Bed morphology and grain size characteristics around a spur dyke

This paper presents an experimental study on the local bed morphology and grain size characteristics around an impermeable spur dyke.A series of experiments are conducted with different types of sedime...     

Experimental study on scour and flow field in a scour hole around a T-shape spur dike in a 90° bend

In this paper results of experimental study on scour and flow field around a T-shape spur dike in a 90 degree channel bend are presented. Experiments were conducted in a laboratory channel to measure the variations of bed topography under a clear water condition. The three dimensional flow field was measured by an Acoustic Doppler Velocimeter in a scour hole due to a T-shape spur dike. It was found that by increasing the Froude number and length of spur dike the amount of scour increases. Increasing the wing length of spur dike decreases the scour. The amount of scour at the upstream of spur dike is much more as compare to that at the downstream of spur dike. By increasing the wing length of spur dike the zone of flow separation expands. As a result the sizes of vortex increase. Details of flow field are also addressed in the paper.     

Three-dimensional modeling of pollutants transportation in the flow field around a spur dyke

Pollutants are carried by sediment-laden flow dissolved in water or adsorbed on sediment particles. A transient three-dimensional model based on a compressible VOF(volume of fluid) method was developed to simulate the transport of dissolved and particulate pollutants.VOF is a numerical technique for acquiring and tracking the free surface of water flow.Local scouring,deposition and re-suspension were simulated and the processes of adsorption and desorption of pollutants on suspended sediment were analyzed.A series of experiments and numerical simulations were performed to study the transportation and dispersion of pollutants in the flow around a non-submerged spur dyke in a straight flume of rectangular shape.The simulation results agreed well with the experimental results.A certain volume of pollutants solution was released into the flow at upstream of the spur dyke.The concentration reduced with time.The concentration reduction was slower in the circumfluence zone than in the main flow.The ratio of adsorption to desorption coefficients was different for pollutants on suspended sediment with different diameter.The peak concentration of dissolved pollutants increased with the ratio of the adsorption to the desorption coefficients.The angle of the spur dyke affected the peak concentration around the dyke.The effect of the angle on dissolved pollutants decreased with the ratio of adsorption to desorption coefficients.The adsorption and desorption coefficients,as well as the saturated adsorption capacity had no effect on the concentration of particulate pollutants.     

Flow near a model spur dike with a fixed scoured bed

Three-dimensional flow velocities were measured using an acoustic Doppler velocimeter at a closely spaced grid over a fixed scoured bed with a submerged spur dike. Three-dimensional flow velocities were measured at 3,484 positions around the trapezoidal shaped submerged model spur dike. General velocity distributions and detailed near field flow structures were revealed by the measurement. Clear differences were revealed between flow over fixed flat and scoured beds. Strong lateral flows were the dominant cause of the observed local scour. Shear stresses were higher for the scoured bed than in the flat bed case. Decreasing rates of scour as the scour hole developed were attributed to increases in critical shear stress in the scour holes caused by the increase in the length and magnitude of adverse slopes associated with the two main scour holes.     

Scour and flow field around a spur dike in a 90° bend

Spur dike is an important element in river training that creates rapid variations in flow field, sediment transport and bed topography. The mechanism of flow and sediment transport in a channel bend is very complex, especially when a spur dike is constructed in a bend. Most of previous investigations on flow behavior and scour around spur dike were carried out in straight channels. In this paper results of experiments on flow field and scour around a spur dike in a 90 degree channel bend are presented. Sand with uniform grain size was used as the bed material. Experiments were conducted for different locations and different lengths of spur dikes at the bend with different values of discharge. The three dimensional flow fields around a spur dike were investigated. The maximum depth of scour was correlated to the Froude numbers, lengths and the locations of spur dike in the bend.     

Scour and flow field around a spur dike in a 90° bend

Spur dike is an important element in fiver training that creates rapid variations in flow field, sediment transport and bed topography. The mechanism of flow and sediment transport in a channel bend is very complex, especially when a spur dike is constructed in a bend. Most of previous investigations on flow behavior and scour around spur dike were carried out in straight channels. In this paper results of experiments on flow field and scour around a spur dike in a 90 degree channel bend are presented, Sand with uniform grain size was used as the bed material, Experiments were conducted for different locations and different lengths of spur dikes at the bend with different values of discharge, The three dimensional flow fields around a spur dike were investigated, The maximum depth of scour was correlated to the Froude numbers, lengths and the locations of spur dike in the bend.     

