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Abstract

The Val Pola rock avalanche began when a sagging slope failed due to toe unloading. The mass then plunged down into a narrow valley from a considerable height, crossed the valley, parted, rebounded, and, in part, ran back up to the source slope. Seven men were killed in this stage. The destabilization of this 34 × 106 m3 rock mass (mostly diorite) was triggered by heavy rainfalls that caused shallow landslides on the Val Pola sides and debris flows along the Val Pola thalweg. These processes resulted in a 35 m deepening of the canyon along the toe of the sagging slope. The morphology of the accumulation is mostly characterized by frequent, aligned hummocks and depressions, by reverse runup ridges, and by a narrow, 900-m-long, tongue-like downstream extension. The interaction between the moving mass and local morphology resulted in a relatively moderate runout and an unusually high degree of spreading. The north arm of the rock avalanche displaced the water of a preexisting landslide-dammed lake, generating a wave that killed 22 people more than 2 km upstream. A new and greater lake was formed as a consequence of the event. The systematic identification of sagging slopes and the establishment of reference geotechnical models are suggested for hazard prevention and emergency management.

Introduction

The Val Pola rock avalanche took place on July 28, 1987, at ~7:25 a.m., six days after a period of heavy rains (Fig. 1). Seven men working at the foot of the slope disappeared in the stream of rock fragments. A village and six hamlets, all previously evacuated, were destroyed. A considerable part of the debris mass plunged into a small landslide lake formed in the previous days, and raised a huge wave of debris-laden water that surged upstream along the main valley (Valtellina) for more than 2 km. An additional four villages were swept away and 22 more people lost their lives.

Figure 1.

Aerial view to west of Val Pola rock avalanche, showing source, descent path, and most of accumulation (August 1, 1987). Several elements mentioned in text can be seen: two surfaces delimiting tailed mass (dip direction/dip, respectively, 070°/45° and 350°/35°); gneiss (G) of Cristallino del Tonale; rock pinnacles of southern alignment (PS); northern and southern reverse runup ridges (RN and RS, respectively); escarpment caused by failure of downstream side of southern reverse runup ridge (left of RS); lacustrine sediments abandoned by return water wave, and deposited as lobes (L) or as mud layer (M); and rock prominence of Plaz (Pl). Photo courtesy of Foto Benini. Sondrio, Italy.

Figure 1.

Aerial view to west of Val Pola rock avalanche, showing source, descent path, and most of accumulation (August 1, 1987). Several elements mentioned in text can be seen: two surfaces delimiting tailed mass (dip direction/dip, respectively, 070°/45° and 350°/35°); gneiss (G) of Cristallino del Tonale; rock pinnacles of southern alignment (PS); northern and southern reverse runup ridges (RN and RS, respectively); escarpment caused by failure of downstream side of southern reverse runup ridge (left of RS); lacustrine sediments abandoned by return water wave, and deposited as lobes (L) or as mud layer (M); and rock prominence of Plaz (Pl). Photo courtesy of Foto Benini. Sondrio, Italy.

The rock avalanche dammed Valtellina from side to side, and caused the formation of a far greater landslide lake. Although mitigation works were rapidly undertaken, it took some months to end the state of emergency, and more than one year to return to normality. Economic losses were heavy.

The rock avalanche and/or its aftermath were considered in several papers (e.g., Chiesa and Azzoni, 1988; Govi, 1988, 1989; Völk, 1989; Maione, 1990; Costa, 1991; Azzoni et al., 1992; Govi and Turitto, 1992; Huber, 1992; Schuster, 1994). However, the prelandslide conditions of the rock mass, the chain of events that triggered the failure, and the morphological and morphome-trical analysis of the debris accumulation deserve further attention, and are the topics dealt with herein.

Geology and Seismicity of the Area

Valtellina, in the Central Italian Alps, is a valley of glacial origin largely remodeled by the main river (Adda) and its tributaries (Pozzi et al., 1990). Val Pola is a minor, right-side tributary canyon of the Adda River (Figs. 2 and 3). The Val Pola rock avalanche came off the east flank of Mount Zandila, a 2936 m peak on the right side of Valtellina, and took its name after the canyon that marked the northern limit of the rock mass.

Figure 2.

Map of Val Pola area in Valtellina (valley of Adda River, in northern Italy), showing villages and physiographic features mentioned in text. Zone devastated by landslide and landslide-generated water wave is outlined.

Figure 2.

