Temporal Evolution of Clear-Water Pier and Abutment Scour

Scour related to bridge hydraulics received much attention in the past decade, including its relation to flood hydrology and hydraulic processes in addition to steady flow. This paper presents new research on bridge pier and abutment scour based on a large data set collected at ETH Zurich, Switzerland. In total six different sediments were tested, of which three were uniform. Also a large variety of scour elements were considered, from 1 to 60% of the channel width, and flow depths ranging from 1 to about 40% of the channel width. Using similarity arguments and the analogy to flow resistance, an equation for temporal scour evolution is proposed and verified with the available literature data. The agreement of the present scour equation with both the VAW data and the literature data were considered sufficient in terms of river engineering accuracy, provided limitations relating to hydraulic, granulometric, and geometrical parameters are satisfied. These limitations are discussed and refer particularly to effects of viscosity, which were excluded in the present scour equation.     

Design Method of Time-Dependent Local Scour at Circular Bridge Pier

A reliable prediction of local scour depth related to hydrological characteristics such as peak discharge, and time corresponding to the equilibrium scour depth is essential for the efficient design of bridge pier foundation. In this paper, a design method to predict the local scour depth with time is proposed. An experimental program was carried out using a cylindrical pier placed in uniform beds under clear-water flows. The pier scour depth was calculated on the basis of a sediment transport equation. Equilibrium local scour depth is reached when the bed-shear stress tends to critical bed-shear stress in the scour hole. Hence, changes to bed-shear stress at the circular bridge pier should be incorporated in the sediment transport theory. The proposed method follows experimental data of various sources.     

Reduction of Local Scour in the Vicinity of Bridge Pier Groups Using Collars and Riprap

The present study examines the use of independent and continuous pier collars in combination with riprap for reducing local scour around bridge pier groups. The efficiency of collars was studied through experiments. The data from the experiments were compared with data from earlier studies on single piers with collars and bridge pier groups without collars. The data showed that in the case of two piers in line, combination of continuous collars and riprap results in the most significant scour reduction of about 50 and 60% for the front and rear piers, respectively. In other cases for two piers in line, independent collars showed better efficiency than a continuous collar around both piers. It was also shown that efficiency of collars is more on a rectangular pier aligned with the flow than two piers in line. Experiments however, indicated that collars are not so effective in reduction of scouring around two transverse piers.     

New Experimental Method to Find Equilibrium Scour at Bridge Piers

A new methodology for the experimental analysis of the equilibrium scour depth at bridge piers is introduced and validated for clear-water conditions. The proposed experimental methodology determines the flow conditions for a given equilibrium scour instead of determining the equilibrium scour for given flow conditions, which is the usual practice. The basic hypothesis is that the shape of the scour hole is essentially related to the scour depth and sediment properties, but not to flow conditions. This hypothesis is checked experimentally. The proposed methodology may drastically reduce the time period required for experiments (from weeks to hours), and avoids the uncertainties due to the equilibrium scour being usually achieved asymptotically. Some preliminary results of the equilibrium scour obtained with the proposed methodology are compared to the expressions given in the literature, showing fair agreement.     

Estimating Equilibrium Scour Depth at Cylindrical Piers in Experimental Studies

Using data from six very long experiments on local scour at single cylindrical piers, some of the existing equilibrium criteria found in the literature are assessed. It is found that common criteria, which consider scour depth increments in 24?h or the observation of horizontal plateaux in records of scour depth time evolution, can incur important errors on the equilibrium scour depth. Using some of the expressions for time evolution found in the literature to fit the experimental data and infer equilibrium scour depth through extrapolation to infinite time, it is found that the equilibrium scour depth cannot be specified, in general, for experiments shorter than one to two weeks.     

Effect of Sediment Size Scaling on Physical Modeling of Bridge Pier Scour

Local pier scour experiments were performed in the laboratory to investigate the effect of relative sediment size on pier scour depth using three uniform sediment sizes and three bridge pier designs at different geometric model scales. When the data from a large number of experimental and field investigations are filtered according to a Froude number criterion, the effect of relative sediment size on dimensionless pier scour depth is brought into focus. The choice of sediment size in the laboratory model distorts the value of the ratio of pier width to sediment size in comparison with the prototype which in turn causes larger values of scour depth in the laboratory than in the field. This model distortion due to sediment size is shown to be related to the scaling of the large-scale unsteadiness of the horseshoe vortex by studying the relevant time scales of its coherent structure upstream of a bridge pier using acoustic Doppler velocimeter measurements. Observations of sediment movement, probability distributions of velocity components, and phase-averaging of velocity measured upstream of a bridge pier reveal properties of coherent motions that are discussed in terms of their contribution to the relationship between dimensionless pier scour depth and the ratio of pier width to sediment size over a large range of physical scales.     

