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1

INTRODUCTION

Landslide hazard along the Campanian carbonateslopes covered by pyroclastic deposits is mainlyconnected to the occurrence of high-velocity debrisavalanches and debris flows. Failures are mainly

controlled by the interaction of numerous factors, both natural and human-induced. Particularly, theresearch results show how the geomorphological set-tings have played a decisive role in locating thesource failures of landslides, which have then de-termined catastrophic effects (Guadagno & PerrielloZampelli 2000, Guadagno 2000, Guadagno et al.2003, Guadagno et al., in press).

Identification of these geomorphological controlfactors causing landsliding is therefore of fundamen-tal importance and constitutes the bases for an accu-rate assessment of the landslide hazard. Moreover,this evaluation should also ascertain the parameters

defining landslide intensity to foresee the scenario interms of possible effects on peoples and properties.

The purpose of this paper is to use the results of previous studies, and modelling, carried out by theauthors in the area of Sarno/Quindici and Cervinaralandslides (in 1998 and 1999 respectively) for thedefinition of susceptibility assessment in a new area.In fact, the similarity of the geomorphological envi-ronment allows the methodologies previouslyadopted in post-landslide analysis to be applied inthe hazard evaluation.

2

THE INSTABILITY OF PYROCLASTICDEPOSITS OF THE CAMPANIA REGION

Historical analyses of landslide occurrence on thewestern side of the Campanian Appennine (Fig. 1)have shown that wide sectors of the carbonate hill-

slopes covered by pyroclastic deposits have fre-quently experienced landslides of debris avalancheand debris flow type in the last two centuries (Rani-eri 1841, Lazzari 1954, Civita et al. 1975, Calcaterraet al. 1997, Del Prete et al. 1998, Fiorillo et al.2001). The convergence of different factors allowsthese slope failures to often induce catastrophic ef-fects on people and properties since they are charac-terised by high energy and the absence of macro-scopic premonitory signs. The historic landslideevents display similar characteristics; analogies con-cern landslide type (initial translational slides evolv-ing in fluid flows), material involved, confinement

in gullies or steep stream channels if present, highvelocity and high erosive capacity. Moreover, theyare generally triggered on the occasion of heavy and

prolonged rainfalls.

2.1 The geological and morphological environment

The environmental context of the Tyrrhenian sectorof Campania (Fig. 1) is characterised by a distinctivegeological setting where an extraordinary conver-gence of geological and morphological factors andof volcanic activity exist. Some of these factors are

Debris avalanche and debris flow susceptibility by using morphologicalfactors and dynamic modelling: a case study in Campania (SouthernItaly)

P. Revellino & F.M. Guadagno Dipartimento di Studi Geologici e Ambientali, Università del Sannio, Benevento, Italy

O. Hungr Department of Earth and Ocean Sciences, University of British Columbia, Vancouver, Canada

ABSTRACT: Landslide hazard of the Campanian carbonate slopes covered by pyroclastic deposits is mainlyconnected to the occurrence of high-velocity debris avalanches and debris flows. Analyses show that flowsinitiate in connection to small translational slides in the pyroclastics. The failure process is controlled by theinteraction of both natural and human-induced factors. Geomorphological settings play a decisive role in lo-cating the source failures. Therefore, in landslide hazard assessment it appears crucial: a) recognize the geo-morphological control factors; b) determine parameters defining landslide intensity (velocity, volume, runout

distance). An approach combining geomorphology and numerical analysis is here adopted. Landslide inten-sity scenarios are simulated predicting the runout behaviour of potential instabilities by using a dynamicmodel previously calibrated on observed events in post-landslide conditions. The selected area is a sector ofthe Avella Mts. falling into the same geomorphological environment of the 1998 Sarno landslides.


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Figure 1. . Comprehensive geological map. Legend: 1) Plio-Pleistocene deposits; 2) Altavilla units (Tortonian MiddlePleistocene); 3) Irpinia units (Langhian-Tortonian); 4) Carbon-ate unit of the Alburno-Cervate unit (Trias-Paleocene); 5) Si-cilide units (Cretaceous-Eocene); and 6) Principal faults. Loca-

tions of the study area (Avella) and of the Sarno/Quindici andCervinara areas are also shown.

significant in conditioning slope evolution and have been described below.