Map of Val Pola area in Valtellina (valley of Adda River, in northern Italy), showing villages and physiographic features mentioned in text. Zone devastated by landslide and landslide-generated water wave is outlined.

Figure 3.

Stereopair of Valtellina-Val Pola area (air photographs 1981). Note northern and southern pinnacle alignments (PN and PS, respectively), Spur (S). and Val Pola canyon (VP). Valley-bottom villages and hamlets are shown: Verzedo (Ve), San Martino Serravallc (SM), Orcaccia (Or), Plaz (Pl), Presure (Pr), Plegne (Pe), Castellaccio (Ca), Morignone (Mo), Foliano (Fo), San Bartolomeo (SB), Sant'Antonio Morignone (SA), Poz (Po), Tirindré (Ti), and Aquilone (Aq). Scale bar is approximate. Photo courtesy of Regione Lombardia (Milan, Italy), published by permission of the Italian Air Force (Concessione Stato Maggiore Aeronautica n. 319 del 9 luglio 1996).

Figure 3.

Stereopair of Valtellina-Val Pola area (air photographs 1981). Note northern and southern pinnacle alignments (PN and PS, respectively), Spur (S). and Val Pola canyon (VP). Valley-bottom villages and hamlets are shown: Verzedo (Ve), San Martino Serravallc (SM), Orcaccia (Or), Plaz (Pl), Presure (Pr), Plegne (Pe), Castellaccio (Ca), Morignone (Mo), Foliano (Fo), San Bartolomeo (SB), Sant'Antonio Morignone (SA), Poz (Po), Tirindré (Ti), and Aquilone (Aq). Scale bar is approximate. Photo courtesy of Regione Lombardia (Milan, Italy), published by permission of the Italian Air Force (Concessione Stato Maggiore Aeronautica n. 319 del 9 luglio 1996).

The east flank of Mount Zandila (Fig. 4) consists of plutonic and metamorphic rocks (Chiesa and Azzoni, 1988; Progetto Finalizzato Geodinamica, 1990). From the slope base upward, these rocks are as follows.

1. The plutonic unit Gabbro di Sondalo (i.e., gabbro of Sondalo) consists of gabbro, gabbrodiorite, diorite, and minor granodiorite;. aplite and pegmatite bodies are also present. The failed rock mass consisted mostly of diorite. The contact between this and the following unit is approximately subhorizontal in the area considered.

2. The metamorphic unit Cristallino del Tonale (i.e., crystalline rocks of Mount Tonale) consists of paragneiss with minor intercalations of amphibolite and orthogneiss. Hornfels is also present. It is in contact with the following through an overthrust surface that dips gently northward.

3. The metamorphic unit Cristallino della Grosina (i.e., crystalline rocks of Grosina Valley) consists mostly of gneiss, phyllite, and mica schist. It crops out west of the area shown in Figure 4, and was not involved in the landslide.

Figure 4.

Geological map of Val Pola area (after Chiesa and Azzoni, 1988; Azzoni et al. 1992 [modified]). Prelandslide configuration is shown (except for boreholes, which were drilled between October and November 1987).

Figure 4.

Geological map of Val Pola area (after Chiesa and Azzoni, 1988; Azzoni et al. 1992 [modified]). Prelandslide configuration is shown (except for boreholes, which were drilled between October and November 1987).

At the bifurcation of the Y-shaped scree (Figs. 3 and 4), between ~1750 and 1880 m elevation, a quartz-rich pegmatite body formed a protruding rock spur ~130 m high and 70 m wide in outcrop (Fig. 5). As discussed in the following, this spur probably had an important role in the failure process, and is called “the Spur” herein.

Figure 5.

Close-up view of Spur (S) (July 27, 1987; view to northwest). Traces of recent rockfall activity from and around it are evident.

Figure 5.

Close-up view of Spur (S) (July 27, 1987; view to northwest). Traces of recent rockfall activity from and around it are evident.

Considerable parts of the prelandslide slope were covered by morainal, alluvial, colluvial, or landslide deposits (Figs. 3 and 4).

The most prominent tectonic features of the area considered are the Mount Zandila and Val Pola faults, both of which strike east-west. Other faults and lineaments strike mostly northwest-southeast, and some strike north-northeast-south-southwest (Fig. 4).

