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112 C H A P T E R 8 8.1 Conclusions This reference guide is based on research conducted for NCHRP Project 24-34, âRisk-Based Approach for Bridge Scour Prediction.â The research accomplished its basic objective of devel- oping a risk and reliability-based methodology that can be used in calculating bridge pier, abut- ment, contraction, and total scour at waterway crossings so that scour estimates can be linked to a probability. The developed probabilistic procedures are consistent with LRFD approaches used by structural and geotechnical engineers. There is widespread belief within the bridge engineering community that unaccounted-for biases and input parameter and hydraulic modeling uncertainty lead to overly conservative estimates of scour depths. The perception has been that this results in the design and con- struction of costly and unnecessarily deep foundations. This reference guide provides risk and reliability-based confidence bands for bridge scour estimates that align the hydraulic design approach with the design procedures currently used by structural and geotechnical engineers. Consequently, hydraulic engineers now have the option and ability to perform scour calcula- tions that incorporate probabilistic methods into the hydraulic design of bridges. The research project developed and implemented a work plan that produced significant results of practical use to the bridge engineering community. The project led to the develop- ment of two approaches that can be used by hydraulic engineers to more efficiently predict bridge scour. The Level I approach makes use of a set of tables of probability values or scour factors to associate an estimated scour depth provided by the hydraulic engineer with a probability of exceedance for simple pier and abutment geometries. For complex founda- tion systems and channel conditions or for cases requiring special consideration, the Level II approach is necessary. A Level II approach also is necessary if the unconditional prob- ability of exceeding design scour depths to meet a target reliability over the life of a bridge is desired. To develop the probability-based estimates or scour factor tables for each scour component and to develop the Level II approach, the project included an examination of the uncertain- ties associated with the prediction of individual scour components. These uncertainties were incorporated into a reliability analysis framework to estimate the probability of scour level exceedance for the service life of a bridge. The reliability analysis for scour is consistent with the reliability analysis procedures developed and implemented by AASHTO LRFD/LRFR for calibrating load and resistance factors for bridge structural components and bridge structural systems as well as foundations. The Level I approach to determine the conditional probability of exceedance of design scour depth for a 100-year design event can be applied using the 27-element matrix presented in Conclusions and Observations
Conclusions and Observations 113 Appendix B if a bridge fits the criteria of one of the 27 bridge categories reasonably well. In total, more than 300,000 HEC-RAS/Monte Carlo simulations were required to produce the statistics on which the 27 tables in Appendix B are based. In addition, more than 300,000 scour calculations were completed off-line for each of the scour equations (resulting in more than 1.2 million off-line scour calculations). The Level II approach consists of a step-by-step procedure that hydraulic engineers can fol- low to provide probability-based estimates of site-specific scour factors. Conducting a Level II analysis implies that the design engineer must implement a HEC-RAS/Monte Carlo simula- tion using software similar to the rasToolÂ© developed for NCHRP Project 24-34. The rasToolÂ© software used in preparing this reference guide is a research-level software engine that requires considerable insight on the part of the user for application of the processes for Level II condi- tional and unconditional probability analyses. Specifically, the Monte Carlo simulation soft- ware was not developed for distribution, nor is it thoroughly documented or supported for general use. It is, however, considered robust and could be applied to a range of bridge and/or open-channel applications. Development of user-friendly HEC-RAS/Monte Carlo simulation software is listed as a research need in the Contractorâs Final Report for NCHRP Project 24-34, which can be found at www.trb.org. It bears repeating that the primary purpose of NCHRP Project 24-34 was to analyze the probability of scour depth exceedance, not the probability of bridge failure. The latter requires advanced analyses of the weakened foundation under the effects of the expected applied loads, which was beyond the scope of the research for this project. 8.2 Observations During the course of NCHRP Project 24-34 a number of issues, considerations, and results were encountered that merit further discussion. 8.2.1 Data Analysis Issues 184.108.40.206 Pier Scour There exists a plethora of data on pier scour from many sources, including both laboratory and field studies. The data sets used in the research that led to this reference guide included both clear-water and live-bed conditions. Both the HEC-18 and Florida DOT pier scour equations were developed as design equations, not best-fit prediction equations, and thus have a degree of conservatism built in. As such, the equations do not underpredict observed scour very often, and the reliability indexes for pier scour compare favorably with those used by structural and geotechnical engineers in LRFD applications for bridges. 