Sean J. Bennett

A model for the entrainment and transport of sediment grains of mixed sizes, shapes, and densities

A model for the entrainment and bed load transport of sediment grains of different sizes, shapes and densities by a unidirectional turbulent flow is developed in terms of (1) sediment types available for transport; (2) the mean and turbulent fluctuating values of fluid forces acting upon the sediment grains; and (3) the nature of the interaction between turbulent fluid forces and available sediment, resulting in entrainment and transport of grains as bed load or in suspension. The behavior of the model is explored extensively, and compared with natural data from flumes and rivers. The predicted threshold of entrainment of individual size fractions within a mixed-size bed agrees well with observations as long as the pivoting angle is specified appropriately as a function of grain size. The rate and size distribution of bed load transport generally agrees well with natural data as long as effective bed shear stress in the presence of bed forms can be defined, bed load transport measurements are reliable, and the size distribution of available sediment is accurately specified.

Velocity structure, turbulence and fluid stresses in experimental gravity currents

Gravity currents are of considerable environmental and industrial importance as hazards and as agents of sediment transport, and the deposits of ancient turbidity currents form some significantly large hydrocarbon reservoirs. Prediction of the behavior of these currents and the nature and distribution of their deposits require an understanding of their turbulent structure. To this end, a series of experiments was conducted with turbulent, subcritical, brine underflows in a rectangular lock-exchange tank. Laser-Doppler anemometry was used to construct a two-dimensional picture of the velocity structure. The velocity maximum within the gravity current occurs at y/d ≈ 0.2. The shape of the velocity profile is governed by the differing and interfering effects of the lower (rigid) and upper (diffuse) boundaries and can be approximated with the law of the wall up to the velocity maximum and a cumulative Gaussian distribution from the velocity maximum to the ambient interface. Mean motion within the head consists of a single large vortex and an overall motion of fluid away from the bed, and this largely undiluted fluid becomes rapidly mixed with ambient fluid in the wake region. The distribution of turbulence within the current is heterogeneous and controlled by the location of large eddies that dominate the turbulent energy spectrum and scale with flow thickness. Turbulent kinetic energy reaches a maximum in the shear layer at the upper boundary of the flow where the large eddies are generated and is at a minimum near the velocity maximum where fluid shear is low.

Spectral analysis of turbulent flow and suspended sediment transport over fixed dunes

Laboratory measurements of turbulent fluctuations in velocity and suspended sediment concentration were obtained synchronously over fixed two-dimensional dunes in a sediment-starved flow. Contour maps of turbulent flow parameters demonstrate that the flow separation cell and a perturbed shear layer are the main sources of turbulence production and that the distribution of suspended sediment is controlled by spatially dependent macro turbulent flow structures. Spectral analysis reveals that peak spectral energies generally occur at 1–2 Hz for the stream wise velocity component and 2–4 Hz for the cross-stream and vertical velocity components. Spectra show larger and better defined energy peaks near the shear layer. Peak spectral energies for suspended sediment concentration occur near 1 Hz throughout the flow. Squared coherency values for cospectral analysis of velocity and sediment concentration are insignificant. Integral timescales for velocity range from 0.20 s for the streamwise component to 0.06 s for the cross-stream and vertical components. Integral length scales for velocity range from 0.065 to 0.135 m for the streamwise component, which is comparable to flow depth, and from 0.020 to 0.030 m for the crossstream and vertical components, which is comparable to dune height. For suspended sediment concentration, integral timescales and length scales are similar to the streamwise velocity component.

Using simulated emergent vegetation to alter stream flow direction within a straight experimental channel

River restoration programs often use vegetation to enhance the biological functionality, recreational opportunities, and aesthetic beauty of degraded stream corridors. Yet, none has used vegetation for the purpose of inducing a straight channel to meander. A flume-based study was designed to alter the flow pattern within a straight, degraded stream corridor by using simulated emergent vegetation of varying density placed at key locations within the channel. Placement of vegetation zones was determined using an empirical relation for equilibrium meander wavelength based on the imposed flow rate. Surface flow velocities were quantified using particle image velocimetry. The study showed that (i) flow velocity can be markedly reduced within and near the vegetation zones, (ii) flow can be diverted toward the opposite bank, and (iii) vegetation density controlled the magnitude of these effects.

Fluid and sediment dynamics of upper stage plane beds

To understand more fully the fluid and sediment dynamics of upper stage plane beds, laboratory experiments were conducted using mobile and fixed beds where turbulent motions of fluid and sediment were measured using laser anemometry. Bed-elevation fluctuations on mobile upper stage plane beds reveal millimeter-high bed waves. Vertical profiles of flow velocity, mixing length, and eddy viscosity (diffusivity) are represented well by the law of the wall. For the mobile bed, von Karman/s κ ≈ 0.33 and equivalent sand roughness to mean bed-grain size varies from 9 to 17 because of the presence of bed load and low-relief bed waves. For fixed beds with no sediment transport, κ ≈ 0.41 and equivalent sand roughness is equal to the mean bedgrain size. The decrease in κ for mobile beds is related to the relative motion of grains and fluid. Mobile-bed turbulence intensities are greater than those for sediment-free fixed beds because of enhanced wake formation from the lee side of near-bed grains and low-relief bed waves. Sediment diffusivities es calculated in a similar way to fluid diffusivities e indicate that es≈e. Sediment diffusivities calculated using the equilibrium balance between upward diffusion and downward settling of sediment are similar to e in near-bed regions (y/d 0.3, suggesting that larger, more energetic turbulent eddies are responsible for sediment suspension higher in the flow.

