Failure of infrastructure embankments induced by ﬂooding and … · 2020. 7. 15. · to...

Nat. Hazards Earth Syst. Sci., 11, 3383–3396, 2011www.nat-hazards-earth-syst-sci.net/11/3383/2011/doi:10.5194/nhess-11-3383-2011© Author(s) 2011. CC Attribution 3.0 License.

Natural Hazardsand Earth

System Sciences

Failure of infrastructure embankments induced by flooding andseepage: a neglected source of hazard

M. Polemio and P. Lollino

Italian National Research Council, Research Institute for Geo-Hydrological Protection, Bari, Italy

Received: 7 January 2011 – Revised: 7 November 2011 – Accepted: 26 November 2011 – Published: 22 December 2011

Abstract. The risk of failure of transportation embankmentsdue to seepage induced by temporary and occasional im-poundments taking place on the upstream side as a conse-quence of exceptional rainfalls is frequently underestimated.These failure events result from a combination of three mainfactors, i.e. the flooding event, the hydraulic weakness andthe geotechnical weakness of the embankment. Based on thecase study of a railway embankment in Southern Italy thatcollapsed in 2005 due to an upstream impoundment that oc-curred after few hours of a very intense rainfall, the paper de-scribes a methodological approach aimed at assessing hazardof failure of transportation embankments induced by flood-ing and seepage. In particular, according to hydrological,hydraulic and geotechnical studies performed to define thefactors affecting the process of the embankment failure, threesubsequent activities are proposed: the historical analysis offlood damages at the watershed scale; and the assessmentof the upstream peak impoundment based on hydrologicalanalysis and the embankment stability analysis, these latterto be carried out at the site specific scale. The approach hereproposed is planned to be further validated and improved bymeans of the application to other case studies, characterisedby different contexts and embankment structures.

1 Introduction

Seepage due to upstream hydraulic head can sometimes rep-resent the cause of general failure of embankments. In gen-eral, hazard associated to this process is well recognised fordam embankments and levees (El Shamy and Aydin, 2008;Serre et al., 2008), but is frequently underestimated fortrans-portation embankmentslocated in critical points, where arailway or a road intersects the drainage network. In par-ticular, for transportation embankments risk associated to

Correspondence to:P. Lollino([email protected])

seepage is related to temporary and occasional impoundmenton the upstream side (JGS, 2006). Upstream impoundmentscan be generally produced by the combination of two differ-ent conditions. The first one can be defined as “flood event”and concerns the occurrence of either peak flood dischargein the drainage network or so-called secondary floods (stag-nancy of rain on low permeability surfaces) as an effect ofheavy rains (Petrucci and Polemio, 2009). Such events canbe generally identified as flash floods, therefore local floodsof high intensity and short duration, generally less than 6 h;these floods can be extremely dangerous or dramatic, al-though being rare (Polemio, 2010). The second conditionis represented by the “hydraulic weakness” of the transporta-tion embankment, which is determined by drains, culverts,bridges, and, in general, hydraulic sections that can be eas-ily obstructed during a flood or are underdesigned from ahydraulic point of view for the specific flood event. The oc-currence of the latter condition is more frequent than whatis generally expected. As such, this is frequently related tohydraulic design criteria defined on the basis of usual or non-exceptional floods, i.e. assessed by accounting for standardlow return periods (20 or 50 yr) or considering low impor-tance and/or short life of the infrastructure embankment. Inboth cases, the risk that the traffic can be interrupted for ashort time as an effect of the water flow with frequency equalto the inverse of the selected flood return period is consid-ered generally acceptable. Such an engineering approach isaimed at the cost optimisation of the transportation work (i.e.the increase of the design flood return period increases theincidence of the construction costs), but its reliability is fre-quently low, since the risk of failure of the embankment dueto upstream water impoundment is generally not consideredin the design procedures, probably because this kind of eventis believed as exceptional, temporary and without relevantconsequences on the embankment stability. In some othercases, a proper hydraulic design is neglected as the drainagecatchment area seems to be small, the drainage network is notevident (as for the case of karstic areas) or floods are rare.

Published by Copernicus Publications on behalf of the European Geosciences Union.

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3384 M. Polemio and P. Lollino: Failure of infrastructure embankments induced by flooding and seepage

Where flood events along with hydraulic weakness are ne-glected, the embankment collapse can produce damages andcasualties (e.g. Ansary and Safiullah, 2002; Tatsuoka et al.,2007; Ten Brinke et al., 2008; Polemio, 2010). This kindof hazard depends both on the probability of flooding eventsand on the embankment characteristics.

In general, seepage within embankments can generate twokinds of processes from a geotechnical point of view: slidingfailures along the embankment slopes and internal erosion in-side the embankment. Both the processes can induce, underparticular conditions, the general failure of the embankment.In particular, sliding failure of the embankment slopes de-rives from the transient seepage process within the embank-ment and the development of pore water pressures, whichare responsible for the reduction of the effective soil shearstrength. On the other side, under specific seepage condi-tions, water flow can gradually induce internal erosion pro-cesses, which may occur under different modes such as (Felland Fry, 2007; FEMA, 2006):

– backward erosion, if seepage exit gradient along thedownstream area of the embankment is sufficiently highto mobilize soil particles and can evolve upstream ifthe hydraulic gradient remains high throughout the em-bankment (Terzaghi, 1939);

– suffusion, represented by the progressive migration ofthe fine grains through the coarser soil particles as aneffect of seepage forces, this being generally possible ifthe soils forming the embankment are internally unsta-ble. Internally unstable soils are usually broadly gradedsoils with particles from silt or clay to gravel size or gap-graded soils (Kenney and Lau, 1985 and 1986; Skemp-ton and Brogan, 1994; Wan and Fell, 2008);

– concentrated leakage erosion, that can initiate throughexisting open cracks or interconnecting voids resultingfrom differential settlement, desiccation, hydraulic frac-ture or poorly compacted materials, when shear stressinduced by concentrated flow exceeds the critical shearstress of the soil (Lane, 1934).

After initiation, all the processes above described can evolveacross subsequent stages, up to the formation of a breachthat corresponds to the embankment failure (Fell et al., 2003;Wan and Fell, 2008). In particular, a breach may occur ac-cording to several mechanisms, as:

– gross enlargement of erosion pipes,

– deep slope instability of the downstream slope,

– unravelling of the downstream slope face,

– overtopping, owing to crest settlement.

