3D near-bed flow field measurements at low sediment transport rates

River morphodynamics and sediment transportMechanics of sediment transpor

Local scouring at bed sills under steady and unsteady conditions

Designers are often required to produce safe and economic structures in rivers with erodible beds, which may frequently induce scouring phenomena as they interfere with the natural stream. In CHAPTER 1, an extensive literature is illustrated on scouring by jets at high and low head structures, and predictive formulae are discussed that estimate the equilibrium scour depth of the scour hole, which are most obtained empirically from field and laboratory data. It is also discussed that different stages occur during local scour development. These stages basically include an initial rapid phase of severe scouring, followed by a stabilization phase approaching equilibrium conditions after a long time. CHAPTER 2 describes the theoretical basis to deal with long-term local scouring at bed sills under clear-water and steady-flow conditions. In gravel bed rivers, bed sills are used to limit bed degradation. Local scouring takes place downstream of each sill in addition to the general erosion pattern, and scour hole dimensions increase with the distance between sills. While overall aggradation can be estimated by using 1D morphological models, local scouring requires a more empirical approach. In fact most scouring phenomena are induced by turbulent jets that diffuse within the scour hole, by resulting in extremely complex flow fields. Many approaches are fully empirical, being based on curve fitting of experimental data that link scour depth to hydraulic quantities and sediment properties. In the most recent attempts, a semi-empirical approach has been followed, based on the identification of appropriate dimensionless groups using dimensional analysis before employing best-fitting procedures. The theoretical derivations proposed, which are based on the assumptions of the Buckingham's ­?-theorem, are discussed by showing some further insights on the nature and role of the dimensionless parameters that ensued form dimensional analysis. CHAPTER 3 presents the results of experimental tests carried out by the author in 2003 about the pattern of local scouring generated at the toe of bed sills. The aim of this study is to evaluate the effect of upstream sediment supply on the scour depth and shape. The experiments simulated conditions of a steady upstream sediment supply which had the same grain size composition as the sediment deposit placed on the bed of the flume. The geometric characteristics of three scour holes developed under conditions of steady-flow discharge and steady upstream sediment supply were measured during 48 different tests. It is shown that the imposed sediment transport does not require the introduction of new dimensionless parameters into existing scour depth and length prediction formulae. The effects of sediment feed are shown to be incorporated in the existing dimensionless parameters. The new data set is used to re-calibrate existing scour depth formula. The influence of jet erosion on scour geometry is also discussed. Most research efforts have focused on predicting scour depth, which may affect the structure at the free overfall, as opposed to volumetric scour dimensions and sediment yield contribution due to local scour, which may affect downstream morphology and water quality as well as the structure but are much more difficult to measure in an actively eroding plunge pool. In CHAPTER 4, shape and volume of equilibrium local scour holes at bed sills in high-gradient streams are investigated by analyzing steady-flow scour profiles at the equilibrium stage. Geometric properties of the scour profile are analytically discussed and verified on the basis of new data from experimental tests described in CHAPTER 3 and data from literature. A model is presented to organize data analysis into a conceptual frame, which is based on the formal relationships for scour depth and length ensued from dimensional analysis, and on the assumption that scours exhibit definite geometric properties. Results show evidence of a universal geometric affinity of the scour. Geometric similarity is also found to occur, but only in a limited range of physical conditions. Prediction formulae are proposed which link the eroded volume to the geometric, hydraulic and sediment properties. Evidence on the scaling nature of the scour hole shapes have important implications on the prediction of the eroded volume. In turn, a correct evaluation of the eroded volume is necessary for the optimization of the design of the inter-sill distance. The temporal development of clear-water local scour depth at bed sills in uniform gravel beds is considered in CHAPTER 5. A new experimental program has been carried out by the author, which started at the end of 2006 and was concluded in March 2007 at the Fluid Dynamics Laboratory of The University of Auckland (NZ). Experiments are presented on the development of scour holes under unsteady hydraulic conditions, with the triangular-shaped hydrographs tested being of different durations and different rates of flow variation. Preliminary observations are discussed on the behavior of scour development, which is compared with the scour evolution for the steady-state case. Based on the experimental results and a theoretical framework, a method is given for the definition and prediction of the scouring process under unsteady flows in terms of a dimensionless temporal parameter. A "flash flood" is defined as an event for which the scour doesn't attain its potential magnitude, i.e. the equilibrium value for the peak hydrograph flow rate. It is shown that this flood nature is dependent on both the characteristics of the flood event itself and the characteristics of the stream. A quantitative measure of what constitutes a flash flood is given in terms of the identified temporal parameter, which represents one of the main goal of this study