Experiments on headcut growth and migration in concentrated flows typical of upland areas

Experiments were conducted to examine soil erosion by headcut development and migration in concentrated flows typical of upland areas. In a laboratory channel, packed sandy loam to sandy clay loam soil beds with preformed headcuts were subjected to simulated rain followed by overland flow. The rainfall produced a well-developed surface seal that minimized surface soil detachment. During overland flow, soil erosion occurred exclusively at the headcut, and after a short period of time, a steady state condition was reached where the headcut migrated at a constant rate, the scour hole morphology remained unchanged, and sediment yield remained constant. A fourfold increase in flow discharge resulted in larger scour holes, yet aspect ratio was conserved. A sediment bed was deposited downstream of the migrating headcut, and its slope depended weakly on flow discharge.

A depth-averaged two-dimensional model for flow, sediment transport, and bed topography in curved channels with riparian vegetation

[1] A depth-averaged two-dimensional numerical model has been developed to simulate flow, sediment transport, and bed topography in river channels with emergent and submerged rigid vegetation and large woody debris. The effect of helical flow in bends is considered by adopting an algebraic model for the dispersion terms in the depth-averaged two-dimensional momentum and suspended-sediment transport equations and by adjusting the bed load transport angle. The governing equations are discretized using the finite volume method on a nonstaggered, curvilinear grid. Model validity has been assessed using experimental data observed in both fixed- and movable-bed laboratory flumes and a natural channel with submerged and emergent rigid vegetation. In general, mean flow velocities, sediment transport rates, and changes in bed topography predicted by the model agree well with the experimental observations. For laboratory and field cases, root-mean-square relative errors for velocities were less than about 13% and 44%, respectively, and about 50% of errors for changes in bed topography were less than 14.5% and 8% of the flow depth, respectively.

Velocity and turbulence structure of density currents and internal solitary waves: potential sediment transport and the formation of wave ripples in deep water

Abstract Laser Doppler anemometry (LDA) was used to measure the instantaneous downstream and vertical velocities in a series of simple and reflected saline density currents in a lock-exchange flume tank. All the currents were turbulent and subcritical. Mean downstream fluid velocities were in excess of the head velocity by up to 30%, and instantaneous velocities were greater by up to 50%. Turbulence intensities were highest within the head, and generally greatest in the middle part of the current, but did not correspond with the level of highest mean velocities. The maximum Reynolds stress also occurred within the head; large negative values were associated with shear along the upper boundary of the current. Peaks of turbulence, Reynolds stress and shear velocity occurred in association with the arrival of reflections. In large-scale turbidity currents, such reflections would be capable of re-entraining and resuspending sediment deposited by the forward current. Some reflections take the form of solitary waves within a residual flow with a velocity vector in the opposite direction. In nature, these could produce symmetrical ripples in environments below storm-wave base.

Characteristics of actively eroding ephemeral gullies in an experimental channel.

The formation of ephemeral gullies can significantly increase soil losses from agricultural lands and severely impact farm productivity, yet few data exist on the physical characteristics of actively eroding gullies. The objectives of the current study were to examine the time variation of gully morphology and sediment load in response to a range of overland flow discharges and the impact of soil moisture content and soil density on erosion rates. Preformed ephemeral gullies were constructed at field scale in a large experimental channel using the same cohesive soil but at two different moisture contents and bulk densities. For the bed with relatively high soil moisture content and bulk density, clear-water flows caused low rates of erosion, detachment-limited flows prevailed, and bed degradation was uniform along the flume. For the bed with relatively low soil moisture content and bulk density, comparable clear-water flows caused high rates of erosion, both detachment-limited and transport-limited flows prevailed, and bed degradation was greater in the upstream reaches. In both experiments, erosion caused the gullies to incise, to significantly increase gully bottom width, and to steepen gully sidewalls. These results compare favorably with field studies of ephemeral gullies, and the hydraulics and sediment transport processes observed are similar to those reported for actively eroding rills.

Effect of slope on the growth and migration of headcuts in rills

Abstract Experiments were conducted to examine soil erosion by headcut development and upstream migration in rills typical of upland areas. Soil material, simulated rain, overland flow discharge, and initial headcut height were held constant in each experiment, but initial slope of the bed varied from 1% to 10%. Air-dried, crushed, and sieved sandy loam to sandy clay loam soil was incrementally packed into a laboratory channel 2 m long and 0.165 m wide to a depth of 0.25 m. Soil bulk density was 1425 kg m−3 in each experiment. A pre-formed headcut 25-mm high was constructed 1.5 m downstream of the entrance of the flume. Simulated rain, applied at 21 mm h−1 for 4 h, produced a well-developed surface seal that minimized the detachment of the surface soil. Following the rainfall, overland flow at a rate of 52 l min−1 was released onto the bed, soil erosion occurred at the pre-formed headcut overfall, and a scour hole developed, enlarged, and migrated upstream. The rate of headcut migration was constant within each experiment, but higher slopes of the bed generally resulted in lower rates of migration. At slopes on the bed of 2% and smaller, the overfall nappe at the headcut brinkpoint remained submerged, and a steady-state condition was achieved: sediment yield and geometry of the scour hole remained constant as the headcut migrated upstream. For slopes on the bed of 3% and greater, the overfall nappe became aerated, and as the headcut migrated upstream, the depth of scour increased. Higher slopes on the bed resulted in deeper scour holes. Mechanisms of soil erosion included the formation of tension cracks and seal removal at the headcut brinkpoint, soil washout along the aerated headcut face, and plunge-pool scour. The slope of the sediment deposit downstream of the migrating headcut was 2.2% for all experiments, and suggests that flow discharge and not the initial slope of the bed controlled transport capacity and downstream adjustment of the bed.