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Figure 1 - Geological scheme of the Apulia Region. Upper rectangle: Southern Italy and 2

provinces, white for Bari Province and lined for the rest of the Apulian provinces. Main 3

rectangle: 1) Recent clastic cover (Pliocene–Pleistocene), 2) bioclastic carbonate rocks 4

(Paleogene) and calcarenites (Miocene), 3) chert-carbonate rocks (Upper Jurassic-5

Cretaceous), 4) carbonate platform rocks (Upper Jurassic-Cretaceous), 5) study area. 6

Fig. 1. Geological scheme of the Apulia Region. Upper rectangle:Southern Italy and provinces, white for Bari Province and lined forthe rest of the Apulian provinces. Main rectangle: (1) recent clasticcover (PliocenePleistocene), (2) bioclastic carbonate rocks (Paleo-gene) and calcarenites (Miocene), (3) chert-carbonate rocks (UpperJurassic-Cretaceous), (4) carbonate platform rocks (Upper Jurassic-Cretaceous), (5) study area.

An area where this type of hazard was underestimated is lo-cated in Murgia, which is a karstic area in Puglia (south-ern Italy) characterised by Mediterranean weather conditions(Fig. 1). Here, rare flood events can trigger huge damagesto the railway and road network. The last flood event oc-curred in 2005 and caused catastrophic damages, includ-ing six deaths, five of them being due to transportation em-bankment failure. Railways and roads were interrupted andseverely damaged in several locations within an area of about1500 km2 (Fig. 2). Therefore, in the present work this areahas been chosen as representative study case in order to ex-plore the factors that can produce such embankment failures(Gabriele et al., 2006; Mossa, 2007; Miglietta and Regano,2008; Polemio, 2010). Based on the study performed on thespecific test-area, this work is aimed at defining a method-ological procedure to investigate the existence of hazard dueto embankment failure induced by flooding. In particular,the approach is based on historical flood analysis, detailedhydrological analysis, embankment soil characterisation, fi-nite element analysis of the seepage process, limit equilib-rium calculations of the embankment slopes and assessmentof the internal erosion likelihood.

Nat. Hazards Earth Syst. Sci., 11, 3383–3396, 2011 www.nat-hazards-earth-syst-sci.net/11/3383/2011/

M. Polemio and P. Lollino: Failure of infrastructure embankments induced by flooding and seepage 3385

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Figure 2 - Schematic map of streams, catchments, railways, roads, and damages. 1) main 3

catchments (natural morphology, not considering artificial channels); 2) selected sub-4

catchment (Scappagrano Stream); 3) benchmark (m asl); 4) urbanised areas; 5) railways; 6) 5

location of main damage of 2005 flood event, 7) rain gauges. 6 7 8

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Fig. 2. Schematic map of streams, catchments, railways, roads, and damages. (1) main catchments (natural morphology, not consideringartificial channels); (2) selected sub-catchment (Scappagrano Stream); (3) benchmark (m a.s.l.); (4) urbanised areas; (5) railways; (6) locationof main damage of 2005 flood event, (7) rain gauges.

2 Methodological approach

Failure hazard assessment of embankments induced byflooding should concern the whole system of existing roadsand railways, but can be more easily performed at the water-shed scale. This kind of analysis, called “verification” here-inafter, can be developed according to three schematic subse-quent stages or activities regarding a specific watershed. Thethree activities are: historical analysis of flood damages; as-sessment of peak impoundment; and embankment stabilityanalysis. The first activity should be carried out consideringthe whole extent of the drainage basin, which generally in-cludes many potential critical points; the remaining activitiesshould instead be detailed for each of these critical points.

The historical analysis on flood damages is focused on theseries of past or historical flood events which caused dam-ages, with specific attention to transportation infrastructures(railways or roads) and particularly embankments. If the ex-istence of past cases is verified, the analysis should high-light the recurrence period of damaging floods and discusseach specific event in order to check if embankment insta-bility events due to upstream impoundment were observedin the past or could be identified for recently built infras-tructures. This activity is based on the collection of flood

data obtained from different information sources, such asnewspapers, historical documents, maps, epigraphic mark-ers on historic buildings, etchings, paintings, scientific ar-ticles, books, and statistical documentation available in thetechnical archives of public institutions (such as municipali-ties). The purpose is to improve the quality of data regardingflood characteristics (peak water levels, flooded areas, dam-age, etc.) and to expand the duration of the study periodto assess flood recurrence period (Herget and Meurs, 2010;Llasat et al., 2006; Petrucci and Polemio, 2009; Polemio,2010; Schmocker-Fackel and Naef, 2010). A small frac-tion of these documents describes the studied phenomenonin detail, but very often such documents deal with the ef-fects, impact or damage (proxy-data) (Benito et al., 2004).Moreover, the information is not always completely reli-able; in such cases, the data require a critical analysis ofthe sources (Glaser, 1996; Petrucci and Polemio, 2002 and2009). Despite these difficulties, a historical analysis mightgive evidence of significant modifications produced by natu-ral disasters or floods (Glaser and Stangl, 2004; Petrucci andPolemio, 2007). Therefore, this activity can represent a re-liable tool in order to assess the existence and the level ofhazard due the effects of flood events on railways and roads

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and, in such case, the specific contribution of the event to em-bankment damages can be considered. If some of these doc-uments include quantitative data concerning rainfall, maxi-mum flood head or stage, discharge yield and/or submergedarea, they might be used for the numerical analysis typicalof the second activity. The historical analysis can be thenintegrated using calculated flood maps, which are usuallyproduced by drainage basin authorities or other local institu-tions, to complete the list of critical points and collect morehydrological information (Cook and Merwade, 2009).

At the end of the aforementioned activity, two schematictypes of results can be hypothesized. The first one is thenonexistence of data concerning damages by floods or ele-ments at risk, particularly in terms of transportation embank-ments. This is the case for areas at very low or negligibleflood risk and/or areas in which transportation embankmentswere built considering all kind of effects due to floods. In thiscase, the activity can be stopped at this step by highlightingthat the risk due to upstream impoundment of transportationembankments can reasonably be assessed as low or negligi-ble.