Clear-Water Scour at Piers in Sand Beds with an Armor Layer of Gravels

Clear-water scour at circular and square piers, embedded in a sand bed overlain by a thin armor layer of gravels, was experimentally studied. Depending on the pier width, flow depth, armor gravel, and bed-sand sizes, three cases of scour holes at piers in armored beds were recognized. A comparison of the experimental data shows that the scour depth at a pier with an armor layer under limiting stability of the surface particles is greater than that without an armor layer for the same bed sediments, if the secondary armoring formed within the scour hole is scattered. The equations of maximum equilibrium scour depths at piers in armored beds for these cases are proposed. On the other hand, the scour depth with an armor layer is less than that without an armor layer for the same bed sediments, when the scour hole is shielded by the compact secondary armor layer.     

Large Scale Clear-Water Local Pier Scour Experiments

Local clear-water scour tests were performed with three different diameter circular piles (0.114, 0.305, and 0.914?m), three different uniform cohesionless sediment diameters (0.22, 0.80, and 2.90?mm) and a range of water depths and flow velocities. The tests were performed in the 6.1?m wide, 6.4?m deep, and 38.4?m long flume at the United States Geological Survey Conte Research Center in Turners Falls, Mass. These tests extend local scour data obtained in controlled experiments to prototype size piles and ratios of pile diameter to sediment diameter to 4,155. Supply water for this flow through flume was supplied by a hydroelectric power plant reservoir and the concentration of suspended fine sediment (wash load) could not be controlled. Equilibrium scour depths were found to depend on the wash load concentration.     

Evolution of Scour Depth at Circular Bridge Piers

Experiments of bridge pier scour are carried out under steady and unsteady clear-water scour conditions with uniform and nonuniform sediments. Around the pier nose, the sediment size variation of surface bed materials is investigated, and a regressed formula is obtained for estimating the mixing layer thickness in terms of median sediment size and geometric standard deviation of grain size distribution. A method based on the mixing layer concept is developed for calculating the equilibrium scour depth in nonuniform sediment. Based on the experimental data of scour rate, a model simulating the scour-depth evolution under steady flow in nonuniform sediment is presented. By analyzing experimental data, a scheme is proposed for computing the scour-depth evolution under unsteady flow.     

Temporal Scales for Live-Bed Scour at Abutments

Live-bed scour at a vertical-wall abutment is experimentally investigated with specific attention paid to the conceptual issues concerning the temporal development of local scour phenomenon. First explored are the time scales for the initial rising phase of the time variation of scour depth. An appropriate identification of such scales and of their normalizing parameters makes it possible to recognize a quantitative dependency of nondimensional time scales on flow intensity. Second, the time scales for the subsequent fluctuations around a mean equilibrium value are considered. Experimental results indicate that the quasiperiodical fluctuations of scour depths do not always correspond to those of bed forms. A conceptual model is outlined to explain this aspect.     

Scour at Submerged Cylindrical Obstacles under Steady Flow

Results of an experimental study on clear-water scour at submerged cylindrical obstacles (circular cylinders) in uniform bed sediments under steady flow are presented. The scour depths at submerged circular cylinders are compared with the scour depths at corresponding unsubmerged cylinders (extended above the free surface of flow) of the same diameters under similar flow and bed sediment conditions. The scour depth decreases with an increase in submergence ratio. A submergence factor is introduced to determine the scour depth at a submerged cylinder from the information of the scour depth at an unsubmerged cylinder of the same diameter. In addition, the flow fields along the upstream vertical plane of symmetry of unsubmerged and submerged cylinders are presented through vector plots, which reveal that the dimension and strength of the horseshoe vortex decreases with an increase in submergence ratio. The horseshoe vortex circulations, which decrease with an increase in submergence ratio, are computed from the vorticity contours by using the Stokes theorem.     

Influence of Sediment Gradation on Scour Downstream of Block Ramps

The scour process downstream of block ramps is an important research topic of value to engineering practice. The object of the present experimental study is to examine the scour mechanism in order to predict the main geometrical parameters of the scour hole downstream of block ramps. The investigations have been conducted using two physical models located in the hydraulic laboratory of the Department of Civil Engineering in Pisa, Italy. Sediments with different values of the nonuniformity parameter were used as channel bed material. Moreover, different ramp slopes, ranging from 1/4 to 1/12, were tested. Equations and graphs demonstrate that the results can be interpreted by means of simple relationships in the case in which a ridge is present downstream of the scour hole.     

