Maximum Migration Distance of Meander Channel in Sand Using Hyperbolic Function Approach

A set of large-scale laboratory experiments were conducted to study the migration of meandering channel. Factors affecting the change of banklines, including the ratio of centerline curvature to channel width, bend angle, and Froude number were tested in the experiments. The effect of each factor on the evolution of channel plan form was evaluated and quantified. The channel bankline displacement was modeled by a hyperbolic function with the inclusion of the initial migration rate and the maximum migration distance. A three-dimensional numerical model was also employed to explain some findings in the laboratory tests. It is found that the maximum migration distance along a bend satisfies a Gaussian distribution. A set of equations were developed for predicting the maximum migration distance. With the maximum migration distance being developed as a function of several geometric and flow parameters, a hyperbolic-function model can be applied to estimate the maximum bankline migration distance when the channel reaches equilibrium.     

Impact of Foliage on the Drag Force of Vegetation in Aquatic Flows

The objective of this study was to determine the contribution of a plant’s foliage to the total plant’s hydrodynamic drag. Experiments were conducted in a laboratory flume using samples of vegetation with different physical forms and biomechanical properties: Branches of pine (Pinus sylvestris) and ivy stipes (Glechoma hederacea). The drag force was measured directly using a strain gauge technique and determined for a series of velocities for each vegetation species with and without foliage. Experimental results revealed a distinct contribution of foliage to the total plant drag. For both plant types, this was particularly marked at lower velocities where the foliage is not streamlined and compressed and, hence, the frontal projected area of the plant is at a maximum. It was found that the flexibility of the plant’s foliage and its ability to streamline with the flow may reduce the overall drag considerably. There was a distinct difference in the CdAP parameter-velocity squared relationship between the “with” foliage plants and nonfoliage counterparts due to the streamlining effect of the foliage with the flow and, hence, the reduction in overall drag associated with the new compressed plant form.     

Behavior of Meandering Overbank Channels with Graded Sand Beds

Measurements of velocity distributions, depth variation, and sediment transport have been made under bankfull and overbank flow conditions in meandering channels with a graded sand bed, using the large-scale U.K. Flood Channel Facility. The overbank conditions depend upon the relative strength of opposing secondary circulation cells generated by shear at the channel crossover and centrifugal forces around the meander bend. Generally the shear-generated secondary flow either reversed or weakened the centrifugal circulation around the next downstream bend. This led to considerable modification of the main channel bed morphology, which, in turn, altered flow distributions. Measurements of the lateral distribution of bed load were made using a ?-scale Helley–Smith sampler. This demonstrated that the bed load was generally concentrated within a limited width of the channel and tended to take the shortest route through the meanders. Comparisons of observed and calculated bed material load gives an indication of how secondary circulation around meanders, under both bankfull and overbank conditions, affects the predictive performance of formulas derived for predominantly one-dimensional flow.     

Experimental Observations of Flow and Bed Processes in Large-Amplitude Meandering Flume

Meanders of large amplitude often exhibit asymmetric planform shape or subsidiary bends. The present work is aimed at improving on understanding of the morphodynamic phenomena affecting the bed evolution of large amplitude meandering channels. Attention is focused on the development of the steady point bar-pool configuration and of the superimposed large-scale migrating bed forms; of particular interest is the role of the changing channel curvature and bed topography variation on flow pattern. A series of experiments was carried out in a sine-generated large-amplitude meandering flume, for two values of width-to-depth ratio. Maps documenting the bed topography and the flow pattern along the meandering bends are reported. Two point bars per bend were observed and seem to be part of a series of damped oscillations developing in response to the changing channel curvature. In response to the bed deformation, the maximum flow velocity moves at the outer bank at the entrance of the bend.     

