Scour Failure of Bridges

Scour failure of bridgesBrian Maddison CEng, MICEIndependent Civil Engineering Consultant, Former Engineering Director ofBridgeway Consulting Limited, Nottingham, UK

In recent years there have been several bridge collapses in the United Kingdom and Republic of Ireland that have been

caused by scour. Towns in the north west of England have been cut off and loss of life occurred. Major railway lines

have been closed for extended periods. Although scour is basically the removal of bed material due to flowing water,

it has a number of different causes and takes different forms. The paper describes the different forms of scour and

looks at a number of case studies to illustrate the different ways in which scour has caused structures to collapse or

require protection. The case studies are of railway bridges and are drawn from official investigation reports and

underwater examinations carried out by the author. The paper concludes by illustrating ways in which failures due to

scour could be avoided by the employment of good bridge management systems.

1. Introduction

Bridge failures are fortunately rare, but every year the pages of

New Civil Engineer contain details of a collapse that has

occurred somewhere in the world. In many cases these

collapses could have been avoided by an adequate bridge

management regime that included good inspection, assessment

and maintenance procedures. One specific type of failure that

from time to time causes sudden catastrophic collapse of

bridges is the undermining of foundations due to bed scour.

Recent examples of collapse due to bed scour are included in

this paper and others can be found in the reference documents

listed.

Bed scour is the transport of bed material by the flow of water

and is present to some degree where the river bed or seabed is

formed of granular material. Scour increases as flow rates

increase and therefore the actual collapse of structures due to

scour often occurs during periods of extreme flow, either due to

flooding or exceptional tides. Of course, this is exactly the time

that direct observation of the foundations of a structure is not

possible and therefore a collapse may be put down to an ‘act of

God’.

A good inspection regime that includes bed measurement and

engineering analysis can find indications of developing scour

before the situation becomes critical. If this is followed up with

well-designed remedial works, undermining of the structure,

even in extreme conditions, may be prevented.

The basic relationship between flow quantity Q, velocity V and

cross-sectional area A is expressed by the equation Q 5 VA.

Thus, velocity increases as the flow quantity increases or the

available cross-sectional area of the watercourse reduces. The

velocity will determine whether bed material of a particular

particle size will be transported. This paper essentially deals

with practical aspects of scour that can be observed during

examination. For a full understanding of scour, it is

recommended that the work of May et al. (2002) is referred

to. Further insight can also be gained from the works of

Hoffmaans and Verheij (1997), Hamill (1999) and Melville and

Coleman (2000).

Different types of scour are dealt with and illustrated with

actual examples, including recent bridge collapses as well as

cases where developing scour problems have been found during

inspection. The paper draws on the author’s own experience as

an engineer/diver in carrying out underwater inspections and

developing an underwater inspection regime for British Rail.

The cost of one single bridge failure can be immense in terms of

disruption to road or rail traffic and even loss of life as well as

in purely monetary terms. A small enhancement to the

inspection and maintenance regimes for vulnerable bridges

can prevent many costly failures.

2. What is scour?

Bed scour is a very simple concept but takes a number of

different forms. It may be a natural occurrence or due to man-

made changes to a river.

2.1 Channel instability (also referred to as natural

scour)

All scour is the result of the transportation of bed material by

the watercourse. Channel instability is a natural phenomenon

and is the result of the erosion and deposition of bed material,

which occurs gradually under normal conditions or very

quickly during floods. Rivers that transport considerable

amounts of bed material are most prone to scour and channel

instability. These include sand-bed rivers and upland gravel

bed rivers. The natural changes that occur will also be affected

Forensic EngineeringVolume 165 Issue FE1

Scour failure of bridgesMaddison

Proceedings of the Institution Civil Engineers

Forensic Engineering 165 February 2012 Issue FE1

Pages 39–52 http://dx.doi.org/10.1680/feng.2012.165.1.39

Paper 1000016

Received 19/12/2010 Accepted 22/07/2011

Keywords: bridges/failures/maintenance & inspection

ice | proceedings ICE Publishing: All rights reserved

39

by a large number of other actions that affect river flows: these

include the placement or removal of artificial obstructions such

as weirs or training walls, exceptional rainfall and increased

runoff due to deforestation or urbanisation.

