Effects of hooked-collar on the local scour around a lenticular bridge pier

This paper presents the results of comprehensive laboratory experiments to investigate the effects of hooked-collar on the scour development around a vertical pier with a lenticular cross section. The flow around the pier was uniform, steady, and under the clear-water condition. The axial scour profiles for cases without and with a lenticular hooked-collar were measured and the effects of hooked-collar dimensions and elevation from the bed were examined. To compute the efficiency of hooked-colla...     

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%.     

Improved bridge pier collar for reducing scour

This study performed a bridge pier collar comparison for the purpose of reducing scour,while introducing a new collar design.The new collar,referred to as Collar Prototype Number 3,was designed based on an equilibrium scour hole and provides a method of controlling the horseshoe vortex.Numerical modelling was utilized to show the flow field and bed shear stress as a result of using Collar Prototype Number 3.A prototype model of Collar Prototype Number 3 and a flat plate collar were constructed f...     

Influence of large woody debris on sediment scour at bridge piers

Large woody debris(LWD) reduces the flow area,deviate the flow and increases the velocity in correspondence of the bridge pier,therefore increases the maximum scour hole depth and accelerates sediment removal.Logs and drifts accumulated on bridge piers are of different dimensions.According to logs characteristics and river morphology,drift accumulations can either extend downstream the bridge pier or they can accumulate totally upstream.This paper aims to analyze the effect of drift accumulation planimetry on bridge pier scour.The experimental investigation has been carried out at the PITLAB hydraulic centre of Civil Engineering Department,University of Pisa,Italy.Drift accumulation was characterized by different relative longitudinal lengths,flow area occlusions,length of longitudinal drift and downstream planimetrical positions relative to the pier center.The experimental investigation has been carried out in clear-water conditions.Several pier sizes,channel widths and sediment materials have been tested.Maximum scour hole in presence of drift accumulation have been compared to the maximum scour hole for an isolated pier.Finally,data were compared with previous literature findings,which highlight the effect of the downstream extension of drift accumulation on bridge pier scour.New relationships have been proposed to predict the effect of drift accumulation on bridge pier scour,both in terms of relative maximum scour and temporal scour evolution.     

EFFECT OF DUNE MIGRATION ON SCOUR AT BRIDGE PIERS

This paper examines the effect of dune migration on local scour around bridge piers. Experiments show that local scour depths fluctuate in response to the translation of dunes past the scour hole. The scour depths measured in a model study conducted in live-bed conditions contain both scour due to the pier and that due to dunes. The con' tribution from scour due to the dunes may form a significant fraction of the total scour depth measured in model investigations. Therefore, it is imperative to separate these two components of scour for analysis and comparison. The study proposes that an equilibrium or time-average scour depth normalized using pier diameter be used for analysis, and the contribution from dunes to the total scour depth be added independently. Dune size, in the absence of field or measured data, may be estimated using published predictive curves. Comparisons between computed and measured scour depths show a good correlation, and 90% of all the data tested fall within a scatter of 15%.     

Collar performance in bridge pier scour with debris accumulation

The mechanism of bridge pier scour becomes more complex in the presence of debris accumulation upstream of the pier. While using countermeasures may be effective in reducing scour, their efficacy could be undermined in such a situation. The current study investigates the effectiveness of using a collar in the presence of different types of floating debris accumulation in reducing scour around a cylindrical bridge pier with non-cohesive bed sediment. The experimental results reveal that using a c...     

Bridge pier scour mitigation under steady and unsteady flow conditions

Watercourse morphology is affected by local scouring when the flow interferes with anthropic structures. Controlling the scour hole size is of predominant importance to guarantee bridge safety as well as to limit the variations of river morphology. A combined countermeasure against bridge pier scour is proposed and tested in order to reduce the maximum scour depth and deviate it away from the bridge foundation. In the first part of the laboratory campaign, combination of two countermeasures (bed-sill and collar) was evaluated for a circular pier under clear-water and live-bed steady flow conditions. The proposed combined countermeasure exhibited an efficiency of about 64% in terms of scour depth reduction. Afterwards, it was tested in unsteady flow conditions, first for a circular pier, then in the case of a rectangular pier with round nose and tail, two circular in-line piers and two rectangular in-line piers, under a hydrograph with a peak flow velocity slightly above the threshold condition of sediment motion. Results showed that the combined countermeasure had an efficiency of about 63% for a single circular pier; however, higher efficiency (about 75%) was obtained in applications to rectangular pier and two in-line circular or rectangular piers.     

