Communicating the Vulnerability of Bridges over Unstable Stream Channels Source: CBS 5 (KPHO Broadcasting Corporation) Peggy A. Johnson Civil and Environmental Engineering Penn State University

Channel instability includes channel widening, bank failure, bed degradation/aggradation, and lateral migration. Instability differs from local and contraction scour in that spatial and temporal scales are different and occur regardless of the bridge. Warrants a separate assessment and analysis. HEC-20 is FHWA manual on this topic.

Unstable stream channels are more difficult to predict and treat locally.

Assessing the relative risk of bridge loss due to stream channel instability is a difficult, but important task.

How can we assess the vulnerability and risk of unsatisfactory conditions at a bridge due to stream channel instability? Vulnerability – the degree to which a system is likely to experience harm as a result of exposure to an event Criticality – consequences of failure or an exposure event Risk – a function of vulnerability (likelihood) and criticality (consequences) Steps: Assign vulnerability rating: Determine criticality: Develop risk matrix. Assess risk and acceptability levels.

Peggy A. Johnson Penn State University Categories of Vulnerability Category Description Low A loss event due to stream channel instability is unlikely. Moderate Given continuing stream conditions, a loss event occurrence is moderate and more likely to occur than not. High Continuing stream instabilities will likely cause a significant loss event. Very High Given the current conditions, continuing stream instability will almost certainly lead to a loss event. Categories of Criticality Category Description Low Costs of failure are low. Moderate Costs of failure are moderate. High Overall loss is high and somewhat costly. Very High Overall loss is very high and costly. Peggy A. Johnson Penn State University

Vulnerability factors, sources, and weights Source of data Range of values Weights Stability assessment rating Johnson (2005; 2006) 12-156 0.33 Channel condition NBI Item 61 0 - 9 Waterway adequacy NBI Item 71 0.20 Scour NBI Item 113 0.13

Peggy A. Johnson Penn State University Vulnerability ratings were modified for integrating the stability assessment ratings with the NBI ratings. Condition NBI rating Assessment rating Reduced Rating Excellent 8-9 12 - 49 1 Good 6-7 50 - 85 2 Fair 4-5 86 - 120 3 Poor 0-4 ≥120 4 Category V Description Low (D) 1-1.75 A loss event due to stream channel instability is unlikely. Moderate (C) 1.76-2.50 Given continuing stream conditions, a loss event occurrence is moderate and more likely to occur than not. High (B) 2.51-3.25 Continuing stream instabilities will likely cause a significant loss event. Very High (A) 3.26-4 Given the current conditions, continuing stream instability will almost certainly lead to a loss event. Vulnerability categories were then developed. Peggy A. Johnson Penn State University

Units or range of ratings Criticality factors, weights and reduced ratings were also developed. Criticality Category Criticality Factor Units or range of ratings Weight Service Increased Travel Time* Hours 0.18 Functional class (#26) 1-19 0.12 ADT (#29) Number Replacement Cost Number of Spans (#45) Roadway Width ( #51) Meters Structure Length (#49 ) *Travel time = f(detour length, detour speed, length of roadway closed, initial speed of roadway) Peggy A. Johnson Penn State University

Reduced scale for criticality Rating Functional Class ADT Number of Spans Roadway Width (m) Structure Length Increased Travel Time (hours) 1 8, 9, 19 0-250 0-7.3 0-14.6 0-0.10 2 7, 16, 17 251-3000 7.4-9.3 14.7-26.8 0.11-0.20 3 6, 14 3001-7500 9.4-11.7 26.9-31.1 0.21-0.50 4 1, 2, 11, 12 > 7500 > 3 > 11.7 > 31.1 > 0.50 Functional Classification Rural 01 Principal Arterial - Interstate 02 Principal Arterial - Other 06 Minor Arterial 07 Major Collector 08 Minor Collector 09 Local Urban 11 Principal Arterial - Interstate 12 Principal Arterial - Other Freeways or Expressways 14 Other Principal Arterial 16 Minor Arterial 17 Collector 19 Local Peggy A. Johnson Penn State University

Peggy A. Johnson Penn State University Categories of criticality Category C Description Low (D) 1-1.75 Damage is not severe enough to warrant any repair or further investigation. Moderate (C) 1.76-2.50 Damage is not as severe and repairs may not necessarily need to be made, although further evaluation is recommended. High (B) 2.51-3.25 Damage is moderate and serious repairs need to be made to the structure and/or costs are moderately high associated with usability. Overall loss is moderate. Very High (A) 3.26-4 Damage is massive and the structure is in need of full replacement and/or costs associated with usability are high in comparison to cost of replacement. Overall loss is very high. Peggy A. Johnson Penn State University

Combining the vulnerability and the criticality leads to a relative approximation of risk.   Criticality Vulnerability Low Moderate High Very High 4A 3A 2A 1A 4B 3B 2B 1B 4C 3C 2C 1C 4D 3D 2D 1D 1A, 1B, 2A, 2B The levels of vulnerability and loss are too high to ignore and must be made a high priority to be controlled or eliminated. 1C, 2C, 3A, 3B, 3C These risks may be unacceptable; however, following further investigation, the bridge owner may choose to accept these risks. 1D, 2D, 3D, 4A, 4B, 4C, 4D These risks may be accepted upon the bridge owner’s review Increasing risk

Peggy A. Johnson Penn State University Three case studies illustrate the process. PA Route 3017 over Bald Eagle Creek, Port Matilda, PA PA Route 26 over Spring Creek in State College, PA Peggy A. Johnson Penn State University

Peggy A. Johnson Penn State University Third example is PA Route 4013 over Bentley Creek, Bentley Creek, PA Peggy A. Johnson Penn State University

Peggy A. Johnson Penn State University Bald Eagle Creek Bentley Creek Spring Creek   Criticality Vulnerability Low Moderate High Very High 4A 3A 2A 1A 4B 3B 2B 1B 4C 3C 2C 1C 4D 3D 2D 1D 1A, 1B, 2A, 2B The levels of vulnerability and loss are too high to ignore and must be made a high priority to be controlled or eliminated. 1C, 2C, 3A, 3B, 3C These risks may be unacceptable; however, following further investigation, the bridge owner may choose to accept these risks. 1D, 2D, 3D, 4A, 4B, 4C, 4D These risks may be accepted upon the bridge owner’s review Peggy A. Johnson Penn State University

Peggy A. Johnson Penn State University Risk-based methods can also be used for selecting bridge scour countermeasures. Peggy A. Johnson Penn State University

Peggy A. Johnson Penn State University Failure Modes and Effects Analysis and Risk Priority Numbers can be used to compute relative likelihood and severity. RPNs for countermeasures can be directly placed in a risk matrix along with confidence levels. Peggy A. Johnson Penn State University

Peggy A. Johnson Penn State University We can also include ability to detect failure. Peggy A. Johnson Penn State University

Peggy A. Johnson Penn State University Risk matrices can provide a simple visual communicating vulnerability and relative risk to conditions at a bridge or scour countermeasure alternatives at the design phase. The visual simplicity of risk matrices provides a tool that enables stakeholders to participate in a risk-informed decision making process. But, this requires rigorously prioritized failure modes or vulnerability/criticality assessment. Alternatives can be explored using the risk matrices, including confidence and detectability, to compare the relative risks of a variety of scour countermeasures and change the perception of decision makers. Peggy A. Johnson Penn State University