As shown in Figure 1, the Campanian Appennineis characterised by calcareous-dolomitic monoclinalsequences dipping towards N-NW (Ippolito et al.1975). As a consequence, the slopes are generally ofstructural origin being alternatively fault and dipslopes.

Beginning from the late Quaternary, air-fall mate-

rial, such as ashes, lapilli and pumices, and ignim- brite-type flows, principally linked with cyclic ex- plosive volcanic activity of the Phlegrean Fields andSomma-Vesuvius Volcanoes, has been broadly man-tled the limestone relief. The result is a rather com-

plex layering stratigraphy deposits dipping as the bedrock surface, due to both the depositionalmechanisms and the subsequent weathering proc-esses and pedogenesis leading to the formation of

buried soil horizons. The deposit thickness is re-markably variable depending on the eruption typeand prevailing wind directions, the former slopemorphology and exposure, as well as the post-deposition effects of erosion and re-mobilization

processes. Generally, it decrease from the valleyfloor (> 10 m) to the top of the hill (up to 2 m), al-though at time laterally and longitudinally discon-tinuous. Moreover, variable density and physico-mechanical properties characterise the sequence ofsoils strongly influencing the development of land-sliding processes (Esposito & Guadagno 1998,Guadagno & Magaldi 2000).

Human activity is very important in modifyingthe morphology of the surficial soil mantle. The fer-tility of the pyroclastic mantle has constantly en-

couraged the use of the slopes for agricultural andforest purposes, so that mechanised forest manage-ment practices have favoured the growth of a densetrack network, significantly unsettling the delicateequilibrium of the slopes (Guadagno et al. 2003).

2.2 Landslide characteristics

Based on studies carried out in different sectors ofthe Campania area (Civita et al. 1975, Guadagno,1991; Calcaterra et al. 1997, Del Prete et al. 1998,Fiorillo et al. 2001), the failure phenomena occur-ring in the pyroclastic soils exhibit a distinctive gen-eral behaviour.

The initial movement typically consists of atranslational slide of portions of the pyroclasticcover, whose failure surface is generally located inthe pyroclastic multilayer cover on the highest andsteepest part of the slope (Guadagno, 2000). The ini-tial sliding mass is able to involve the pyroclasticcover downslope of the initial failures, which could

be subjected to liquefaction by means of rapidundrained loading (cf. Johnson 1984, Sassa, 1984).As a result of this, the initial slides soon transforminto extremely rapid, fluid debris avalanches (cf.Hungr et al. 2001), increasing in volume by erodingthe slope soil cover (Fig. 2).

The avalanches can become confined into gulliesor steep stream channels, narrowing considerably,

but still eroding the pyroclastic and colluvial coverfrom the slope and transforming in debris flows.Analyses have shown that maximum velocity canreach 20 m/sec (Revellino et al. 2002, 2004). At the

slope bases, the flows spread out over depositionalfans or aprons. The landslide material is often re-worked by large amounts of flowing water and hy-

per-concentrated flows (“debris floods”) that de-velop downstream of the main deposition zones.

2.3 Source area features

Studies on recent case histories (Guadagno 2000,Guadagno et al. 2003, Guadagno et al., in press)show that the controlling causes of the initial failureare strictly connected to morphological, strati-

graphic, hydrogeological, geotechnical and pe-dological factors. Nevertheless, we believe thatslope morphological features have played a pre-dominant role, as a predisposing cause, with regardto the other factors. The initial translational slides inmost cases were triggered upslope of morphologicaldiscontinuities and clearly linked to kinematic free-dom conditions of the pyroclastic masses in suchmorphological contexts. A further specification con-cerns the instability occurring immediatelydownslope of cuts for trackways and involving thefilling materials.


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Figure 2. Caption. A typical topographic profile of a Cam-

panian landslide as analysed here. The evolution of the move-ment is shown along the debris flow path (after Revellino et al.2004).

