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1 SUMMARY Scour at Wide Piers and Long Skewed Piers Current methods for predicting local scour at bridge piers, including those described in Federal Highway Administration (FHWA) Hydraulic Engineering Circular No. 18 (Richardson and Davis 2001), were developed on the basis of small-scale laboratory studies and do not consider factors relevant to wide piers and long piers that are skewed to the flow. Because of these limitations, the current methods generally overpredict local scour at such piers, leading to the use of unwarranted and costly foundations or countermeasures. Research was needed to evaluate current methods for predicting local pier scour and their applicability to wide piers and long skewed piers and to develop improved methods for highway agencies to use in the design, operation, and maintenance of highway bridges. The objective of this research, conducted under National Cooperative Highway Research Program (NCHRP) Project 24-32, was to develop methods and procedures for predicting time-dependent local scour at wide piers and at long skewed piers that are suitable for consid- eration and adoption by American Association of State Highway and Transportation Officials (AASHTO). The research was limited to noncohesive soils and steady flow. This objective was accomplished by (1) conducting an extensive literature and information search that included emailing questionnaires to 176 researchers, federal and state department of transportation (DOT) engineers, and practicing engineers; (2) identifying and acquiring published predictive equations/methods and pertinent laboratory and field data for testing the equations/methods; (3) developing methods for evaluating the equations and data; and (4) modifying and evaluating the equations using the compiled laboratory and field database. For the purpose of this study, "wide pier" was defined as piers equal to or greater than 10 ft in width and having a pier width to sediment diameter ratio greater than 100. Thirty-seven responses to the questionnaire were received. The literature search revealed that there is very little information (predictive equations and data) explicitly on scour at wide piers and long skewed piers. However, the predictive equations in the literature are intended to apply equally well to large as well as small piers. For this reason all local scour equations located by the information search were considered in this study. Twenty-three of the more recent and commonly used equilibrium local scour equations were identified and assembled. As part of the initial screening process, the scour depths pre- dicted by these equations for a wide range of laboratory and field conditions were compared. Six of the equations yielded unrealistic (extremely large or negative) results and were elimi- nated, leaving seventeen equations for further analysis in this project. The data search resulted in a significant quantity of both laboratory and field equilibrium scour data. However, only a portion of these data was relevant to this study. Researchers have devoted less effort to the understanding and prediction of local scour evolution rates than to equilibrium scour depths. Only 10 scour evolution methods/equations
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2 were located and obtained. These equations/methods were evaluated for a range of hypo- thetical laboratory and field conditions for the purpose of identifying those that produce unrealistic results. This exercise showed significant differences between the methods; four methods were eliminated from further consideration. A total of 195 laboratory local scour evolution data sets were located and obtained. All of these data are for single, simple shaped piers founded in cohesionless sediments and subjected to constant-in-time flow velocities. Only one completed local scour field study, where scour evolution rates and the associated flow parameters were measured, was obtained. The struc- ture was a small, 2 ft square pile located on the seaward end of a bent on a bridge over a tidal inlet on the northern Gulf Coast of Florida. The predictive equations/methods were tested using both laboratory and field data. Because the maturity of the scour hole at the time of measurement for the field data was unknown, the field data were only used to evaluate underprediction by the equations. If the measured scour depth was that produced by the measured flow (and not from a previous, more severe event), the predicted depths should be greater than the measured values. There is little information regarding the measurement and data reduction techniques (e.g., how local scour was distin- guished from the other forms of scour, how the measurements were made, etc.) for two of the data sets (Zhuravlyov 1978 and Gao et al. 1993). The Zhuravlyov report contains both labo- ratory and field data from several sources but without any information regarding methods used to obtain the data. Field data from one of the sources were eliminated as being inconsis- tent with data for similar conditions from other sources that document instrumentation and data acquisition information. After the initial evaluation of both the equilibrium scour and scour evolution equations, improvements were made to the best-performing equations/methods. A melding of Sheppard and Miller's (2006) and Melville's (1997) equilibrium equations resulted in the single best- performing equation, referred to here as the Sheppard/Melville or S/M equation. Melville and Chiew's (1999) scour evolution equation, which contains an expression for equilibrium scour depth, was modified by adjusting some of the coefficients and replacing the equilibrium scour term with the S/M equation. This equation, referred to here as the Melville/Sheppard or M/S equation, proved to be the best-performing scour evolution equation with the laboratory data. No scour evolution field data for steady (or quasi-steady) flows was located in the data search. Findings This research produced (1) comprehensive equilibrium scour and scour evolution quality- controlled laboratory and field databases, (2) a compilation of predictive equations for equilibrium local scour depth and scour evolution rates, (3) an evaluation of the accuracy of the equations and the data in the databases, (4) an evaluation of the methods used to account for the effect of flow skew angle on local scour depth, (5) a recommended equation for equilibrium scour prediction, (6) a best-performing equation for scour evolution prediction, and (7) a list of data gaps and recommendations for future local scour research. Conclusions The ability to predict equilibrium scour depths at structures of relatively simple geometry is relatively good. There are, however, practical conditions where little or no laboratory data exist. In particular, experiments with large structures founded in relatively fine sand and subjected to high-velocity, live-bed flows are needed. More research and data are needed to improve the accuracy of scour evolution predictions. In addition, equilibrium scour and scour evolution data are
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3 needed (1) for long skewed piers subjected to a range of water depths, flow velocities, and sediment sizes; (2) to better determine the effects of sediment gradation on equilibrium scour and scour evolution rates; (3) for local scour at low-velocity flows; and (4) for local scour with unsteady flows. Recommended Predictive Equations This study has produced recommendations for predicting (1) equilibrium local scour depths at piers with simple geometries of all widths, (2) the effect of flow skew angle on equilibrium scour depth, and (3) scour evolution rates at structures with simple geometries. This information in conjunction with the methods described in the Florida DOT Scour Manual (Sheppard and Renna 2005) can be used for estimating local scour depths at piers with more complex geometries. The recommended equilibrium local scour equation resulting from this study is a melding of equations developed by Sheppard and Miller (2006) and Melville (1997). This equation, which performed best of those tested with both laboratory and field data, contains parameters that account for the primary scour mechanisms. The best-performing scour evolution equation is a modification and combination of previous works by Melville and Chiew (1999) and Sheppard and Miller (2006). However, the data used in the development and testing of this equation in the live-bed scour range are limited to experiments with small laboratory structures and, therefore, the predicted scour evolution rates may not be appropriate for use in the development of design scour depths at this time. The recommended equilibrium scour depth equation, the best-performing scour evolution equation, and the method to account for flow skew angle are presented in this report.