The river channel may change both in plan and longitudinal

section. Changes in section will occur as bed material is

transported along the river. Areas of deposition will change to

areas of erosion, either naturally or as a result of artificial

actions, and the river bed level will be lowered accordingly,

affecting the stability of any structure founded on that bed (see

Figures 1 to 3). The effect of channel instability on a bridge or

waterside structure may be to undermine foundations and

direct flows towards or behind the structure (Figure 4).

The same actions that cause changes in the longitudinal section

of a river will also cause changes in the plan. In flood plains,

erosion will cause a river to become sinuous. The banks of a

river on the outside of curves will erode and silt will be

deposited on the inside of curves where flows are slower. Thus,

the route of the main river will change over years or,

occasionally, dramatically in times of flood (Figure 5). Spans

of a bridge originally designed to be for flood-relief purposes

only may eventually become the main river spans and often the

piers involved will be founded at a shallower depth than those

of the original river spans (Figure 6).

2.2 Contraction scour

A local reduction in the width of the river can cause a general

lowering of the bed level by decreasing the cross-sectional area

of the river and increasing flow velocity, bed shear stress and

frequency of bed movement. A similar effect can also be caused

when river levels rise to above the soffit of a bridge or other

obstruction. Scour may occur in three ways.

(a) Channel contraction. The normal channel of the river is

reduced at a bridge by the presence of piers, abutments

extending into the channel and/or training walls, etc.

Local increase in sedimentsupply

Deposition of sediment

River bed

Figure 1. Natural raising of the river bed

Local reduction in sediment supply

Bed erosion (scour)

River bed

Figure 2. Natural lowering of the river bed

Distance downstream

Depositing phaseEroding phase

Stable

Cha

nnel

slo

pe

Figure 3. Longitudinal section of river bed

Bridge pier built in areaof deposition

Deposition

Erosion

Bridge pier becomesundermined when

river bed changes and deposition changes to

erosion

Figure 4. Bridge pier undermined by natural bed changes



40

(Figure 7). The situation can be worsened by the presence

of heavy debris in the water.

(b) Flood plain and estuary contraction. Typically this will

occur when a railway embankment is built across a flood

plain. In times of flood, runoff from the flood plain is

channelled through a bridge opening with resultant

increase in flow velocity as the same quantity of flood

water is channelled through a relatively small cross-

sectional area. In this situation, flood water will seek any

opening in the flood bank and, as well as contributing to

scour in the main channel, scour may occur at flood-relief

arches, road bridges and similar structures. The same

effect happens when embankments are constructed across

tidal estuaries (Figure 7).

(c) Surcharging. Where the soffit of a bridge is lower than

high water level during a flood period, surcharging will

occur. Water will flow though the bridge opening, which

has a reduced cross-section not only due to the available

width of the river but also limited by the height of the

soffit. Using the equation Q 5 VA, the velocity will be

greater than if the height of the water was not restrained

by the soffit. As the level rises higher above the soffit, the

head of water increases the velocity.

When the increase in flow velocity through a bridge opening

because of channel contraction becomes sufficient to transport

bed material, the result will be an overall lowering of the bed

level at the bridge, probably across the full width of the river,

Original course

River meanders due to erosionMeander

StraightenedOriginal course

Bends are straightened due to deposition

Slow-moving river becomes braded

Figure 5. Changes to rivers in plan



41

although this will depend on variations in the bed material.

This effect may be increased by the addition of flood plain

contraction.

Contraction scour may be observed after flooding has ceased

or silt may be deposited as the river slows, making the scour

difficult to detect.