There is no reliable information on local seismicity for the period preceding 1887. After 1887 (Postpischl, 1985), 31 earthquakes with intensity equal to or greater than the IV degree of the Mercalli, Cancani, and Sieberg (MCS) scale were recorded within a 70 km radius of the study area (the MCS intensity scale [Sieberg, 1932] coincides within 0.5 degree with the Modified Mercalli scale). The strongest shock occurred in 1961 and attained the VII degree. In any case, no seismic activity is directly related to initiating the landslide.

Precipitation

Between 1962 and 1987, the meteorological station of Arnoga, located at 1874 m elevation 10 km northwest of the landslide site (Fig. 2), recorded a mean annual rainfall of 1111 mm. On average, the ground is snow covered more than 190 days/yr at elevations above 2000 m. From 1977, an increasing trend is evident in the local rainfall pattern (Fig. 6). Until 1976, the average cumulative January–July and January–December precipitations were 551 and 1015 mm, respectively, whereas between 1977 and 1987 they were 749 (+36%) and 1244 (+23%), respectively (Ponte, 1995). This, together with a degree of permafrost degradation (Dramis et al., 1995), may have had a role in destabilizing the rock mass. Note that in 1977 and 1981 the January–July rainfall was greater than in 1987, and that it was nearly equal in 1983 (Fig. 6).

Figure 6.

Cumulative rainfall recorded at Arnoga rain gauge from January to July in period 1962–1987.

Figure 6.

Cumulative rainfall recorded at Arnoga rain gauge from January to July in period 1962–1987.

In addition to a long-term increase in precipitation, significant short-term amounts occurred in 1987 prior to the failure. The January–July rainfall was 859 mm in 1987, compared to the average of 640 mm (Figs. 6 and 7). In the 30 days before failure (June 29–July 28), the rainfall showed fair continuity and remarkable intensity. Both continuity and intensity were particularly pronounced in the 10 days before failure (July 19–28) (Fig. 7). In the 30 days and 10 days the area received, respectively, 34% and 16% of the cumulative rainfall amount from January 1 to July 28. The highest single-day amount of the entire seven-month period was 99 mm (12%) on July 19, i.e., 10 days before failure. The day before, it had been preceded by the second-highest amount, 70 mm (8.5%), with a total of 169 mm (20.5%) in two days. Shallow landslides and debris-flow activity were first noticed in Val Pola in the early evening of July 18.

Figure 7.

Daily and cumulative rainfall patterns at Arnoga station from January to July, 1987.

Figure 7.

Daily and cumulative rainfall patterns at Arnoga station from January to July, 1987.

With respect to the period 1962–1987, the monthly rainfalls of both June and July attained their maximum values in 1987.

These were, respectively, 213 mm (222% of the average) and 290 mm (254% of the average).

In summary, the precipitation recorded at Arnoga in 1987 results above average (and with a conspicuous amount concentrated in only two days) if the seven-month period January–July is considered, while it was at the maximum in terms of monthly amount for June and July. However, precipitation in mountain regions shows particularly high spatial variability, and Arnoga and Val Pola are 10 km apart. Thus, the actual rainfall pattern at Val Pola remains largely unknown, and the preceding discussion is acceptable within the limits of an extrapolation.

Morphology Before July 1987

Between peak (2936 m) and thalweg, the eastern flank of Mount Zandila has a mean slope angle of ~31° and the maximum slope is ~45° or greater. The sector of slope where the July 28 landslide developed had a maximum elevation of ~2370 m, and the Adda thalweg at its foot was between 1050 m (north) and 1000 m (south) (Figs. 3, 8, and 9). The head of this sector of slope consisted of an arcuate and bare escarpment, inclined at 45° and ~100 m in height. Below this was the debris of two (or possibly one) ancient overlapping slides ~20–30 m thick. Part of these deposits was on the lower section of the escarpment, but most of them extended eastward with gentle dip until they intersected, at ~2100 m, the rim of a second escarpment.

Figure 8.

Morphology and mass movement before July 1987.

Figure 8.

Morphology and mass movement before July 1987.

Figure 9.

Area on east flank of Mount Zandila involved in 28 July rock avalanche, as it appeared 24 h earlier (view to northwest). Val Pola (VP) canyon is in middle of image, and traces of landslide and erosion processes of previous days are clearly seen (cf. Fig. 3). Such processes resulted in emplacement of debris fan (DF) across Adda; as consequence, small lake is being impounded (I) and Morignone (Mo) is partly flooded. Large Y-shaped scree that originated from rockfall accumulation is visible left of Val Pola. Stem of Y is particularly pronounced, as consequence of large number of rockfalls generated in those days by Spur (S) (pegmatite body, lozenge shaped in exposure. crops out above stem top). Approximate outline of rock mass that failed on July 28 is shown. In foreground are rock prominence of Plaz (Pl) and houses of San Martino Serravalle. Photo courtesy of Azienda Energetica Municipale, Milan. Italy.

