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ASSESSMENT, PRIORITIZATION, AND COMMUNICATION OF RISKS OF FAILURES OF A BRIDGE SCOUR COUNTERMEASURE OVER A RANGE OF FLOW EVENTS
ABSTRACT
Assessment, prioritization, and communication of risks of failures of countermeasures over a wide range of flow events can be very effective in the selection and the design of an appropriate countermeasure considering the uncertainty in the performance of a countermeasure in an uncertain complex river system. In this study, a risk-based approach is developed for assessing, prioritizing, and communicating the risks of failure modes of a bridge scour countermeasure using Failure Modes and Effects Analysis (FMEA) tool. In FMEA, the risks of failure modes are derived from the severity (S) and occurrence (O) ratings. Widely used approach of Risk Priority Number (RPN), product of the risk ratings, is improved to assess risks overcoming the mathematical limitations of Risk Priority Numbers (RPNs). Another simple and effective approach of assessing risks, Risk Priority Code (RPC), is used to make a comparison between the RPN and RPC results, and to find out which approach is comparatively more rigorous. The appropriate risk assessment approach is then used to build risk matrix. Risk matrix is a table that maps severity (S) and occurrence (O) ratings to the corresponding risk priority level. The developed risk matrix will be able to assess and prioritize the risks of failure modes of a countermeasure using only the qualitative information represented by S and O ratings. An approach of defining and incorporating confidence level in risk matrix is developed to provide more comprehensiveness and/or completeness to the risk matrix. Such an approach of developing risk matrix will enable bridge engineers to use risk matrix as a basis for making riskinformed decision. To determine the relative change in a risk priority level over a range of flow conditions, a study is then carried out focusing on the backwater in the vicinity of a bridge. The backwater is simulated using one dimensional model Hydrologic Engineering Centers River Analysis System (HEC-RAS). The effects of backwater are defined based on the monitoring survey data. Then, the developed risk matrix is used to determine any change in the risk priority level considering the backwater effects. Such an approach is simple enough to implement in the planning phase to account for the effects of flood events in the risk-based approach of selection and design of a countermeasure. Given the unavailability of quantitative information and the limited resources and time, the study results have important implications in the selection of an appropriate countermeasure and/or to pay special attention to certain components of a countermeasure which are at a higher risks in some situations.
TABLE OF CONTENTS
List of Figures………………………………………………………………………………….vi
List of Tables………………………………………………………………………………….viii
Acknowledgements…………………………………………………………………………….ix
Chapter 1 Introduction………………………………………………………………………….1
Chapter 2 Risk Assessment for the Failure Modes of a Bridge Scour Countermeasure………..6
2.1 Introduction…………………………………………………………………………………6
2.2 Selection of Bridge Scour Countermeasure…………………………………………………7
2.3 Failure Modes and Effects Analysis (FMEA)………………………………………………8
2.3.1 Prioritizing the Failure Modes using RPNs………………………………………9
2.3.2 Proposed Approach of Prioritizing the Failure Modes using RPNs…………….10
2.3.3 Alternative Approach of Prioritizing the Failure Modes using RPCs……………12
2.4 Illustrative Example……………………………………………………………………….16 2.4.1 Traditional RPN Approach………………………………………………………17
2.4.2 Proposed RPC/SO Code Approach………………………………………………18
2.4.3 Comparison………………………………………………………………………24
2.5 Conclusion…………………………………………………………………………………27
2.6 References…………………………………………………………………………………29
Chapter 3 Communicating the Risk of Failure of a Bridge Scour Countermeasure…………..32
3.1 Introduction………………………………………………………………………………..32
3.2 Risk Matrices and Their Applications……………………………………………………..33
3.2.1 Assessment and Prioritization of Risks in Risk Matrices……………………….34
3.2.2 Risk Matrices using FMEA Data………………………………………………..35
3.2.3 Communicating Confidence Level in Risk Matrices……………………………37
3.2.4 Limitations of Risk Matrices…………………………………………………….38 3.3 Analysis Approach for Constructing a Risk Matrix……………………………………….39
3.3.1 Define Risk Factors and Rating Scales in FMEA……………………………….40
3.3.2 Risk Assessment and Risk Prioritization………………………………………..41
3.3.3 Building Risk Matrices using FMEA Data………………………………….…..42
3.3.4 Defining Confidence Levels Associated with Risk Ratings…………………….45 3.3.5 Risk Informed Decisions based on Risk Matrices………………………………49
3.4 Illustrative Example……………………………………………………………………….49
3.4.1 Using Risk Matrices to Assess and Prioritize Failure Modes……………………50
3.4.2 Defining Confidence Level in S and O Ratings…………………………………53
3.4.3 Incorporating Confidence Level in Risk Matrix…………………………………55
3.5 Conclusion…………………………………………………………………………………59
3.6 References…………………………………………………………………………………61
Chapter 4 Risk Assessment for the Failure of an In-stream Bridge Scour Countermeasure Postflood Events……………………………………………………………………………………66
4.1 Introduction………………………………………………………………………………..66
4.2 In-stream Countermeasure and the Associated Risks……………………………………..67
4.2.1 Flood Events and In-stream Countermeasure……………………………………68
4.2.2 Laboratory, Numerical, and Field Studies of In-stream Countermeasures………69
4.3 Risk Assessment using Risk Matrix……………………………………………………….70
4.4 Methodology………………………………………………………………………………71
4.4.1 Step 1. Develop a Risk Matrix…………………………………………………..71
4.4.2 Step 2. Compute Backwater……………………………………………………..72
4.4.3 Modify the Risk Matrix………………………………………………………….72
4.5 Case Studies……………………………………………………………………………….73
4.5.1 Pennsylvania Test Site…………………………………………………………..73 4.5.2 Using the Proposed Approach in Design………………………………………..76 4.5.2.1 Step 1: Construction the Risk Matrix……………………………………….76
4.5.2.2 Step 2: Developing the HEC-RAS Model……………………………….….76
4.5.2.3 Step 3: Modify Risk Matrix based on Model Results………………………78
4.6 Conclusion…………………………………………………………………………………81
4.7 References…………………………………………………………………………………82
Chapter 5 Conclusion………………………………………………………………………….86
Appendix A Rating Scales of Severity (S), Occurrence (O), and Detection (D), Chapter 2, 3,
and 4……………………………………………………………………………………………88
Appendix B Easton road over Chippewa creek in Wayne County, Ohio, HEC-RAS inputs,
outputs, and sediment data, Chapter 4………………..………………………………………..90
Appendix C White Deer Creek in Northern Union County, Pennsylvania, HEC-RAS inputs
and outputs, Chapter 4……………………………………………………………………..…177
Chapter 1 Introduction
Maintenance at bridges is frequently required to protect the foundations against significant erosion. Channel instabilities, such as aggradation, degradation, widening and lateral migration, are apt to change over time and can act to initiate bridge scour, which can then undermine the safety of bridge foundations. Bridge scour countermeasures can limit the degree of channel instability, thus reducing the risk of failure of the bridge due to scour.
Although there are many types of bridge scour countermeasures, a strong interest is directed towards the in-stream flow alteration countermeasures that are commonly used in United States for stream restoration and bed or bank stabilization projects. In-stream countermeasures are intuitively designed to consider both the form and the function of the channel promoting stream ecology, and therefore, they are less likely to create imposing regulatory burdens. Moreover, the science and practice of in-stream countermeasures have evolved significantly over past few decades.
The challenge associated with applying in-stream countermeasures is that bridge engineers are often faced with the task of selecting, designing and predicting the performances of the countermeasures, even when there exists no quantitative information regarding the random variability of the hydrologic and hydraulic parameters. Both the limited allocated resources and incapability in completely evaluating the complex interactions of stream hydrodynamics with the streambed sediments make it almost impossible in completely covering the complete nature of uncertainty in the performance of in-stream countermeasures. The problem gets aggregated when limited resources are allocated in detailed design analyses of certain components of countermeasures based on the subjective judgment of the bridge engineers without any effort to classify, sort and analyze the uncertainty in the used information; and this leads to an ambiguous and vague process of selecting and designing of a countermeasure. Consequently, there always remains the risk of failure of a countermeasure in meeting the design intent that is, protecting the bridge from scour.
In the planning phase, qualitative risk analysis is the only best option in identifying and prioritizing the potential failures of a countermeasure considering the unavailability of the quantitative information. Such a risk-based approach can also facilitate in making risk-informed decisions regarding selection of an appropriate alternative, or to identify which component of the countermeasure require detailed design analysis to attain desired certainty.
Due to simplicity of usage and the requirement of only qualitative information, Failure Modes and Effects Analysis (FMEA) is one of the most widely used qualitative risk-based approaches, which identifies the effects, frequencies, and detection methods of the failure modes for a specific system. Risk assessment in FMEA is performed by developing a Risk Priority Number (RPN), which is a product of severity (S), occurrence (O), and detection (D) ratings associated with individual failure modes. Risk assessment in FMEA can be also performed by calculating Criticality (C), product of S and O ratings, since multiple failure modes with different S, O, or D ratings might have similar RPNs. However, it is recommended that, D should not be considered when prioritizing the failure modes since it is not a risk component (Society of Automotive Engineers, Bowles 1998, Palady et al. 1994), which favors the use of Cs in FMEA. However, unavailability of appropriate tools for prioritizing Cs gives no option to the risk analysts but to use RPNs in FMEA.
Although FMEA provides an opportunity for being an effective risk analysis tool, spurious interpretations of RPNs in prioritizing the risks prohibits its usage as a potential tool for making risk-informed decisions. Rank order of risks, based on RPNs, implies that trade-offs can be made between S, O, and D ratings, although rating scales do not tell us how changes along one dimension relate to changes along another dimension. Thus, the RPNs lack rational comparison and prioritization technique raising concerns about the performance of RPNs in actually improving the risk-informed decisions. A detailed study is required to establish rationalisms for making decisions using RPNs. It is also necessary to compare the performance of RPNs with other simple risk assessment approach, such as Risk priority code (RPC) (Franceschini and Galetto 2001), as a basis for making risk-informed decisions.
On the other hand, risk matrices, one of the most practiced risk assessment tools (Cox 2008), are usually used to convey the risk information to the decision makers. In risk management standards and reference books, risk matrix is promoted as a tool to decide which problems to work on. Risk matrices are very popular with terrorism risk analysis, highway construction project management, office building risk analysis, climate change risk management, and enterprise risk management (Cox 2008). U.S. Federal Highway Administration (2006), U.S. Federal Aviation Administration (2007), and Australian Government (2006) are some of the many risk organizations and risk consultants that use risk matrices to assess risks and set priorities in addressing issues. In risk matrix the risks are, in general, assessed based on the severity (S) and occurrence (O) ratings of risk events.
However, risk matrices are criticized with regard to the unreliable risk assessment approach, such as using the product of S and O ratings, that is, criticality (C) as a basis for assessing the relative risks. Moreover, there are no standard reliable risk matrix that can be used in any risk management project. A study must be carried out enhancing the use of risk matrices overcoming the mathematical and logical limitations. It is also necessary to provide more completeness and/or comprehensiveness to the risk matrix to increase the reliability of risk matrix as a basis for risk-informed decisions.
The goal of the proposed work is to aid making risk-informed decisions in the selection and design process of bridge scour countermeasures. Specific objectives to be addressed that will contribute to the attainment of the goal are: (1) to assess and prioritize the relative risks of failure modes of a bridge scour countermeasure using FMEA overcoming the limitations of RPNs; (2) to present complex risk data in a concise visual fashion using an accurate and reliable risk matrix, which can be used as a basis for risk-informed decisions; and (3) to increase the accuracy of risk matrix over a range of flow conditions. In order to achieve the objectives, the following three hypotheses are proposed.
Hypothesis 1. Not all values of RPNs can be compared in a rigorous way.
Hypothesis 2. A rigorous approach of risk assessment and incorporation of confidence level will enable to build an accurate and comprehensive risk matrix, which can be used as basis for riskinformed decisions.
Hypothesis 3. Predicting changes in the relative risk of a failure mode of a countermeasure over a range of flow conditions can be done using a simple approach of incorporating mathematical model results into a risk matrix.
Specific contributions of individual chapters are as follows.
Chapter two illustrates why all RPNs cannot be compared in a way that higher RPNs have higher S and O ratings. The study will use FMEA to identify the failure modes and the associated local and system-wide effects for a bridge scour countermeasure. Failure modes will be compared based on RPNs. Comparison and prioritization of failure modes will be conducted, ensuring that the higher RPNs have comparatively higher ratings. With the hypothesis that all RPNs cannot be compared in such a rigorous way, the approach of RPC will be implemented to compare all the failure modes. The results of RPNs and RPCs will then be compared. Finally the most appropriate risk assessment approach will be suggested based on the relative simplicity and effectiveness of the approach.
Chapter three describes how an accurate and reliable risk matrix can be built. An appropriate risk assessment approach will be used to build a rigorous risk matrix. The built risk matrix will provide prioritization of the risks of failure of a countermeasure based on a rigorous analysis. Once the matrix is constructed, an approach will be developed for defining and incorporating the confidence level into the risk matrix. The developed complete and comprehensive risk matrix will be able to provide the basis for risk-informed decisions.
Chapter four describes an approach of assessing the risks of the failure of an in-stream countermeasure in the vicinity of a bridge over a range of flow events. The focus will be on the backwater that is resulted in the vicinity of some bridges during flood events. The onedimensional model, HEC-RAS, will be used to simulate backwater under different flow conditions. The effects of backwater will be assessed based on the monitoring survey data using the risk matrix developed in Chapter 2. The developed approach will enable to integrate HECRAS model results in a risk matrix for the relative risk assessment of a failure mode of an instream countermeasure.
Appendix A includes the rating scales of severity (S) and occurrence (O) used for assessing and prioritizing the failure modes of a countermeasure in Chapter three and four.