2.3 Local scour

Local scour is a consequence of the presence of a structure in a

watercourse. The turbulence caused as water flows by a pier

will generally cause an uplifting effect at the nose, resulting in

erosion of the bed. The material removed from this area will be

deposited behind the pier as eddies form and the flow slows.

The mechanics of this action have been carefully studied by

hydraulics institutions in Britain, America and Europe and the

general result is as shown in Figure 8. Details of some of the

studies are shown in the literature (Hamill, 1999; Hoffmaans

and Verheij, 1997; May et al., 2002; Melville and Coleman,

2000). As the water strikes the nose of the pier, the direction of

flow is directed downwards causing material to be removed

from the bed at this point.

The shape of local scour shown in Figure 8 may be modified by

many factors. Most significant among these will be the shape

of the nose of the pier and the direction of flow relative to the

pier. If the pier is not aligned with the flow, local scour will

occur along the side of the pier that faces the flow. Hamill

(1999), Melville and Coleman (2000) and May et al. (2002) all

provide more detailed information.

In tidal waters, scour may occur on both ebb and flow of the

tide, resulting in scour at both ends of a pier. Scour may also

occur at ‘downstream’ ends of river piers when there is

significant backflow.

Local scour also occurs to weirs and protection inverts that

cross a watercourse. Typically, scour will start to develop at the

downstream end of an invert and, as the scour develops, the

weir effect increases until a deep hole develops. The scour then

gets under the invert and the trailing edge of the invert breaks

off. This process is progressive and can continue until the

invert breaks back to the bridge and the piers become

undermined (Figure 9).

The rapid movement of water caused by boat propellers and

water jet engines can also cause scour. A particular problem for

harbour walls can be the use of side-thrusters that are often

directed at the walls in close proximity.

Viaduct at time of construction

Viaduct after change of river position

River spans

Foundations exposed and undermined

River spans

Figure 6. Bridge piers at risk due to changes in the course of the

river

Flood plainFlood plain

Flood plain Flood plainEm

bank

men

tE

mba

nkm

ent

Con

tract

ed fl

ood

plai

n w

idth

Con

tract

ed ri

ver w

idth

Training wall

River flowArea of bed lowering

Figure 7. Contraction scour



42

2.4 Scour sub-divisions

All three types of scour may be sub-divided into two types.

(a) Clear water scour. This often occurs at piers in relatively

low flows and is the simple removal of bed material by

flowing water. Upstream of a bridge, the water is not

transporting significant amounts of bed material (hence

the term clear water). At the bridge, bed material is

removed and transported away but no material from

upstream is deposited at the same time. Therefore, scour

holes formed remain present when flows subside and can

be seen during underwater inspection.

(b) Live bed scour is the continuous erosion and deposition

of bed material during periods of flooding. In its worst

case, the bed under a pier foundation will become fluid as

material is constantly removed and replaced. This type of

scour may be very difficult to detect. By the time a diver is

sent down to inspect the pier, the flow will have reduced

and the bed stabilised at a much higher level than the

maximum scour level during flood.

3. Case studies and investigations

3.1 Glanrhyd 1987: local scour, channel instability

and live bed scour

On 19 October 1987, the bridge carrying the Central Wales line

over the River Towy collapsed during a period of heavy rain and

flooding. At about 07:15, a passenger train ran on to the bridge

and fell into the swollen river. Four people died. The full report

of the accident was published by the railway inspectorate (DfT,

2009). The investigations undertaken included the following.

(a) A detailed underwater survey of the remaining parts of the

damaged piers undertaken by a diving team led by a

chartered civil engineer. The results of this survey and

other investigations concluded that the collapse was caused

by scour to the downstream end of pier 3 that undermined

the foundations, allowing the pier to settle and eventually

break its back (Figure 10). The survey showed that the pier

was originally constructed by driving timber piles to form a

cofferdam, making a base for the bridge foundations

within the cofferdam of ‘cemented river gravel’ and then

placing stone foundation slabs. Many of the timber piles

were missing and this had allowed the undermining to

progress below the foundation slabs.

