Effects of Inclination of Bridge Piers on Scouring Depth

For a safe design of a bridge pier footing, it is important to estimate the maximum depth of scour as accurately as possible. The aim of this experimental study is to investigate the effects of inclination of bridge piers on local scour depths around bridge piers. Single circular piers inclined toward the downstream direction were founded in a uniform bed material. Near threshold conditions were employed. The results of this study indicate that the local scour depth decreases as the inclination of the pier increases.     

Riprap Protection at Bridge Piers

Although riprap is the most commonly employed countermeasure against scouring around bridge piers, few studies exist of riprap performance under live-bed conditions. In this study, failure mechanisms, stability, and placement level effects for riprap at bridge piers are considered experimentally. Under clear-water conditions, riprap is subject to shear, winnowing, and edge failure. Under live-bed conditions, a fourth failure mechanism, destabilization by bed-form progression, becomes important. Destabilization by bed-form progression is dependent on the destabilizing influence of bed-form troughs as they pass the pier. Experiments were used to assess the ability of riprap stones to protect bridge piers under a wide range of flow conditions. The effects of placing the riprap layer at depth within the sediment bed, rather than level with the bed surface, were investigated also. The study showed that, as the flow velocity increases, the ability of riprap stones to protect a pier decreases asymptotically until the scour depth in the riprap layer reaches that of an equivalent unprotected pier. In addition, it was found that the deeper the placement level the less exposed the riprap was to destabilizing bed forms and the better the protection against local scour. Lowering the placement level also meant that the riprap performed better than for surface-placed layers as the flow velocity increased. The mode of riprap failure is also changed as the placement level below the bed surface is lowered. A pier riprap size-prediction equation is proposed, including a parameter to account for placement level.     

Local Scour and Riprap Stability at Bridge Piers in a Degrading Channel

The experimental study examines local scouring and riprap stability at bridge piers in rivers subject to bed degradation. The data show that the equilibrium bed profile associated with that with or without a pier is essentially the same, except for the obvious section around the pier. Total scour depth is shown to be the sum of bed degradation and pier scour depth. The latter can be computed from the time-average live-bed scour depth associated with the undisturbed velocity ratio before bed degradation. The experimental data also show that pier-scour depth is invariant with time, for t ≥ 24?h. In a degrading channel, riprap around a pier will eventually develop into a stable mound when the bed shear stresses reduce with bed degradation. An auxiliary test shows that the mound is very vulnerable to another designed flood flow accompanied by large dunes. This type of riprap instability may be called bed-degradation induced failure.     

Bridge Pier Scour in Clay-Sand Mixed Sediments at Near-Threshold Velocity for Sand

Local scour at circular bridge piers embedded in a clay-sand-mixed bed was investigated in laboratory flume experiments. The effects of clay content, water content, and sand size on maximum equilibrium scour depth, equilibrium scour hole geometry, scouring process, and time variation of scour were studied at velocities close to the threshold velocities for the sand in the clay-sand mixture. It was observed that clay content and water content were the key parameters that effect the scouring process, scour hole geometry, and maximum equilibrium scour depth. The bridge pier scouring process in clay-sand mixtures involved different dominating modes for removal of sediment from scour hole: chunks-of-aggregates, aggregate-by-aggregate, and particle-by-particle. Regression-based equations for estimation of nondimensional maximum scour depth and scour hole diameter for piers embedded in clay-sand mixtures having clay content of <40% and water content of <40% were proposed as functions of pier Froude number, clay content, water content, and bed shear strength.     

Wire Gabion for Protecting Bridge Piers

As a new alternative countermeasure to riprap for scour protection around bridge piers, wire gabions were investigated experimentally for failure mechanisms, effects of significant parameters on failure and its sizing in a clear-water condition. The dominating failure mechanism was found to be a shear failure. Based on the experimental data, the controlling factors for the stability of wire gabions as a scour countermeasure at the pier are flow depth relative to pier diameter, length to thickness ratio, coverage, alignment and placement depth of wire gabions. An equation for sizing of a wire gabion is proposed in terms of Froude number and factors reflecting both the effect and limit of significant parameters. Comparison of the equation with those of ripraps shows that smaller wire gabions than ripraps provide an equivalent protection implying cost effective and improved stability.     

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.     

