A comparison of the UK and Italian national risk-based guidelines for assessing hydraulic actions on bridges

Figures & data

Figure 1. A schematic representation of the UK method and indication assessment process for the three aspects considered; details can be found in Takano and Pooley (2021).

Table 1. Factors and range of values for the Priority Factor in Equation (1)(1) $$P_F=F·M·H_S·T_R·C·C_{AF}·D$$(1) .

Factor	Description	Values
F	Foundation type factor	0.75 (piled) − 1.25 (masonry arch)
M	Ground material factor	0.5 (bedrock) − 1 (granular)
HS	History of scour factor	1 (no scour) − 1.5 (scour)
TR	Type of river factor	1 (estuarine) − 1.5 (mountainous)
C	Communities importance factor	0.7 (unclassified road or footbridge) −1 (motorway)
CAF	Communities additional factor	1 (no disruption) − 1.3 (essential services/links)
D	Debris accumulation factor	1 (debris unlikely) − 1.3 (debris highly likely)

Table 2. Factors used in Equation (2)(2) $$D_T=D_C+D_L=D_C+1.5W_P{f}_{PS}{f}_{\mathrm{P}A}{f}_y{f}_d$$(2) and typical ranges.

Factor	Description	Values
DC	Contraction scour (m)	Any
WP	Pier width (m)	Any
fPS	Pier shape factor	0.7 (lenticular) −1.4 (rectangular)
fPA	Angle of attack factor	≥1 (typical range 1-3)
fy	Depth of water factor	≤1 (typical range 0.7-1)
fd	Debris accumulation factor	≥1 (typical range 1-3)

Figure 2. Scour risk assessment chart for CS 469, interpolating the Priority factor P_F on the horizontal axis and the relative scour depth D_R on the vertical axis. Each score is identified by a category (i.e. High, Medium, Low) and a score (i.e. 10 to 100).

Figure 3. A schematic representation of the hydraulic risk assessment for the Italian method. Details can be found in CSLP. (2020).

Figure 4. Combination tables to combine partial ACs.

Figure 5. The selected structures for the case study: (a) Staverton Bridge in the UK; (b) Borgoforte Bridge in Italy.

Table 3. Input data for the two case studies and the two methodologies; data were obtained from private reports.

	Input parameters	Borgoforte Bridge	Staverton Bridge
Italian method	Width of the riverbed occupied by the bridge	18 m	47.9 m
	Width of the floodplains occupied by the bridge	18 m	10.8 m
	Total width of the floodplains	188 m	46.9 m
	Span length	63.50 m	≈ 6 m
	Clearance (200 years return period flood)	< 0.80 m	1.60 m
both	Pier width	1.5 m	3.65 m
	Debris accumulation	Yes	Yes
	Foundation depth	18 m	Unknown Italian method: assumed 2 m UK method: assumed 1 m
	Mean daily traffic	16667 vehicles/day	<1000 vehicles/12 hours
	Width of the riverbed	270 m	58.65 m
	Longitudinal slope of the channel	0.00004	0.0034
	Manning’s coefficient	0.025 s/m^1/3	0.045 s/m^1/3
UK method	Average bed material size	0.5 mm	70 mm
	Flood peak flow (with climate change adjustment)	15720 m^3/s	764 m^3/s
	Width of bridge waterway	244.3 m	40.3 m
	Width of upstream left floodplain	300 m	70 m
	Width of upstream right floodplain	50 m	0 m
	Pier geometry (shape / width / length)	Circular, 1.5 m, 1.5 m	Triangular, 3.65 m, 7.39 m
	Width of abutments	N/A*	2.0 m
	Length of abutments	N/A*	6.1 m
	Average depth of the main channel	30.4 m	3.3 m
	Soffit depth	27.1 m	5.25
	Wetted perimeter	283.1 m	24.3 m
	Angle of attack	0°	0°

* Abutments well outside channel flow

Table 4. Summary of methodological similarities and differences between the Italian and UK methods (authors’ critical analysis).