Table 1 illustrate some statistical analyses on thelocation of the source areas carried out by Guadagnoet al. (in press) for the large cluster of debris ava-lanches (176) from the Sarno/Quindici area of 1998.The study confirms that the 61% of the slides oc-curred at short distance above or below the cut slopeof a trackway, even in different morpho-structuralsettings, following the mechanism displayed in Fig-

ure 3. This demonstrates that slope undercutting and,more in general, human-induced changes are an im-

portant factor in forming the initial failures. Thesame authors, taking into account the recurrent tri-angular shape in plan view of the debris avalanchessource zones, shown that specific parametrical rela-tionships seems link up the source area geometrywith some local morphological and stratigraphiccharacteristics. More particularly, they specificallyanalysed the apex angle width (on which the volumeinvolved along the open slopes depends) of the tri-angular-shaped avalanche scars, relating it to the

slope angle and the height of the natural or man-made scarps. From this study is inferred that thevariability of the apex angles, which range from spe-cific values for the different morphological settingsof the source areas (Table 2), seems to be connected

principally to the impact energy of the initial failuresand to the geometry of the pyroclastic mantle.

2.4 Dynamic back-analyses

In order to calibrate a model, with a limited range of

Figure 3. . Schematic cross-sections along a pyroclastics-mantled slope: A) Instability mechanism at the location ofnatural scarps. B) Instability mechanism at a location of man-made cuts along a trackway. C) Instability mechanism due to arock fall (after Guadagno & Revellino, in press).

input parameters, simulating the dynamic behaviour

of potential first-order landslides that occur in asame environmental context, Revellino et al. (2002,2004) carried out a dynamic analysis of theSarno/Quindici and Cervinara debris avalanches anddebris flows. The model selected was the “DAN”(Dynamic Analysis), developed by Hungr (1995),

based on an explicit Lagrangian solution of theequations of unsteady non-uniform flow in a shallowopen channel. The relationship to simulate therheological behaviour of the moving masses was theVoellmy fluid (Voellmy 1955),used for the whole

path of the flows. The following equation was thusconsidered:

ξ

vγ )µ

g

caαγ H( τ

2

cos ++= (1)

where, τ is the resisting stress at the base of the flow,depending on unit weight (γ) of the flowing material(approx. 16 kN/m3), flow depth (H), dynamic fric-tion coefficient of the material (µ) and a turbulencecoefficient with dimensions of m/s2 (ξ). These pa-rameters represent the input data to be inserted in the

program. The others (slope angle α and centrifugalacceleration ac) came directly from the geometrical

Table 1. Recurrence of the morphological settings recognised in the failure areas of the 1998 Sarno/Quindici landslide event (afterGuadagno et al., in press). ______________________________________________

LocationMorphological conditions of initial failures

Sarno(57)*

Quindici(88)*

Siano(11)*

Bracigliano(20)*

Total(176)*

Above natural scarp 21 (37%) 17 (20%) 8 (73%) 5 (25%) 51(29%)

Below natural scarp 2 (3%) 0 (0%) 1 (9%) 0 (0%) 3 (2%)

Above man-made cut 18 (31%) 57 (65%) 0 (0%) 11 (55%) 86 (49%)

Involving fill 8 (14%) 11 (12%) 2 (18%) 0 (0%) 21 (12%)

Without morphological control 8 (14%) 3 (3%) 0 (0%) 4 (20%) 15 (8%)* Number of initial failures


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Table 2. Width range of the apex angle (ω) as calculated byGuadagno et al. (in press) for different morphological settingsof the source areas.