(b) Evidence taken from Welsh Water Authority staff who

stated that the bed levels of the river in the vicinity of the

bridge changed from year to year and that a gravel bank

near the bridge was an intermittent and changing feature.

(c) Evidence taken from British Rail that indicated that the

depth and type of construction of the foundations of the

bridge were not known and had not been considered when

repairs were undertaken to the piers some 7 years previously.

(d) A study of the river flow was carried out by Hydraulics

Research Limited, some of the conclusions of which were

as follows.

(i) There was a ‘re-circulating zone’ or eddy at the

downstream end of pier 3.

(ii) Up to 17 000 t of sediment may have passed the

bridge during the 3 h of peak flows, indicating major

live bed scour with both erosion and deposition at

the bridge.

(iii) The depth of any anticipated scour at pier 3 could

have been between 0?75 and 2?2 m.

Overall, it seems that local scour at the downstream end of pier

3 was the main cause of the collapse, but elements of the report

suggest that channel instability may also have been a factor.

The results of the investigation into the Glanrhyd collapse was

the start of a complete review of scour risk to railway bridges in

the UK. The conclusions drawn included the following.

(a) The engineers responsible for the safety of the bridge

lacked thorough knowledge of the complex behaviours of

rivers such as the Towy.

(b) Foundation depths were not known.

(c) Remedial works previously carried out to defective bridge

piers increased the likelihood of scour damage because

the piers were widened and the shape of the cutwaters was

changed.

Elevation

Pier

Nominal bed levelScour hole

Scour hole

Flow

Pier Deposition of silt

Foundations

Plain

Figure 8. Local scour to a pier



43

(d) The scour was probably live bed scour with the scour hole

refilling with sand/gravel as flows subsided. This made it

difficult for divers to find it during routine inspections.

Just 2 years later, there was another major railway bridge collapse

at Inverness, again due to scour. This did not involve loss of life

but severed the only railway that runs north from Inverness for

many months. A completely new bridge had to be built.

3.2 Beighton 2003: contraction scour

In 2003, a member of the public, walking along the bank of the

River Rother near Beighton (South Yorkshire) reported a

collapse of the railway bridge (Figure 11). It was found that the

pier and arch of one span on the bridge had partially collapsed.

Fortunately, arch bridges are inherently strong in compression

and even though the arch was severely damaged, trains had

been passing over the bridge without incident. However, all rail

traffic was diverted onto the track that was not affected by the

damage until temporary supports could be installed. Detailed

underwater inspections and investigations were undertaken

and the following information was ascertained.

(a) The incident was reported in the summer months but it is

likely that the scour occurred during flooding in the

Stage 1 _ as constructed

Foundations

Pier

Stage 2 _ scour development

Foundations

Pier

Invert

Invert

InvertFlow

Flow

Flow

FlowBed level

Minor scour toupstream face

Leading edge of invert undermined

Leading edge of invert undermined

Downstream edge of invert collapsesand scour undermines pier

Downstrean edge of invert collapses into

scour hole

Major scour hole develops at

downstreamend

Invert

Stage 3 _ damage to invert

Foundations

Pier

Stage 4 _ pier undermined

Foundations

Pier

Figure 9. Progressive collapse of invert



44

previous winter. There is no public footpath under the

bridge so visits by the public are rare and the underside of

the bridge cannot be seen from the railway line.

(b) The bridge was constructed in three parts at different

times and with slightly different forms of construction

(Figure 12). At the point where the scour occurred, the

soffit level of the bridge dropped by about 300 mm.

(c) Local information provided by members of the public

and railway staff indicated that in times of flood, water

levels were seen to rise above the upstream soffit level.

There were also some indications on the bank to support

this.

(d) A detailed survey of the collapsed structure and the

surrounding river bed found that the bed of the river under

the bridge was composed of a thin layer of clay

(approximately 150 mm) overlaying sand and gravel.

