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50 where extremely limited and yet many, if not most, bridge piers ys = S/M equilibrium scour equation in the United States fall into this category (i.e., relatively C1 = -0.04 large structures and high-velocity design flows). C2 = 200 2. Experiments to obtain equilibrium scour depths, and C3 = 127.8 scour evolution rates, at rectangular piers, aligned with te = reference time and skewed to the flow, under controlled laboratory con- t90 = time to reach 90% of equilibrium scour depth ditions. Reported scour data for piers skewed to the flow is extremely limited. These tests could be performed with The M/S equation is the best performing of the equations/ moderate size structures. methods analyzed. The accuracy and range of structure pier 3. Similar experiments to those described in the first item widths for the clear-water scour data far exceeds that for the above, but with much larger piers. These tests will have to be live-bed scour data. For this reason there is greater confidence in the prediction of scour evolution rates in the clear-water conducted in a large stream where the flows are sufficiently scour range. high and controlled and the sediment cohesionless. There are many situations where the pier is large and the 4. Experiments to investigate equilibrium scour depths and design flow velocity is relatively small and/or of short duration. scour evolution rates at complex and multiple piers. The vast Scour evolution rates are important for these cases because majority of local scour experiments have been performed equilibrium scour depths are not likely to be achieved during with circular or square cylinders while most prototype the design event. Substantial bridge foundation cost savings piers are more complex in shape. While methods exist could be realized if scour evolution rate were considered when (Richardson and Davis 2001, Sheppard and Renna 2005) predicting design scour depths. However, due to the lack of for estimating local scour depths at piers with complex data for even moderate size structures in the live-bed scour shapes, more laboratory data are needed to test the accuracy range even the best-performing scour evolution equation of these methods. (M/S equation) should not be used for design at this time. 5. Experiments to investigate influences of sediment gradation, It can be used for estimating the level of conservativeness g, on equilibrium scour depths. Armoring of the bed in the of design scour depths based on equilibrium scour values. vicinity of a structure due to large sediment size distributions More research, including controlled, live-bed tests with larger (large g) can have a substantial impact on equilibrium structures, is required before scour evolution rate predictions scour depths. More data are needed before this effect can can be used in the development of design scour depths. be predicted to sufficient accuracy for use in design. 6. Experiments to investigate local clear-water scour at low values of V1/Vc (V1/Vc < 0.7). Some reported laboratory Recommendations for and field scour depth data for low values of V1/Vc are larger Future Research than would be expected. This investigation will improve The recommended equations for predicting equilibrium scour depths for typical daily flows (which are sometimes scour depths and scour evolution rate are empirical, although used for ship impact analyses). they are based on the physics of the sediment scour processes. 7. Experiments associated with testing the theory regarding As such, they can be no better than the data on which they are pressure gradientinduced local scour (Sheppard 2004). based. Because there are gaps in both the laboratory and field There are several explanations for why equilibrium scour data, there are practical combinations of structure, sediment, depends on a/D50. Understanding the underlying mecha- and flow conditions where the equations have not been nisms responsible for this dependence is important for the tested. Several experiments are proposed to address this extrapolation of predictive equations to conditions where problem. Detail regarding the recommended research is given laboratory data cannot be easily obtained due to flume size in Appendix B (available on the NCHRP Report 682 summary limitations, etc. web page: www.trb.org/Main/Blurbs/164161.aspx). A list of these experiments is presented below, in ranked order: The experiments described above are self-contained and can be conducted in parallel. Some could be conducted at more 1. Laboratory live-bed equilibrium scour and scour evolution than one location while others, such as the one with large rate experiments with uniform, fine sediment, and larger- structures in an outside open channel, will require special model pile structures. The data for these conditions are channels and there will be limited location options.