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 estimate 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 values of the model parameters suggest that the maximum sour depth predicted by the deterministic HEC-18 Sand and HEC-18 Clay models tend to be conservative. Evidence is also found that the applicability of the HEC-18 Clay method is not limited to clay but can also be used for other soil types. The main advantage of the HEC-18 Clay method with respect to the HEC-18 Sand method is that it predicts the depth of scour as a function of time and can be used to estimate the final scour at the end of the design life of a structure. The paper addresses model uncertainties for given hydrologic variables. Hydrologic uncertainties have been presented in a separate paper.

The engineering design of a hydraulic structure such as a river bridge requires consideration of the factors that affect the safety of the structure. Among them, one of the most important variables is bridge foundation scour. However, engineering experience seems to indicate that computation of scour depth using current scour formulas tends to overpredict scour in comparison to field measurements. The result can be an overdesigned bridge foundation that increases the cost of the bridge. One possible reason for the overprediction is the current practice of adding separate estimates of contraction scour and pier scour when in fact these processes occur simultaneously and interact. During the occurrence of a flood, velocities and depths increase but they are affected by changes in the distribution of discharge between the main channel and floodplain. In addition, the time history or time development of contraction scour and local pier scour is not the same. As a result, the influence of contraction scour on pier scour, for example, is time dependent. Laboratory experiments are proposed using a 1:45 scale hydraulic model of the Ocmulgee River bridge at Macon, Georgia. Initially, the contraction scour will be measured without the bridge piers in place. In this experiment, the time history of the scour and the velocity distributions at the equilibrium state will be measured. Then the piers will be placed at the bridge cross-section in the flume, and the same measurements will be made. The sensitivity of the measurements to small changes in depth at the same discharge will also be determined, and comparisons will be made with field measurements of scour depth. The results will be used to assess the relative contribution of contraction scour and local pier scour to the final design of the bridge foundation depth.

Bridge scour, which is the removal of bed materials from near the bridge foundations, is observed to be the most predominant cause of bridge failures in the United States. Scour in cohesive soils is greatly different from scour in cohesionless soils owing to the differences in critical shear stresses, scour extents and the time taken to reach the maximum scour depth in the scour process. The present solutions available for the cohesionless soils cannot be applied to cohesive soils because of the above crucial reasons. Also, a compound channel model with main channel and flood plain arrangement represents more closely the field stream conditions rather than a simple rectangular prismatic model. In this study, a systematic investigation of the scour process due to flow contractions in a compound channel with cohesive soil bed is made by conducting a series of flume tests representing typical field conditions. The effect of the most crucial factors causing contraction scour namely flow velocity, depth of flow and the shape of the abutment is examined. Correction factors are developed for changes in flow geometries incorporating simulation results from the one dimensional flow simulation model HEC RAS. Most importantly, a methodology to predict the depth of the deepest scour hole and its location in the vicinity of the contraction structure is developed for compound channels through an extension of the presently available methodology to predict maximum scour depths in simple rectangular channels. A prediction method to identify the extent of the uniform scour depth is also developed. Finally, an investigation of precision of the proposed methodology has been carried out on the field data from a number of real life contraction scour cases. The results obtained from this study indicate that depth of flow and geometry of the contraction section significantly influence final scour depth in cohesive soils with deeper flows and harsh contractions resulting in increased scour depths. However, corrections for different contraction inlet skew angles and long contractions need to be further explored in future studies.

Bridge scour, which is the removal of bed materials from near the bridge foundations, is observed to be the most predominant cause of bridge failures in the United States. Scour in cohesive soils is greatly different from scour in cohesionless soils owing to the differences in critical shear stresses, scour extents and the time taken to reach the maximum scour depth in the scour process. The present solutions available for the cohesionless soils cannot be applied to cohesive soils because of the above crucial reasons. Also, a compound channel model with main channel and flood plain arrangement represents more closely the field stream conditions rather than a simple rectangular prismatic model. In this study, a systematic investigation of the scour process due to flow contractions in a compound channel with cohesive soil bed is made by conducting a series of flume tests representing typical field conditions. The effect of the most crucial factors causing contraction scour namely flow velocity, depth of flow and the shape of the abutment is examined. Correction factors are developed for changes in flow geometries incorporating simulation results from the one dimensional flow simulation model HEC RAS. Most importantly, a methodology to predict the depth of the deepest scour hole and its location in the vicinity of the contraction structure is developed for compound channels through an extension of the presently available methodology to predict maximum scour depths in simple rectangular channels. A prediction method to identify the extent of the uniform scour depth is also developed. Finally, an investigation of precision of the proposed methodology has been carried out on the field data from a number of real life contraction scour cases. The results obtained from this study indicate that depth of flow and geometry of the contraction section significantly influence final scour depth in cohesive soils with deeper flows and harsh contractions resulting in increased scour depths. However, corrections for different contraction inlet skew angles and long contractions need to be further explored in future studies.