However, the clay layer had been damaged. From the point

where the clay was found to be missing and the gravel was

exposed, an extensive scour hole had developed.

(e) The scour hole was deepest at the construction joint

between upstream and centre parts of the pier. The

foundations of the centre part of the pier were about

300 mm higher than the upstream and downstream parts

(Figure 13).

(f) As a consequence of the shallower foundations, the centre

part of the pier was undermined by the scour and had

collapsed into the scour hole. The arch above also

partially collapsed.

(g) Previous underwater reports showed that there was no

evidence of a distinct scour hole although the bed was

very slightly lower at this location.

(h) Side spans of the bridge had become silted up, reducing

their use as flood-relief spans and increasing the like-

lihood of contraction scour.

From the above information it was concluded that the most

likely cause of the undermining was due to contraction scour

during a period when the river was surcharging. At the bridge,

the width of the river is reduced by the piers of the bridge and

Downstream half of pierpart buried in sand deposition

Undermining beneathpier present

Remains of timber cofferdam

Upstream section pier 3

Water level

Random pier masonry

Downstream

Bed level 1 m from pier edge

Toe level of cofferdam

Bed level on pier

Figure 10. Glanrhyd, pier 3

Figure 11. Upstream face of River Rother bridge



45

the silted-up side spans. During periods of flood when the

height of the river is above the level of the arch soffit, the

available cross-sectional area A is further constricted when

compared with the area of the open channel clear of the bridge.

Furthermore, at the point where the scour occurred, the soffit

of the bridge dropped, creating a funnelling effect and

increasing the head of water. As the same quantity Q of water

sought to pass through the restriction, the velocity V increased,

resulting in bed scour.

Whether or not the clay layer on the bed of the river was

natural or installed as an anti-scour measure is unclear, but it

had certainly been performing this function prior to the

incident. However, once it was breached, the granular material

below would quickly scour away and the clay layer would be

progressively removed by the flow.

The scour hole was sufficiently deep to undermine the centre part

of the pier but did not reach the level of the foundations of the

upstream or downstream parts of the pier, which remained intact.

The collapsed section of pier was eventually capped off and the

adjacent arches replaced with concrete beams (Figure 14).

3.3 Holyhead c. 1990: scour caused by bed lowering

and ship propellers

Holyhead harbour was developed by the London and North

Western railway over many years as their main port for

ferries to the Republic of Ireland. British Rail took over the

port upon nationalisation and further developments took

place. One part of the harbour was known as the cattle dock

and, as the name suggests, was used for the import of cattle.

By 1990, the trade in live cattle had ceased and the dock was

used for the temporary berthing of ferries that were awaiting

repair or their next duty. In January 1990 the dock wall

collapsed.

Following a detailed underwater examination, evidence indi-

cated that the wall, which was constructed of stone masonry,

had been built off the granite bed of the harbour. However, at

the time of the collapse it was founded on a narrow shelf of

rock above bed level (Figure 15). This shelf was showing signs

of erosion.

Investigations revealed that the bed level of the harbour had

been lowered many years prior to the incident to accommodate

larger vessels. Anecdotal evidence was provided by one of the

divers who had carried out this operation using explosives. The

operation had been successful but had left some of the harbour

walls founded on a shelf of rock about 500 mm above the bed

of the harbour. Furthermore, because explosives had been

used, the shelf was somewhat irregular in shape. It would

appear that as ships became larger and made more use of bow

thrusters, scour occurred at the base of the wall until a section

slid off the shelf and into the harbour.

Flow

Construction joint

Construction joint _ soffit lowers

Scour

River Rother

AbutmentAbutment

Pier

Pier

Pier

Pier

Figure 12. River Rother Bridge plan at river level



46

3.4 Feltham 2009: scour caused by obstruction of the

river flow

On 14 November 2009, a small railway bridge carrying the

Windsor to London (Waterloo) line over the River Crane

collapsed. The incident was first reported at 22:03 when the

driver of a train crossing the bridge reported a dip in the

track. The incident was investigated and it was found that

the east abutment at the upstream end of the bridge had

collapsed into the river (Figure 16), taking with it a part of

the arch and leaving the railway track suspended above a

void (Figure 17).