Clear-Water Scour below Underwater Pipelines under Steady Flow

Experiments on clear-water scour below underwater pipelines (initially laid on the sediment bed) in uniform and nonuniform sediments under steady flow were conducted. Equilibrium scour profiles were modeled by a cubic polynomial. The experimental results are examined to describe the influence of various parameters on equilibrium scour depth. The equilibrium scour depth ds increases with increase in approach flow depth h for shallow flow depths, becoming independent of higher flow depths when h/b>5, where b=pipe diameter. However, the curves of scour depth versus sediment size d and Froude number Fb have a maximum value of ds/b = 1.65 at b/d = 27 and Fb = 0.6. The influence of sediment gradation on scour depth is prominent for nonuniform sediments, which reduce scour depth to a large extent due to the formation of armor layer within the scour hole. The influence of different shaped cross sections of pipes on the scour depth was investigated, where the shape factors for circular, 45° (diagonal facing) and 90° (side facing) square pipes obtained as 1, 1.29, and 1.91, respectively. Using the data of scour depths at different times, the time variation of scour depth is scaled by an exponential law, where the nondimensional time scale increases sharply with increase in Froude number characterized by the pipe diameter. In addition, clear-water scour below circular pipelines laid on a thinly armored sand bed (the sand bed is overlain by a thin armor layer of gravels) was experimentally studied. Depending on the pipe diameter, armor gravel, and bed-sand sizes, three cases of scour holes were recognized. The comparison of the experimental data reveals that the scour depth below a pipeline with an armor layer under limiting stability of the surface particles (approach flow velocity nearly equaling critical velocity for surface particles) is greater than that without armor layer for the same sand bed, if the secondary armoring formed within the scour hole is scattered. In contrast, the scour depth with an armor layer is less than that without armor layer for the same sand bed, when the scour hole is shielded by the secondary armor layer.     

Similitude of Large-Scale Turbulence in Experiments on Local Scour at Cylinders

The writers’ experiments on local scour at vertical cylinders placed in a sand bed show that similitude of large-scale turbulence is an important consideration influencing equilibrium depth of local scour. For the range of cylinder diameters used in their experiments, the writers identify a direct trend between equilibrium scour depth (normalized with cylinder diameter) and the intensity and frequency of large-scale turbulence shed from each cylinder; values of normalized scour depth increased when cylinder diameter decreased. The writers offer a scour-depth adjustment factor to account for this trend, which essentially is a scale effect incurred with experiments involving three independent length scales: cylinder diameter, bed-particle diameter, and flow depth. The consequent similitude consideration, or scale effect, has general significance for laboratory studies of local scour associated with hydraulic structures in sediment beds.     

Stochastic Flow Analysis for Predicting River Scour of Cohesive Soils

Damage to bridge crossings during flood events endangers the lives of the traveling public and causes costly disruptions to traffic flow. The most common causes of bridge collapse are scouring of the streambed and banks and erosion of highway embankments. This study couples a synthetic river flow simulation technique with a scour model for cohesive soils and determines the expected scour depth for a given lifetime of the bridge. A fractionally differenced autoregressive integrated moving average model generates synthetic streamflow sequences of the same length as the expected lifetime of the bridge. The scour model predicts the progression of scour depth through time in a multilayered soil. The model is used to determine the scour depth associated with different replicates of the synthetic flow sequences of the same length as the lifetime of the bridge. The probability distribution of scour depth is estimated by repeating this simulation procedure over a number of independent realizations of streamflow series for a given life of the bridge. This approach provides a framework for the probabilistic design and risk analysis of bridge foundations subjected to scour.     

Maximum Local Scour Depth at Bridge Piers under Unsteady Flow

The temporal effect of hydrograph on local scour depth is investigated under clear-water scour condition. By analyzing the characteristics of scour-depth evolutions at bridge piers for different rising hydrographs, a relation for estimating the maximum scour depth in uniform sediment is proposed. In the relation, the flow unsteadiness effect is taken into account by an unsteady flow parameter combining the peak-flow intensity and time-to-peak factors. For nonuniform sediment with d84 employed as the effective sediment size, this relation can yield reasonably good results of the maximum scour depth under rising hydrograph.     