If the existence of past damages to embankments is insteadverified, the second activity of the methodology here pro-posed should be performed. This second activity regards thequantitative assessment of observed impoundments or peakfloods at specific embankments, with the maximum level ofaccuracy allowed by the available data. Therefore, the peakimpoundment assessment is aimed at defining the impound-ment effects on the upstream side of embankments. Valuesof peak water head and impoundment duration should be de-termined on the basis of the historical research and/or floodmaps based on geomorphological, hydrological or hydraulicflood studies, if existing. At this stage, the quantitative ex-tension of historical analysis is pursued by using stream flowrouting (the stream flow hydrograph in a stream section oralong a whole stream), which denotes mathematical or nu-merical 1-D to 3-D formulations and techniques for flow sim-ulation in natural rivers and channels (Cunge et al., 1980;Herget and Meurs, 2010; Ortega and Garzon, 2009; Polemio,2010). The basic purpose of this analysis is to quantify thepeak water head, the discharge yield and/or the submergedarea by using one or two of these types of data as input toassess the more reliable missing information as output. Withregard to this subject, many hydraulic or hydrologic meth-ods are available in the literature and can be easily applied(ASCE, 1996). The study case hereafter described offers anexample of such calculations based on procedures verifiedworldwide. Standardisation of this part of the activity is notstraightforward, as it depends both on the type of availabledata and the method applied (Cook and Merwade, 2009). Se-lected historical data, treated by means of hydraulic and/orhydrological methods, may allow assessment of the waterhead variation on the upstream embankment face as a func-tion of time; as an alternative, low-detail historical data canbe used to assess the recurrence period of damaging floods.

The stream flow routing permits calculation of hydrographsin each stream section for floods of selected return period,this being assigned as not lower than the recurrence periodof damaging historical floods.

The knowledge acquired at the end of this activity interms of damaging floods characteristics can be applied tothe whole studied drainage basin, assessing hydraulic criticalconditions for each critical point and considering embank-ments that have been recently built upstream or downstreamof embankments damaged in the past. At the same time,the results of the first two activities, i.e. historical analysisof flood damages and peak impoundment assessment, can beapplied to embankments that have been built after the studiedhistorical events.

For each of the critical situations detected in the drainagebasin, the third activity of the methodology should be carriedout. This is based on the geotechnical stability analysis of theembankment as subjected to the boundary and loading con-ditions imposed by the impoundment event, as reconstructedaccording to the previous activities. The geotechnical assess-ment includes:

1. stability analyses of the two embankment slopes bymeans of either limit equilibrium analyses or eventuallymore sophisticated numerical simulations;

2. analyses aimed at assessing the stability of the embank-ment soil material against the risk of internal erosion.

Regarding the latter, the susceptibility of the embankment tointernal erosion according to all the main erosion processesabove described, i.e. backward erosion, suffusion and con-centrated leakage erosion, should be eventually assessed. Inparticular:

2a. for backward erosion, the corresponding susceptibilityshould be verified by means of a comparison betweenthe exit hydraulic gradient and the critical value of gra-dient for piping (Terzaghi, 1939; Schmertmann, 2000);

2b. for concentrated leakage erosion, the analysis shouldconsider the comparison between the shear stress in-duced by the flow through the potential leakage and thecritical shear stress for erosion to occur (Lane, 1934);

2c. for suffusion, the corresponding susceptibility shouldbe examined by means of geometric criteria aimed atdefining if the soils are internally unstable (Kezdi, 1979;Kenney and Lau, 1985; Burenkova, 1993), whereas theconditions for the onset of suffusion should be definedby means of hydraulic criteria (Skempton and Brogan,1994; Wan and Fell, 2008).

In general, the assessment of the embankment stability (thirdactivity), which should be restricted only to the embank-ments that were found to be critical based on the watershed-scale hydraulic analysis, should be based on the embank-ment geometry and soil physical and hydraulic property data,



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Figure 3. View of the breach developed in the Acquaviva railway embankment (point 4 in 2

Figure 4) after 5 October 2005 flooding (picture taken from the upstream side) . 3

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Fig. 3. View of the breach developed in the Acquaviva railway em-bankment (point 4 in Fig. 4) after 5 October 2005 flooding (picturetaken from the upstream side).

which can be generally gained from the results of field and/orlaboratory investigations performed at the time of design andconstruction of the embankment, available from the owneragency of the specific embankment. These data can be even-tually augmented by specific in-situ and/or laboratory test-ing, the latter to be carried out on samples taken from bore-holes or shallow trenches, which can be performed for theaim of the risk assessment.

3 The study case

The study case is located in Apulia, southern Italy, wherethe bedrock is largely karstic carbonate rocks. The region isdominated by three carbonate structural domains: Gargano,Salento, and Murgia (Cotecchia et al., 2005) (Fig. 1).

The altitude of Murgia, where the study area is located,varies from sea level to 680 m a.s.l. The karst environ-ment consists of platform Cretaceous limestone and dolo-stone covered with thin layers of Plio-Quaternary rocks andsoils. The carbonate rock is bedded, jointed and subject tokarst phenomena (Cotecchia et al., 2005). The landscape istypically low-relief karst, characterised by dolines and flat-bottom valleys (locally calledlame) covered by thin allu-vial deposits, mainly residual clayey deposits from karst pro-cesses. The climate is typically Mediterranean and semi-arid,where mean annual rainfall is 550 to 650 mm and mean an-nual temperature 14.3 to 16.8◦C.

During recent centuries, the Murgia was rarely hit by dam-aging flood events, but from the beginning of the twentieth-century, flood events have been frequently observed insome basins surrounding Bari and other minor towns hav-ing mouths on the Adriatic Sea and the Ionian Sea coastlines(Polemio, 1998; Mossa, 2007; Polemio, 2010).

On 22 October 2005, a wide breach of the embankmentof the national railway connecting Bari and Taranto occurredas a consequence of a flash flood (Fig. 3). The embankment

collapsed after about 6 h from the beginning of the rainfallevent, which produced a maximum upstream head of the im-poundment of about 6.3 m from the ground level (the peakwas defined based on to the debris observed close to the de-stroyed embankment; the head was determined by means oftopographical measurements). After about 2 to 3 h the reser-voir had reached the maximum level, the embankment failedwith the dispersion of soil and rock blocks all over the down-stream area. Evidences of shallow instability processes alongboth the two embankment slopes as well as of internal ero-sion processes were observed throughout the remaining por-tions of the embankment during field surveys performed bythe authors after the failure event. A detailed back-analysis ofthis study case has been performed according to the method-ology described in Sect. 2 in order to define the most likelycauses of embankment failures and are described hereafter.