The investigations revealed that the abutment collapsed due to

scour and a number of factors were involved (DfT, 2010).

(a) The abutment that collapsed was the original abutment

constructed in 1848. Foundations of this abutment were

shallower than the west abutment, which was constructed

later (in 1858). Core drilling carried out in 1991 showed the

east abutment to be founded at 0?65 m below river bed

level while the west abutment was 1?5 m below bed level.

(b) The shape of the river channel directed the flow towards

the east abutment, making it vulnerable to scour.

(c) In August, a member of the public took a photograph

showing the east span of the bridge to be obstructed by

floating debris.

On 28 October, an Environmental Agency inspector saw a

major blockage of the watercourse (Figure 18).

Approximate water level during flood periods

Soffit level of centre and downstream arches

Collapsing pier

Water level at time of inspection

Thin clay layer

Granular bed below clayScour hole

Foundations ofupstream arch

Foundation level of centre arch

Section through upstream arch

Centre arch

Change in soffit level

Upstream arch

Part plan of affected arch

Scour hole

Figure 13. River Rother Bridge



47

At the time of the collapse, 5?5 m (65%) of the abutment’s

length was found to be unsupported where the material below

the foundations had been removed by scour.

The conclusions of the investigations were that the abutment

was already vulnerable to scour because it was on the outside

of a bend of the river and had substantially shallower

foundations that other parts of the structure. The debris in

Figure 14. River Rother bridge; collapsed centre pier removed

Rock bed

Bed level following blasting

Original harbour bed level

Masonry wall

Concrete slab

Rubble fill

Possible wash out of fill

Figure 15. Cross-section through Holyhead harbour wall prior to

collapse

Figure 16. River Crane bridge; collapsed abutment

Figure 17. River Crane bridge; track unsupported

Figure 18. River Crane bridge; debris blockage prior to collapse



48

the river then caused river flow to be channelled directly

towards the abutments and the scour occurred during a period

of high river flows.

3.5 Cefyn viaduct: local scour

Situated on the Welsh borders near Wrexham, Cefyn viaduct

carries the railway across a wide valley and the River Dee. It is

a major multi-span masonry arch viaduct. During the course of

a three-yearly underwater inspection, it was found that the pier

located in the river was severely undermined by local scour.

There was no indication of contraction scour and the damage

had been caused by a typical form of local scour that had

removed bed material from below the upstream foundations

and caused the collapse of the cutwater though, fortunately, it

had not extended far below the main body of the pier and

repairs were able to be made. This viaduct is a major piece of

Victorian engineering and a collapse would have been a

catastrophe. The scour could have been prevented by the

provision of adequate bed protection (Figure 19).

3.6 Ruddington: scour caused by dredging

South of Ruddington in Nottinghamshire, the former Great

Central Main line to London crosses a small watercourse

known as Fairham brook. The Great Central Line was not

built until about 40 years after other main lines in the country;

the civil engineering is of the highest standard and the

foundations of their structures are generally large and deep.

However, when inspected, divers found that the pier of this

two-span bridge was in danger of collapse due to deepening of

the river bed (Figure 20). One span of the bridge crosses a farm

track and is over 1000 mm above the bed of the river in the

other span. When inspected, it was found that the foundations

of the south abutment and pier were exposed. The bottom of

the foundations could be seen but, fortunately, undermining

had not taken place.

Investigations showed that there was no local scour but that

the level of the river bed was consistently low for a significant

distance away from the bridge and that there were signs that

dredging had taken place. Contacts with the local drainage

authority were made and it transpired that a drainage and

flood-relief scheme had been undertaken. Although not carried

out at the bridge itself, dredging of the watercourse had caused

the bed levels at the bridge to be lowered by 1000 mm, putting

the bridge at risk. The risk was not only of undermining but of

the pier sliding sideways because of lack of horizontal

resistance to the forces transmitted from the farm track. Steel

sheet piling has now been driven around the pier and abutment

to stabilise the structure.