Clear-water scour around semi-elliptical abutments with armored beds

Experimental study has been carried out under a clear-water scour condition to explore the local scour around semi-elliptical model bridge abutments with armor-layer bed, compared with the local scour process around semi-circular abutment. Two types of model bridge abutments, namely semi-elliptical and semi-circular abutments have been used in this experimental study. The model abutments had a ratio of streamwise length of abutment to the length of abutment transverse to the flow of 2 for semi-circular abutments and 3 for semi-elliptical abutments. In total, 50 Experiments have been designed and conducted under different flow conditions such as bed shear velocities, flow depth, and dimensions of bridge abutment model, as well as grain size of the bed material. Based on these experiments, the scour process around bridge abutments has been assessed. The dependence of the equilibrium scour depth of the scour hole on hydraulic variables has been studied. Empirical equation describing the equilibrium scour depth of the scour hole around bridge abutments has been developed.     

Numerical model of local scour considering the unsteady sediment inflow and sediment sorting: Application to scour upstream of slit weir

The current study proposes a novel framework for the numerical model for estimating the temporal scour considering unsteady sediment inflow and the sediment sorting process. The framework was applied to local scour upstream of a slit weir. The scour model is based on an ordinary nonlinear differential equation derived from sediment continuity and scour rate equations. A one-dimensional(1-D)Boussinesq-type model coupled with nonequilibrium sediment transport was incorporated in the scour model to...     

IMPACTS OF DIMENSION AND SLOPE OF SUBMERGED SPUR DIKES ON LOCAL SCOUR PROCESSES - AN EXPERIMENTAL STUDY

1 INTRODUCTION In alluvial streams bed scour often occurs if the sediment load is less than the transport capacity of the flow. Two types of scour are identified, namely local scour and channel bed scour. Channel bed scour can be further classified accord…     

Turbulent burst around experimental spur dike

Three-dimensional turbulent flow field was measured around an experimental spur dike by using a micro-acoustic Doppler velocimeter.Time fractions of turbulent burst events including outward interaction,ejection,inward interaction,and sweep were analyzed in each quadrant at the neighbor of the dike before and after the formation of scour hole.Over 80%of burst events near the bed have lower-order of magnitudes for both flat and scoured bed surfaces.Ejections and sweeps are prevalent before the local scour was initiated,and then outward interactions are dominant after the scour hole was formed.Conditional Reynolds stresses and high-order moments of turbulent velocities were analyzed along the thalweg.The magnitudes of u’-w’ pair were much larger than those of v’-vv’ pair in the scour zone.Among four burst events,ejections and sweeps are the higher order events contributing to the Reynolds stresses.Since sediment entrainment and transport are closely associated with turbulent bursts near bed,the development of scour hole greatly depends on the higher order event near the bed.     

Experimental assessment and prediction of temporal scour depth around a spur dike

Spur dikes are river training structures that have been extensively used worldwide for towards enhancing flood control and the stability of embankments and riverbanks.However,scour around spur dikes can be a major problem affecting their stability and hydraulic performance.The precise computation of temporal scour depth at spur dikes is very important for the design of economical and safe spur dikes.This study focuses on experimentally assessing the temporal variation of scour depth around a vertical wall spur dike and identifying the parameters,which mostly influence spur dike performance for a channel bed surface comprised of sand-gravel mixtures.In the current study,the authors did physical experiments in a flume based study to obtain new data,aimed at deriving a new predictive model for spur dike scour and comparing its performance to others found in the literature.It was found that the dimensionless temporal scour depth variation increases with an increase in(i)the threshold velocity ratio,(ii)the densimetric Froude number of the bed surface sediment mixture,(iii)the flow shallowness(defined as the ratio of the approach flow depth,y,to the spur dike’s transverse length,l),and(iv)the flow depth-particle size ratio.It is also concluded that the temporal scour depth variation in the sediment mixture is influenced by the non-uniformity of sediment and decreases with an increase in the non-uniformity of the sediment mixture.A new mathematical model is derived for the estimation of temporal scour depths in sand-gravel sediment mixtures.The proposed equation has been calibrated and validated with the experimental data,demonstrating a good predictive capacity for the estimation of temporal scour depth evolution.     

Experimental study on local scour around single oblong pier fitted with a collar in a 180 degree flume bend

In this work, investigation on the development of local scour around an oblong pier in a 180 degree flume bend is presented. Scour hole can cause failure of the bridge especially during the river floods. In this study, the use of oblong collars for reducing the effects of local scour at a bridge pier is presented together with the time aspect of the scour development. Tests were conducted using one oblong pier in positions of 60degree under one flow conditions. The study was conducted using a physical hydraulic model operated under clear-water conditions in cohesionless bed material. In this study, the time development of the local scour around the oblong pier fitted with and without collar plates was studied. Investigated was the effect of size and elevation collar on the time development of scour and its efficacy at preventing scour at a bridge pier. The time development of the scour hole around the model pier with and without a collar installed was compared with similar studies on bridge piers. The results of the model study indicated that the maximum depth of scour is highly dependent on the experimental duration. It was observed that, as the minimum depth of scour occurs for the square collar at width of 3B placed at elevation of 0.1B below the bed and the size of a collar plate increases, the scour decreases. Measuring depth of scouring based on experimental observation, an empirical relation is developed with regression coefficient 95%.     

