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.     

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.     

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.     

Riprap Failure at Circular Bridge Piers

Riprap of bridge piers is placed to prevent scour and to secure the pier from failure. Riprap is therefore an addition to a pier to increase its performance against scour. The present research intends to present three basic scour mechanisms associated with circular-shaped bridge piers in rivers first, to introduce then a number of selected experiments for a range of hydraulic, geometric, and sedimentologic conditions, and finally to describe a novel procedure for assessing the safety of these river elements against failure. This procedure is based on the Shields diagram relating to sediment entrainment in a uniform and flat sediment bed subjected by a water flow. The Shields approach is extended for the presence of a circular-shaped pier that is protected by a circular-arranged riprap layer of equal size elements. The design procedure presented in the following thus reduces to the entrainment condition of a pier for equal riprap and the sediment sizes and to the Shields entrainment condition when the pier diameter degenerates to 0.     

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.     

Field Measurements and Simulation of Bridge Scour Depth Variations during Floods

An understanding of bridge scour mechanisms during floods in a fluvial river is very important for cost-effective bridge foundation design. Reliable bridge scour data for flood events are limited. In this study, field experiments were performed at the Si-Lo Bridge in the lower Cho-Shui River, the longest river in Taiwan, to collect scour-depth data using a sliding magnetic collar, a steel rod, and a numbered-brick column. By separating each scour component, a methodology for simulating the temporal variations of the total scour depth under unsteady flow conditions is proposed. The proposed total-scour model integrates three scour components, namely general scour, contraction scour, and local scour. The collected field data, comprising both general scour and total scour depths, are used to validate the applicability of the proposed model. Based on the peak flow discharges during floods, a comparison of the local scour depths calculated using several commonly used equilibrium local scour formulas indicates that most equations may overestimate the local scour depth.     

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.     

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.     

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.     

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.     

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.     

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.     

Control of Scour at Bridge Piers by a Downstream Bed Sill

Results are presented from laboratory experiments to investigate the effectiveness of bed sills as countermeasures against local scouring at a smooth circular bridge pier, for flow conditions near the threshold of uniform sediment motion. The bed sill was located downstream of the pier, and its effectiveness with the distance between pier and sill was evaluated. The dependence of the scour depth on different dimensionless groups was defined. The results showed that a bed sill placed at a short distance downstream of the pier reduces the scour depth, area, and volume. In particular, the smaller the distance between the two structures, the larger the effectiveness of the countermeasure. The bed sill seems to take effect some time after the beginning of the test, as the scour hole downstream of the bridge pier develops sufficiently and interacts with the countermeasure.     

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.     

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.     

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.     

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.     

Bridge Pier Scour under Flood Waves

The effect of a single-peaked flood wave on pier scour is investigated both theoretically and experimentally. The conditions considered involve clear-water scour of a cohesionless material of given median sediment size and sediment nonuniformity, an approach flow characterized by a flow depth and velocity, a circular-shaped cylindrical bridge pier, and a flood hydrograph defined by its time to peak and peak discharge. A previously proposed formula for scour advance under a constant discharge was applied to the unsteady approach flow. The generalized temporal scour development along with the end scour depth are presented in terms of mainly the densimetric particle Froude number based on the maximum approach flow velocity and the median sediment size. The effect of the remaining parameters on the end scour depth is discussed and predictions are demonstrated to be essentially in agreement with model observations.     

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.     

Experimental Investigation of Clear-Water Local Scour of Compound Piers

Local scour around complex piers under steady clear-water condition was studied experimentally for a variety of configuration, including different sizes and shapes of complex piers. A total of 70 experiments were carried out. Three sets of experiments were performed over the entire range of possible pile cap elevations for complex piers with different geometrical characteristics. The collected data are used to quantify the pile cap elevations that maximize or minimize the local scour depth. Some of the available methodologies to estimate the maximum local scour depth around such complex piers are evaluated. The predictions of the scour depths improved by using the revised methods of Hydraulic Engineering Circular Number 18 and Coleman.