Figure 9.

Area on east flank of Mount Zandila involved in 28 July rock avalanche, as it appeared 24 h earlier (view to northwest). Val Pola (VP) canyon is in middle of image, and traces of landslide and erosion processes of previous days are clearly seen (cf. Fig. 3). Such processes resulted in emplacement of debris fan (DF) across Adda; as consequence, small lake is being impounded (I) and Morignone (Mo) is partly flooded. Large Y-shaped scree that originated from rockfall accumulation is visible left of Val Pola. Stem of Y is particularly pronounced, as consequence of large number of rockfalls generated in those days by Spur (S) (pegmatite body, lozenge shaped in exposure. crops out above stem top). Approximate outline of rock mass that failed on July 28 is shown. In foreground are rock prominence of Plaz (Pl) and houses of San Martino Serravalle. Photo courtesy of Azienda Energetica Municipale, Milan. Italy.

The second escarpment (2100–2000 m) was also inclined at 45° and was indented by many gullies. Below it, from ~2000 to 1750 m, the slope steepness gradually reduced, but the surface was still rugged. From 1750 m down, the slope was more or less rectilinear to the junction with the valley-bottom deposits. The mean slope angle between 2370 m and the valley bottom was 32°.

Between the south ends of the two escarpments, a double alignment of rock pinnacles with an interposed depression formed a trench. The depression, partly debris filled, was 20–30 m wide and ~50 m deep (as could be seen after the failure) (Figs. 3 and 8). This trench, together with some small scarps and other features uphill, is interpreted as evidence of a sagging slope (Hutchinson, 1988, 1995; Dramis and Sorriso-Valvo, 1994); i.e., the rock mass was, or had been, slowly sagging toward the north.

The slope had some screes, the largest of which (Y-shaped) received material from the 2000–2100 m escarpment and, above all, from the Spur. Rockfall activity from the Spur was observed for centuries.

Two bedrock prominences were the most important features of the opposite valley slope (Figs. 3 and 8). Both consisted of materials from the Gabbro di Sondalo, with surficial deposits generated by glacial, fluvial, and mass-movement processes. The southern prominence, named Plaz, culminated at 1333 m and was 300–400 m from the river channel; the other, named San Bartolomeo, was lower (1219 m) and closer to the river. The slope-between the prominences was concave and was crossed by two torrents. The San Bartolomeo prominence caused a conspicuous narrowing of the valley bottom.

Failure: Predisposing and Triggering Factors

Understanding and modeling the failure mechanisms are essential aspects of the problem, but require further investigation. Here the sagging, rock-mass conditions, and groundwater conditions are outlined, together with a reference picture of the geotechnical characteristics of the rock.

The rock mass that failed on July 28 was surficially delimited on the north by the Val Pola incision, on the east by the slope face from ~1700 to 2300 m, on the south by the depression between the double alignment of pinnacles, and on the west by the upper escarpment (Figs. 8 and 9).

Following Hutchinson's (1988) classification, and on the basis of the evidence now available, this rock mass can be qualified as a single-sided, biplanar sagging (Fig. 10). On plotting relevant morphometrical parameters (Table 1) in the diagrams by Hutchinson (1995), one finds that, both in terms of deep-seatedness (Vmax) and of deep form (δ/L), this gravitational slope deformation compares well with other known cases. As in many other alpine slopes (e.g., Abele, 1974; Forcella, 1984; Forcella and Orombelli, 1984; Mortara and Sorzana, 1985), this deformation is thought to have begun at the end of the Würm, ca. 10 ka. If this is the case, mean deformation rate can be estimated as 2–3 mm/yr. The prelandslide topography also suggests possible bulging effects at the toe of the sagging slope.

Figure 10.

Longitudinal section of Val Pola sagging. Drawing shows northern and southern alignments of pinnacles (PN and PS, respectively). original topographic surface (T), postslide topographic surface. which largely coincides with slip surface (Ss; dashed when inferred because buried by debris), material removed from Val Pola prior to July 28 failure (gray tone), and relevant morphometrical parameters defined in Table 1 (Hutchinson, 1995). See Figure 8 for location.

