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41.1 Introduction Load and resistance factor design (LRFD) incorporates state-of-the-art analysis and design methodologies with load and resistance factors based on the known variability of applied loads and material properties. These load and resistance factors are calibrated from actual bridge statistics to ensure a uniform level of safety over the life of the bridge. LRFD allows a bridge designer to focus on a design objective or limit state, which can lead to a similar probability of failure in each component of the bridge. Bridges designed with the LRFD specifications are intended to have relatively uniform safety levels, which should ensure superior serviceability and long-term maintainability. A widespread belief within the bridge engineering community has been that unaccounted- for biases and input parameter and hydraulic modeling uncertainty lead to overly conservative estimates of scour depths. The perception also has been that this results in design and con- struction of costly and unnecessarily deep foundations. NCHRP Report 761: Reference Guide for Applying Risk and Reliability-Based Approaches for Bridge Scour Prediction is intended to close the gap between perception and reality and provide risk and reliability-based confidence bands for bridge scour estimates that will align the hydraulic design approach with the design procedures currently used by structural and geotechnical engineers. Bridge hydraulic engi- neers now have the option of and ability to perform scour calculations that incorporate similar probabilistic methods. This reference guide provides a risk and reliability-based methodology that can be used in calculating bridge pier, abutment, contraction and total scour at waterway crossings so that scour estimates can be linked to a probability. The probabilistic procedures are consistent with LRFD approaches used by structural and geotechnical engineers. This document provides two approaches to estimating and predicting bridge scour. The Level I approach uses sets of tables of probability values (scour factors) that associate the estimated scour depth provided by the hydraulic engineer with a probability of exceedance for a given design event. The Level I approach is illustrated by a series of examples in Chapter 7 of this document. For complex foundation systems and channel conditions, or for cases requiring special con- sideration, the site-specific Level II approach is necessary. The Level II approach consists of a step-by-step procedure that hydraulic engineers can follow to develop probability-based esti- mates of site-specific scour factors. The Level II approach also is described in this document using an illustrative example. When using the Level I probability-based estimates or scour factor tables for each scour component or applying the Level II approach, the methodology requires an understanding of C H A P T E R 1 Introduction and Applications
Introduction and Applications 5 the uncertainties associated with the prediction of individual scour components. This refer- ence guide incorporates these uncertainties into a reliability analysis framework to estimate the probability of scour level exceedance for the service life of a bridge. The service life reli- ability 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 primary purpose of NCHRP Report 761 is to enable practitioners to analyze the prob- ability of scour depth exceedance, not the probability of bridge failure. Addressing the prob- ability of bridge failure requires advanced analyses of the weakened foundation under the effects of the expected applied loads, which is beyond the scope of this reference guide. 1.2 Applications This reference guide is based on the results of NCHRP Project 24-34, âRisk-Based Approach for Bridge Scour Predictionâ (Lagasse et al. 2013). The goals of NCHRP Project 24-34 were to develop a methodology that can be used to link scour depth estimates at a river crossing to a probability and extend this methodology to provide a preliminary approach for determining a target reliability for the service life of the bridge consistent with LRFD approaches used by structural and geotechnical engineers. The probability linkage considered the propagation of uncertainties among the parameters used to quantify the confidence of scour estimates for a design event based on uncertainty of input parameters and considering model uncer- tainty and bias. Although the focus of NCHRP Project 24-34 was refining hydraulic design approaches for bridges, the tools developed also can be applied to other situations for which a more precise evaluation of risk and reliability associated with flooding would improve public safety. 1.2.1 Transportation Facilities Bridge scour applications are the focus of this reference guide; however, the techniques described in this document can be used to assess potential threat and risk of failure for any existing or proposed transportation facility. These techniques also can be used to evaluate and justify the need for structural solutions or countermeasures to inhibit scour or channel instability in proximity to existing or proposed transportation facilities. Bridge appurtenant structures such as guide banks and roadway approach embankments, flow-control structures such as spurs or bendway weirs, and roadway alignments trending parallel to an active channel or on a floodplain could all benefit from a risk and reliability analysis using the techniques in this document. As a specific example, accumulations of vegetative debris (or drift) on bridges during flood events constitute a continuing threat to bridges nationally. Debris accumulations can obstruct, constrict, or redirect flow through bridge openings, resulting in flooding, damaging loads, or excessive scour at bridge foundations (see Figure 1.1). NCHRP Report 653: Effects of Debris on Bridge Pier Scour provides an approach to computing the increased scour potential at piers with debris (Lagasse et al. 2010). That study also provides an extensive data base from laboratory studies of debris clusters with a range of shapes, geometry, and locations in the water column. The scour equations developed from the debris study are deterministic and essentially provide a transform from a pier with debris to an equivalent wider pier. With these equations and the available data set, it is possible to use the techniques introduced in this reference guide to con- duct a detailed probability analysis of the results of the calculation procedures developed for debris loading on bridge piers.
