Comparison of risk-based methods for bridge scour management

Figures & data

Figure 1. Reliability and risk concepts in bridge integrity management. (Abbreviations: p_f = probability of failure, c_i = indirect consequences, c_d = direct consequences).

Table 1. Scoring system for risk-based methods. N/A means that the factor has not been considered. (Abbreviations: h_f = scour probability; p_s|f = failure probability, c_i = indirect consequences, c_d = direct consequences).

Factor	Description	Estimated	score	Assessed	score
TANGIBLE VALUES
${\mathit{h}}_{\mathit{F}}$ Hazard	Probability that an event that can trigger scour (e.g., a flood) occurs within a given time frame	The hazard is estimated using indicators which relate e.g., to the geometry of the river profile	1	The hazard is assessed using hydraulic modelling to determine likely flows, water levels, discharges and velocities	2
${\mathit{p}}_{\mathit{S}|\mathit{F}}$ Vulnerability	Probability of scour damage given the occurrence of an event that can trigger scour (e.g., a flood)	The vulnerability is estimated using indicators e.g., related to the geometry of the foundation	1	The vulnerability is assessed through analytical or numerical models of the structural behaviour	2
Ci Indirect consequences	Consequences resulting from the loss of functionality of the bridge (e.g., rerouting), including social and environmental consequences	e.g., the impact of rerouting is estimated as low/medium/high according to the road type	1	e.g., the consequence of rerouting is assessed analytically in economic terms	2
${\mathit{C}}_{\mathit{d}}^{\mathit{t}\mathit{a}\mathit{n}}$ Direct consequences (tangible values)	Consequences related to the physical damage of the structure or of its components (e.g., repair and replacement)	e.g., the cost of rebuilding is estimated as low/medium/high according to the bridge type	1	e.g., the cost of rebuilding the bridge is assessed analytically	2
INTANGIBLE VALUES
${\mathit{C}}_{\mathit{d}}^{\mathit{i}\mathit{n}\mathit{t}}$ Direct consequences (intangible values)	Consequences related to injuries, casualties, historical assets, damage to the environment	Not included	0	Included	1

Table 2. Score ranking for risk-based methods; reliability-based methods not considered.

Risk-based ranking	Score (/9)	Description
basic	0–3	The method has a lower level of refinement and sophistication. Most factors are not considered or are estimated using empirical indicators. The method may provide a preliminary screening and ranking of interventions.
intermediate	4–6	The method has a medium level of refinement and sophistication. Most factors are considered in the analyses, although not many accurately assessed. The method may provide a reasonable screening but should be improved to provide consistent results from a cost-benefit analysis.
advanced	7–9	The method has a higher level of refinement and sophistication. All factors are considered, just a few empirically estimated, and uncertainties are accounted for. The method may provide a refined scour risk assessment.

Table 3. Summary of the review of scour management methods. Acronyms: Cerema - Centre d’études et d’expertise sur les risques, l’environnement, la mobilité et l’aménagement; MIT - Ministero delle Infrastrutture e dei Trasporti; MOTC - Directorate General of Highways; Mn DOT - Minnesota Department of Transport. Final scores (‘TOTAL SCORE’) were associated with the following ranking for risk-based methods (Table 2): 0–3 basic (BAS); 4–6 intermediate (INT); 7–9 advanced (ADV).

Country	Type	Agency commissioning publication	Reliability indicators	${\mathit{h}}_{\mathit{F}}$	${\mathit{p}}_{\mathit{S}|\mathit{F}}$	Consequences indicators	${\mathit{C}}_{\mathit{i}}^{\hspace{0.17em}}$	${\mathit{C}}_{\mathit{d}}^{\mathit{t}\mathit{a}\mathit{n}}$	${\mathit{C}}_{\mathit{d}}^{\mathit{i}\mathit{n}\mathit{t}}$	TOTAL SCORE	Monitoring	Reference
UK	Rail	Network Rail	Foundation depth Riverbed Floodplain Catchment properties Pier/abutment geometry Water height Water velocity	2	1	N/A	N/A	N/A	0	N/A	Not integrated	CIRIA Report C742 Kirby et al. (2015) NR/L2/CIV/295 NR (2011) NR (2019)
UK	Road	National Highways	Foundation depth Riverbed Floodplain Catchment properties Pier/abutment geometry Scour records Scour protection	2	1	Traffic level	1	N/A	0	4 (INT)	Not integrated	HA (2012) - BD 97/12
France	Road Rail	Cerema	Flow rate Riverbed Catchment properties Pier/abutment geometry Construction period Foundation type Scour records	2	2	Detours Bridge importance Traffic level Bridge value Human losses	1	1	1	7 (ADV)	Not integrated	Cerema (2019)
Italy	Road Rail	MIT	Scour depth Foundation depth Riverbed Floodplain Pier/abutment geometry	2	1	Detours Bridge importance Traffic level Activities (“entity”) under the bridge Bridge length (proxy for human losses) Dangerous goods on the bridge	1	1	1	6 (INT)	Integrated (in Level 4)	CSLP (2020)
Taiwan	Road	MOTC	Scouring Stability Index (SSI) Deterioration degreeDeterioration extentRelationship between deterioration and safetyRepair urgency	2	1	Maintenance cost Rebuilding cost	0	2	0	5 (INT)	Not integrated	Cheng et al. (2019) Liao, Lu, and Wang (2015)
Japan	Rail	Japanese National Railways Japan Railways Group	Water height Geological and hydraulic conditions Pier inclination	1	2	Transportation operation Detours Indirect consequences of bridge closure	1	0	0	4 (INT)	Integrated	Abé, Shimamura, and Fujino (2014) Suzuki et al. (2008) C. W. Kim et al. (2020)
South Korea	Road Rail	N/A	Scour risk (not better specified)	2	1	Detours Traffic Level Bridge importance Construction costs	2	1	1	7 (ADV)	Not integrated	D. Kim, Lee, and Lee (2018)
Minnesota (USA)	Road	Mn DOT	Scour records Scour protection Foundation type Debris Riverbed Geological and hydraulic conditions Flow rate Aerial/topographic maps	2	2	Traffic Level	1	0	0	5 (INT)	Integrated	Mn DOT (2009)
New Zealand	Road	Transfund	Water height Pier type Foundation type Bridge geometry Riverbed Scour records Catchment properties	1	1	Bridge importance Detours Utilities carried by the bridge Route importance	1	0	0	3 (BAS)	Not integrated	Coleman and Melville (2001)

Figure 2. Graphical summary of the results for the reviewed methods, as an example of visualisation of the scores of the scoring system. The total score is considered as follows: 0–3 basic (BAS); 4–6 intermediate (INT); 7–9 advanced (ADV). Abbreviations: h_f = scour probability; p_s|f = failure probability, c_i = indirect consequences, c_d = direct consequences.

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Data availability statement

No data, models, or code were generated or used during the study.