Scour Vulnerability of River Bridge Piers

A simple procedure is proposed to assess the vulnerability of bridge piers in rivers, taking into account the phenomena governing fluvial dynamics during flood events. The procedure requires an estimation of the maximum scour depth of the soil surrounding both the pier and the foundation as well as an analysis of the bearing capacity of the pier–foundation–soil geotechnical system. The scour depth is determined in terms of the physical and mechanical properties of the streambed soil, the shape of the pier foundation and the destabilizing effects induced by hydrodynamic forces. The coupling of both the hydraulic and geotechnical analyses enables to identify the most significant factors characterizing scour depth and affecting pier vulnerability. Two levels (low, medium) of allowable vulnerability, bounded by an extreme condition of high vulnerability, are defined and analytically determined in function of the maximum scour depth and the foundation depth. Specific diagrams corresponding to each category of foreseen actions allow a quick evaluation of the vulnerability of a bridge pier.     

Clear-Water Scour at Abutments in Thinly Armored Beds

Experiments on local scour at short abutments (ratio of abutment length to approaching flow depth less than unity), namely vertical-wall, 45° wing-wall, and semicircular, embedded in a bed of relatively fine noncohesive sediment overlain by a thin armor-layer of coarser sediment, were conducted for different flow conditions, thickness of armor-layers, armor-layer, and bed sediments. The abutments were aligned with the approaching flow in a rectangular channel. The armor-layer and the bed underneath it were composed of different combinations of uniform sediments. In the experiments, the approaching flow velocities were restricted to the clear-water scour condition with respect to the armor-layer particles. Depending on the approaching flow conditions, three cases of scour at abutments in armored beds were identified. Effects of different parameters pertaining to scour at abutments are examined. The comparison of the experimental data shows that the scour depth at an abutment with an armor-layer in clear-water scour condition under limiting stability of the surface particles (approaching flow velocity nearly equaling critical velocity for the threshold motion of surface particles) is always greater than that without armor-layer for the same bed sediments. The characteristic parameters affecting the maximum equilibrium nondimensional scour depth (scour depth-abutment length ratio), identified based on the physical reasoning and dimensional analysis, are excess abutment Froude number, flow depth-abutment length ratio, armor-layer thickness-armor particle diameter ratio, and armor particle-bed sediment diameter ratio. The experimental data of clear-water scour condition in thinly armored beds under limiting stability of surface particles were used to determine the equation of maximum equilibrium scour depth through regression analysis. The estimated scour depths were in agreement with the experimental scour depths. Also, an equation of maximum equilibrium scour depth in uniform sediments was obtained.     

Shields’ Entrainment Criterion in Bridge Hydraulics

Two related problems of sediment hydraulics are addressed: (1) Inception of sediment transport for nearly uniform flow of both uniform and nonuniform sediment beds for two different sediment densities and grain sizes ranging from sand to gravel, and (2) generalized inception conditions if elements are inserted in a plane sediment bed. The Shields’ criterion is formulated with basic quantities involving gravity, viscosity, and densities of the two-phase flow. The results of the analysis relate to the viscous, the transition, and the fully turbulent regimes. The transition regime is verified with extended laboratory experiments. Then, these conditions are used as a basis for formulating a general stability criterion for loose bed hydraulics, and compared to detailed experiments involving pier and square elements located either at the channel side or at its axis. In addition, a generalized densimetric particle Froude number is introduced that accounts for both uniform sediments and mixtures. The engineering application of the present results is straightforward, given that basic parameters of hydraulics, sediment, and fluid are involved.     

Scour in Long Contractions

Experimental results on local scour in long contractions for uniform and nonuniform sediments (gravels and sands) under clear-water scour are presented. An emphasis was given to conduct the experiments on scour in long contractions for gravels. The findings of the experiments are used to describe the effects of various parameters (obtained from dimensional analysis) on equilibrium scour depth under clear-water scour. The equilibrium scour depth increases with decrease in opening ratio and with increase in sediment size for gravels. But the curves of scour depth versus sediment size have considerable sag at the transition of sand and gravel. The scour depth decreases with increase in densimetric Froude number, for larger opening ratios, and increases with increase in approaching flow depth at lower depths. However, it becomes independent of approaching flow depth at higher flow depths. The effect of sediment gradation on scour depth is pronounced for nonuniform sediments, which reduce scour depth significantly due to the formation of armor layer in the scour hole. Using the continuity and energy equations, a simple analytical model for the computation of clear-water scour depth in long contractions is developed with and without sidewall correction for contracted zone. The models agree satisfactorily with the present and other experimental data. Also, a new empirical equation of maximum equilibrium scour depth, which is based on the experimental data at the limiting stability of sediments in approaching channel under clear-water scour, is proposed. The potential predictors of the maximum equilibrium scour depth in long contractions are compared with the experimental data. The comparisons indicate that the equations given by Komura and Lim are the best predictors among those examined.