Bed-Load Effects on Hydrodynamics of Rough-Bed Open-Channel Flows

The extent to which turbulent structure is affected by bed-load transport is investigated experimentally using a nonporous fixed planar bed comprising mixed-sized granular sediment with a d50 of 1.95?mm. Three different sizes of sediment (d50 = 0.77, 1.99, and 3.96?mm) were fed into the flow at two different rates (0.003 and 0.006?kg/m/s), and subsequently transported as bed load. Particle image velocimetry (PIV) was used to determine the turbulence characteristics over the fixed bed during clear water and sediment feed cases. Mean longitudinal flow velocities at any given depth were lower than their clear water counterparts for all but one of the mobile sediment cases. The exception was with the transport of fine grains at the higher feed rate. In this case, longitudinal mean flow velocities increased compared to the clear water condition. The coarse grains tended to augment bed roughness, but fine grains saturated the troughs and interstices in the bed topography, effectively causing the influence of bed irregularities to be smoothed. The PIV technique permitted examination of both temporal and spatial fluctuations in flow variables: therefore many results are presented in terms of double-averaged quantities (in temporal and spatial domains). In particular, the form-induced stress, which arises from spatially averaging the Reynolds averaged Navier–Stokes equations and is analogous to the Reynolds turbulent stress, contributed between 15 and 35% of the total measured shear stress in the roughness layer. Flow around protrusive roughness elements produced a significant proportion of the turbulent kinetic energy shear production, suggesting that this process is highly intermittent near rough beds.     

Assessment of the Effectiveness of a Constructed Compound Channel River Restoration Project on an Incised Stream

Compound channels are often constructed in restoration projects on rivers and streams that have been channelized or are deeply incised. This design allows for flow over a wider cross-sectional area during high flows and is expected to reduce both flow velocities and bed-shear stresses in the channel during high flows. Using a compound channel restoration project on Tassajara Creek as a case study, the effectiveness of a constructed compound channel in reducing channel velocities and bed-shear stresses during high flow events was tested in two ways. First, since this is an a posteriori analysis, postproject surveys and assessments of the project are used to demonstrate the geomorphic and ecological benefits of the constructed compound channel for reducing further channel incision, improving channel stability, and enhancing native riparian vegetation, while still providing conveyance capacity for design flood flows. Second, the effectiveness of a constructed compound channel in reducing channel velocities and bed-shear stresses during high flow events is evaluated using both the one-dimensional (1D) model, HEC-RAS, and the three-dimensional (3D) numerical model, UnTRIM. This analysis demonstrates that the 1D analysis does not accurately portray the benefits of the compound channel, and is therefore not a suitable tool for evaluating the effectiveness of compound channel designs. These results demonstrate the advantages of using a 3D model and make a strong case for the implementation of more detailed hydrodynamic modeling in evaluating the suitability of restoration alternatives to improve the planning and design of river restoration projects.     

Effects of Velocity Gradients and Secondary Flow on the Dispersion of Solutes in a Meandering Channel

Two contrasting mechanisms, created by channel curvature which strongly affect longitudinal dispersion of solutes in rivers are examined. In natural channels the large cross-sectional variability of the primary velocity component tends to increase longitudinal dispersion by providing a large difference between adjacent fast and slow moving zones of fluid. By contrast secondary circulation tends to decrease longitudinal dispersion by enhancing transverse mixing. A series of tests have been carried out in a very large flume containing a meandering water-formed sand bed channel to measure the longitudinal dispersion coefficient at various locations around a meander. These experimental observations are compared with experimental data obtained from meandering channels with smooth, fixed sides and regular cross-sectional shapes. All the data has been compared against predictions from two current modeling approaches. Finally, the significance of the two competing mechanisms in curved channels is discussed with regard to their relative influence on longitudinal mixing.     

Modeling the Effects of Macrophytes on Hydrodynamics

A computer model was created as a scientific and management tool for understanding the effects of macrophytes on hydrodynamics and water quality. A model was required that could simulate macrophytes in a complex water body and could be coupled to a multicompartment water quality model of phytoplankton, dissolved oxygen, nutrients, pH, and organic matter. This would permit the investigation of water resource issues where macrophyte growth, phytoplankton growth, nutrient loadings, and flood control were all contributing factors. The model was added as a compartment to the U.S. Army Corps of Engineers two-dimensional, laterally averaged, dynamic water quality model, CE-QUAL-W2 (Corps of Engineers, water quality, width averaged, two dimensional) and applied to the Columbia Slough, Ore. Features of the macrophyte model include the capability to simulate multiple submerged macrophyte species; transport of nutrient fluxes between plant biomass and the water column and/or sediments; growth limitation due to nutrient, light and temperature; simulation of the spatial distribution of macrophytes vertically and horizontally; the modeling of light attenuation in the water column caused by macrophyte concentration; and the modeling of open channel flow with channel friction due to macrophytes. The macrophyte model was tested through mass balances and sensitivity analyses. The modeling of channel friction was evaluated by comparing predicted water levels with data from tests conducted in a laboratory flume. Use of the model in the Columbia Slough showed reasonable predictive capability regarding estimated biomass and water level dynamics.     

