flows. Despite several attempts, reversal has rarely been recorded in field measurements. We propose that, instead of being reversed, maxima and minima in shear stress are phase‐shifted with respect to the pool–riffle sequence bedform profile, so that maximum shear stress occurs upstream of riffle crests at high flow, and downstream at low flow. Such phase‐shifts produce gradients of shear stress that explain riffle deposition, and pool scour, at high flow, in accord with sediment continuity. The proposal is supported by results of a one‐dimensional hydraulic model applied to the surveyed bathymetry of a pool–riffle sequence in a straight reach of a gravel‐bed river. In the sequence studied, the upstream phase‐shift in shear stress at high flow was associated with variations in channel width, with width minima occurring upstream of riffle crests, approximately coincident with shear stress maxima, and width maxima occurring downstream of riffle crests. Assuming that the width variation is itself the result of flow deflection by riffle crests at low flow, and associated bank‐toe scour downstream, low and high flow can be seen to have complementary roles in maintaining alluvial pool–riffle sequences. Copyright © 2004 John Wiley & Sons, Ltd.

Numerical simulation of scour by a free falling jet

In this paper the numerical simulation of scour by a free falling jet is presented. It is assumed that the flow is two-dimensional, the alluvium is cohesionless, and that at the beginning of a run, i.e. at t = 0, the movable bed is flat. The scour simulation involves three basic steps: simulation of a turbulent flow in the stilling basin of a free falling jet, determination of distribution of sand concentration, and computation of bed deformation. The flow simulation rests on the momentum equations, the continuity equation, and on the k-ε equations for turbulent flows. A general 2-D non-orthogonal curvilinear computational domain is used; the solution is by the finite volume method, with a non-staggered grid arrangement. The SIMPLE scheme is used for pressure correction and the checkerboard problem is solved with a momentum interpolation scheme. The distribution of sand concentration is determined on the basis of the convection-diffusion equation. An appropriate boundary condition for concentration at the bed, which takes into account the effect of bed-load, is implemented. The bed deformation is computed with the aid of the sediment continuity equation. The aforementioned steps are repeated until the equilibrium bed surface (i.e. the equilibrium scour hole) is reached. The results of the numerical simulation appear to compare favourably with experiment.

Three-dimensional modelling of ship-induced flow and erosion

Ship-induced erosion by propeller jets causes considerable damage in canals, harbours, locks and waterways. Jet velocity reaching the bed could be as high as 15 m/s depending on the ship and the machinery. The objectives of the present study were to investigate the nature of the propeller jet flow and to estimate the rate of erosion caused by the jet. Three-dimensional numerical models can be used to simulate ship-induced propeller flows provided the propeller can be replaced with a simpler geometry. The study modelled the propeller flow by one lower and one upper jet flow. The turbulent characteristics, such as the turbulence intensities and the energy balance of the lower jet, are similar to a flat wall boundary layer flow. The application of the three-dimensional flow model was extended by numerically solving the sediment continuity equation. The computations were based on the lower part of the jet velocity profiles. The method enables the scour cavity induced by a stationary ship with a running engine to be calculated. The computed scour cavity agreed well with the field data.

Three-dimensional modelling of ship-induced flow and erosion

Ship-induced erosion by propeller jets causes considerable damage in canals, harbours, locks and waterways. Jet velocity reaching the bed could be as high as 15 m/s depending on the ship and the machinery. The objectives of the present study were to investigate the nature of the propeller jet flow and to estimate the rate of erosion caused by the jet. Three-dimensional numerical models can be used to simulate ship-induced propeller flows provided the propeller can be replaced with a simpler geometry. The study modelled the propeller flow by one lower and one upper jet flow. The turbulent characteristics, such as the turbulence intensities and the energy balance of the lower jet, are similar to a flat wall boundary layer flow. The application of the three-dimensional flow model was extended by numerically solving the sediment continuity equation. The computations were based on the lower part of the jet velocity profiles. The method enables the scour cavity induced by a stationary ship with a running engine to be calculated. The computed scour cavity agreed well with the field data.

Sediment yield modelling for small agricultural catchments: land‐cover parameterization based on remote sensing data analysis

AbstractVegetation and soil properties and their associated changes through time and space affect the various stages of soil erosion. The island of Ishigaki in Okinawa Prefecture, Japan is of particular concern because of the propensity of the red‐soil‐dominated watersheds in the area to contribute substantial sediment discharge to adjacent coastal areas. This paper discusses the application of remote sensing techniques in the retrieval of vegetation and soil parameters necessary for the distributed soil‐loss modelling in small agricultural catchments and analyses the variation in erosional patterns and sediment distribution during rainfall events using numerical solutions of overland flow simulations and sediment continuity equations. To account for the spatial as well as temporal variability of selected parameters of the soil‐loss equations, a method is proposed to account for the variability of associated vegetation cover based on their spectral characteristics as captured by remotely sensed data. To allow for complete spatial integration, modelling the movement of sediment is accomplished under a loose‐coupled GIS computational framework. This study lends a theoretical support and empirical evidence to the role of vegetation as a potential agent for soil erosion control. Copyright © 2003 John Wiley & Sons, Ltd.

