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.     

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.     

Erosion Function Apparatus for Scour Rate Predictions

Scour is the number one cause of bridge failures. Scour in coarse grained soils (sand, gravel) is relatively well known, but scour in fine grained soils (silt, clay) and weak rock is not. In coarse grained soils, scour takes place very rapidly and the scour rate is rarely an issue because one flood is likely to create the maximum scour depth. In fine grained soils, the scour process is much slower; as a result, even after a hundred years, a bridge may not experience the maximum depth of scour. Therefore, in fine grained soils it becomes necessary to predict the rate at which scour takes place. A new apparatus called the EFA (Erosion Function Apparatus; 〈http:∕/tti.tamu.edu∕geotech∕scour〉) has been built and tested to measure the erosion rate of fine grained soils; the EFA can also be used to measure the erosion rate of coarse grained soils if necessary. The end of a Shelby tube sample from the bridge site is fitted through a tight opening at the bottom of a pipe with a rectangular cross section. Water flows through the pipe and erodes the soil sample, which protrudes 1 mm above the bottom of the pipe. The rate at which the sample erodes is measured, and the shear stress imposed by the water on the soil is calculated. The plot of erosion rate versus shear stress is the result of an EFA test. It indicates the critical shear stress at which erosion starts and the rate of erosion beyond that shear stress. This function can be used to predict the rate of scour at a bridge.     

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.     

Multiflood and Multilayer Method for Scour Rate Prediction at Bridge Piers

The SRICOS method was proposed in 1999 to predict the scour depth versus time curve at a cylindrical bridge pier for a constant velocity flow, a uniform soil, and a deep-water condition. In this article, the method is extended to include a random velocity-time history and a multilayer soil stratigraphy; it is called the Extended-SRICOS or E-SRICOS. The algorithms to accumulate the effects of different velocities and to sequence through a series of soil layers are described. The procedure followed by the computer program to step into time is outlined. A simplified version of E-SRICOS called S-SRICOS is also presented; calculations for the S-SRICOS method can easily be done by hand. Eight bridges in Texas are used as case histories to compare predictions by the two new methods (E-SRICOS and S-SRICOS) with measurements at the bridge sites.     

Flow Velocity and Pier Scour Prediction in a Compound Channel: Big Sioux River Bridge at Flandreau, South Dakota

The two-dimensional (2D) depth-averaged river model Finite-Element Surface-Water Modeling System (FESWMS) was used to predict flow distribution at the bend of a compound channel. The site studied was the Highway 13 bridge over the Big Sioux River in Flandreau, South Dakota. The Flandreau site has complex channel and floodplain geometry that produces unique flow conditions at the bridge crossing. The 2D model was calibrated using flow measurements obtained during two floods in 1993. The calibrated model was used to examine the hydraulic and geomorphic factors that affect the main channel and floodplain flows and the flow interactions between the two portions. A one-dimensional (1D) flow model of the bridge site was also created in Hydrologic Engineering Centers River Analysis System (HEC-RAS) for comparison. Soil samples were collected from the bridge site and tested in an erosion function apparatus (EFA) to determine the critical shear stress and erosion rate constant. The results of EFA testing and 2D flow modeling were used as inputs to the Scour Rate in Cohesive Soils (SRICOS) method to predict local scour at the northern and southernmost piers. The sensitivity of predicted scour depth to the hydraulic and soil parameters was examined. The predicted scour depth was very sensitive to the approach-flow velocity and critical shear stress. Overall, this study has provided a better understanding of 2D flow effects in compound channels and an overall assessment of the SRICOS method for prediction of bridge pier scour.     

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.     

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.     

3D Unsteady RANS Modeling of Complex Hydraulic Engineering Flows. II: Model Validation and Flow Physics

A chimera overset grid flow solver is developed for solving the unsteady Reynolds-averaged Navier-Stokes (RANS) equations in arbitrarily complex, multiconnected domains. The details of the numerical method were presented in Part I of this paper. In this work, the method is validated and applied to investigate the physics of flow past a real-life bridge foundation mounted on a fixed flat bed. It is shown that the numerical model can reproduce large-scale unsteady vortices that contain a significant portion of the total turbulence kinetic energy. These coherent motions cannot be captured in previous steady three-dimensional (3D) models. To validate the importance of the unsteady motions, experiments are conducted in the Georgia Institute of Technology scour flume facility. The measured mean velocity and turbulence kinetic energy profiles are compared with the numerical simulation results and are shown to be in good agreement with the numerical simulations. A series of numerical tests is carried out to examine the sensitivity of the solutions to grid refinement and investigate the effect of inflow and far-field boundary conditions. As further validation of the numerical results, the sensitivity of the turbulence kinetic energy profiles on either side of the complex pier bent to a slight asymmetry of the approach flow observed in the experiments is reproduced by the numerical model. In addition, the computed flat-bed flow characteristics are analyzed in comparison with the scour patterns observed in the laboratory to identify key flow features responsible for the initiation of scour. Regions of maximum shear velocity are shown to correspond to maximum scour depths in the shear zone to either side of the upstream pier, but numerical values of vertical velocity are found to be very important in explaining scour and deposition patterns immediately upstream and downstream of the pier bent.     

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.     

Sediment Erosion Characteristics in the Anacostia River

Four in situ experiments on sediment erosion characteristics were conducted at the Anacostia River that runs through Washington, D.C. Supplemental erosion rate data were also obtained by carrying out five laboratory experiments using sediment samples collected at the field. In laboratory experiments, the sediment samples were mixed with tap water and placed in the flume to form beds for finding the difference in terms of erosion characteristics caused by different sediment composition among the five samples. This approach enables the finding of erosion characteristics for the entire tidal Anacostia River with limited resources. The in situ measured critical bed-shear stresses τcr for erosion at the water-sediment interface z = 0 varies from 0.03 to 0.08?Pa. Field results indicated that τcr(z) increases with the depth z and becomes more than 0.6 to 0.7?Pa with an erosion thickness of less than 1?cm. Sediment beds prepared at a laboratory appear having an upper limit on how much τcr(z) can be developed.     

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.     

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.     

Influence of Wood Debris Accumulation on Bridge Pier Scour

This note deals with the influence of debris accumulation on scour around bridge piers. Clear-water experiments in different hydraulic conditions have been carried out with three wood debris shapes: rectangular, triangular, and cylindrical. A wide range of debris thickness and width were studied in order to determine their influence on the maximum scour hole depth temporal evolution. The ratio of the pier diameter to the channel width was varied between 0.05 and 0.12 with total bridge contractions up to 20%. A proposed relation presents a simple design procedure to predict the increase in scour depth, which mainly depends on the flow contraction due to the debris accumulation.     

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.     

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.     

Geometry of Scour Holes Associated with 90° Spur Dikes

The stable pools formed by spur dikes can be a significant factor for improving aquatic habitat in unstable degraded streams. Prediction of the volume and geometry of the scour hole associated with a spur dike yields information that is valuable for assessing the potential benefit to the aquatic habitat. Volumes of the scour hole in the vicinity of model spur dikes were measured in a laboratory flume under clear-water overtopping flows. Spur dike length, flow depth, and shear velocity ratio were varied in the experiments and found to significantly influence the volume of the scour hole. For overtopping flows, the ratio of the flow depth to the spur dike height was found to be an important control on the geometry of the resulting scour hole. At higher flow depth to spur dike height ratios, the location of the maximum depth of scour changed from the upstream point of the dike toward the channel bank and caused a secondary scour zone to form downstream of the spur dike. A preliminary technique is proposed to predict the volume of scour for spur dikes perpendicular to the bank.     

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.