DISCONTINUOUS FLOW OF TURBID DENSITY CURRENTS Ⅱ. INTERNAL HYDRAULIC JUMP

Traveling and stationary internal hydraulic jumps in density currents with positive or negative entrainment coefficients were analyzed based on simple assumptions. An expression of internal hydraulic jumps with entrainment coefficients was derived. Experimental data, published in literature, of stationary internal hydraulic jumps in turbid, thermal and saline density currents including measured values of water entrainment were used to compare with theory. Comparison was also made of traveling internal hydraulic jumps between measured data and theory.     

DISCONTINUOUS FLOW OF TURBID DENSITY CURRENTS I. CHANNEL EXPANSION AND CONTRACTION

Laboratory experiments on turbid density currents were conducted to observe the flow features of these currents with abrupt contracted and expanded reaches. Experimental data were used to determine water entrainment coefficients for both channel expansion and contraction. Expressions for turbid density currents with water entrainment coefficients in abrupt contracted and expanded reaches were derived, and compared with experimental data.     

Turbulent shear stresses in hydraulic jumps,bores and decelerating surges

In an open channel, a sudden rise in water level induces a positive surge, or bore, that may develop as a hydraulic jump in translation. When the surge propagates against an adverse slope, it decelerates until it becomes a stationary hydraulic jump. Both hydraulic jumps and decelerating surges induce some intense turbulent mixing and have some major impact on the sediment transport in natural systems. Herein, a physical investigation was conducted in a relatively large rectangular channel. Hydraulic jumps and surges were generated by the rapid closure of a gate at the channel downstream end. The turbulent shear stresses were measured with high temporal and spatial resolution (200 Hz sampling rate) in the jump flow. A comparison between the stationary hydraulic jump, hydraulic jump in translation and decelerating surge measurements showed some marked differences in terms of turbulent mixing. The results highlighted some intense mixing beneath the jump front and roller for all configurations. The levels of turbulent stresses were one to two orders of magnitude larger than a critical threshold for sediment motion. The findings provide some insights into the hydraulic jump migration processes in mobile bed channels, and the complex transformation from a moving jump into a stationary jump. Copyright © 2010 John Wiley & Sons, Ltd.     

NUMERICAL SIMULATION OF TURBIDITY CURRENT IN RESERVOIR

I. INTRODUCTIONWhen a sediment--laden flow reaches the backwater zone of a reservoir, the suddenreduction of flow velocity causes sediment particles to settle toward the river bed. Undercertain circumstsnces, it will plunge and form a layer of sediment--water mixture flowingbeneath the water surface. This flowing layer is called the turbidity current. A turbiditycurrent is relatively stable and has important impacts on reservoir sedimentation.In the case of deep reservoirs, due to temper…     

MODELING UNSTEADY TURBIDITY CURRENTS IN A STEEP NARROW RESERVOIR

1.IN~DUCTIONTurbiditycurrentisoneclassofflowsnameddensitycurrentorgravitycurrent(therHunterRouse(Yih(1980)),whichdePictstheintmsionofheaVyfluidintoalighterone.Usually,thedensitydifferencebetWeentWonuidisrelativelysmallandmixingacrosstheimerfaceoccurs.ThedrivingforceofdensitycurrentsisnotdensitydifferenceitselfbutthedifferenceinspeCmcweights.Turbiditycurrentisnamedwhenthedensitydifferenceisespeciallycausedbysuspendedfinesedimentparticles.Sincesediment-ladenflowcaninteraCtwiththelowerbou…     