3.7 Staythorpe: changes to the course of the river

Staythorpe viaduct is a good example of how the exact amount

of bed scour can be detected by ‘forensic engineering’. This

railway viaduct, carrying the Nottingham to Lincoln line over

the River Trent near Staythorpe power station, had been

rebuilt in 1973 and new piers were constructed, each of which

consisted of concrete bored piles that were unlined below

ground but extended above bed level within pre-cast concrete

rings. The viaduct is located just downstream of a weir and is

subject to fast flows during flood periods.

Some 15 years later, during the course of routine underwater

inspections, it was discovered that the main flow of the river

was dividing and that a gravel bank had built up in the centre

of the river. The pattern of scour was complex and a full

contour plan of the bed was produced in order to fully

understand the situation. The worst area of scour was in fact

immediately downstream of the bridge on the outside of the

bend in the river. The bed at this point was over 4 m lower

than the average bed level. Up to 2 m of scour was present at

the bridge itself.

These results were confirmed by divers who took detailed

measurements of the piles. They were able to dive under a layer

of concrete that extended between the piles. At first, this was

found to be puzzling until it was realised that, below this layer,

the concrete of the piles was uneven and contained inclusions

of large gravel indicating that they had originally been

underground. Above the layer, the piles were cast within

concrete rings.

Pier 2

Bed level

Stones movedLoose stones lyingon bed

Scour hole

Flow

Void

Figure 19. Cefyn viaduct



49

Fortunately, the original piling logs had been retained as part

of the as-built information for the viaduct and these showed

that the piles extended about 9 m below the original bed level

so the 2 m of scour could be reviewed in that context

(Figure 21). In this case, a thorough investigation proved that

there was no need to take emergency action. However, repairs

were subsequently carried out both to restrict further scour and

to provide protection to the exposed piles. The incident

demonstrated the benefit of deep piled foundations for river

bridges and the value of retaining as-built information.

3.8 Malahide: failure of causeway and protection

apron

On 21 August 2009, pier number 4 of Malahide viaduct

collapsed into the estuary (Figure 22). This viaduct carries the

main line between Dublin and Belfast. The collapse was

reported by the driver of a train that passed over the damaged

viaduct but fortunately crossed immediately before complete

collapse occurred.

Detailed investigations were able to prove that the masonry

piers of the viaduct were built on top of a stone causeway that

acted as a weir (RAIU, 2010). This causeway was maintained

in a fair condition for over 100 years by a regular regime of

replenishment of the stones, although during that period the

causeway elongated seaward due to migration of the stones. In

1967–1968, a major grouting scheme was undertaken to fill

voids in the weir. This scheme was reasonably successful but

more stones were discharged to fill scour holes on a number of

occasions up until 1996.

A hydraulic model of the bridge was built to investigate the

failure mechanism. The grouted layer, which was about

1500 mm thick, acted as an invert but, as scour occurred at

the seaward side of the causeway on the ebb tides, this became

undermined. The undermining continued in the manner shown

in Figure 9 until the invert between piers 4 and 5 collapsed, at

which time the scour began to undermine pier 4 until it failed.

Abutment Abutment

Farm track

Pier

Foundation

Water level

Original bed level

River bed

Timber piles

Figure 20. Ruddington Bridge: bed scour due to dredging

Steel trestle

Concrete pile cap

Pre-cast concrete rings

Bed level at time of construction

Exposed concrete piles

Bed level

Concrete piles below bed

Concrete spillage

2 m

Figure 21. Staythorpe Bridge pier



50

4. ConclusionsWhat conclusions can be drawn from the investigations into

scour problems?