Risk-Based Method for Selecting Bridge Scour Countermeasures

Bridge engineers are often faced with the task of selecting and designing effective bridge scour countermeasures. The selection of an appropriate countermeasure is dependent on whether the problem is local scour at the pier or abutment, contraction scour across the bed at the bridge opening, reach-wide channel degradation, or lateral channel movement. Confidence in a given countermeasure depends on prior experience in using the measure, cost, maintenance, and the ability to detect failure. The use of countermeasures often introduces uncertainty due to a lack of systematic testing and unknown potential for failure. In this paper, a risk-based method for ranking, comparing, and choosing the most appropriate scour countermeasures is presented using failure modes and effects analysis and risk priority numbers (RPN). Failure modes and effects analysis incorporates uncertainty in the selection process by considering risk in terms of the likelihood of a component failure, the consequence of failure, and the level of difficulty required to detect failure. Risk priority numbers can provide justification for selecting a specific countermeasure and the appropriate compensating actions to be taken to prevent failure of the countermeasure.     

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.     

Use of Sacrificial Piles as Pier Scour Countermeasures

Laboratory studies on the use of sacrificial piles as a pier scour countermeasure are reported. Sacrificial piles, which previous studies have shown to result in significant scour depth reductions under clear-water conditions, were found to be rather less effective under live-bed conditions due to the passage of bed forms. Sacrificial piles are not recommended unless the flow remains aligned and the flow intensity is relatively small.     

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.     

Probability of Exceedance Estimates for Scour Depth around Bridge Piers

Scour at a bridge pier is the formation of a hole around the pier due to the erosion of soil by flowing water; this hole in the soil reduces the carrying capacity of the foundation and the pier. Excessive scour can cause a bridge pier to fail without warning. Current predictions of the depth of the scour hole around a bridge pier are based on deterministic models. This paper considers two alternative deterministic models to predict scour depth. For each deterministic model, a corresponding probabilistic model is constructed using a Bayesian statistical approach and available field and experimental data. The developed probabilistic models account for the estimated bias in the deterministic models and for the model uncertainty. Parameters from both prediction models are compared to determine their accuracy. The developed probabilistic models are used to estimate the probability of exceedance of scour depth around bridge piers. The method is demonstrated on an example bridge pier. The paper addresses model uncertainties for given hydrologic variables. Hydrologic uncertainties have been presented in a separate paper.     

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.     

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.     

Clearwater Local Scour at Complex Piers

A new methodology to predict local scour depth at a complex pier is presented herein that combines existing expressions for scouring respectively at uniform piers, caisson-founded piers, pile groups with debris rafts, and pile groups alone. The method recognises the relative scouring potentials of the components of complex piers and the transition of scouring processes occurring for varying pile-cap elevation. The validity of the method is confirmed herein using the present and also historical measurements of local scour at complex piers. The proposed methodology has the advantages of being conceptually consistent with observed scour behaviours, relatively simple to apply, applicable to wide ranges of flow and sediment conditions (through incorporation into a more general analysis framework), and applicable over the entire range of possible pile-cap elevations. For design purposes, the present method highlights respective pile-cap elevations that maximize (i.e., to be avoided over the pier life) and minimize local scour at complex piers. The present method reinforces that where the pile-cap elevation relative to the bed can vary with time at a bridge site, potential local-scour depths need to be assessed over the range of possible pile-cap elevations for the pier.     

SRICOS: Prediction of Scour Rate in Cohesive Soils at Bridge Piers

A new method called SRICOS is proposed to predict the scour depth z versus time t around a cylindrical bridge pier of diameter D founded in clay. The steps involved are: (1) taking samples at the bridge pier site; (2) testing them in an erosion function apparatus to obtain the scour rate ? versus the hydraulic shear stress applied τ; (3) predicting the maximum shear stress τmax, which will be induced around the pier by the water flowing at vo before the scour hole starts to develop; (4) using the measured ? versus τ curve to obtain the initial scour rate ?i corresponding to τmax; (5) predicting the maximum depth of scour zmax for the pier; (6) using ?i and zmax to develop the hyperbolic function describing the scour depth z versus time t curve; and (7) reading the z versus t curve at a time corresponding to the duration of the flood to find the scour depth that will develop around that pier. A new apparatus is developed to measure the ? versus t curve of step 2, a series of advanced numerical simulations are performed to develop an equation for the τmax value of step 3, and a series of flume tests are performed to develop an equation for the zmax value of step 5. The method is evaluated by comparing predictions and measurements in 42 flume experiments.     

