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4 CHAPTER 1 Introduction Background and reports including HEC-18. This project is limited to the structure-induced component of the scour, referred to here Current methods for predicting local scour at bridge piers, as local scour. Several approaches have been taken to predict including those described in Hydraulic Engineering Circular equilibrium and evolution rates of local scour depths including No. 18 (HEC-18; Richardson and Davis 2001), were devel- analytical, dimensional analysis (using laboratory and/or field oped on the basis of small-scale laboratory studies and do not data to determine the functional relationships between the consider factors relevant to wide piers and long piers that are dimensionless groups), and numerical analysis (two- and three- skewed to the flow. Because of these limitations, the current dimensional modeling of the flows and sediment transport). methods generally overpredict local scour at such piers, leading The most successful attempts to date, from a practitioner's to the use of unwarranted and costly foundations or counter- point of view, have been those based on dimensional analysis measures. Research was needed to evaluate current methods techniques and laboratory data. for predicting local pier scour and their applicability to wide This project is concerned with two aspects of the local piers and long skewed piers and to develop improved methods scour problem, namely (1) the prediction of equilibrium scour for highway agencies to use in the design, operation, and main- depths at wide and long skewed piers and (2) the rate at which tenance of highway bridges. NCHRP Project 24-32 was initiated scour occurs at these piers. Historically, these problems have to address this need. been addressed separately. Most local scour research has been directed at predicting equilibrium scour depths at structures Objectives with simple geometries, with some work in recent years on structures with more complex shapes. Less work has been The objective of this research was to develop methods and reported on the rate at which local scour occurs. Most of the procedures for predicting time-dependent local scour at wide scour rate (scour evolution) predictive methods require piers and at long skewed piers that are suitable for considera- knowledge of the equilibrium scour depths as well as the flow, tion and adoption by the AASHTO. The research was limited sediment, and structure parameters. to noncohesive soils and steady flow. 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 Project Description predicted by some of the equations developed using small- Some level of awareness of sediment scour at bridge piers scale laboratory data. Researchers have attempted to account is as old as bridges themselves. The piers on many early stone for this discrepancy in different ways. One attempt (Johnson arch bridges over rivers were rounded or pointed on the and Torrico 1994) was to single out and analyze the larger-scale upstream end to streamline the flow and reduce scour. It was laboratory and field data and apply a correction factor to the not until the 20th century, however, that engineers attempted predictive equation currently in HEC-18. Other researchers to quantify and predict scour depths. The lowering of the have included terms in the equation to account for scour sediment bed at a bridge pier can be caused by a number depth dependence on structure size (and have conducted large- of mechanisms including degradation, channel or stream scale laboratory experiments to establish this relationship) migration, man-made or natural stream contraction, and local and concluded that their empirical equations are applicable structure-induced scour as described in a number of papers for the full range of structure sizes (Sheppard et al. 2004). The