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5 data from the larger-structure laboratory tests clearly show accurate rate of erosion measurements in both the laboratory a decreasing dependence of equilibrium scour depth on and the field. A reasonable quantity of scour evolution labo- structure size as the structure size increases. The physics of ratory data exists in the literature (Oliveto and Hager 2002, why this occurs remains unproven. Some attempts have been Rajasegaran 1997, Grimaldi 2005, Melville and Chiew 1999, made to explain this phenomenon (e.g., Ettema et al. 2006 Sheppard et al. 2004, and Sheppard and Miller 2006). and Sheppard 2004). Ettema et al. investigated the differences A number of methods for computing local scour evolution in the scale of the turbulence in the wake region with increasing rates are also available (Chang et al. 2004, Melville and Chiew pier size and associated this with the decreasing dependence of 1999, Mia and Nago 2003, Miller and Sheppard 2002, Roulund scour on increasing pier width. Sheppard gave a theoretical et al. 2005, and Zaghloul and McCorquodale 1975). The more explanation involving the pressure-gradient field surrounding promising methods were identified and evaluated using a labo- the structure. According to this hypothesis, pressure gradients ratory scour evolution data set assembled as part of this project. in the vicinity of the structure due to the presence of the struc- ture are much larger for smaller structures than for larger ones. Report Organization The forces on the sediment grains produced by these pressure gradients are larger near small structures than for larger The general approach to the project is described in Chapter 2. prototype structures. This explains why predictive equations Equilibrium scour and scour evolution data acquisition based on small-scale laboratory data overpredict scour depths and analyses are covered in Chapter 3. Chapter 4 covers the at prototype-scale structures. Stated another way, for a given initial screening, modification, and final evaluation of the sediment size, some local scour mechanisms diminish in equilibrium scour prediction equations. Chapter 5 covers magnitude with increasing structure size. Therefore, predictive the initial screening, modification, and final evaluation of the equations, based on laboratory data, that do not consider this scour evolution equations. Chapter 6 covers the predictive decreased influence overpredict scour depths at prototype- equations for scour at piers skewed to the flow. Chapter 7 sum- scale structures. This has implications regarding (1) estimating marizes the results of this project regarding the best equations/ prototype scour depths from physical model test data and methods for predicting equilibrium local scour depths and (2) interpreting laboratory-scale scour rate data. scour evolution rates for wide piers and long skewed piers and The rate at which local scour occurs depends on all the outlines recommendations for future research. The appendices parameters that control sediment transport rate over a flat bed (available on the NCHRP Report 682 summary web page: as well as the structural parameters (size, shape, orientation www.trb.org/Main/Blurbs/164161.aspx) contain detailed to the flow). Improvements in instrumentation technology information that supplements and supports the information (miniature video cameras, high frequency, narrow beam presented in the report including the equilibrium scour data acoustic transponders, etc.) in recent years have led to a more compiled as part of this project.