Temporal Variation of Scour Depth at Nonuniform Cylindrical Piers

The paper proposes a semiempirical model to estimate the temporal development of scour depth at cylindrical piers with unexposed foundations. A cylindrical pier with a foundation is considered as nonuniform pier. The concept of primary vortex and the principle of volumetric rate of sediment transport are used to develop a methodology to characterize the rate of evolution of the scour hole at nonuniform cylindrical piers. The model also simulates the entire scouring process at nonuniform cylindrical piers having the discontinuous surface located below the initial bed level. The scouring process includes three zones; viz Zone 1 having the scouring phenomenon similar to that of a uniform pier, Zone 2 in which the scour depth remains unchanged with its value equal to the depth of the top level of foundation below the initial bed level while the dimensions of the scour hole increase, and in Zone 3 the geometry pier foundation influences the scouring process. A concept of superposition using an effective pier diameter is proposed to simulate the scouring process in Zone 3. In addition, the laboratory experiments were conducted to utilize the laboratory results for the validation of the model. The simulated results obtained from the proposed model are in good agreement with the present experimental results and also other experimental data. Also, the effect of unsteadiness of flow is incorporated in the model and the results of the model are compared with the experimental data. The model agrees satisfactorily with the experimental data.     

Reduction of Local Scour around Bridge Piers Using Slots and Collars

The present study examines the use of pier slots and collars for reducing local scour at bridge piers. The efficacy of slots, of different lengths and at different angles of attack, was studied through experiments. The reduction of scour due to the placement of circular collars, of different sizes and at different elevations, was also investigated. Analysis of the data from the experiments as well as data from earlier studies led to an equation for the maximum scour depth around circular bridge piers fitted with collars. The equation applies to local scour of uniform-sized sediment in clear-water flow.     

Characteristics of Horseshoe Vortex in Developing Scour Holes at Piers

The outcome of an experimental study on the turbulent horseshoe vortex flow within the developing (intermediate stages and equilibrium) scour holes at cylindrical piers measured by an acoustic Doppler velocimeter (ADV) are presented. Since the primary objective was to analyze the evolution of the turbulent flow characteristics of a horseshoe vortex within a developing scour hole, the flow zone downstream of the pier was beyond the scope of the investigation. Experiments were conducted for the approaching flow having undisturbed flow depth ( = 0.25?m) greater than twice the pier diameter and the depth-averaged approaching flow velocity ( = 0.357?m/s) about 95% of the critical velocity of the uniform bed sand that had a median diameter of 0.81?mm. The flow measurements by the ADV were taken within the intermediate (having depths of 0.25, 0.5, and 0.75 times the equilibrium scour depth) and equilibrium scour holes (frozen by spraying glue) at a circular pier of diameter 0.12?m. In order to have a comparative study, the ADV measurements within an equilibrium scour hole at a square pier (side facing the approaching flow) of sides equaling the diameter of the circular pier were also taken. The contours of the time-averaged velocities, turbulence intensities, and Reynolds stresses at different azimuthal planes (0, 45, and 90°) are presented. Vector plots of the flow field at azimuthal planes reveal the evolution of the characteristics of the horseshoe vortex flow associated with a downflow from intermediate stages to equilibrium condition of scour holes. The bed-shear stresses are determined from the Reynolds stress distributions. The flow characteristics of the horseshoe vortex are discussed from the point of view of the similarity with the velocity and turbulence characteristic scales. The imperative observation is that the flow and turbulence intensities in the horseshoe vortex flow in a developing scour hole are reasonably similar.     

Supercritical Flow over a Dentated Sill

Experimental analysis concerning supercritical flow over a dentated sill in nonsubmerged conditions and the local scour downstream of this sill is presented. In this case, the energy dissipation is incomplete so that a scour control downstream of the hydraulic structure could be required. The use of this type of dissipation device could be needed in cases where the maximum flow depth upstream of the sill must be limited to satisfy physical or structural conditions. This could happen, for instance, in the restoration of stilling basin in gravity dams and at the outlet of diversion systems. Experimental and theoretical results, useful in the design of this type of stilling basin and scour control, are presented in the paper, with an application to the case of the Molato Dam in Italy.     

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.