Figure 10.

Longitudinal section of Val Pola sagging. Drawing shows northern and southern alignments of pinnacles (PN and PS, respectively). original topographic surface (T), postslide topographic surface. which largely coincides with slip surface (Ss; dashed when inferred because buried by debris), material removed from Val Pola prior to July 28 failure (gray tone), and relevant morphometrical parameters defined in Table 1 (Hutchinson, 1995). See Figure 8 for location.

Table 1.

Val Pola Sagging—Morphometrical Data

Vmax (m)*α (°)δ (m)§L (m)#δ/L**
16528858850.096
Vmax (m)*α (°)δ (m)§L (m)#δ/L**
16528858850.096

Note: See Figure 10. All definitions below are from Hutchinson (1995).

*Deep-seatedness measure: it is the maximum vertical depth from ground surface to basal slip surface. 165 m is the maximum depth in the section considered, which is that with the highest δ/L ratio, but absolute maximum vertical depth (190 m) is in another section,

§Maximum depth.

Angle to the horizontal of the line connecting crown and toe of the sagging mass. Maximum depth of slip surface below the line defined above.

#Length of the line defined above.

**Form index of deep slides: the higher the index, the deeper the form.

The failed mass had at the top a minor amount of paragneiss from the Cristallino del Tonale, but was mostly formed by diorite from the Gabbro di Sondalo, which is the predominant rock type in the rock-avalanche debris. Diorite is also present in the large escarpment below Sassavin, which was in morphological continuity with the no longer existing escarpment located at 2000–2100 m. In addition, diorite was encountered in two boreholes, more than 100 m deep, a few hundred meters west of Sassavin (Fig. 4) (Chiesa and Azzoni, 1988). The Spur was part of a quartz-rich pegmatite body, and was ~130 m high and 70 m wide in outcrop.

Diorite in the borehole cores is generally medium grained, and sometimes fine grained, and very closely to moderately closely jointed (International Society for Rock Mechanics Commission on Standardization of Laboratory and Field Tests, 1978). In particular, spacing of joints ranges from 5 to 30 cm, and with greater frequency from 10 to 20 cm. Clay coatings, 1–5 mm thick, are often present on joint walls. Other coatings or bare, stained walls are occasionally observed. Gabbrodiorite, granodiorite, and contact-metamorphic rocks are also locally present in the cores.

The two main joint sets are listed in Table 2 (Chiesa and Azzoni, 1988). The spacing of exposed joints in the field is usually not greater than a few tens of centimeters, and is similar to borehole data. The two surfaces delimiting the detachment cavity, with attitude 350°/35° (dip direction/dip) and 070°/45° respectively, belong to these two sets (Fig. 1). The Joint Roughness Coefficient (JRC) indicates that these joints were smooth, undulating to smooth and nearly planar (Barton, 1973).

Table 2.

Main Joint Sets

Joint setDip directionDipJoint Roughness Coefficient
S1050°–070°45°7
S2350°35°–50°7.5
Joint setDip directionDipJoint Roughness Coefficient
S1050°–070°45°7
S2350°35°–50°7.5

Table 3 summarizes the Rock Quality Designation (RQD) data from the cores based on the classes proposed by Deere et al. (1967). Average values are almost identical in the two holes, and on the whole suggest a rock mass of fair quality, while the weighted average indicates good quality. The grade of weathering of the rock in the cores ranges from class I (fresh rock) to class III (moderately weathered rock) (Geotechnical Control Office, 1988; Gullà and Matano, 1994). Overall, these data suggest that the conditions of the rock mass outside the failed portion are fair. However, the actual conditions of the failed portion must have been relatively worse due to the sagging.

Table 3.

Rock Quality Designation Data From Boreholes

BoreholeClass 0–25Class 26–50Class 51–75Class 76–90Class 91–100TotalMinimumMaximumMeanStandard deviationWeight average
(n)(%)(n)(%)(n)(%)(n)(%)(n)(%)(n)(%)(%)(%)(%)(%)
110141217182515211623710100633267
213151011202225282124890100663189
1 + 2231522143823402537231600100653178
BoreholeClass 0–25Class 26–50Class 51–75Class 76–90Class 91–100TotalMinimumMaximumMeanStandard deviationWeight average
(n)(%)(n)(%)(n)(%)(n)(%)(n)(%)(n)(%)(%)(%)(%)(%)
110141217182515211623710100633267
213151011202225282124890100663189
1 + 2231522143823402537231600100653178

Along Val Pola, the failed mass was overlain by a debris cover several tens of meters thick, which was presumably similar in grain size to the debris encountered in the boreholes. The latter, essentially morainal in nature, consisted of boulders, pebbles, gravel, and sand, with a fine-grained fraction from silty to weakly silty-clayey. This composition varied irregularly with depth, and the fine-grained fraction was not always present.