3.1 Historical analysis of flood damages

Regarding the examined watershed, historical and technicalsources have been analysed in order to create a databaseof flood events covering a period as long as possible. Thegoal was to broaden the study period and gather more detailsabout event recurrences (Polemio and Sdao, 1996; Petrucciand Polemio, 2002, 2009). In particular, data were gath-ered from several kinds of sources (Polemio, 2010): histor-ical documents and archives, scientific papers and books onflood events, a public database for natural disasters, docu-mentation available in the technical archives of public in-stitutions, newspapers, direct observations of recent floodevents, books, public statistical databases, historical archivesand maps in public libraries and municipalities, which werecollected in order to define the anthropogenic land modifica-tions.

The research allowed the identification of 13 flood eventsfrom 1567 to 2007, each causing damages of different typeand level; in particular, seven events caused damages to em-bankments (Table 1). It was noted that the landscape of thestudy area was modified many times by anthropogenic ac-tivities during the last millennia. The main anthropogenicmodifications – deforestation, development of sheep grazingand cereal growing areas, changes in population and devel-opment of urban and rural areas – had effects on the floodrecurrence period and the level of damages. The effects ofthese modifications were analysed to consider their impactin terms of damaging flood occurrences and hydrologicalcharacteristics. The construction of the first embankmentstook place in Italy at the end of the nineteenth-century dueto the railway network development. From the beginningof the twentieth-century, damaging flood events have beenfrequently observed in Murgia. The 1926 flood, which isthe first extremely catastrophic event recorded, caused manycausalities and enormous damages, including damages tosome embankments (Alfieri, 1927). The flood completelydestroyed the bridge and the embankment at location A in



Table 1. Flood events triggering damages to embankments with bridges and/or culverts.

Year StreamDamages

Deaths Injured Others damages Level

1567 Picone (?) low1833 Picone ? low1914 Valenzano Railway embankment medium1925 Lamisanata medium1926 Picone, Valenzano 20 Tens Town flooded, houses, factories, infrastructure railway extremely high1957 Picone Road culvert, narrow area very low2005* Picone, S.Giorgio 6 Tens Town flooded, houses, roads, factories, railway, infrastructures high

* The location of the main damages of the 2005 flood event are shown in Fig. 2.

Fig. 4, which were later rebuilt on the basis of an efficienthydraulic design, as confirmed by the 2005 flood that didnot cause any damage to the new bridge. Based on a seriesof maximum flood head observations and applying simplestream flow routing procedures, the peak flow was calculatedin a number of stream sections; in the area of location A, thepeak value was 225 m3 s−1, which is equal to a mean spa-tial contribution of 2.5 m3 km−2 of drainage basin (Alfieri,1927). The 1957 flood caused very low damages, mainly thedestruction of a paved road and a small road culvert (locationB in Fig. 4), located upstream of the embankment hit in 2005(location D in Fig. 4).

In 2005, the Picone Stream again produced disastrousflood events, as in 1926. The event of October 2005 causedsix deaths, numerous injuries and many other types of dam-age (Table 1). The whole road network was heavily hit(Fig. 2); the flood event damaged bridges, culverts, embank-ments, walls and carriageways at 34 points. Many of thesecasualties and accidents were associated with new trans-portation works that were built in the previous 20 yr. Fivepeople died in a car while crossing the drainage network inlocation C of Fig. 4, due to the rapid collapse of a culvert andof a road embankment. These works date to the 1990s andwere intended to modernise a secondary road that forded thestream (generally dry due to the karstic nature of the area).Two remarks should be highlighted: the work design did notexploit the experience of the bridge collapse that occurredabout 1 km downstream during the 1926 flood (location A inFig. 4); no damage occurred in that critical point before theroad “modernisation” (Polemio, 2010).

In location D of Fig. 4 (see also Fig. 3), where the em-bankment here examined is located, almost all of the injuredpeople were passengers who miraculously survived a rail-way disaster: a train went off the rails at high velocity (over120 km h−1) after crossing a 21 m long portion of the railsthat was left unsupported due to the collapse of the under-lying 7 m high embankment. The embankment and the cor-responding culvert were constructed in the 1990s as part ofa new double-track railway, which was built to replace the

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2 Figure 4 - Detailed map of selected sub-catchment: 1) Alluvial deposits, clay and silty clay 3

with pebbles; 2) calcarenite, secondly sand and conglomerate; 3) limestone and dolostone; 4) 4

Picone Stream catchment divide; 5) selected sub-catchment (Scappagrano Stream); 6) 5

drainage network; 7) Isohyet contours (mm) of total rainfall (from 2:30 AM on 22 October to 6

6:30 AM on 23 October); 8) isohypse (in the selected sub-basin); 9) urbanised areas; 10) 7

railway; 11) road; 12) location of 2005 main damage; 13) location of relevant historical and 8

recent damages. 9

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Fig. 4. Detailed map of selected sub-catchment: (1) alluvial de-posits, clay and silty clay with pebbles; (2) calcarenite, secondlysand and conglomerate; (3) limestone and dolostone; (4) PiconeStream catchment divide; (5) selected sub-catchment (ScappagranoStream); (6) drainage network; (7) Isohyet contours (mm) of to-tal rainfall (from 02:30 LT on 22 October to 6:30 a.m. on 23 Octo-ber); (8) isohypse (in the selected sub-basin); (9) urbanised areas;(10) railway; (11) road; (12) location of 2005 main damage; (13) lo-cation of relevant historical and recent damages.

old single-track railway and to provide a shorter and high-speed path. The old railway instead crosses the Scappagranostream with a small bridge and a high embankment locatedabout 700 m downstream from the new line.

Therefore, for the application here described, the upstreamportion of the Picone stream was selected due to the highdensity and frequency of relevant flood events, and a sub-catchment was specifically considered for the relevant effectsof 2005 flood (Fig. 2).



3.2 Peak impoundment assessment

In environments dominated by karstic features, the streamflow is very often so rare as to be considered exceptionalwhen it is observed, regardless of the peak flow value(Ford and Williams, 2007). In these cases, research thatinvolves the role of stream discharge must operate witha lack of time series on stream yield and, therefore, ahydro-geomorphological approach can be used. The hydro-geomorphological approach is mainly based on two types ofdata: rainfall and geomorphological characteristics of stud-ied drainage basins.