Dispersal and air entrainment in unconfined dilute pyroclastic density currents

Unconfined scaled laboratory experiments show that 3D structures control the behavior of dilute pyroclastic density currents (PDCs) during and after liftoff. Experiments comprise heated and ambient temperature 20 μm talc powder turbulently suspended in air to form density currents within an unobstructed 8.5?×?6?×?2.6-m chamber. Comparisons of Richardson, thermal Richardson, Froude, Stokes, and settling numbers and buoyant thermal to kinetic energy densities show good agreement between experimental currents and dilute PDCs. The experimental Reynolds numbers are lower than those of PDCs, but the experiments are fully turbulent; thus, the large-scale dynamics are similar between the two systems. High-frequency, simultaneous observation in three orthogonal planes shows that the currents behave very differently than previous 2D (i.e., confined) currents. Specifically, whereas ambient temperature currents show radial dispersal patterns, buoyancy reversal, and liftoff of heated currents focuses dispersal along narrow axes beneath the rising plumes. The aspect ratios, defined as the current length divided by a characteristic width, are typically 2.5–3.5 in heated currents and 1.5–2.5 in ambient temperature currents, reflecting differences in dispersal between the two types of currents. Mechanisms of air entrainment differ greatly between the two currents: entrainment occurs primarily behind the heads and through the upper margins of ambient temperature currents, but heated currents entrain air through their lateral margins. That lateral entrainment is much more efficient than the vertical entrainment, >0.5 compared to ～0.1, where entrainment is defined as the ratio of cross-stream to streamwise velocity. These experiments suggest that generation of coignimbrite plumes should focus PDCs along narrow transport axes, resulting in elongate rather than radial deposits.     

On the general character of coastal flow

It is well-known that some coastal currents such as the Florida Current have a recirculating region on their coastal side, while other currents such as the East Australian Current have no such region. Under the hypothesis that the form drag on a coastal current by an irregular coastal topography reduces the momentum transport of the current to a minimum for the given flow within the current, the hydraulic theory of coastal currents is used to explain this behaviour.     

An entrainment model for non‐uniform sediment

A model was developed for the prediction of the entrainment rate of non‐uniform sediment considering the movement of bedforms. Laboratory experiments were conducted to advance the formulations of the proposed model and to validate and estimate the model parameters. The model parameters were related to the hydraulic conditions of the flow and the properties of the sediment mixtures using dimensional analysis and gene expression programming. The model incorporated four parameters on its formulation, namely the Shields stress and critical Shields stress to describe the hydraulic and sediment conditions of the flow, the Kramer coefficient of uniformity to describe the grain size distribution of a particular sediment mixture, and the relative position of a particular grain size fraction to the geometric mean to describe the entrainment rate of that fraction within the sediment mixture. The proposed model provided satisfactorily predictions with a deviation less than 25% between the measured and predicted values for most of the fractions, which confirms the validity of the proposed approach and model in predicting of the entrainment rates of various fractions. The model predictions were also compared with other models available for the prediction of the entrainment rate of non‐uniform sediment. The model predictions were within the same order of magnitude of the other models’ predictions. Copyright © 2015 John Wiley & Sons, Ltd.     

Incipient motion conditions for single rock fragments in simulated rill flow

Laboratory experiments using simulated tilting rill channels with different bed roughnesses have been conducted to determine hydraulic conditions for incipient motion of single rock fragments lying on rill beds. These threshold conditions can be described by a modified Shields' entrainment parameter (θ_m) and relative depth (Z), adjusted for rill conditions. The negative relation found between θ_m and Z also fits steep (stream) channel data collected by other investigators. Plots of the classical Shields' entrainment parameter (θ) versus both relative depth and relative particle size also showed clear negative trends. Special care has been taken to avoid false relations that can arise between compound parameters. This occurs when the latter are not independent from each other because of their definitions.     

A three-dimensional model study of near-inertial motion: generation and influence of an along-shelf flow

