1. 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

2. 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.

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

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

5. 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.

6. 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

7. 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

8. 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 1
Good
6-7 2
Fair
4-5 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)
Given continuing stream conditions, a loss event occurrence is moderate and more likely to occur than not.
High (B)
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

9. 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

10. 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 3
6, 14 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
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11. 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)
Damage is not as severe and repairs may not necessarily need to be made, although further evaluation is recommended.
High (B)
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

12. 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

13. 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
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14. Peggy A. Johnson Penn State University
Third example is PA Route 4013 over Bentley Creek, Bentley Creek, PA
Peggy A. Johnson Penn State University

15. 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

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

17. 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

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

20. 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