In the specific case study, the hydro-geomorphologicalanalysis considered rainfall data, qualitative hydrogeologi-cal characteristics of outcropping rocks and soils, and geo-morphological data of the drainage basin within a GIS envi-ronment. Main sources of data were: topographical maps(1:50 000 to 1:5000), geological map, soil map, land-usemap, DTM (digital terrain model, 30 m spacing), aerial pho-tos, authors’ survey and rainfall data. All maps were used indigital formats; remaining data were processed to be anal-ysed in a GIS environment. The Scappagrano stream, asmall, upstream sub-basin of Picone Stream, which is closedby the upstream section of the selected embankment culvert(Figs. 2 and 4), was selected as our study area.

During the 2005 flood event, the flood flow was hinderedby the small embankment culvert, thus giving rise to theupstream impoundment. At the culvert section the catch-ment surface is 23.0 km2 and the maximum stream length is11.9 km, whereas the altitude ranges from 272 (culvert bot-tom) to 475 m a.s.l. and the mean altitude is 356 m a.s.l. Theconcentration time,tc, i.e. the time of water flowing from themost remote point of the drainage basin to the outlet section,was assessed using methods suggested by Ferro (2006) andwas determined to be roughly equal to 3 h.

Rainfall data of the specific event from 17 rain gauges(managed by the Regional Civil Protection, Air Force andUCEA, the Italian Central Office of Agriculture Ecology)were available in terms of rainfall cumulative values every0.5 h. These gauges were located from 4 to 30 km from theselected sub-catchment (Fig. 2). Using the kriging method,110× 106 m3 of rain fell on a ground surface approximatelyequal to 1840 km2, from 02:30 local time (LT hereinafter) on22 October to 06:30 LT on 23 October (Deutsch and Journel,1992).

Long time series of the annual maximum rainfalls of shortduration were used to assess the return period of the highintensity rainfall event that caused the 2005 flood (Polemio,2010). Time series of the annual maximum of cumulativerainfall for 1, 3, 6, 12, and 24 h are available since 1929. Themaximum rainfall return period was assessed by consider-ing a confidence interval of 0.95 and a series of probabilitydistribution function and methods for parameter assessment(Polemio and Sdao, 1996; ASCE, 1996; Polemio, 2010). The

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2 Figure 5 - Rainfall and flood of the 2005 event in the selected catchment. 1) Actual and 2) net 3

rainfall (portion of rainfall available for runoff), 3) flood discharge at the catchment bottom, 4

4) culvert outflow. 5

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Fig. 5. Rainfall and flood of the 2005 event in the selected catch-ment. (1) Actual and (2) net rainfall (portion of rainfall availablefor runoff), (3) flood discharge at the catchment bottom, (4) culvertoutflow.

return period of the maximum 3-h cumulative rainfall was es-timated in the range 70–113 yr.

In the Scappagrano basin upstream of the selected em-bankment culvert, 97 to 140 mm (116 mm mean spatialvalue) and a total rain volume of 2.7× 106 m3 fell, mostlyover the time interval of 3 h (Fig. 4). The peak cumulativerainfall was observed at a gauge located 4 km westward ofthe basin, where 162 mm of rain fell. Plotting the 3-h cu-mulative rainfall with 0.5-h steps, for each map the spatialmean rainfall was calculated for the selected catchment; themaximum was H= 112.7 mm, observed between 01:30 and02:00 LT on 23 October. A rough assessment of the returnperiod of H was in the range of 50–100 yr. The runoff inthe study area was defined based on a direct transformationof the net rainfall excess with respect to the infiltration rate.The studied flood was generated by very high intensity 3-h rainfall. The observed rainfall defined the actual rainfallcurve in Fig. 5.

The standard procedure of the Curve Numbermethod (CN) was used to assess the net rainfall of thecatchment. CN was combined with the geomorphic instan-taneous unit hydrograph method (GIUH), which permits thecalculation of the hydrograph in the stream outlet sectionfor which discharge measurements of the studied floodare unavailable (USDA, 1986). The output was comparedwith known data on the observed flood for calibration andvalidation of the model. Based on the rainfall observedbefore the flood event, the antecedent moisture conditionswere classified as type II. The dimensionless CN parameter,defined in the range 0–100, was estimated consideringboth soil infiltration characteristics and land use. TheCN of the catchment was assessed by distinguishing twosoil permeability classes: class A for soils with very highmaximum infiltration rate, such as limestone and dolostone;and class B for soils with lower maximum infiltration rate,



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Figure 6 – Schematic cross-section of the Acquaviva embankment 8

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Fig. 6. Schematic cross-section of the Acquaviva embankment.

such as calcarenite (Fig. 4). The outcrops of alluvial depositswere included in the surrounding of limestone/dolostone orcalcarenite. The land use was determined using up-to-dateCORINE land-cover maps obtained from satellite data(EEA, 1994); the main classes were vineyard, complexcultivation patterns (due to greenhouses for example), wood,olive grove and grazing, in decreasing order. The spatialmean value of CN was 67 for the catchment. Assessingnet rainfall, a time step of 10′ (instead of 30′ of availablerainfall data) was used by linearizing the CN method outputto refine the GIUH method application (Fig. 5). The GIUHmethod accounts for the influence of the channel networkon the delay and the shape of the hydrograph (Nash, 1957;Rodriguez-Iturbe and Valdes, 1979; USDA, 1986). The coreof the method is to define the instantaneous unit hydrograph(IUH) on the basis of geomorphological classification crite-rion of the drainage network, the so-called Horton-Strahlercriterion (Horton, 1945; Strahler, 1952). Using this method,we calculated the flood discharge hydrograph of Fig. 5.

Validating the flood hydrograph and reducing the uncer-tainty could be useful for flood discharge assessment sincestream flow measurements were unavailable for the studyarea. This limitation was solved by taking advantage of the2005 event. In this case, a small concrete culvert (Fig. 3,hydraulic section of 9 m2) could not accommodate the out-flow of peak discharge, creating a large flooded area and anarea of water storage on the upstream side of the embank-ment. Seepage across the embankment caused the rapid col-lapse of a 21-m long portion of the structure, where the max-imum impounded water head was reached, causing a veryquick emptying of the flooded area. The maximum waterhead and the relationship between water altitude and watervolume (190 000 m3 peak value) were defined in detail withspecific in-situ topographic measurements, while the tempo-ral sequence of water stages was defined based on officialinformation from the railway authorities. The discharge out-flow of the culvert was assessed with well-known formulasthat consider the culvert geometry and the upstream waterhead (peak value 48 m3 s−1). The results in Fig. 5 are consis-tent with all measurements and information collected due tothe railway disaster, thus confirming the accuracy of the ap-plied method. The upstream flood peak flow was 87 m3 s−1,which occurred at about 03:40 LT on 23 October, about 4 hafter the high intensity rain began. The total volume of theflood was equal to 1.1× 106 m3.