A three-dimensional baroclinic model of the Balearic Sea region is used to examine the processes influencing the distribution of near-inertial currents and waves in the region. Motion is induced by a spatially uniform wind impulse. By using a uniform wind, Ekman pumping due to spatial variability in the wind is removed with the associated generation of internal waves. However, internal waves can still be produced where stratification intersects topography. The generation and propagation of these waves, together with the spatial distribution of wind-forced inertial oscillations, are examined in detail. Diagnostic calculations show that in the near-coastal region inertial oscillations are inhibited by the coastal boundary. Away from this boundary the magnitude of the inertial oscillations increases, with currents showing a 180° phase shift in the vertical. The inclusion of an along-shelf flow modifies the inertial currents due to non-linear interaction between vorticity in the flow and the inertial oscillations. Prognostic calculations show that besides inertial oscillations internal waves are generated. In a linear model the addition of an along-shelf flow produces a slight reduction in the energy at the near-inertial frequency due to enhanced viscosity associated with the flow and changes in density field. The inclusion of non-linear effects modifies the currents due to inertial oscillations in a manner similar to that found in the diagnostic model. A change in the effective inertial frequency also influences the propagation of the internal waves. However, this does not appear to be the main reason for the enhanced damping of inertial energy, which is due to the along-shelf advection of water of a different density into a region and increased viscosity and mixing associated with the along-shelf flow.Responsible Editor: Phil Dyke     

Two-dimensional density currents in a confined basin

We present new experimental results on the mechanisms through which steady two-dimensional density currents lead to the formation of a stratification in a closed basin. A motivation for this work is to test the underlying assumptions in a diffusive “filling box” model that describes the oceanic thermohaline circulation (Hughes, G.O. and Griffiths, R.W., A simple convective model of the global overturning circulation, including effects of entrainment into sinking regions, Ocean Modeling, 2005, submitted.). In particular, they hypothesized that a non-uniform upwelling velocity is due to weak along-slope entrainment in density currents associated with a large horizontal entrainment ratio of E _eq ?～?0.1. We experimentally measure the relationship between the along-slope entrainment ratio, E, of a density current to the horizontal entrainment ratio, E _eq, of an equivalent vertical plume. The along-slope entrainment ratios show the same quantitative decrease with slope as observed by Ellison and Turner (, 6, 423–448.), whereas the horizontal entrainment ratio E _eq appears to asymptote to a value of E eq?=?0.08 at low slopes. Using the measured values of E _eq we show that two-dimensional density currents drive circulations that are in good agreement with the two-dimensional filling box model of Baines and Turner (Baines, W.D. and Turner, J.S., Turbulent buoyant convection from a source in a confined region, J. Fluid. Mech., 1969, 37, 51–80.). We find that the vertical velocities of density fronts collapse onto their theoretical prediction that U =-2^?2/3 B ^1/3 E _eq ^2/3 (H/R) ζ, where U is the velocity, H the depth, B the buoyancy flux, R the basin width, E _eq the horizontal entrainment ratio and?ζ?= z/H the dimensionless depth. The density profiles are well fitted with?Δ?= 2^?1/3 B ^2/3 E _eq ^?2/3 H _-1 [ln(ζ )?+?τ ], where?τ?is the dimensionless time. Finally, we provide a simple example of a diffusive filling box model, where we show how the density stratification of the deep Caribbean waters (below 1850?m depth) can be described by a balance between a steady two-dimensional entraining density current and vertical diffusion in a triangular basin.     

Numerical modeling of lock-exchange gravity/turbidity currents by a high-order upwinding combined compact difference scheme

This study presents two-dimensional direct numerical simulations for sediment-laden current with higher density propagating forward through a lighter ambient water.The incompressible NavierStokes equations including the buoyancy force for the density difference between the light and heavy fluids are solved by a finite difference scheme based on a structured mesh.The concentration transport equations are used to explore such rich transport phenomena as gravity and turbidity currents.Within the framework of an Upwinding Combined Compact finite Difference(UCCD)scheme,rigorous determination of weighting coefficients underlies the modified equation analysis and the minimization of the numerical modified wavenumber.This sixth-order UCCD scheme is implemented in a four-point grid stencil to approximate advection and diffusion terms in the concentration transport equations and the first-order derivative terms in the Navier-Stokes equations,which can greatly enhance convective stability and increase dispersive accuracy at the same time.The initial discontinuous concentration field is smoothed by solving a newly proposed Heaviside function to prevent numerical instabilities and unreasonable concentration values.A two-step projection method is then applied to obtain the velocity field.The numerical algorithm shows a satisfying ability to capture the generation,development,and dissipation of the Kelvin-Helmholz instabilities and turbulent billows at the interface between the current and the ambient fluid.The simulation results also are compared with the data in published literatures and good agreements are found to prove that the present numerical model can well reproduce the propagation,particle deposition,and mixing processes of lock-exchange gravity and turbidity currents.     