Firstly, it is noticeable that when a bridge collapses due to

scour, the causes of the collapse become clear quite quickly to

the team carrying out the forensic investigations. Although it is

often thought prudent to carry out detailed hydrological

investigations, in fact the causes are often almost self-evident.

If the same effort could be put into bridge management

regimes, collapses could be avoided.

Secondly, it is noticeable that on a number of occasions,

knowledge of the structures concerned was inadequate. In the

case of the Malahide viaduct, for example, the investigation

report (RAIU, 2010) states:

The dearth of documents available [to the responsible engineers]

meant they were not fully aware of the construction of the

Malahide Viaduct, and incorrectly assumed that the structure was

founded on bedrock… at the time of the accident, [they] were

unaware of the routine discharge of stones along the viaduct as this

process was not formally recorded, and there was an apparent loss

of corporate memory of knowledge.

Despite this statement, it did not take investigators long to

discover a wealth of information about the structure. Their

report includes original drawings of the viaduct and details of

recent remedial works. Had this information been made

available to the railway engineers, the collapse could have

been averted.

Thirdly, information made available to staff carrying out the

examination of underwater structures is often incomplete.

Similarly, when reviewing reports, it is essential that the

engineer has all known information about the bridge available.

When all such data was in the form of drawings and paper

records it is perhaps understandable that it was not reviewed

after every examination. However, computers now make it

easy to rectify this problem by taking the following steps.

(a) Establish, record and have available to the examining

team full details of foundation depths. If these are not

available from ‘as-built’ drawings, core drilling may be

required. Once this information is available, ensure that it

becomes routine to consider it when looking at under-

water reports and scour data.

(b) Develop a more sophisticated way of reviewing river bed

soundings. These are routinely taken at each underwater

inspection but for them to really predict the likelihood of

a bridge being affected by scour then the data should be

well handled. The results should show changes in time (by

comparisons with previous soundings) and geographical

changes. The latter will indicate if bed levels at the bridge

are lower than might be expected when compared with

upstream levels and will also show any migration of the

main channel. In the case of Staythorpe, for instance,

only by plotting bed contours over a significant length of

the river could the pattern of scour be established.

(c) Carry out a review of the inspection reports. This task

should be done by engineers with particular expertise in

scour.

Finally, it is important that repairs to scoured river beds and

scour protection works are not carried out without proper

engineering design. Whilst major anti-scour works such as steel

sheet piling will of course be correctly designed, simple tasks

such as filling scour holes are sometimes left to local teams to

carry out without any design input. This has been seen to result

in too small stones being placed and these usually end up some

distance downstream after the first flood.

REFERENCES

DfT (Department of Transport) (2009) Report on the Collapse of

Glanrhyd Bridge on 19th October 1987 in the Western

Region of British Railways. HMSO, London. See www.

railwaysarchive.co.uk for further details (accessed 02/09/

2011).

DfT (2010) Failure of Bridge RDG1 48 between Whitton and

Feltham. Railway Investigation Branch, DfT, Derby,

Report no. 17/2010. See www.raib.gov.uk for further

details (accessed 02/09/2011).

Hamill L (1999) Bridge Hydraulics. E & FN Spon, London.

Hoffmaans GJCM and Verheij HJ (1997) Scour Manual.

Balkema, Rotterdam.

May RWP, Ackers JC and Kirby AM (2002) Manual on Scour at

Bridges and Other Hydraulic Structures. CIRIA, London.

Figure 22. Malahide viaduct; two spans collapsed



51

Melville BW and Coleman SE (2000) Bridge Scour. Water

Resources Publications, Highlands Ranch, CO.

RAIU (Railway Accident Investigation Unit) (2010) Malahide

Viaduct Collapse on the Dublin to Belfast Line on the 21st

August 2009. RAIU, Blackrock. Report no. R2010-004. See

www.raiu.ie for further details (accessed 02/09/2011).

WHAT DO YOU THINK?

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52

Scour Failure of Bridges

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