SIMILARITIES (S)
S1)	to assess scour risk separately from the other hydraulic actions.
S2)	to account for the history of scour problems, if recorded via past inspections.
S3)	to consider indirect consequences, within a network-level perspective: the UK PF includes a community severance or disruption factor, i.e., relevance of the bridge for the served community, rerouting or access to critical services, while the Italian method considers the daily traffic level, the presence of alternative routes, and the importance of the bridges during an emergency.
S4)	to produce risk classes as a final outcome.
S5)	to recommend similar actions in relation to the risk class, i.e., to gather more information. However, the UK guidelines include urgent actions for High Risk bridges scoring 100 in the assessment (see D4).
S6)	to consider effects due to debris - although the UK guidelines include effects of debris quantitatively, whilst the Italian guidelines only do so qualitatively.
S7)	to overlook direct consequences: both approaches do not consider the cost of repair/replacement.
DIFFERENCES (D)
D1)	the UK guidelines assess risk due to scour actions only; for hydraulic actions and channel stability, a vulnerability assessment is proposed. The Italian guidelines encompass risk assessment for four hazards (seismic, landslide, stability, and hydraulic); all the risk types are combined to obtain the final rating.
D2)	regarding hydraulic actions, the UK guidelines assess various risks separately (i.e., scour actions, debris forces, hydraulic actions, and channel stability); scour is assessed independently from hydraulic actions, e.g., if the bridge results as submerged, more calculations are prescribed to measure hydraulic loads for deck uplift (no risk rating). For the Italian guidelines, no action can be taken considering only one type of risk (e.g., hydraulic).
D3)	the UK guidelines prescribe calculations for local and constriction scour, water depth, and velocity at several cross-sections; with reference to Level 2, the Italian guidelines are based on records and visual inspections, and do not involve calculations. For example, the UK method calculates local scour via a multi-factor empirical equation accounting for pier shape, angle of attack, and debris accumulations; while the Italian method assumes scour as two times the pier width in the absence of targeted inspections.
D4)	The UK guidelines account for climate change by considering a 20-30% allowance for a 200-year return period flood peak flow (based on the geographical area; Takano & Pooley, 2021); the Italian guidelines do not explicitly account for climate change (water levels used in the computation of the vertical clearance are retrieved from flood maps where climate change is accounted for according to the current technical legislation, D.Lgs. 49, 2010).
D5)	the outcome of the UK method’s risk assessment procedure could also include operative actions, e.g., bridge closure or traffic limitations when at high risk; the outcome of the risk assessment for the Italian method is limited to gathering more information or carrying out in-depth investigations in the case of a bridge at high risk (Level 3-4) (see S4).
D6)	The UK method does not include damage to humans and the environment (and any direct consequences at all), while the Italian method assumes the bridge length as a proxy of human casualties (see S7) and includes as a risk factor the presence of dangerous materials, i.e. those substances (carried along the bridge) which, due to their particular nature, are capable of producing significant damage to people and the environment.
D7)	As for the minimum clearance, the UK method considers whether the bridge is submerged or not during the 200-year (plus climate change correction) return period flood peak flow (see also D1). For the Italian method, the hazard AC for insufficient vertical clearance is determined considering the magnitude of the minimum clearance relating to floods of different return periods.

Table 5. Final risk rating for the two methods and the two bridges considered in this study (L = low, M = medium, H = high).

		Borgoforte Bridge (Italy)		Staverton Bridge (UK)
		Risk level	Actions	Risk level	Actions
UK method	Scour	M	Implement scour risk reduction plan and regular monitoring	H	Sub-standard structure, to be managed in accordance with CS 470 (e.g., speed limitation, bridge closure). Plan of scour protection measures as a priority.
	Hydraulic actions	No risk	No further actions required	Debris forces - impact (intermediate)	Include in a vulnerability assessment report suggesting actions.
	Channel stability	L	No further actions required	M	Include in a vulnerability assessment report suggesting actions.
Italian method	Minimum clearance	H	–	H	–
	Scour (general and local)	M-H		H
	Global hydraulic risk	H	Carry out further analysis to assess structural safety (following NTC. (2018)) *	H	Carry out further analysis to assess structural safety (following NTC. (2018)) *.

* The AC for hydraulic risk should be combined with the ACs relevant to the other three risk types. In the table, it is assumed that the AC for hydraulic risk and the general AC are the same.

Figure 6. Application of the UK method (CS 469) to the two case studies.

Figure 7. Application of Italian method to the two case studies.

Figure 8. Sensitivity analysis for Staverton bridge considering the UK method for the Priority factor P_F for piers and abutments. For Borgoforte Bridge no variation was observed.

Figure 9. Sensitivity analysis for the Italian method applied to Borgoforte Bridge; for Staverton Bridge no variation was observed. MC = Minimum Clearance, GS = General Scour, LS = Local Scour.

Takano, H., & Pooley, M. (2021). New UK guidance on hydraulic actions on highway structures and bridges. Proceedings of the Institution of Civil Engineers - Bridge Engineering, 174(3), 231–238. doi:10.1680/jbren.20.00024

 Web of Science ®Google Scholar

CSLP. (2020). Linee Guida per la Classificazione e Gestione del Rischio, la Valutazione della Sicurezza ed il Monitoraggio dei Ponti Esistenti. Ministero delle Infrastrutture e dei Trasporti. Consiglio Superiore dei Lavori Pubblici. Retrieved from https://bit.ly/3qiwRDR (In Italian)

 Google Scholar

D.Lgs. (2010, February 23). n. 49. Attuazione della direttiva 2007/60/CE relativa alla valutazione e alla gestione dei rischi di alluvioni. . Retrieved from https://bit.ly/309dL8K.

 Google Scholar

NTC. (2018). Ministry of Infrastructure Decree, DM 17 gennaio 2018: Aggiornamento delle Norme tecniche per le costruzioni, Suppl. or. n.30 alla G.U. n.29 del 4/2/2008 (in Italian)

 Google Scholar