32

1

2

3

4

0

10

20

30

40

50

60

70

80

90

100

0,001 0,01 0,1 1 10 100

log D (mm)

% p

assi

ng

inner rockfill

fine soil

5

Figure 7 - Grain size distribution curves of the inner rockfill and finer soil along the slopes for 6

the Acquaviva railway embankment 7

8

9

10

11

12

Fig. 7. Grain size distribution curves of the inner rockfill and finersoil along the slopes for the Acquaviva railway embankment.

3.3 Assessment of the embankment stability

A schematic cross-section of the embankment under studyin the centre of the valley is shown in Fig. 6. The inner por-tion of the embankment was formed of a widely graded rock-fill that was confined by two layers of finer soils, with vari-able thickness between 80 and 150 cm, along the embank-ment slopes. The grain size distribution curves of the rock-fill, as defined in accordance with the measurements carriedout during the construction of the embankment by the ItalianRailway Agency, are shown in Fig. 7. The curves indicatethat the inner rockfill was composed of coarse gravel (25 %),medium gravel (45 %), fine gravel (15 %) and sands (15 %);the sand component was formed of 6 % of fine and mediumsands and 9 % of coarse sand. Unfortunately, direct measure-ments on the grain size curve of the finer soil along the slopesare not available, although this soil can be classified to be inthe range between silt and sand. Therefore, in the analysisdescribed later on, a representative grain size curve typicalfor this class of soil has been assumed (Fig. 7).

3.3.1 Seepage analysis

A finite element analysis of the transient seepage processwithin the embankment, taking into account unsaturated con-ditions of the soil, has been carried out in order to investigatethe variation with time of the flow regime as an effect of theprovisional impoundment on the upstream side. The resultshave been then used both to evaluate the hydraulic condi-tions capable of producing the onset of the piping processand to back-calculate the sliding processes observed alongthe embankment slopes. The mesh adopted for the calcu-lation is reported in Fig. 8. A boundary condition variable



33

1

2

3

Figure 8 - Mesh and boundary conditions adopted for the seepage analysis. Green elements 4

indicate low-permeability soil strata along the slopes; yellow elements represent rockfill. Red 5

circles indicate nodes with a prescribed hydraulic head; blue empty triangles indicate free 6

draining nodes. The inclined red line in the downstream toe area indicates the line of 7

numerical monitoring of the pore pressure (see Figure 12); the horizontal blue line in the 8

downstream toe area represents the line of numerical monitoring of the hydraulic gradient (see 9

Figure 13). 10

11

12

13

14

upstream downstream

Fig. 8. Mesh and boundary conditions adopted for the seepage anal-ysis. Green elements indicate low-permeability soil strata alongthe slopes; yellow elements represent rockfill. Red circles indicatenodes with a prescribed hydraulic head; blue empty triangles indi-cate free draining nodes. The inclined red line in the downstreamtoe area indicates the line of numerical monitoring of the pore pres-sure (see Fig. 12); the horizontal blue line in the downstream toearea represents the line of numerical monitoring of the hydraulicgradient (see Fig. 13).

34

1

2

3

0

2

4

6

8

0 2 4 6 8

time (hours)

upst

ream

wat

er le

vel (

m)

4

Figure 9 - Hydraulic head condition variable with time applied in the seepage analysis to the 5

upstream slope boundary 6

7

8

9

Fig. 9. Hydraulic head condition variable with time applied in theseepage analysis to the upstream slope boundary.

with time in accordance with the impoundment level data re-ported in Sect. 3.2 has been imposed on the upstream face(Fig. 9). A free-draining boundary condition has been pre-scribed along the downstream face, whereas an impermeableboundary condition has been imposed along the bottom ofthe embankment. A saturation line at the bottom of the em-bankment has been assumed as initial condition (t = 0). Bothof the materials of the embankment have been assumed tohave isotropic hydraulic properties. Since no field test couldbe performed on the material as placed in-situ, the value ofthe coefficient of permeability of the saturated soil formingthe inner rockfill within the embankment has been derivedfrom the corresponding grain-size distribution. According toChapuis and Aubertin (2003), this method can be used for aquick but quite confident estimate of thek value, althoughit can be affected by limitations especially for the investiga-tion of eventual anisotropic properties. The value of the satu-rated coefficient of permeability for the inner rockfill was es-timated to beK1 = 5×10−3 m s−1, whereas for the fine soilalong the slopes was equal toK2 = 4×10−7 m s−1. Based

35

1

2

3

4

Figure 10 - Saturation levels in the embankment at different times in the hypothesis of low 5

permeability soil strata along the two embankment slopes. 6

7

8

9

Fig. 10.Saturation level in the embankment at different times in thehypothesis of low permeability soil strata along the two embank-ment slopes.

36

1

2

3

4

Figure 11 - Saturation level in the embankment at different times in the hypothesis of 5

homogeneous embankment. 6

7

8

Fig. 11.Saturation level in the embankment at different times in thehypothesis of homogeneous embankment.

on the corresponding grain size distributions, the soil-watercharacteristic curve of the rockfill has been derived by meansof the Van Genuchten (1980) relationship, whereas the soil-water characteristic curve of the fine soil has been assumedbased on the Arya and Paris relationship (1981), which isgenerally considered as more suitable for sandy soils; thelaws of variation of the coefficient of permeability with suc-tion have instead been defined by means of the Green andCorey approach (1971).

The results of the seepage analysis show that the satura-tion line reaches the maximum level, which in the rockfillremains about horizontal, 3 m above the base of the embank-ment, during the raising of the impoundment level (Fig. 10).The geometry of the saturation line within the upstream finesoil stratum is directly related to the variation of the upstreamboundary condition. Figure 10 also shows that the evolutionwith time of the saturation line is significantly affected bythe difference of permeability of the two soils. In particu-lar, the influence of the fine soil strata along the embank-ment slopes is highlighted if such seepage domain is com-pared with that deriving from a similar analysis simulatinghomogeneous embankment conditions, as shown in Fig. 11.Different pore pressure distributions within the embankmentresult from these two flow processes. In particular, the valuesof the pore pressures calculated along the red line indicated inFig. 8 for the case of homogeneous embankment and thosepredicted for embankment with two fine soil strata at timet = 6 h (i.e. after 3 h of constant maximum reservoir level)are compared in Fig. 12. The figure shows that for the caseof fine soil slopes, values of pore pressures higher than forthe homogeneous case develop at embankment heights lowerthan 3 m.