Simulation of Flow and Mass Dispersion in Meandering Channels

This paper reports the development of an enhanced two-dimensional (2D) numerical model for the simulation of flow hydrodynamics and mass transport in meandering channels. The hydrodynamic model is based on the solution of the depth-averaged flow continuity and momentum equations where the density of flow varies with the concentration of transported mass. The governing equation for mass transport model is the depth-averaged convection and diffusion equation. The dispersion terms arisen from the integration of the product of the discrepancy between the mean and the actual vertical velocity distribution were included in the momentum equations to take into account the effect of secondary current. Two laboratory experimental cases, flow in mildly and sharply curved channels, were selected to test the hydrodynamic model. The comparison of the simulated velocity and water surface elevation with the measurements indicated that the inclusion of the dispersion terms has improved the simulation results. A laboratory experiment study of dye spreading in a sine-generated channel, in which dye was released at the inner bank, centerline, and outer bank, respectively, was chosen to verify the mass transport model. The simulated concentration field indicated that the Schmidt number can be used as a calibration parameter when dispersion is computed using a 2D approach with a simplified turbulence model.     

Temporal Development of Scour Holes around Submerged Stream Deflectors

Deflector structures used in many fish habitat rehabilitation schemes are frequently overtopped, yet few studies have examined the scour patterns created around submerged models. Furthermore, laboratory studies typically test smooth-surfaced structures, whereas those installed in natural rivers are generally made of logs or boulders. This study uses rough-surfaced paired deflectors to investigate the temporal evolution of scour for three overtopping ratios in identical approach flow conditions in a flume. Results show that maintaining identical discharge and raising the deflector height, which reduces the overtopping ratio (i.e., flow depth divided by structure height), generates increased depth and volume of scour next to the structures. The location of maximum depth and the rate of scouring with time is similar for the two highest deflectors (overtopping ratios of 1.22 and 1.83), but different for the lowest deflector model (overtopping ratio of 3.67). To improve the success rate of river restoration projects using in-stream structures, the overtopping ratio should be considered in equations that predict the scour depth evolution with time.     

Preliminary Assessment and Rating of Stream Channel Stability near Bridges

The primary cause of bridge failure in the United States is scour and channel instability around the bridge foundations. The ability to assess channel stability in the vicinity of bridges is needed to alert engineers to possible unstable conditions at the bridge foundations, to design stable road crossings, and to mitigate against erosion at those structures. This information is valuable for stream stabilization projects as well, particularly for cases where the reach to be restored includes a bridge. However, a systematic methodology for rapidly assessing channel stability that is applicable at bridges located in the various regions of the country does not currently exist. In this study, an assessment method for the preliminary documentation and rating of channel stability near bridges was developed, based on prior stability assessment methods as well as observations at bridges in 13 physiographic regions of the continental United States. This method provides an assessment of channel stability conditions as they affect bridge foundations. It is intended for a quick assessment of conditions for the purpose of documenting conditions at bridges and for judging whether more extensive geomorphic studies or complete hydraulic and sediment transport analyses are needed to assess the potential for adverse conditions developing at a particular bridge in the future.     

Incipient Motion of Bivalve Shells on Sand Beds under Flowing Water

A model is presented to determine the critical shear stress for the incipient motion of bivalve shells on a horizontal sand bed, under a unidirectional flow of water. Hydrodynamic forces on a solitary bivalve shell, resting over a sand bed, are analyzed for the condition of incipient motion including the effect of turbulent fluctuations. Hydrodynamic forces such as drag and lift are taken into consideration. Three types of bivalve shells, namely Coquina Clam, Cross-barred Chione and Ponderous Ark, were tested experimentally for the condition of incipient motion. The shape parameter of bivalve shells is defined appropriately. Experiments were conducted in a flume with a horizontal bed, and the critical shear stresses were computed using Vanoni’s side-wall correction. The experimental data are used to calibrate the model making lift coefficient a free parameter. The results obtained using the present model agree satisfactorily with the experimental data.     