ω Morphological setting of thesource areas Min Max

Above natural scarp 9° 35°

Above man-made cut 17° 58°

Involving fill 5° 72°

characteristics of the landslide.Starting from the assumption that the landslides

are remarkably similar to each other in terms ofstyle, scale, mode of triggering, material and thegeological and topographic setting, a motion analy-ses was implemented on real cases. The largest de-

bris flows/avalanches from the two areas were se-lected for analysis, resulting in 17 cases (14 fromSarno-Quindici and 3 from Cervinara). It was as-sumed that the source volumes consisted of a slab ofconstant depth (1.5 m) and that, downslope of thesource area, the debris avalanches were eroding thesame constant thickness of material. By means of atrial and error procedure, a couple of the distinctiverheological parameters (µ and ξ) was found that pro-vided a good correspondence whit the observed be-haviour of the flows. In particular, it was determinedthat most of the landslides could be simulated usingthe Voellmy model, with a friction coefficient of0.07 and a turbulence coefficient of 200 m/sec2. The

back-analyses of the real cases allowed several land-slide intensity parameters, such as runout distance,flow velocity and thickness of the deposit, to be

evaluated and compared with real data obtained byin situ measurements, as shown in Table 3.The results obtained demonstrate that the DAN

program and the Voellmy model are capable of suc-cessful simulation of the observed behaviour of thedebris avalanches and debris flows, with a narrowlyconstrained selection of the set of input parameters(Revellino et al 2004). A range of accuracy of ap-

proximately ± 4% was obtained for the overall land-slide displacements of 16 out of 17 cases by usingonly a single general pair of flow resistance parame-ters. Simulations of velocities and thickness of de-

posits were also excellent. As a result, it may beconcluded that the model can be considered cali-

brated for the physiographic setting of the Campaniaregion. It can therefore be used to predict the runout

behaviour of potential slides, in the given setting, aslong as realistic approximations of locations and ge-ometries of instabilities can be made.

3

ANALYSIS OF A NEW AREA

The general methodology explained earlier was applied to a new test area in the same region.The performed analyses had the main objective totest and validate a procedure which aims to:− assess the landslide susceptibility of potential

source areas by identifying the morphologicalconditions leading to debris avalanche initiation;

−

predict the runout behaviour of potential debrisavalanches-debris flows by using the model calibrated on observed events; and

− simulate landslide intensity scenarios by identify-

ing maximum runout distances, volumes, velocityand deposit distribution of potential landslides.

Therefore, to reach the goal, a five-step analysis wasfollowed: a) the landslide inventory; b) mapping ofthe susceptible sites and terrain to landslide initia-tion c) a recognition of potentially instable locations;d) an estimation of the more likely flow path; e) theevaluation of the runout scenarios for different typi-cal failure sites and failed masses (Revellino 2004).

The prediction of landslide hazard for areas notcurrently subjected to landsliding is generally basedon the assumption that hazardous landslide eventsthat have occurred in the past can provide useful in-formation for the prediction of future occurrences(Soeters & van Westen 1996), even if instabilitiescan occur in areas that have never experienced land-slides. Therefore, mapping these phenomena and thefactors considered to be of influence is a key aspectin evaluating the landslide susceptibility of an area.Following this assumption and the considerations ofParagraph 2.3 and by means of fieldwork, aerial

photographic support and historical data, a landslideinventory was made and all the morphological fea-tures controlling landslide initiation (natural scarps,artificial cuts, "sagging rope" roads, road bends)identified.

Prediction of the starting point of potential land-slides has been made in a deterministic manner andthrough field observations, by identifying some criti-cal locations, such as open cracks, points of prefer-ential water run-off or seepage, soil masses jutting atthe scarp edges, within the typical morphologicalsettings recognised in the failure areas.

The geometry of the source area of potential ini-

tial debris avalanches was defined assuming theminimum and maximum opening of apex angle ofeach typical morphological setting (Table 2) com-

puted during the statistic analyses (Guadagno et al.,in press). Depending on the width of the apex angle,several different geometries of the initial slides werethus derived, in order to evaluate the influence on


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Table 3. Summary of the analysis of measured cross-sections, velocity, runout, and deposit thickness and their comparison withDAN model analysis for the 17 flows selected from Sarno-Quindici and Cervinara landslide events. The model calculations are based on the general Voellmy parameters µ = 0.07 and ξ = 200 m/sec2 (after Revellino et al. 2004, modified).