6 Reference Guide for Applying Risk and Reliability-Based Approaches for Bridge Scour Prediction 1.2.2 Floodplain Risk and Flood Control Facilities In many areas along larger river systems and in close proximity to large urban areas, flood control facilities such as levees, dikes, and flood-relief structures are used to protect the public from major floods (see Figure 1.2). The U.S. Army Corps of Engineers (USACE) has developed a National Levee Data Base (NLD) to provide a focal point for comprehensive information about the nationâs levees. The data base contains information to facilitate and link activities such as flood risk communication, levee system evaluation for the National Flood Insurance Program (NFIP), levee system inspections, flood plain management, and risk assessments (Military Engineer 2012). The techniques developed in this reference guide, particularly the unique linkage between the fundamental hydraulic model supporting the NFIPâthe USACEâs Hydrologic Engineering Center River Analysis System (HEC-RAS)âand accepted statistical methods to quantify risk through simulation of the hydraulic parameters for a large number of flood events, enable a quantitative evaluation of risk to flood control facilities from a single flood event or over the remaining service life of a facility. Specifically, these techniques can Figure 1.1. Drift accumulation on a single bridge pier (photo courtesy of Ayres Associates). Figure 1.2. California State Highway 160 on Sacramento River levee (photo courtesy of Google EarthTM).
Introduction and Applications 7 establish the probability of exceeding design flood elevations or determine the probability of occurrence of critical flow velocities in excess of failure thresholds used as a basis for design. Nationally, roughly 2,000 federal levees extend over some 13,000 miles; more than 20,000 non- federal levees also exist whose extent has yet to be fully identified. Although an agency such as the USACE has a broad base of highly qualified hydrologists, hydraulic engineers, and scientists to assess the reliability and evaluate risks to federal levees, other levee owners do not have a commensurate level of technical support. For private levee districts, smaller municipalities, and private owners, the risk assessment techniques presented in NCHRP Report 761 offer an approach to identify, prioritize, evaluate, and counter possible threats to flood control infra- structure within their districts and on critical river reaches. 1.2.3 Channel Restoration and Rehabilitation Works Many stream bank stabilization and rehabilitation measures have failed because the designer was unable to establish the risk of failure during a design flood or the reliability of the struc- ture over its design life. In addition, there is increasing interest in the use of environmen- tally sensitive biotechnical approaches to channel restoration and stream bank protection as an alternative to more traditional âhardâ engineering techniques (see Figure 1.3). Design of both traditional and biotechnical measures requires accounting for hydrologic, hydraulic, geo- morphic, geotechnical, vegetative, construction, and maintenance factors. Many biotechni- cal measures (e.g., root wads, engineered log jams, and vegetated riprap) have been deployed for channel restoration and have survived for a number of years, but considerable skepticism remains within the engineering community regarding performance of these measures when subjected to flood event magnitudes typically experienced over the design life desired for res- toration or rehabilitation projects. In particular, very little information is available regarding the durability and service life expectations and maintenance requirements for biotechnical countermeasures. The techniques developed in this reference guide for simulating the hydraulic conditions for a large number of flood events would enable a quantitative evaluation of risk to channel restoration installations from a single flood event or over the remaining service life of the struc- ture. These techniques can establish the probability of exceedance (or of non-exceedance) of critical hydraulic design parameters such as flow velocity and shear stress in relation to failure thresholds used as a basis for design. Because many rehabilitation projects require establishing a desired sinuosity and protecting the resulting bendways from erosion (as shown in Figure 1.3), the ability to determine the probability of exceeding the design scour depth at protected mean- der bends is one obvious application of the techniques presented in this reference guide. 1.3 Organization of the Reference Guide Chapter 2 of this reference guide provides a discussion of the various types and sources of uncertainty that must be considered in the assessment of bridge scour. Citations from the literature provide relevant background information on the current state of practice. Hydrol- ogy and hydraulics both introduce uncertainties in the determination of the variables that are subsequently used as input to the various scour equations. That is, the three components of scour addressed (pier, contraction, and abutment scour) are fundamentally linked to both the hydrologic estimation of the magnitude of a design flood event and the anticipated hydraulic conditions associated with that event. The scour equations themselves involve uncertainty, as evidenced by the fact that even under controlled laboratory conditions the equations do not precisely predict the observed scour. Lastly, the scour problem is framed in the context of
8 Reference Guide for Applying Risk and Reliability-Based Approaches for Bridge Scour Prediction (a) Blunn Creek, Austin, TX, before restoration (b) During construction (c) After construction Figure 1.3. Typical biotechnical river restoration project protecting an eroding bendway (photo courtesy of City of Austin, TX, Watershed Protection Department).
