Current practice for determining the total scour prism at a bridge crossing involves the calculation of the various individual scour components (e.g., pier scour, abutment scour, contraction scour, and long-term channel changes). Then, using the principle of superposition, these individual components are considered to be purely additive and the total scour prism is then drawn as a single cumulative line for various frequency flood events (e.g., 50-year, 100-year, and 500-year flood events). The scour equations are generally understood to be conservative in nature and, with the exception of the contraction scour equations, have been developed as envelope curves for use in design. This approach does not provide an indication of the uncertainty involved in the computation of any of the individual components. Uncertainties in hydrologic and hydraulic models and the resulting uncertainty of relevant inputs (e.g., design discharge, velocity, depth, and flow distribution between the main channel and the floodplain) to the scour calculations will all have a significant influence when evaluating the risk associated with scour prediction. To develop an overall estimate of confidence in the calculated scour depths, one must use engineering judgment and examine the level of confidence associated with the results of the hydrologic analysis, the level of confidence associated with the hydraulic analysis, and the level of confidence associated with each of the scour components. Scour reliability analysis involves quantification of the uncertainties in each of these steps and then combines them in such a way that the overall estimate of confidence is known for the final prediction of scour. Research was performed under NCHRP Project 24-34 by Ayres Associates with the assistance of the City College of the City University of New York and the Pennsylvania State University. The objective of NCHRP Project 24-34 was to develop a risk-based methodology that can be used in calculating bridge pier, abutment, and contraction scour at waterway crossings so that scour estimates can be linked to a probability consistent with Load and Resistance Factor Design (LRFD) approaches used by structural and geotechnical engineers. This Reference Guide is oriented toward the practitioner. The research agency’s final report documenting the complete results of the research is not published but is available online at www.trb.org by searching “NCHRP Project 24-34.” (Author/publisher)
Samenvatting