Although the failure process was largely controlled by the two delimiting discontinuities, the size of the rock mass compared with the reduced spacing of the internal discontinuities suggests that the mass may be considered an “equivalent continuum medium” (Ribacchi, 1993; Tommasi and Rotonda, 1995) if the stability conditions are to be modeled. To this aim, the geotechnical parameters representative of the rock mass have to be evaluated; this is particularly difficult when a great volume is involved. A preliminary literature survey resulted in the picture of Table 4, which gives the ranges and computed average values of relevant geotechnical parameters for rock types comparable to those in the study area (Lama and Vutukuri, 1978). All parameters show wide variations, making it difficult to validate a geotechnical model. However, due to the slow deformation connected with the sagging, it can be hypothesized that the geotechnical parameters of the Val Pola rock mass were at or close to the lower boundaries of the ranges shown in Table 4. For the same reason, it can be assumed that the strength parameters of the two delimiting discontinuities were close to the basic friction angle (Hoek and Bray, 1981).

Table 4.

Geotechnical Parameters of Reference Rock Types

ParameterValueGabbro*Diorite*Quartzite*Granite*Plutonic r.§
γ(kN/m3)Min.26.519.925.123.525.6
Max.31.329.739.927.031.0
Mean29.327.125.726.027.1
E Lab. (GPa)Min.32.521.410.74.67.8
Max.125.0106.7119.078.399.4
Mean81.365.759.644.956.6
E Site (GPa)Min.N.D.2.61.90.3N.D.
Max.N.D.17.147.968.9N.D.
MeanN.D.8.920.714.2N.D.
G (Gpa)Min.14.912.80.27.1N.D.
Max.48.044.340.655.0N.D.
Mean34.631.627.619.4N.D.
vMin.0.110.010.080.020.05
Max.0.380.320.330.390.39
Mean0.220.220.200.190.20
sc(Mpa)Min.126.264.177.212.848.8
Max.317.0333.0357.0324.0324.0
Mean216.1192.5216.2157.8146.4
st (MPa)Min.13.85.23.52.62.6
Max.16.450.335.338.612.2
Mean15.131.423.412.49.1
c Lab. (MPa)Min.N.D.56.220.00.116.5
Max.N.D.176.070.656.0176.0
MeanN.D.N.D.N.D.N.D.56.1
c Site (MPa)Min.N.D.N.D.0.40.1N.D.
Max.N.D.N.D.2.02.5N.D.
MeanN.D.N.D.N.D.N.D.N.D.
φ Lab. (°)Min.N.D.24262923.8
Max.N.D.55646456.0
MeanN.D.N.D.N.D.N.D.45.6
φ Site (°)Min.N.D.N.D.4341N.D.
Max.N.D.N.D.5662N.D.
MeanN.D.N.D.N.D.N.D.N.D.
ParameterValueGabbro*Diorite*Quartzite*Granite*Plutonic r.§
γ(kN/m3)Min.26.519.925.123.525.6
Max.31.329.739.927.031.0
Mean29.327.125.726.027.1
E Lab. (GPa)Min.32.521.410.74.67.8
Max.125.0106.7119.078.399.4
Mean81.365.759.644.956.6
E Site (GPa)Min.N.D.2.61.90.3N.D.
Max.N.D.17.147.968.9N.D.
MeanN.D.8.920.714.2N.D.
G (Gpa)Min.14.912.80.27.1N.D.
Max.48.044.340.655.0N.D.
Mean34.631.627.619.4N.D.
vMin.0.110.010.080.020.05
Max.0.380.320.330.390.39
Mean0.220.220.200.190.20
sc(Mpa)Min.126.264.177.212.848.8
Max.317.0333.0357.0324.0324.0
Mean216.1192.5216.2157.8146.4
st (MPa)Min.13.85.23.52.62.6
Max.16.450.335.338.612.2
Mean15.131.423.412.49.1
c Lab. (MPa)Min.N.D.56.220.00.116.5
Max.N.D.176.070.656.0176.0
MeanN.D.N.D.N.D.N.D.56.1
c Site (MPa)Min.N.D.N.D.0.40.1N.D.
Max.N.D.N.D.2.02.5N.D.
MeanN.D.N.D.N.D.N.D.N.D.
φ Lab. (°)Min.N.D.24262923.8
Max.N.D.55646456.0
MeanN.D.N.D.N.D.N.D.45.6
φ Site (°)Min.N.D.N.D.4341N.D.
Max.N.D.N.D.5662N.D.
MeanN.D.N.D.N.D.N.D.N.D.