220.127.116.11 Contraction Scour In contrast with the pier scour equations, the HEC-18 contraction scour equations are essen- tially predictive, given that they are derived from sediment transport principles and theory. Therefore, underpredictions of observed scour are much more common, and the resulting reli- ability is very low compared to typical target values used in LRFD applications. Only studies that used long-contracted sections were analyzed, because short contractions include an abutment scour effect. Available data were limited to the clear-water condition. 18.104.22.168 Abutment Scour The Contractorâs Final Report for NCHRP Project 24-20, âEstimation of Scour Depth at Bridge Abutmentsâ (Ettema et al. 2010) was published as this study was beginning. The results
114 Reference Guide for Applying Risk and Reliability-Based Approaches for Bridge Scour Prediction of that research have been formally incorporated into the 5th edition of HEC-18 (Arneson et al. 2012). Many data sets in the literature deal with abutment scour. Unfortunately, most of those data sets do not contain sufficient information regarding the distribution of flow between the main channel and the overbank area to allow analysis using the NCHRP Project 24-20 approach. The equations for live-bed abutment scour (Scour Condition A) and clear-water abutment scour (Scour Condition B) both use a calculation for contraction scour and then apply an amplification factor to account for the additional scour caused by local effects at the tip of the abutment. Therefore the scour predicted by this method is the total scour at the abutment. Because the amplification factors were developed as envelope curves to the observed scour depths, the equations are considered to be design equations and therefore have a degree of built- in conservatism. The reliability of the abutment scour equations was found to be intermediate between those of the pier scour and contraction scour equations. 8.2.2 Importance of Hydrologic and Hydraulic Uncertainty The HEC-RAS/Monte Carlo simulations proved to be very enlightening with respect to quantifying the effect that hydrologic and hydraulic uncertainties have on scour estimates. Using standard Water Resources Council Bulletin 17-B methodology, the uncertainty in the design discharge is easily quantified using the upper and lower 95% confidence limits. Risk increasesâand the confidence interval decreasesâwith increasing periods of record. Using the confidence limits from flood frequency analyses showed that hydrologic uncertainty can have a major influence on scour variability. Given any particular discharge, a hydraulic model (such as HEC-RAS) is necessary to develop hydraulic conditions such as depth and velocity, which are then used as input to the scour equations. A striking result of the research for NCHRP Project 24-34 was the effect of the Manning n resistance coefficient on the distribution of flow between the main channel and the overbank areas, and the resulting effect on the different types of scour. For pier scour, both the HEC-18 and Florida DOT equations were shown to be relatively insensitive to changes in flow distribution. In contrast, the contraction and abutment scour equations were very sensi- tive to this effect. Calibrating a hydraulic model to high water marks observed for various floods is crucial to reducing hydraulic uncertainty and thus reducing uncertainty in contrac- tion and abutment scour depths. 8.2.3 Roadway Overtopping When roadway overtopping is incorporated in the hydraulic model, contraction scour is considerably reduced. Roadway overtopping results in road closure and often results in damage to the approach embankments and possibly to the road surface. However, the bridge itself benefits from the relief of flow afforded by the overtopping condition. This effect has important implications for the design of new bridges as well as the analysis of existing bridges. Where overtopping is likely, the hydraulic model should reflect this as accurately as possible because of the benefit it provides in reducing contraction scour. For develop- ing the scour factors used in Chapter 5 and the service life target reliability analysis used in Chapter 6 of this reference guide, however, the effects of roadway overtopping were not included. The total discharge was routed through the bridge opening in all the Monte Carlo simulation runs.
Conclusions and Observations 115 8.2.4 Total Scour The combined effect of pier scour plus contraction scour was investigated to develop reliabil- ity indexes for the probability that the total design scour would be exceeded during the design life of the bridge. As first noted in Chapter 4, the NCHRP Project 24-20 abutment scour equations predict total scour at the abutment. NCHRP Project 24-37, currently underway, will examine whether total scour can be accurately estimated as simply a superposition of the individual com- ponents. Presumably, that study will include examination of the accuracy of estimates when a pier is within the abutment scour zone. The results of NCHRP Project 24-37 will have implica- tions for the probability-based total scour procedures presented in this reference guide.