山体遮挡对滇池风生流的影响初探

用二维风生流数值模型模拟滇池湖流运动。滇池在主导风向西南风作用下,假定湖面风场是均匀的,数值模拟的湖流流态与实测湖流结果相差很大。而考虑山体遮挡影响,根据实测湖流期间现有的风情资料,在湖面上构造一非均匀风场,数值模拟结果与实测值基本一致。山体遮挡对滇池风生流的影响是不容忽视的。建议进一步进行湖流和湖面风向、风速监测,并建立过山气流数学模型,深入研究山体遮挡对湖泊风生流的影响。     

Forward-modeling of co-evolution of turbidity currents,sediment transport,and cyclic steps in the Rio Muni Basin

Previous quantitative studies of field-scale cyclic steps are mostly based on analysis of field data. Such studies have shed light on the erosion/deposition patterns over these morphological features as well as the magnitudes of the turbidity current parameters back estimated using the measured geometry data. However, it remains unclear to what extent such back estimated hydraulic features and erosion/deposition patterns can be numerically reproduced by process-based numerical models. Here, a tw...     

Effect of seepage on initiation of cohesionless sediment transport

This paper presents theoretical analyses and experimental results of seepage effects, especially downward seepage, on the initiation of cohesionless sediment particles. The theoretical analysis examines how the additional seepage force acts to modify the critical shear stress for sediment entrainment. Laboratory experiments were conducted using medium sand with diameter of 0.9 mm with downward seepage to quantitatively show suction effects on sediment entrainment. The critical shear stresses with different suction rates were calculated using the experimental results. The measured data together with published results provide an overall view on seepage effects on the initiation of cohesionless sediment transport. Depending on whether seepage is in the form of injection or suction, it will either increase or decrease the critical shear stress. The result reveals that the ratio of drag force at the threshold condition with seepage to that without seepage is dependent on the ratio of the hydraulic gradient with seepage to its value at the quick condition.     

Flow and deposition of pyroclastic granular flows: A type example from the 1975 Ngauruhoe eruption,New Zealand

Small-volume pyroclastic density currents (PDCs) are generated frequently during explosive eruptions with little warning. Assessing their hazard requires a physical understanding of their transport and sedimentation processes which is best achieved by the testing of experimental and numerical models of geophysical mass flows against natural flows and/or deposits. To this end we report on one of the most detailed sedimentological studies ever carried out on a series of pristine small-volume PDC deposits from the 1975 eruption of Ngauruhoe volcano, whose emplacement were also witnessed during eruption. Using high-resolution GPS surveys, a series of lateral excavations across the deposits, and bulk sedimentological analysis we constrained the geomorphology, internal structure and texture of the deposits with respect to laterally varying modes of deposition.     

Seismic passive resistance with vertical seepage and surcharge

Present paper focuses on the computation of the seismic passive earth pressure acting on a vertical rigid retaining wall by a soil mass subjected to vertical steady-state seepage and a uniform surcharge load. Based on the basic assumptions of Coulomb's theory and a pseudo-static method of analysis, a general solution for the passive earth pressure containing two coefficients is presented. In the solution, many parameters, such as unit weight of saturated soil, soil effective internal friction angle, soil/wall friction angle, water/soil unit weight ratio, surcharge intensity coefficient, horizontal and vertical seismic acceleration coefficients, Poisson's ratio of soil mass, hydraulic gradient, and coefficients of pore water pressure, are considered. The effects of hydraulic gradient and seismic forces on passive earth pressure coefficient and passive earth pressure distribution are investigated. The results indicate that passive earth pressure increases with increasing hydraulic gradient for downward water flow case, but decreases for upward water flow case, and that the presence of seismic forces induces a reduction in passive earth pressure.     