37

1

2

0

0,5

1

1,5

2

2,5

3

3,5

0 5 10 15 20 25 30 35 40

u (kPa)

emba

nkm

ent h

eigh

t (m

)

homogeneous

fine soil slope

3

Figure 12 – Pore water pressures calculated at the downstream toe area (see red line in Figure 4

8) at time t = 6 h (see Figure 9) for different embankment configurations 5

6

Fig. 12. Pore water pressures calculated at the downstream toe area(see red line in Fig. 8) at timet = 6 h (see Fig. 9) for different em-bankment configurations.

38

1

2

0

0,2

0,4

0,6

0,8

1

1,2

1,4

1,6

1,8

2

012345678

distance from downstream toe (m)

ih

homogeneous embankment

actual critical gradient

ideal critical gradient

embankment with fine soil slope

3

Figure 13 – Outflow horizontal gradient calculated at the downstream toe (see blue line in 4

Figure 8) at time t = 6 h (see Figure 9) for different embankment configurations and 5

comparison with gradient values critical for the onset of piping.6

Fig. 13. Outflow horizontal gradient calculated at the downstreamtoe (see blue line in Fig. 8) at timet = 6 h (see Fig. 9) for differentembankment configurations and comparison with gradient valuescritical for the onset of piping.

The values of the horizontal hydraulic gradient at thenodes along the blue line in Fig. 8 have also been calculatedat t = 6 h and compared for the two configurations of the em-bankment in Fig. 13. The hydraulic gradient for the homo-geneous rockfill embankment reaches the maximum valueat the downstream face (ih = 0.65) and then reduces grad-ually to values less thanih = 0.3 in the very inner portionof the embankment. For the case of a rockfill embankmentwith low-permeability soil along the embankment faces, the

gradient reaches the highest values within the fine soil layer(ih = 1.2 to 1.9), then reduces in the adjacent rockfill area tolower but still significant values (ih = 0.4 to 1.2) and finallybecomes negligible in the innermost areas.

3.3.2 Analysis of the failure process of the embankment

Slope stability

Several shallow sliding or unravelling processes, mainly in-volving the fine soil strata along the embankment slopes,were observed along the remaining portion of the embank-ment during field surveys performed after the failure event(Fig. 14). Based on the pore pressure distributions calculatedby means of the transient seepage analysis, limit equilibriumback-analyses have been carried out using the Morgestern-Price method (Morgenstern and Price, 1965) with respect tosliding surfaces along the embankment slopes that resemblethose observed in the field (Fig. 15a and b). In particular,the strength parameters of the rockfill have been assigned tobe equal toc′

= 0 kPa andϕ′= 36◦, since these parameters

can be assumed as representative of a lower boundary of theshear strength of the examined rockfill material, whereas themobilised strength parameters of the fine soil strata along theembankment slopes have been derived as output of the anal-ysis.

Assuming the pore pressure distribution at timet = 6 hfrom the beginning of the raising of the impoundment leveland two values of the mobilised cohesion of the fine soilrespectively equal to 0 and 5 kPa, the results of the back-analysis for the sliding surface along the downstream slopein Fig. 15a indicate values of the mobilised friction angle ofthe fine soil that are equal to:

c′m = 0 : ϕ′

m = 36◦

c′m = 5 kPa: ϕ′

m = 25◦

whereas higher values of the mobilised friction angle havebeen obtained for the sliding surface along the upstreamslope indicated in Fig. 15b, e.g.:

c′m = 0 : ϕ′

m = 42◦

c′m = 5 kPa: ϕ′

m = 38◦

These quite high mobilised strength values should confirmthat shallow slope instability processes were likely to haveoccurred along both downstream and upstream slope of theembankment due to the very unfavourable flow conditionsthat took place in the embankment during the flooding event.Moreover, flow sliding might also have occurred since thesoils involved are in the range of soils potentially suscepti-ble to liquefaction and flow sliding according to their parti-cle size distributions, as indicated by Hunter and Fell (2003).Unravelling was also likely to occur along the downstreamslope due to very high seepage flow existing at the down-stream toe.



39

1

2

3

4

Figure 14 – Unravelling/sliding failures observed along the slopes of the remaining portion of 5

the Acquaviva embankment. 6

7

Fig. 14. Unravelling/sliding failures observed along the slopes of the remaining portion of the Acquaviva embankment.

40

1

2

3

4

Figure 15 – Back-analysis of the slope stability of the embankment at time t = 6h. Sliding 5

surface assumed along the downstream slope (a) and along the upstream slope (b). The blue 6

dashed line represents the saturation level calculated at the same time. 7

8

a)

b)

Fig. 15. Back-analysis of the slope stability of the embankmentat time t = 6 h. Sliding surface assumed along the downstreamslope(a) and along the upstream slope(b). The blue dashed linerepresents the saturation level calculated at the same time.

Internal erosion

Quantitative assessments aimed at verifying the likelihood ofoccurrence of the different processes of internal erosion havebeen carried out for the specific examined embankment.

The procedure for the assessment of the susceptibility tosuffusion has been based on the evaluation of the shape curveH-F, as described by Kenney and Lau (1985, 1986), which isto verify if such curve lies for some portion under the criti-cal lines provided by the same authors. Thereafter, the valueof the hydraulic gradient as determined from the finite ele-ment seepage analysis (see Sect. 3.3.1.) has been comparedwith the value of the critical hydraulic gradient calculatedaccording to Skempton and Brogan (1994). Based on thegrain size distribution curve defined for the inner rockfill,the corresponding “shape curve” in the H-F plot has been

41

1

2

0

0,1

0,2

0,3

0 0,1 0,2 0,3F

H

H/F=1.0

H/F=1.3

grain size data

3

Figure 16 – H/F shape curves calculated for the grain size distribution curves in Figure 7 and 4

comparison with critical H/F curves. 5

6

Fig. 16. H/F shape curve calculated for the rockfill grain size curvein Fig. 7 and comparison with critical H/F curves.

derived (Fig. 16). The curve has been defined for F valuesbetween 0 and 0.2, since the material is characterised by auniformity coefficientCu higher than 3 (see Kenney and Lau,1985). Figure 16 shows that the curve representative of theembankment rockfill is typical of a material highly unsta-ble for two ranges of diameters: in fact, a minimum valueof the ratio H/F= 0.57 is obtained for a diameterD in therange [0.1,0.3] and also a relative minimum H/F= 1.0 cor-responds to the range ofD = [1,1.5]. These values of thestability index clearly confirm the susceptibility of the mate-rial to be internally unstable according to the criterion pro-vided by Kenney and Lau (1985). Moreover, the materialalso is shown to be unstable according to the Kezdi criterion(1979), since the value of the ratiod ′

85/D′

15 is higher than4. The corresponding value of the critical gradient has beenthen estimated based on the experimental works of Skemp-ton and Brogan (1994). In particular, considering a range of



42

1

2

3

Figure 17 – Photos of the concentrated erosion pipe at the contact between the wall of the 4

concrete conduit and the soil material. 5

6

Fig. 17. Photos of the concentrated erosion pipe at the contact be-tween the wall of the concrete conduit and the soil material.