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.     

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.     

Simultaneous use of cable and collar to prevent local scouring around bridge pier

Various methods to control scour around bridge piers have been proposed.In the present study the application of cable or collar and a combination of cable and collar were examined experimentally,as countermeasures against local scouring at a smooth circular bridge pier,close to threshold flow conditions of initiation of uniform sediment motion.The results show that the simultaneous use of cable and collar has high efficiency in reducing the scour depth.The best configuration was found for a cable-pier diameter ratio of 0.15 and thread angle of 15°,in which the scour depth in upstream of the pier reduced to about 53%.In the case of a pier protected with cable and collar the scouring postponed more than pier protected with collar and the rate of scouring is less than in pier protected with collar.These advantages can reduce the risk of pier failure when the duration of flood is short. The results indicate that the scour reduction increases as the cable diameter increased and the thread angle decreased.     

ESTIMATION OF SCOUR HOLE PROPERTIES AROUND VERTICAL PILE USING ANNs

Correct estimation of the scour around vertical piles in the field exposed to oscillatory waves is very important for many offshore structures and coastal engineering projects. Conventional predictive formulas for the geometric properties of scour hole, however, are not able to provide sufficiently accurate results. Artificial Neural Networks (ANNs) are simplified mathematical representation of the human brain. Three-layer normal feed-forward ANN is a powerful tool for input-output mapping and has been widely used in civil engineering problems. In this article the ANNs approach is used to predict the geometric properties of the scour around vertical pile. Two different ANNs including multilayer perceptron (with four different learning rules) and radial basis functions neural networks are used for this purpose. The results show that a three-layer normal feed-forward multilayer perceptron with quick propagation (QP) learning rule can predict the scour hole properties successfully.     

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…     

高承台下自由桩长对双薄壁墩连续刚构桥地震响应的影响

为探究高承台下自由桩长对双薄壁墩连续刚构桥地震响应的影响,基于OpenSees程序建立了实桥有限元模型并进行弹塑性时程分析,通过对比不同自由桩长模型的时程曲线、峰值响应及滞回特性,分析了自由桩长对桥梁地震响应的影响.结果表明:自由桩长增加会减小桥梁刚度;地震作用下,随自由桩长增加,主梁、支座及自由桩顶的水平位移增大,且...     

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...     

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…     

Splitter plate as a flow-altering pier scour countermeasure

Results of an experimental study on the countermeasure of scour depth at circular piers are presented. Experiments were conducted for pier scour with and without a splitter plate under a steady, uniform clear-water flow condition. The results of pier scour without splitter plate were used as a reference. Different combinations of lengths and thicknesses of splitter plates were tested attaching each of them to a pier at the upstream vertical plane of symmetry. Two different median sediment sizes (d ₅₀ = 0.96 and 1.8 mm) were considered as bed sediment. The experimental results show that the scour depth consistently decreases with an increase in splitter plate length, while the scour depth remains independent of splitter plate thickness. In addition, temporal evolution of scour depth at piers with and without a splitter plate is observed. The best combination is found to be with a splitter plate thickness of b/5 and a length of 2b. Here, b denotes the pier diameter. An empirical formula for the estimation of equilibrium scour depth at piers with splitter plates is obtained from a multiple linear regression analysis of the experimental data. The flow fields for various combinations of circular piers with and without splitter plate including plain bed and equilibrium scour conditions were measured by using an acoustic Doppler velocimeter. The turbulent flow fields for various configurations are investigated by plotting the velocity vectors and the turbulent kinetic energy contours on vertical and horizontal planes. The splitter plate attached to the pier deflects the approach flow and thus weakens the strength of the downflow and the horseshoe vortex, being instrumental in reducing the equilibrium scour depth at piers. The proposed method of pier scour countermeasure is easy to install and cost effective as well.     