Modeling the Evolution of Incised Streams. III: Model Application

Incision and the ensuing widening of alluvial stream channels represent important forms of channel adjustment. Two accompanying papers have presented a robust computational model for simulating the long-term evolution of incised and restored or rehabilitated stream corridors. This work reports on applications of the model to two incised streams in northern Mississippi, James Creek, and the Yalobusha River, to assess: (1) its capability to simulate the temporal progression of incised streams through the different stages of channel evolution; and (2) model performance when available input data regarding channel geometry and physical properties of channel boundary materials are limited (in the case of James Creek). Model results show that temporal changes in channel geometry are satisfactorily simulated. The mean absolute deviation (MAD) between observed and simulated changes in thalweg elevations is 0.16?m for the Yalobusha River and 0.57?m for James Creek, which is approximately 8.1 and 23% of the average degradation of the respective streams. The MAD between observed and simulated changes in channel top width is 5.7% of the channel top width along the Yalobusha River and 31% of the channel top width along James Creek. The larger discrepancies for James Creek are mainly due to unknown initial channel geometry along its upper part. The model applications also emphasize the importance of accurate characterization of channel boundary materials and geometry.     

Effect of Floodwater Extraction on Mountain Stream Morphology

Floodwater is often extracted for consumptive purposes from western mountain streams in the United States. The long-term extraction of floodwater may alter the morphological and ecological balance of such streams. Scale model experiments based on eight mountain gravel-bed streams in Idaho were conducted to test the effects of floodwater extraction on stream morphology. The model channel transported a poorly sorted mix of model “gravel,” as well as copious amounts of model “sand.” The channel had a discontinuous floodplain, developed its own bar morphology, and contained large model colluvium as well as a bedrock platform. A mobile-bed equilibrium was first developed using a repeated hydrograph. The experiment was then repeated using a sliding cutoff discharge. The discharges in the hydrograph that were below a given cutoff discharge were reduced to 30% of bankfull discharge. By raising the cutoff discharge, it was possible to study the effect of increasing severity of floodwater extraction on stream morphology. The experiments indicated an increase in sand content on the bed surface and a decrease in the standard deviation of fluctuations in bed elevation with increasing severity of floodwater extraction.     

Compatibility of Reservoir Sediment Flushing and River Protection

In this paper, we propose a system of numerical models for the compatibility assessment of reservoir sediment flushing and protection of downstream river environments. The model system is made up of two simulation models. The first model simulates soil erosion in watershed slopes and sediment transport in the tributary of the reservoir by means of a weighted essentially nonoscillatory (WENO) method, which is conservative and fourth-order accurate in space and time. The second model simulates velocity and suspended solid concentration fields in the reservoirs. This model is based on the three-dimensional (3D) numerical integration of motion and concentration equations, expressed in contravariant form on a generalized boundary-conforming curvilinear coordinate system by using a conservative and higher-order accurate numerical scheme. The proposed system of models is applied to the Pieve di Cadore (Veneto, Italy) reservoir and to its catchment area. By comparing suspended solid concentrations that are discharged through the bottom outlets during flushing operations with suspended solid concentrations in the main river during natural flooding, we perform an assessment of the compatibility between sediment flushing and the protection of the river ecosystem downstream.     

Test of a Method to Calculate Near-Bank Velocity and Boundary Shear Stress

Detailed knowledge of the flow and boundary shear stress fields near the banks of natural channels is essential for making accurate calculations of rates of near-bank sediment transport and geomorphic adjustment. This paper presents a high-resolution laboratory data set of velocity and boundary shear stress measurements and uses it to test a relatively simple, fully predictive, numerical method for determining these distributions across the cross-section of a straight channel. The measurements are made in a flume with a fairly complex cross-section that includes a simulated, cobble-roughened floodplain. The method tested is that reported by Kean and Smith in Riparian Vegetation and Fluvial Geomorphology in 2004, which is modified here to include the effects of drag on clasts located in the channel. The calculated patterns of velocity and boundary shear stress are shown to be in reasonable agreement with the measurements. The principal differences between the measured and calculated fields are the result of secondary circulations, which are not included in the calculation. Better agreement with the structure of the measured streamwise velocity field is obtained by distorting the calculated flow field with the measured secondary flow. Calculations for a variety of narrower and wider configurations of the original flume geometry are used to show how the width-to-depth ratio affects the distribution of velocity and boundary shear stress across the channel.     