Numerical modeling of local scour of non-uniform graded sediment for two arrangements of pile groups

A three-dimensional(3D) non-hydrostatic numerical model is established to investigate local scour around four aligned circular piles in uniform and non-uniform sediment mixtures and to provide information for improving scour countermeasures design. In the current study, unsteady Reynolds averaged Navier-Stokes(URANS) equations along with a Re-normalization Group(RNG) k-ε model were applied to simulate the flow field. A non-uniform sediment transport model was applied to estimate the bedload tran...     

Characterization of horseshoe vortex in a developing scour hole at a cylindrical bridge pier

Since local scour at bridge piers in rivers and estuaries is a major cause of bridge failure, estimation of the maximum local scour depth is of great importance to hydraulic and coastal engineers. Although numerous studies that focus on scour-depth prediction have been done and published, understanding of the flow and turbulence characteristics of the horseshoe vortex that drives the scour mechanism in a developing scour hole still is immature. This study aims to quantify the detailed turbulent flow field in a developing clear-water scour hole at a circular pier using Particle Image Velocimetry (PIV). The distributions of velocity fields, turbulence intensities, and Reynolds shear stresses of the horseshoe vortex that form in front of the pier at different scour stages (t=0, 0.5, 1, 12, 24, and 48 h) are presented in this paper. During scour development, the horseshoe vortex system was found to evolve from one initially small vortex to three vortices. The strength and size of the main vortex are found to increase with increasing scour depth. The regions of both the maximum turbulence intensity and Reynolds shear stress are found to form at a location upstream of the main vortex, where the large turbulent eddies have the highest possibility of occurrence. Results from this study not only provide new insight into the complex flow-sediment interaction at bridge piers, but also provide valuable experimental databases for advanced numerical simulations.     

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. Copyright © 2011 John Wiley & Sons, Ltd.     

AN INTEGRATED APPROACH FOR SIMULATING AND PREDICTING SCOUR AROUND ABUTMENTS

1 INTRODUCTION The local scour around bridge piers and abutments has always had heightened interest to hydraulic engineers. An accurate and efficient method to estimate the local scour is greatly desired. However, the behavior of flows passing through bri…     

Experimental investigation on scour topography around high-rise structure foundations

The current study aims to investigate the characteristics of scour topography around High-Rise Structure Foundations(HRSFs)via physical modeling tests.Clear-water scour tests with a uniform non-cohesive bed are modeled under the action of unidirectional steady flows.Time variations of the erosion and deposition topography are measured.The results show that deposition downstream of the first dune behind the HRSF is not located on the centerline of the wake.The deposition pattern indicates that a long steady wake region exists behind the permeable foundation.The scour depth around an HRSF is much less than that around a monopile because of the structural permeability,which gives rise to the bleed flow and a weakened downflow and horseshoe vortex.Additionally,the asymmetry of the HRSF affects the scour rate but not the final equilibrium scour depth.The average scour slope decreases along the direction of the flow.On the contrary,the scour radial distance increases along the direction of the flow,with the average value changing from 1.36De to 2.35De(where De is the equivalent diameter of the foundation).Furthermore,the scour hole around the HRSF is serrated rather than smooth owing to the presence of multiple piles.Empirical formulae are suggested for estimating the evolution of scour depth and volume.These laboratory experiments provide reference information for relevant numerical modeling studies and can be applied to guide engineering designs in an ocean area.     

A comparison between artificial neural network algorithms and empirical equations applied to submerged weir scour evolution prediction

Estimating the time evolution of a local scour hole downstream of submerged weirs can help determine the maximum scour depth and length and is essential to designing submerged weir foundations.In the current study, artificial neural networks with a backpropagation learning algorithm were used to estimate the temporal variation of scour profiles downstream of submerged weirs under clear water conditions. Physical factors, such as the flow condition, weir size, and sediment characteristics, are ge...     

SCOUR HOLE CHARACTERISTICS BELOW FREE OVERFALL SPILLWAY

1 INTRODUCTION Flow through hydraulic structures often issues in the form of jets. The jet velocities are usually high enough to produce sizable, even dangerous scour hole. The extent of the resulting scour depends on the nature of bed material and flow characteristics. The erosion process is quite complex and depends upon the interaction of hydraulic and morphological factors. Scouring may lead to: endangering the stability of the structure by structural failure or increased seepage, end…     

NUMERICAL SIMULATION OF HEAD-CUT WITH A TWO-LAYERED BED

1INTRODUCTION The rate of gully erosion is dominated by the upstream migration of existing nick-points called headcut.Due to the shape of the headcut,the flow from the upstream channel impinges into the pool of the scour hole and forms a complex three-dimensional flow structure.The turbulent flow deepens the scour hole,transports the eroded material downstream,undercuts the headcut wall and creates gravitational slumping of the gully head material.In reality,the occurrence of a head cut i…