A New Theoretical Framework to Model Incipient Motion of Sediment Grains and Implications for the Use of Modern Experimental Techniques

none2noneAndrea Marion;Matteo TregnaghiMarion, Andrea; Tregnaghi, Matte

Affinity and similarity of local scour holes at bed sills

Excessive erosion of gravel bed rivers represents a hazard, which is often controlled by the construction of bed sills. Shape and volume of equilibrium local scour holes at bed sills in high-gradient streams were investigated by analyzing steady flow experimental data. Scour profiles were normalized by scaling the depths and the longitudinal distance from the upstream sill with the maximum dimensions of the scour hole. Results exhibited a universal geometrical affinity of the scour. Geometrical similarity was also found to occur, but only in a limited range of physical conditions. Evidence on the scaling nature of the scour hole shapes has important implications on the prediction of the eroded volume. In turn, a correct evaluation of the eroded volume is necessary for the optimization of the design of the intersill distance. Prediction formulae are proposed which link the eroded volume to the geometrical, hydraulic, and sediment properties

Scouring at bed sills as a response to flash floods

The temporal development of clear-water local scour depth at bed sills in uniform gravel beds is considered. Experiments are presented on the development of scour holes under unsteady hydraulic conditions, with the triangular-shaped hydrographs tested being of different durations and different rates of flow variation. Based on the experimental results and a theoretical framework, a method is given for the definition and prediction of the scouring process under unsteady flows in terms of a dimensionless temporal parameter. A “flash flood” is here defined as an event for which the scour doesn’t attain its potential magnitude, i.e., the equilibrium value for the peak hydrograph flow rate. This flood nature is dependent on both the characteristics of the flood event itself and the characteristics of the stream. A quantitative measure of what constitutes a flash flood is given in terms of the identified temporal parameter. Results show that the ratio between the final scour depth and the potential scour depth at a bed sill for a given hydrograph can be estimated as a function of the identified temporal parameter

Stochastic determination of entrainment risk in uniformly sized sediment beds at low transport stages: 2. Experiments

Fluvial sediment transport is caused by a complex interaction of interdependent grain and fluid processes many of which are stochastic in nature and cannot be adequately represented by deterministic equations. Random variable analysis has been used previously but limited data are available to describe the variability of grain resistance combined with particle arrangements, and thus validate such analysis. In this study low to medium bed load transport tests were carried out in a flume where sediment movement was monitored using a three-camera 3D PIV system. Simultaneous grain motion and flow velocity measurements were made on a plane located slightly above and parallel to the sediment bed. Detailed statistical velocity information was acquired to model the velocity distribution at the bed level. This was combined with the joint probabilistic distribution of particle exposures and grain resistance to motion, which were obtained from discrete particle modeling (DPM) simulations. DPM simulations were used to provide a stochastic mathematical description of the risk that a stationary particle is entrained by the flow. Predictions from the stochastic model equations replicated the observed pulsation in sediment transport. This demonstrates that it is possible to simulate sediment entrainment and transport at a high resolution by adequately modeling all the sub-processes. A number of flow patterns were identified that caused large fluctuations of the entrainment rate. These all exhibit high velocity flow structures, but they selectively cause the dislodgement of individual particles located at different positions. This selective behavior follows from the variability of the interaction between the near-bed flow and the particles having different exposure