Introduction and Applications 9 current guidance from FHWA and AASHTO LRFD statistical methods and procedures used in bridge structural design from the perspective of a hydraulic engineer. Chapter 3 describes an approach to evaluating the uncertainty of the three scour compo- nents. The approach is based on Monte Carlo simulation linked directly with the most common and widely accepted hydraulic model used in current practice, HEC-RAS. For each individual scour component, the parameters that were allowed to vary in the Monte Carlo simulation are discussed along with a matrix of other factors and considerations that were not addressed. The chapter provides a discussion of model uncertainty and the definitions of bias and coef- ficient of variation (COV) in relation to the scour equations. Chapter 3 also provides a discus- sion of the linkage between the hydraulic model HEC-RAS and the Monte Carlo simulation software. Chapter 4 presents a brief summary of the data sets used in developing model bias and COV for each of the three individual scour components. For pier scour, both the HEC-18 and Florida Department of Transportation (Florida DOT) equations are assessed. The equations are from the 5th edition of FHWAâs Hydraulic Engineering Circular (HEC) No. 18 (Arneson et al. 2012). The equations are assessed using comprehensive data sets from both laboratory and field stud- ies. Contraction scour uses the HEC-18 equation for clear-water scour with laboratory data only. Abutment scour uses the total scour approach recommended in the most recent edition of HEC-18 with laboratory data only. Chapter 5 provides two approaches for assessing the conditional probability that the design scour depth will be exceeded for a given design flood event. Either approach can be used to estimate this probability for each of the three individual scour components. The first approach (Level 1) assumes that the practitioner can categorize a bridge based on three general condi- tions: (1) the size of the bridge, channel, and floodplain (small, medium or large); (2) the size of the piers (small, medium, or large); and (3) the hydrologic uncertainty (low, medium, or high). This Level I approach provides scour factors that can be used to multiply the estimated scour depth to achieve a desired level of confidence based on the reliability index, b, commensurate with standard LRFD practice. Scour factors are provided in tabular format for each of the indi- vidual scour components for all 27 combinations of the three category conditions for simple pier and abutment geometries (Appendix B). When the practitioner cannot match a particular site to the categories described for Level 1, a Level II approach is required. Necessarily site-specific, the Level II approach is illustrated in this reference guide using data from a bridge on the Sacramento River. The discussion includes the results for pier, contraction, abutment, and total scour considering hydrologic uncertainty, hydraulic uncertainty, and scour prediction (model) uncertainty. A step-by-step summary of the Level II procedure is also provided. Chapter 6 presents a methodology to determine the unconditional probability that a scour estimate will not be exceeded over the remaining service life of an existing bridge or the design life of a new bridge. The proposed methodology uses the conditional probabilities of the design scour depth being exceeded for a limited number of return-period flood events. The condi- tional probabilities are then integrated to determine the unconditional probability of exceed- ance over the entire service life. The integration method is implemented for pier scour (both HEC-18 and Florida DOT methods), contraction scour (HEC-18 method), combined pier and contraction scour, and abutment scour using bridge-specific data. Chapter 7 provides five illustrative examples using the Level I approach to: (1) categorize a bridge site; (2) estimate pier, contraction, abutment, and total scour; and (3) identify the appro- priate scour factors for a desired level of confidence using the results provided in Chapter 5 and
10 Reference Guide for Applying Risk and Reliability-Based Approaches for Bridge Scour Prediction the information in Appendix B. Examples are presented for a range of bridge configurations and hydrologic/geomorphic settings where hydraulic input is developed from both 1-D and 2-D models. Chapter 8 briefly summarizes the procedures and applications and discusses topics beyond the scope of this document that would extend the results and usefulness of these procedures. A list of the references cited in NCHRP Report 761 follows Chapter 8. Appendix A provides a glossary of terms used in the reference guide that will be helpful to the practitioner not completely familiar with the statistical approaches that underpin the pro- cedures of this guidance document. Appendix B presents a summary of scour factors in tabular and graphical form for use with the Level I approach described in Chapter 5.