Velocity Runout Deposit ThicknessSlide

Cross-section

run-up(m/s)

Supereleva-tion(m/s)

Model(m/s)

Actual(m)

Model(m)

Availablesite

Actual (min./max)(m)

Model(m)

1 993 1003 m 0.2/0.6 0.4

2hi

l 15.3

8.37.1

7.18.4

14.2

1883 1923n

o

2.5/4.0

5.0/8.0

4.1

4.83 480 518 p 0.4/0.8 0.4

4 bg

5.512.8

6.110.2

3397 3280 a 1.5/3.0 2.8

5 2560 2591 b 0.7/1.4 1.5

6 1895 1995

7 1860 1890cd

0.8/1.00.4/0.5

0.60.2

8 2051 2074ef

0.4/0.70.6/0.8

0.40.5

9 a 10.3 9.4 1535 1589 g 0.4/1.0 2.3

10 1955 2069 h 0.2/0.4 0.2

11 2028 207712 1122 1145

13 1052 1170

14 1965 2058

15cd

5.96.1

6.25.9

3210 2990

16 e 10.9 13.9 1234 1250 i 0.4/0.8 0.4

17 f 10.3 14.2 736 760 l 0.6/1.0 1.5

the runout distance of different amounts of materialmobilised in the landslide event.

Subsequently, the landslide path was outlined onthe basis of the flow character of the land-slides.More specifically, by using the CAD Vector-Works software, a morphometric analysis was car-ried out by means of a vector analysis of the slopegradient. On a 1:5,000 scale topographic map, thesub-catchment basin area, involved in the potentiallandslide event, was divided into 20x20 metre cells.For each cell, the average gradient was estimatedand consequently the preferential flow directionswere determined. The debris avalanche-debris flow

path and depositional area path were defined follow-ing the flow lines thus obtained.

The thickness of the initiating debris slides anderosion depth downslope from the source area wasdeduced from field measurements of the pyroclasticcover thickness, assuming the involving of the slopeand gully material. The downstream limit of the ero-sion zones was assumed to be placed at the gullymouth.

As above mentioned, the strong similarity ingeomorphological setting between the area of AvellaMts and those of Sarno/Quindici and Cervinara ac-counts for the use of previously-determined parame-ters used in past DAN analysis by Revellino et al.

(2002, 2004) (friction coefficient, µ, of 0.07 and turbulence coefficient, ξ, 200 m/s2).

At first, the runout analysis was applied to threedebris avalanches-debris flows that occurred in May1998 (Pareschi et al. 2002), involving the slopes of atest area. The geometry of the landslides were out-lined on a 1:5,000 scale topographic map, based onfield surveys and aerial photography, following the

procedure previously described. The pre-existingchannel bed topography, over which the flows wereto run, and the landslide path were, therefore, givenas input initial conditions. Outputs consist of frontlocation, front velocity and deposit thickness. Onlymaximum runout distances acquired with the modelhave been used for the purpose of comparison. De-

posit thickness measurements were not possiblesince they had been removed or obliterated. The re-sults of the simulation are in Table 4 showing theinitial and final material volume calculated by DAN,while a comparison of calculated and actual runoutdistance is given in Table 5.

All the three cases show an over-estimation of therunout distance. This could be explained by the ef-fect of the large amount of rocky clasts and debriscarried by the flows and that could influence theirmobility. However, the results are well comparablewith those obtained during the calibration of the


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model and the divergence never exceed +7% of thetrue value in the worst case. It was thus verified that

Table 4. Initial and final volumes calculated by DAN on threeevents of the test area.

Slide Initial volume(m3)

Final volume(m3)

1 103 35,953

2 175 9566

3 323 25,661

Table 5. Summary of the analysis of measured runout distance.The model calculations are based on the Voellmy parameters µ = 0.07 and ξ = 200 m/s2.

RunoutSlide

Actual (m) Model (m)

1 1865 1938

2 928 1001

3 1474 1572

the DAN program and the Voelmy model, as calibrated, are able to replicate field observation also inthis case. Moreover, the over-estimation of therunout distance obtained could be considered as afavourable condition in the hazard evaluation.

4 EXAMPLE APPLICATION TO AN UNFAILEDSLOPE

In order to obtain runout distance, landslide velocityand deposit thickness of first-order events in the se-lected sub-basin, location of the potential source ar-eas and dynamic modelling of the connected flowswere carried out. In the test areas, the initial slidesare associated with the following conditions: a) fail-ures above a man-made cut; b) failures involving afill; c) failures above a natural scarp.