A simple model of unsteady sheet-flow sediment transport

A simple, quasi-steady, one-dimensional, vertical (1DV) model of unsteady sheet flow is presented. The central aim is to provide greater realism in the near-bed, high-concentration layer, than is possible using models based on the classical reference-concentration approach. This is achieved by tracking erosion and deposition at the bottom of the mobile sediment layer in relation to the amount of sediment present in the sheet-flow and suspension layers. The model relies on empirical assumptions for the time-varying sheet-flow layer thickness ( δ) and time-varying equivalent bed roughness ( k s). The formulations adopted yield realistic instantaneous vertical profiles of velocity and sediment concentration from the stationary bed, through the high-concentration sheet-flow layer up into the outer suspension layer. The suspension layer itself is modelled using a standard k– ε turbulence-closure scheme, together with the sediment continuity equation. Matching conditions are applied at the interface between the sheet-flow and suspension layers. Preliminary results presented here include initial validation comparisons with the data set of Horikawa et al. [Horikawa, K., Watanabe, A., Katori, S., 1982. Sediment transport under sheet flow conditions. Proceedings of the 18th International Conference on Coastal Engineering, Cape Town. ASCE, Reston VA, USA, 1335–1352.]. Further comparisons with the experiments of Dohmen-Janssen et al. [J. Geophys. Res. 106 (2001) 27103] cover a wide range of wave–current conditions and sand grain sizes. The results are satisfactory with respect to the measured velocity and concentration profiles, apart from cases involving fine sand for which the effects of flow unsteadiness are not accounted for in the formulation. In addition, the new model provides satisfactory predictions of sand transport rates.

A Geomorphic Explanation for a Meander Cutoff Following Channel Relocation of a Coarse-Bedded River

The Veteran's Fishing section of the Blackledge River in central Connecticut was relocated in the late 1950s. The relocation resulted in an unstable channel despite extensive efforts to prevent erosion. Overbank erosion and meander cutoffs were investigated using detailed survey data, characterizations of sediment deposits, flow modeling, and a moment-stability analysis. Limited reworking of revetment boulders indicate that riprap bank material was immobile during a 1979 flood event responsible for the formation of the cutoff channel. A moment-stability analysis factor-of-safety value of 1.1 supports the conclusion that riprap was not directly eroded from the banks. Alluvial particles with d(95) values ranging up to 120 mm were deposited along a bar downstream from the cutoff channel at flows estimated to be below a 1.5-year recurrence interval flow. Development of the bar deposit resulted in locally elevated water surfaces at high flow. The resulting overbank flow across the meander neck to the adjacent downstream bend led to the creation of an upstream migrating knickpoint, the erosion of approximately 16,000-year-old sediments, and the subsequent meander cutoff. The results of the study indicate that traditional erosion-control measures cannot prevent extreme channel adjustments if the geomorphic processes that control sediment continuity also are not considered.

Vertical sorting in bed forms: Flume experiments with a natural and a trimodal sediment mixture

Two sets of flume experiments were conducted to examine grain size selective transport and vertical sorting in conditions with migrating bed forms and bed load transport. In the two sets of experiments we used a sediment mixture from the river Rhine and a trimodal mixture, respectively. The vertical sorting profiles showed a downward coarsening trend within the bed forms, and in some experiments an essentially immobile coarse bed layer formed underneath the migrating bed forms. Three mechanisms contribute to the formation of such a coarse bed layer: (1) the avalanching process at the lee face, (2) conditions of partial transport in which a relatively large amount of coarse material does not participate in the transport process, and (3) the winnowing of fines from the trough surface and subsurface. The experiments show that vertical sorting fluxes not only occur through net degradation or aggradation but also through the migration of bed forms and through the variability in trough elevations. This is contradictory to the way vertical sorting processes are modeled in most existing sediment continuity models for nonuniform sediment. The present study is therefore also a plea for modifying existing sediment continuity models to account for vertical sorting processes other than through net aggradation or degradation.

Numerical simulation of morphological changes in rivers and reservoirs

For the computation of the morphological changes in river or reservoir beds, due to erosion or deposition, a hydrodynamic model is combined with the sediment continuity equation. The hydrodynamic model is based upon the equations of mass and momentum conservation. For the solution of the above system of three partial differential equations, the explicit numerical schemes of MacCormack and Lax-Wendroff are tested. The whole mathematical model is applied to two experimental cases and to a real case. The first experimental case concerns the aggradation in a laboratory channel due to sediment overloading, and the second one the sediment release from a reservoir after a dam break. The real case concerns the morphological changes in the torrent Mallero (Northern Italy) during a flood event.