H/F values equal to 0.6 to 1.0, the mean value of the actualcritical gradient of such unstable material is approximatelyequal to:

icr = 0.25÷0.3× ic = 0.35

where ic represents the critical gradient deduced from theTerzaghi theory (ic = 1.15).

It turns out that, for the outer portion of the embankment,the hydraulic gradient calculated in the case of fine soil alongthe embankment slope is significantly higher than criticalgradient for unstable material and is also higher than that forstable material, but it reduces down to negligible values inthe inner areas of the embankment (Fig. 13). However, thevalue of the hydraulic gradient calculated under conditionsof homogeneous embankment, which were presumably es-tablished after shallow instability processes and unravellingtook place both upstream and downstream, is of the same or-der of magnitude as the critical gradient in the inner areasof the embankment. These results should explain that suffu-sion was very likely to have occurred within the embankmentduring the flooding event. Therefore, this process can repre-sent a possible cause for a settlement of the embankment inthe failed portion, with the consequent overtopping and theglobal failure of the embankment.

The comparison of the calculated values of the hydraulicgradient with the critical gradient for piping, as defined ac-cording to the Terzaghi theory, instead makes backward ero-sion more unlikely than suffusion. Evidence presumed toindicate concentrated erosion holes at the contact betweenthe concrete conduit and the nearby rockfill areas, the latterbeing generally exposed to the risk of poor compaction andhigh permeability, was also observed during field surveys af-ter the failure event. In Fig. 17 two pictures of the effectsof an erosion pipe within the soil material close to a wallof the conduit are shown. In order to perform a preliminaryevaluation of the likelihood of initiation and propagation ofconcentrated erosion in a hole of the same size, an assess-ment of the hydraulic shear stress in a cylindrical pipe as afunction of the pipe length, assuming a mean pipe diame-ter of D = 0.5 m and a mean water head loss due to friction

43

1

2

3

0

100

200

300

400

500

10 15 20 25 30 35

L (m)

τ (P

a)

4

Figure 18 – Estimated hydraulic shear stress in a cylindrical pipe as a function of the pipe 5

length (pipe diameter of D = 0.5 m; water head loss of Hf = 4 m) 6

Fig. 18. Estimated hydraulic shear stress in a cylindrical pipe as afunction of the pipe length (pipe diameter ofD = 0.5 m; water headloss of Hf = 4 m)

of Hf = 4 m, has been carried out. The results are shown inFig. 18 and indicate values of the shear stress that are higherthan the critical shear stress provided for coarse grained soilsby Wan and Fell (2004).

4 Discussion

The methodology here described represents an example ofa preliminary analysis aimed at evaluating the hazard exist-ing for embankments located in specific areas where transientupstream reservoirs due to intense flooding can develop, withthe consequent risk of internal erosion and catastrophic fail-ure of the same structures. The hydraulic analysis aimed atevaluating the history of the upstream boundary conditioninduced by flooding as well as the methodology adopted todetermine the internal erosion susceptibility or the instabil-ity processes along the embankment slopes have been pre-sented in the previous sections with respect to the specificcase study. The methodology proposed can be used to assessa scale of hazard among the embankments existing within awatershed. As such, the hydraulic analysis should be aimedat defining the hydrograph for those sections in the whole wa-tershed under study that are shown to be at risk; and, in par-ticular, the return period of specific extreme rainfall eventsand the peak impoundment induced at the considered sec-tions should be determined. For those embankments that aredetermined to be critical according to the hydraulic analy-sis, the internal geometry of the embankment and the corre-sponding soil physical and hydraulic properties should be as-sessed either by means of the information available from thedesign of the embankment or by means of appropriate field



or laboratory testing, the latter to be carried out on samplestaken from boreholes or shallow trenches. Then, the geotech-nical analysis aimed at assessing both the susceptibility tointernal erosion of the embankment and the slope stabilityconditions should be performed. It follows that the embank-ments that are determined to be characterised by a high levelof failure hazard should be identified and proper strategies ofremediation should eventually be considered accordingly.

The main limitations of the methodology proposed regardthe rather high level of uncertainty related to the difficultyin defining accurate soil properties of the embankment aswell as the most likely modes of failure. Therefore, sincethe physical processes involved in the failure of an embank-ment are often complex and not precisely quantifiable, suchan approach can act as a guide to the likely behaviour of theembankment under extreme flooding conditions.

5 Concluding remarks

The paper proposes a methodology to approach the problemof assessing hazard associated with failure of existing trans-portation embankments due to flooding. The methodology isbased on a historical analysis of flood damages at the water-shed scale, a peak impoundment analysis at the single em-bankment scale and a consequent analysis of the embank-ment stability.

In this paper, a specific back-analysis of the collapse of anational railway embankment induced by a flash flood eventhas been carried out as an example of the possible method-ological approach.

The results of the analysis indicate that the failure of therailway embankment is thought to be induced by internal ero-sion processes, probably consisting of suffusion of the soilforming the embankment, together with concentrated pipeerosion along the contact between the rockfill and the con-crete culvert, that led within a few hours to the global failureof a large portion of the structure.

Seepage and limit equilibrium analyses have also shownthat the shallow slope failures observed along both the down-stream and upstream slopes were induced by high pore wa-ter pressures induced by the transient flow process withinthe embankment. However, these shallow slope failures arethought to be not the primary cause of the embankment fail-ure, but rather they likely enhanced the internal erosion pro-cesses.

Edited by: D. KeeferReviewed by: two anonymous referees

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http://dx.doi.org/10.5194/hess-14-1581-2010

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