The dynamic analysis (Figures 4 and 5) were per-formed considering an initial movement as describedin case b). In fact, even if in the test area slope mor-

phology is generally regular and slope angles exceed40° in correspondence of some morphological dis-continuities (Figure 6), in some specific zones, thenatural setting is modified by the presence of track-way cuts. At an elevation of about 900 m a.s.l., a"sagging rope" segment of a pathway was identifiedas worst starting point of a potential landslide.Therefore, the landslide path initiations were out-lined using an apex angle (ω) of 5°, for the minimumwidth, and of 72°, for the maximum width (as de-rived from the analyses shown in Table 2), thesource area of the potential debris avalanche being

below a road-cut, involving the filling material. Thein situ deposit thickness measurements allowed theaverage erosion rate to be fixed, so that dynamicanalyses of the two example cases were carried outassuming an erosion thickness of 1.5 m for the de-

bris avalanche path (up to an elevation of about 800m a.s.l.) and of 1.0 m for the gully path. The resultsof the dynamic analyses for the minimum (Event A)and maximum (Event B) width of apex angle are

shown in Figures 4 and 5 respectively.

200

400

600

800

1000

E l e v a t i o n ( m )

0

5

10

15

20

V e l o c

i t y ( m / s )

0

100

200

W i d t h ( m )

FRONT

TAIL

WIDTH

0 400 800 1200 1600

Distance (m)

0.5

1.5

0.0

1.0

2.0

A v g .

D e p o s i t

T h i c k n e s s ( m )

Figure 4. . Result of dynamic analysis of the Event A (ω = 5°)using the Voellmy model within DAN, with µ, of 0.07 and ξ,of 200 m/s2. The flow profiles are plotted at 20-second inter-vals. Flow depth and erosion depth are multiplied by 10. Ve-locity-distance relationship is shown both for the front and the

tail of the landslide. The average deposit thickness along the path is also plotted.

200

400

600

800

1000

E l e v a t i o n ( m )

0

5

10

15

20

V e l o c

i t y ( m / s )

0

100

200

300

W i d t h ( m )

FRONT

TAIL

WIDTH

0 400 800 1200 1600 2000

Distance (m)

0.5

1.5

0.0

1.0

2.0

A v g .

D e p o s i t

T h i c k n e s s ( m )

Figure 5. . Result of dynamic analysis of the Event B (ω =72°). For explanation see Figure 4.


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Figure 6. . Simulated path geometry of Event A and Event B events as obtained from the analyses. Velocity flow profiles along thelandslide axes are also shown for both cases. Legend: 1) Landslide area; 2) Event A path; 3) Event B path; 4) Potential landslide ar-eas 5) Event A velocity distribution in m/s; 6) Event B velocity distribution in m/s; 7) Secondary watersheds; 8) Gullies cut in bed-rock; 9) Trackways scarps; 10) Natural scarps (height > 5 m); 11) Natural scarps (height < 5 m).


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As expected, the landslide displacement increaseswith the landslide volume. For Event A prediction,a maximum runout distance of 1645 m is the resultof 8208 m3 of volume involved; for Event B, 21,023m3 of volume produce a maximum runout distanceof 1879 m. A maximum final deposit thickness of 1m is simulated by the model in both cases. Figure 6shows the simulated landslide path geometriesand the velocity flow distributions along the land-

slide axes, as predicted by the model. As can be ob-served, flow velocities at the mouth of the gully arein the order of about 9 m/s, for Event A event andgreater than 10 m/s for Event B event.

5 CONCLUSION

The results described above, compared with thoseobtained in the previous studies, show that the DANcode, the Voellmy rheology and the procedureadopted could represent a useful tool to evaluate thedynamic behaviour of potential debris avalanches

and debris flows in the Campania region. If the dy-namic model gives essential information on thelandslide intensity, the knowledge of the geo-mophological characteristics, especially the recogni-tion of the potential source area, appear to be fun-damental in hazard evaluation.

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