Dynamic model for constriction scour caused by large woody debris

AbstractLarge woody debris (LWD) can have a significant impact upon local channel morphology by creating scour pools and zones of reduced shear stress in which sediment is deposited. It is important to predict scour depths associated with LWD, as it is becoming increasingly common for debris to be added into river channels to improve sediment retention and create pools for aquatic habitat. Engineered log‐jams should therefore be designed using factor of safety engineering analysis, which includes estimates of associated scour and deposition rates. However, the rate and total depth of scour associated with LWD have not been modelled comprehensively, with authors resorting to the use of generic local and constriction scour models to predict scour depths. Also, constriction scour models presented, to date, do not calculate the rate of scour development. In this paper a model is presented for predicting the rate and total depth of scour associated with a channel constriction. The model is one‐dimensional and is based upon the sediment continuity equation, the calculation of specific head changes through the constricted reach and also allows for a variable free surface elevation above the bed at the constriction. This model could be applied to any channel constriction problem but here is used to determine scour rates and depths associated with deflector‐type LWD jams. Deflector jams are one category of jam type presented in a debris jam classification scheme, in which jam type is a function of the ratio of average riparian tree height to average channel width. Deflector jams, as the name implies, partially block the flow and therefore act as a channel constriction, which results in constriction scour. Copyright © 2002 John Wiley & Sons, Ltd.

Hydrological simulation of a conservation bench terrace system in a subhumid climate

A finite element model to simulate runoff and soil erosion from agricultural lands has been developed. The sequential solutions of the governing differential equations were found: Richards' equation with a sink term for infiltration and soil water dynamics under cropped conditions; St Venant equation with kinematic wave approximation for overland and channel flow; and sediment continuity equation, for soil erosion. The model developed earlier has been improved to simulate erosion/deposition in impoundments and predicted and observed soil loss values were in reasonably good agreement when the model was tested for a conservation bench terrace (CBT) system. The finite element model was extensively applied to study the hydrological behaviour of a CBT system vis-à-vis the conventional system of sloping borders. The model estimates runoff and soil loss reasonably well, under varying conditions of rainfall and at different crop growth stages. The probable reasons for discrepancies between observation and simulation are reported and discussed. Sensitivity analysis was carried out to study the effect of various hydrological, soil and topographical parameters, such as ratio of contributing to receiving areas, weir length, depth of impoundment, slope of contributing area, etc. on the flow behaviour in a CBT system.

Closure to “Probabilistic Exner Sediment Continuity Equation for Mixtures with No Active Layer” by Gary Parker, Chris Paola, and Suzanne Leclair

Closure to “Probabilistic Exner Sediment Continuity Equation for Mixtures with No Active Layer” by Gary Parker, Chris Paola, and Suzanne Leclair

Discussion of “Probabilistic Exner Sediment Continuity Equation for Mixtures with No Active Layer” by Gary Parker, Chris Paola, and Suzanne Leclair

Discussion of “Probabilistic Exner Sediment Continuity Equation for Mixtures with No Active Layer” by Gary Parker, Chris Paola, and Suzanne Leclair

Numerical Modeling of Bed Evolution in Channel Bends

A two-dimensional numerical model is developed to predict the time variation of bed deformation in alluvial channel bends. In this model, the depth-averaged unsteady water flow equations along with the sediment continuity equation are solved by using the Beam and Warming alternating-direction implicit scheme. Unlike the present models based on Cartesian or cylindrical coordinate systems and steady flow equations, a body-fitted coordinate system and unsteady flow equations are used so that unsteady effects and natural channels may be modeled accurately. The effective stresses associated with the flow equations are modeled by using a constant eddy-viscosity approach. This study is restricted to beds of uniform particles, i.e., armoring and grain-sorting effects are neglected. To verify the model, the computed results are compared with the data measured in 140° and 180° curved laboratory flumes with straight reaches up- and downstream of the bend. The model predictions agree better with the measured data tha...

Simulation of flow around piers

FLUENT CFD was used to predict the three-dimensional flow field around a circular cylinder. Solutions were obtained for rigid beds and for scour holes of different sizes resulting from different time-durations. The numerical results were used to obtain the variation of bed shear-stress around the cylinder. These results were used in the sediment continuity equation to obtain an expression for the variation of scour depth with time. The asymptotic scour depth was found to depend on three dimensionless numbers: the pile number, the sediment size number and the duration time number. The theoretical relationship was calibrated using various laboratory and field results.

La continuité sédimentaire et sa rupture

Click to increase image sizeClick to decrease image sizeThe idea of hydraulic continuity has been accepted some 20 years ago and is now accounted in river planning and training, e.g. in flood retention by upstream storage. By comparaison, the effectivness of sediment continuity is still often neglected. The main reason is a difference in its perception, which is due to much longer time scales of associated effects. The presentation is intended at demonstrating the conditions and various effects of sediment continuity. Special attention is paid to how the river answers to perturbations or breaking of this continuity.