Effects of Background Water Composition on Stream–Subsurface Exchange of Submicron Colloids

While very fine sediments (colloids) are normally assumed to be readily transported downstream without deposition, recent evidence suggests that these particles will often deposit into streambeds due to a combination of physical and chemical mechanisms. This study investigates a regime of particle deposition where settling is unimportant and thus where particle deposition can only result from advective stream–subsurface exchange followed by deep-bed filtration. Laboratory flume experiments were conducted to examine the deposition of 0.45 μm diameter silica colloids into a silica sand bed. This system was selected for study because submicron sized colloids will not settle and silica colloid filtration by silica sand is generally quite low. Despite the lack of settling and the weak particle–particle interactions, the ongoing interfacial flux of colloids to the subsurface still produced significant filtration of silica colloids over the course of the experiments. Variation of the background ionic strength caused significant modification of filtration behavior and silica colloid deposition. In addition, cleaning the sand surface with mild acid and base washes reduced both filtration and net colloid exchange. These experimental results are interpreted in terms of a fundamentally based physicochemical model which predicts net particle deposition based on stream and subsurface hydrodynamic conditions and subsurface filtration. These results show that both particle surface chemical conditions and background water chemistry play a critical role in controlling the net transport and deposition of fine sediments. It is important to recognize the effects of physicochemical processes both when designing laboratory experiments and when analyzing environmental particle transport.     

Bank-Attached Vanes for Bank Erosion Control and Restoration of River Meanders

The effectiveness of a novel approach of using vanes, installed at a low angle and attached to the bank, for bank protection and for the restoration of river meanders has been investigated in a laboratory study. Experiments were carried out in a large-scale meandering mobile-bed channel with graded sediment. The bed topography, three-dimensional flow pattern, and turbulence characteristics in the meandering channel with or without structures are analyzed. When a single or an array of such vanes is installed, the scour hole at the base of the outer bank is infilled and the thalweg is relocated toward the center of the river. The structures induce a secondary flow cell near the outer bank which counteracts the main spiral flow in the bend. In contrast to common spurs and bendway weirs, large-scale horizontal vortices are not generated behind the structures. Vanes which grade to the bed from bankfull level at the bank show better performance than low level ones, whereas multiple structures show positive effects as far downstream as the crossover section.     

Numerical Model for Sediment Transport and Bed Degradation in the Yangtze River Channel Downstream of Three Gorges Reservoir

This paper presents a two-dimensional morphological model for unsteady flow and both suspended-load and bed-load transport of multiple grain size to simulate transport of graded sediments downstream from the Three Gorges Reservoir. The model system includes a hydrodynamic module and a sediment module. The hydrodynamic module is based on the depth-averaged shallow water equations in orthogonal curvilinear coordinates. The sediment module describing nonuniform sediment transport is developed to include nonequilibrium transport processes, bed deformation, and bed material sorting. The model was calibrated using field observations through application to a 63-km-long alluvial river channel on the middle Yangtze River in China. A total of 16 size groups and a loose layer method of three sublayers were considered for the transport of the nonuniform bed materials in a long-term simulation. Predictions are compared with preliminary results of field observations and factors affecting the reliability of the simulated results are discussed. The results may be helpful to the development of more accurate simulation models in the future.     

Inception Point and Air Entrainment on Flows under Macroroughness Condition

Rock chutes or block ramps are fishway passages with low environmental impact. They also contribute to reaeration of rivers with low dissolved oxygen content, owing to the turbulence enhanced by their three-dimensional macroroughness conditions. This paper analyzes the air entrainment inception in flows over beds in macroroughness condition and the self-aerated flow features of the developing flow downstream of the inception point. Air concentration, inception point locations, and water depth elevations have been measured on two different scaled chutes for slopes ranging between 1V:5.88H and 1V:2.17H. Moreover, two different ogee crest lengths have been tested to assess the role of the inlet conditions on the location of the inception point. New equations have been developed to estimate the location of the point of inception and the respective water depth. Longitudinal variations of the mean air concentration downstream from the inception point have been studied and compared with data from the literature. An expression is presented to estimate the optimum length of the block ramp in natural rivers for maximizing air-water mixing.