Geometry of obstacle marks at instream boulders—integration of laboratory investigations and field observations

Obstacle marks are instream bedforms, typically composed of an upstream frontal scour hole and a downstream sediment accumulation in the vicinity of an obstacle. Local scouring at infrastructure (e.g. bridge piers) is a well-studied phenomenon in hydraulic engineering, while less attention is given to the time-dependent evolution of frontal scour holes at instream boulders and their geometric relations (depth to width, and length ratio). Furthermore, a comparison between laboratory studies and field observations is rare. Therefore, the morphodynamic importance of such scour features to fluvial sediment transport and morphological change is largely unknown. In this study, obstacle marks at boulder-like obstructions were physically modelled in 30 unscaled process-focused flume experiments (runtime per experiment ≥ 5760 min) at a range of flows (subcritical, clear-water conditions, emergent and submerged water levels) and boundary conditions designed to represent the field setting (i.e. obstacle tilting, and limited thickness of the alluvial layer). Additionally, geometries of scour holes at 90 in-situ boulders (diameter ≥ 1 m) located in a 50-km segment of the Colorado River in Marble Canyon (AZ) were measured from a 1 m-resolution digital elevation model. Flume experiments reveal similar evolution of local scouring, irrespective of hydraulic conditions, controlled by the scour incision, whereas the thickness of the alluvial layer and obstacle tilting into the evolving frontal scour hole limit incision. Three temporal evolution phases—(1) rapid incision, (2) decreasing incision, and (3) scour widening—are identified based on statistical analysis of spatiotemporal bed elevation time series. A quantitative model is presented that mechanistically predicts enlargement in local scour length and width based on (1) scour depth, (2) the inclination of scour slopes, and (3) the planform area of the frontal scour hole bottom. The comparison of field observations and laboratory results demonstrates scale invariance of geometry, which implies similitude of processes and form rather than equifinality.     

Seismic response of rocking isolated railway bridge piers with sacrificial components

In this study, sacrificial components were incorporated into self-centering railway bridge piers to improve the lateral stiffness. The seismic response of this new detail was investigated. First, the method to compute the initial uplift moment of the self-centering pier is given. In addition, shaking table tests were conducted on a free-rocking pier without sacrificial components, which was used to validate a two-spring numerical model. Good agreement was obtained between the numerical results and experimental data. Furthermore, the validated model was employed to investigate the influence of sacrificial components on the seismic response of rocking piers. For this purpose, two models were developed, with and without sacrificial components. Nonlinear response history analysis was then performed on both models under three historical motions. The results showed that compared to the one without sacrificial components, the rocking pier with sacrificial components has comparable displacement at the top of the pier, and maximum uplift moment at high amplitude motion. Therefore, incorporating sacrificial components into the rocking pier can increase the lateral stiffness at service load and low amplitude frequent earthquakes but can produce comparable response at high seismic excitation. These results provide support for performance-based seismic design of self-centering rocking piers.

     

Effects of riverbed scour on seismic performance of high-rise pile cap foundation

To explore the seismic performance of a high-rise pile cap foundation with riverbed scour, a finite element model for foundations is introduced in the OpenSees finite element framework. In the model, a fiber element is used to simulate the pile shaft, a nonlinear p-y element is used to simulate the soil-pile interaction, and the p-factor method is used to reflect the group effects. A global and local scour model is proposed, in which two parameters, the scour depth of the same row of piles and the difference in the scour depth of the upstream pile and the downstream pile, are included to study the influence of scour on the foundation. Several elasto-plastic static pushover analyses are performed on this finite element model. The analysis results indicate that the seismic capacity (or supply) of the foundation is in the worst condition when the predicted deepest global scout depth is reached, and the capacity becomes larger when the local scour depth is below the predicted deepest global scout depth. Therefore, to evaluate the seismic capacity of a foundation, only the predicted deepest global scout depth should be considered. The method used in this paper can be also applied to foundations with other soil types.     

USE OF SURFACE GUIDE PANELS AS PIER SCOUR COUNTERMEASURES

1 INTRODUCTION Protection of bridge piers against local scour is of major importance to bridge maintenance. The safety of bridges is seriously threatened by the river flood during typhoons or thunderstorms. In Taiwan typhoon’s floods are often so strong in possession of high energy to transport a large amount of bed sediment in river. The average velocity in a flood river can go typically over 3 m/s even up to 5 m/s, several times of the regular flow speed. The peak discharge of flood i…