Stochastic determination of entrainment risk in uniformly sized sediment beds at low transport stages: 1. Theory

The entrainment of sediments in rivers exhibits an intermittent behavior. Incipient motion should therefore be described as a random process requiring a stochastic predictive approach. The effect of near-bed turbulence on grain entrainment and the variation in bed particle stability due to local surface heterogeneity are included into a probabilistic framework. The intuitive evidence that hydrodynamic forces acting on the sediment bed and the resistance to motion of the bed particles are two mutually dependent aspects of a unique process is modeled by introducing a conditional independence hypothesis. Based on this concept, new insights into the stochastic aspects of incipient motion are obtained. For low ratios of the boundary shear stress to the critical shear stress, by including the mutual dependence of different processes the new theoretical development predicts up to 50% larger probabilities of grain removal from the bed surface compared to Grass\u2019 original formulation. This follows from the entrainment risk being not only dependent on the distributions of fluid forces and grain resistance, but also on the correlation that these distributions exhibit in relation to the geometrical configuration of the sediment bed. This complex interaction is neglected in existing probabilistic models of sediment transport. It is then demonstrated that such additional contribution explains better the influence of both flow turbulence and particle arrangement on key features of the overall grain entrainment process

Modelling time varying scouring at bed sills

In this paper a modelling approach is presented to predict local scour under time varying flow conditions. The approach is validated using experimental data of unsteady scour at bed sills. The model is based on a number of hypotheses concerning the characteristics of the flow hydrograph, the temporal evolution of the scour and the geometry of the scour hole. A key assumption is that, at any time, the scour depth evolves at the same rate as in an equivalent steady flow. The assumption is supported by existing evidence of geometrical affinity and similarity of scour holes formed under different steady hydraulic conditions. Experimental data are presented that show the scour hole development downstream of bed sills due to flood hydrographs follow a predictable pattern. Numerical simulations are performed with the same input parameters used in the experimental tests but with no post-simulation calibration. Comparison between the experimental and model results indicates good correspondence, especially in the rising limb of the flow hydrograph. This suggests that the underlying assumptions used in the modelling approach are appropriate. In principle, the approach is general and can be applied to a wide range of environments (e.g. bed sills, step-pool systems) in which scouring at rapid bed elevation changes caused by time varying flows occurs, provided appropriate scaling information is available, and the scour response to steady flow conditions can be estimated

Statistical Description on the Role of Turbulence and Grain Interference on Particle Entrainment from Gravel Beds

A complete understanding of the role of grain-scale particle-flow interaction in sediment entrainment and transport has still not been achieved in spite of recent technological advancement in measurement capabilities. In this study, the initial motion of natural sediment particles in a gravel deposit was detected and combined with simultaneous local measurements of the velocities on a horizontal plane located above the bed surface using a three-component stereoscopic PIV. A series of experimental tests with increasing low values of boundary shear stress were conducted. The acquisition system allowed coupling between streamwise and vertical near-bed velocity and the entrainment of more than 900 individual grains. Initial analysis agreed with previous observations on the predominance of sweeps (Quadrant IV), and to a lesser extent, of outward interactions (Quadrant I) in entraining gravel particles. However, the latter were found to move sediments just as efficiently as sweeps impacting on particles that had long periods of rest and so were exhibiting higher levels of stability. This behavior suggests that sweep-induced lift based on Bernoulli’s principle does not entirely explain the generation of vertical forces on highly stable bed particles. Closer inspection of the data revealed that many entrainments were correlated to occasions when stable bed grains interacted with grains travelling in their close vicinity. Approximately 30% of the entrained population was observed to initiate motion in this type of situation. For this subsample of entrainment events, outward interactions were found to be comparatively more effective than for the noninterference case, whereas the relative contribution of sweeps exhibited an opposite trend