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4Background Current methods for predicting local scour at bridge piers, including those described in Hydraulic Engineering Circular No. 18 (HEC-18; Richardson and Davis 2001), were devel- oped 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 counter- measures. 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 main- tenance of highway bridges. NCHRP Project 24-32 was initiated to address this need. Objectives The objective of this research was to develop methods and procedures for predicting time-dependent local scour at wide piers and at long skewed piers that are suitable for considera- tion and adoption by the AASHTO. The research was limited to noncohesive soils and steady flow. Project Description Some level of awareness of sediment scour at bridge piers is as old as bridges themselves. The piers on many early stone arch bridges over rivers were rounded or pointed on the upstream end to streamline the flow and reduce scour. It was not until the 20th century, however, that engineers attempted to quantify and predict scour depths. The lowering of the sediment bed at a bridge pier can be caused by a number of mechanisms including degradation, channel or stream migration, man-made or natural stream contraction, and local structure-induced scour as described in a number of papers and reports including HEC-18. This project is limited to the structure-induced component of the scour, referred to here as local scour. Several approaches have been taken to predict equilibrium and evolution rates of local scour depths including analytical, dimensional analysis (using laboratory and/or field data to determine the functional relationships between the dimensionless groups), and numerical analysis (two- and three- dimensional modeling of the flows and sediment transport). The most successful attempts to date, from a practitionerâs point of view, have been those based on dimensional analysis techniques and laboratory data. This project is concerned with two aspects of the local scour problem, namely (1) the prediction of equilibrium scour depths at wide and long skewed piers and (2) the rate at which scour occurs at these piers. Historically, these problems have been addressed separately. Most local scour research has been directed at predicting equilibrium scour depths at structures with simple geometries, with some work in recent years on structures with more complex shapes. Less work has been reported on the rate at which local scour occurs. Most of the scour rate (scour evolution) predictive methods require knowledge of the equilibrium scour depths as well as the flow, sediment, and structure parameters. Both researchers and practitioners have observed that local scour depths at prototype structures with large projected widths (i.e., wide piers or long skewed) are less than those predicted by some of the equations developed using small- scale laboratory data. Researchers have attempted to account for this discrepancy in different ways. One attempt (Johnson and Torrico 1994) was to single out and analyze the larger-scale laboratory and field data and apply a correction factor to the predictive equation currently in HEC-18. Other researchers have included terms in the equation to account for scour depth dependence on structure size (and have conducted large- scale laboratory experiments to establish this relationship) and concluded that their empirical equations are applicable for the full range of structure sizes (Sheppard et al. 2004). The C H A P T E R 1 Introduction
5data from the larger-structure laboratory tests clearly show a decreasing dependence of equilibrium scour depth on structure size as the structure size increases. The physics of why this occurs remains unproven. Some attempts have been made to explain this phenomenon (e.g., Ettema et al. 2006 and Sheppard 2004). Ettema et al. investigated the differences in the scale of the turbulence in the wake region with increasing pier size and associated this with the decreasing dependence of scour on increasing pier width. Sheppard gave a theoretical explanation involving the pressure-gradient field surrounding the structure. According to this hypothesis, pressure gradients in the vicinity of the structure due to the presence of the struc- ture are much larger for smaller structures than for larger ones. The forces on the sediment grains produced by these pressure gradients are larger near small structures than for larger prototype structures. This explains why predictive equations based on small-scale laboratory data overpredict scour depths at prototype-scale structures. Stated another way, for a given sediment size, some local scour mechanisms diminish in magnitude with increasing structure size. Therefore, predictive equations, based on laboratory data, that do not consider this decreased influence overpredict scour depths at prototype- scale structures. This has implications regarding (1) estimating prototype scour depths from physical model test data and (2) interpreting laboratory-scale scour rate data. The rate at which local scour occurs depends on all the parameters that control sediment transport rate over a flat bed as well as the structural parameters (size, shape, orientation to the flow). Improvements in instrumentation technology (miniature video cameras, high frequency, narrow beam acoustic transponders, etc.) in recent years have led to a more accurate rate of erosion measurements in both the laboratory and the field. A reasonable quantity of scour evolution labo- ratory data exists in the literature (Oliveto and Hager 2002, Rajasegaran 1997, Grimaldi 2005, Melville and Chiew 1999, Sheppard et al. 2004, and Sheppard and Miller 2006). A number of methods for computing local scour evolution rates are also available (Chang et al. 2004, Melville and Chiew 1999, Mia and Nago 2003, Miller and Sheppard 2002, Roulund et al. 2005, and Zaghloul and McCorquodale 1975). The more promising methods were identified and evaluated using a labo- ratory scour evolution data set assembled as part of this project. Report Organization The general approach to the project is described in Chapter 2. Equilibrium scour and scour evolution data acquisition and analyses are covered in Chapter 3. Chapter 4 covers the initial screening, modification, and final evaluation of the equilibrium scour prediction equations. Chapter 5 covers the initial screening, modification, and final evaluation of the scour evolution equations. Chapter 6 covers the predictive equations for scour at piers skewed to the flow. Chapter 7 sum- marizes the results of this project regarding the best equations/ methods for predicting equilibrium local scour depths and scour evolution rates for wide piers and long skewed piers and outlines recommendations for future research. The appendices (available on the NCHRP Report 682 summary web page: www.trb.org/Main/Blurbs/164161.aspx) contain detailed information that supplements and supports the information presented in the report including the equilibrium scour data compiled as part of this project.