The bridge scour depths in cohesive soil have been predicted using the scour equations developed for cohesionless soils due to scarce of studies about cohesive soil. The scour depths predicted by the conventional methods will result in significant errors. For the cost effective design of bridge scour in cohesive soil, the Scour Rate In COhesvie Soil (SRICOS) for the singular circular pier in deep water condition was released in 1999, and has been developed for complex pier and contraction scour. The present study is the part of SRICOS-EFA method to predict the history of contraction scour, and local scours, such as abutment scour and pier scour. The main objective is to develop the prediction methods for the maximum and the uniform contraction scour depth, the maximum pier scour depth and the maximum abutment using flume test results. The equations are basically composed with the difference between the local Froude number and the critical Froude number. Because the scour happens when the shear stress is bigger than the critical shear stress, which is the maximum shear stress the channel bed material can resist from the erosion, and continues until the shear stress becomes equal to the critical shear stress. All results obtained from flume tests for pier scour have been conducted in Texas A&M University from 1997 to 2002 are collected and reanalyzed in this study. Since the original pier scour equation did not include soil properties. The effect of water depth effect, pier spacing, pier shape and flow attack angle for the rectangular pier are studied and correction factors with respect to the circular pier in deep water condition were newly developed in present study. For the abutment scour, a series of flume tests in large scale was performed in the present study. Two types of channel - rectangular channel, and compound channel - were used. The effect of abutment length, shape and alignment of abutment were studied and the correction factors were developed. The patterns of velocity and of scour were compared, and it was found that the maximum local scour occurred where the maximum turbulence was measured. For the contraction scour, the results obtained from a series of flume tests performed in 2002 and a series of flume tests for the abutment scour in the present study are analyzed. The methodologies to predict the maximum contraction scour and the uniform contraction scour in the compound channel was developed. Although all prediction methods developed in the present study are for the cohesive soils, those methods may be applicable to the cohesionless soils because the critical shear stress is included in the methods. All prediction methods were verified by the comparison with the databases obtained from flume test results and field data.

No description available.

Bridge piers within moving water are exposed to an additional failure mechanism known as scour. Upon the scour depth reaching the foundation of the pier, the structural integrity of the pier, and consequently the bridge, can be jeopardized. Bridge pier scour is the result of a three-dimensional flow separation consisting primarily of the horseshoe vortex, flow acceleration along the sides of the pier, and wake vortices. There are numerous factors that can affect bridge pier scour, of which many of them have been studied extensively. However, there are still some factors where the knowledge base is limited: one example is the presence of an ice cover around bridge piers. In order to reduce the risk of failure induced by scour, regardless of the cause, a preferred option is to use scour countermeasures. However, an ideal countermeasure does not exist. Therefore, the purpose of this research is to design and test an improved bridge pier scour countermeasure, while also better understanding the effects an ice cover has on scour. Achieving a new countermeasure design consisted of a hybrid approach that combined both numerical and experimental modelling. The numerical model was used in an iterative manner to expedite the design process, as well as to reduce experimental costs. Upon testing and improving the initial collar design numerically, physical models were constructed for the purpose of testing experimentally. Experimental tests were performed at a 1:30 scale in the presence of a sand bed. The same experimental setup was used to investigate bridge pier scour under an ice cover, except a rigid structure was constructed to replicate an ice cover. The artificial ice cover possessed either a smooth or a rough underside and was installed in such a way to replicate a floating or fixed (pressurized) ice cover. The purpose of the new countermeasure design was to improve on the flat plate collar by guiding the horseshoe vortex in a novel manner. By doing so, the quantity of erosive forces contacting the bed was greatly reduced. In order to reach a final design, a series of prototype designs were tested, and are outlined in this thesis, as they provide valuable insight into the scour problem. The final countermeasure design resembles a contoured collar but is made of riprap, where it was found to reduce the scour depth and volume by 81.0% and 92.3%, respectively, while using 18% less riprap than the conventional flat riprap countermeasure. Upon investigating scour in the presence of an ice cover, it was found that the quantity of scour increases as the ice cover becomes rougher and as the flow becomes more pressurized beneath. Specifically, the scour depth under the rough ice cover and the most pressurized condition increased by 412%. It was demonstrated that implementing any device which increases the width of the pier has inherent limitations for reducing scour. Instead, having a depression around the pier, especially made of riprap, such that it is flush with the bed and can help guide the horseshoe vortex, was found to greatly reduce scouring. Furthermore, it was observed that the presence of any ice cover on the surface of the water generates greater pier scour, therefore necessitating that ice cover always be taken into consideration when designing bridges in cold climates.

Thesis (Ph.D.)--Michigan Technological University, 2005. / Includes bibliographical references. Also available online via the Michigan Technological University Library website (http://www.lib.mtu.edu/). Also available on the World Wide Web.

Thesis (M.C.E.)--University of Delaware, 2009. / Principal faculty advisor: Jack A. Puleo, Dept. of Civil & Environmental Engineer. Includes bibliographical references.

Thesis (M.S. - Civil Engineering and Engineering Mechanics)--University of Arizona. / Includes bibliographical references.