Hydraulic conditions for rill incision under simulated rainfall: A laboratory experiment

A series of controlled laboratory experiments were conducted in order to obtain precise data on the hydraulic and sediment transport conditions during rill formation. Tests were carried out using a crusting-prone binary mixed soil in a 15 m long flume at an average slope of 0·087 under simulated rainfall. Rainfall intensities varied from 30–35 mm h^?1 and developed about 70 per cent of the kinetic energy of natural rainfall of similar intensity. Runoff and sediment discharge measured at the downstream weir were strongly influenced by rill forming processes. Essentially, rill incision reduced runoff discharge as a result of increased percolation in rill channels but increased sediment discharge. Secondary entrainment processes, such as bank collapse, also increased sediment discharge at the weir. Knickpoint bifurcation and colluvial deposition, however, decreased sediment discharge. Rills always developed through the formation of a knickpoint. The critical condition for knickpoint initiation was the development of supercritical flow and waves which mould and incise the bed. Prior smoothing of the soil surface by entrainment and redistribution of sediment facilitated supercritical flow. Statistical analysis showed that hydraulic and sediment transport conditions differed significantly in rilled and unrilled flows. The relationship between sediment discharge, rill erosion, and flow hydraulics was found to be nonlinear, conforming to a standard power function in the form y = ax^b. Rills were also associated with significantly increased sediment transport capacities. However, rill initiation was not clearly defined by any specific hydraulic threshold. Instead, rilled and unrilled flows were separated by zones of transition within which both types of flow occur.     

Predicting wetted width in any river at any discharge

Coefficients describing at‐a‐station power‐law relationships between discharge and width were calculated by applying multilevel models to field data collected during routine hydrological monitoring at 326 gauging stations across New Zealand. These hydraulic geometry coefficients were then estimated for each of these stations using standard stepwise multiple‐linear regression models. Analysis was carried out to quantify how the relationship between width and discharge changed in relation to several available explanatory variables. All coefficients describing the at‐a‐station hydraulic geometry were found to have statistically significant relationships with catchment area. Statistically significant relationships between each of the coefficients were also found with the addition of catchment climate as an explanatory variable. Further statistically significant relationships were found when station elevation and channel slope, as well as hydrological source of flow and landcover of the upstream catchment were added to the explanatory variables. The level of confidence that can be associated with estimates of width at ungauged sites, and sites with limited data availability, was then assessed by comparing model predictions with independent paired data on observed width and discharge from 197 sites. When compared against these independent data, model predictions of width were improved with the addition of predictor variables of the hydraulic geometry coefficients. The greatest improvements were made when climate was added to catchment area as predictor variables. Minor improvements were made when all available information was used to predict width at these independent sites. Although the analysis was purely empirical, results describing relationships between hydraulic geometry coefficients and catchment characteristics corresponded well with knowledge of the processes controlling at‐a‐station hydraulic geometry of river width. Copyright © 2010 John Wiley & Sons, Ltd.     

Limits to scale invariance in alluvial rivers

Assumptions about fluvial processes and process–form relations are made in general models and in many site-specific applications. Many standard assumptions about reach-scale flow resistance, bed-material entrainment thresholds and transport rates, and downstream hydraulic geometry involve one or other of two types of scale invariance: a parameter (e.g. critical Shields number) has the same value in all rivers, or doubling one variable causes a fixed proportional change in another variable in all circumstances (e.g. power-law hydraulic geometry). However, rivers vary greatly in size, gradient, and bed material, and many geomorphologists regard particular types of river as distinctive. This review examines the tension between universal scaling assumptions and perceived distinctions between different types of river. It identifies limits to scale invariance and departures from simple scaling, and illustrates them using large data sets spanning a wide range of conditions. Scaling considerations and data analysis support the commonly made distinction between coarse-bed and fine-bed reaches, whose different transport regimes can be traced to the different settling-velocity scalings for coarse and fine grains. They also help identify two end-member sub-types: steep shallow coarse-bed ‘torrents’ with distinctive flow-resistance scaling and increased entrainment threshold, and very large, low-gradient ‘mega rivers’ with predominantly suspended load, subdued secondary circulation, and extensive backwater conditions. © 2020 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd