Assessment, prioritization, and communication of risks of failures of a bridge scour countermeasure over a range of flow events

Open Access
Ashraf, Fahmidah U
Graduate Program:
Civil Engineering
Doctor of Philosophy
Document Type:
Date of Defense:
June 18, 2015
Committee Members:
  • Peggy Ann Johnson, Dissertation Advisor
  • Xiaofeng Liu, Committee Member
  • Alfonso Ignacio Mejia, Committee Member
  • Mort D Webster, Committee Member
  • Risk assessment
  • Bridge scour countermeasure
  • Failure Modes and Effects Analysis
  • Risk Matrix
  • Backwater Effect
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 risk-informed 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.