Note: Lab = Laboratory; Min. = minimum; Max. = maximum; N.D. = no data.

*Basic data after Lama and Vutukuri, 1978.

†The quartz-rich rock body forming the Spur is no longer exposed. It has been assumed that quartzite is sufficiently similar and that its mechanical properties may be representative of those of the Spur.

§Basic data after Kulhawy, 1975, in Lama and Vutukuri, 1978.

Springs were present on the slope south of the canyon, at the Spur and at the lower tip of the Y-shaped scree, as well as along the Val Pola canyon, at ~2100 and 1650 m (Fig. 4). Except for the spring at the Spur, all springs were essentially fed by water percolating in the debris cover. The activity of the spring at the Spur, which was the most abundant, closely followed the rainfall pattern. After the landslide, no important spring manifested in the denuded slope below the detachment cavity, i.e., between ~1700 and 1150 m, while a number of small springs manifested on the 070°/45° surface. All this seems to indicate that the 350°/35° surface was a permeability threshold, and that a perched aquifer existed above it.

A period of heavy rainfall affected the Valtellina area from July 15 to 22, 1987, with a marked worsening on July 18 and 19 (Fig. 7). Extensive damage was reported throughout the area (Botta, 1987; Brunetti and Moretti, 1987; Smiraglia, 1987). As a consequence of rainfall, beginning in the early evening of July 18, shallow landslides occurred along both sides of Val Pola. They were probably caused by the formation of perched water tables in the debris cover and/or by the sharp reduction of negative pore pressures. The landslide material mixed with flowing water, resulting in debris flows with a strong erosive power, and a feedback effect may have been set up at this point. When the process ended, about noon of July 19, the Val Pola thalweg had been deepened 35 m between 2000 and 1600 m elevation, i.e., along the toe of the sagging mass, and to 40 m between 1300 and 1100 m (Figs. 9 and 11). Between 1600 and 1300 m the thalweg was in bedrock and did not change significantly.

Figure 11.

State of sites in early morning of July 28, before rock avalanche, and rock-avalanche kinematics.

Figure 11.

State of sites in early morning of July 28, before rock avalanche, and rock-avalanche kinematics.

Beginning at 7 p.m. on July 18, the Val Pola torrent, together with a torrent from the opposite slope, placed a 0.6 × 106 m3 debris fan across the Adda. Although mitigation measures were quickly undertaken, a lake formed behind this blockage. By the early morning of July 28, the day of the landslide, the lake contained an estimated 0.5 × 106 m3 (Figs. 9 and 11).

On July 25, surveillance personnel observed a 600-m-long semicircular crack that, although discontinuous, extended from south-southeast to north-northwest along the foot of the summit escarpment with a maximum width of 1 m (Fig. 12). On July 26 and 27 the crack extended and widened, the final length and maximum width being 900 m and 5 m, respectively. In the same period, the rockfall activity from the lower escarpment, and particularly from the Spur, increased: in the 24 h preceding the ultimate failure, 98 rockfalls were counted.

Figure 12.

Uppermost part of rock mass (July 25, 1987; view north). Pale line zigzagging along escarpment foot (arrowed) is perimetrical crack observed that day for first time, when it was 600 m long and 1 m (maximum) wide; it represented clear symptom of destabilization.

Figure 12.

Uppermost part of rock mass (July 25, 1987; view north). Pale line zigzagging along escarpment foot (arrowed) is perimetrical crack observed that day for first time, when it was 600 m long and 1 m (maximum) wide; it represented clear symptom of destabilization.

The sagging deformation and the continual rockfall activity indicate that slope stability had become precarious long before July 1987. However, had the boundary conditions (i.e., the Val Pola sides and thalweg) remained unchanged, it is probable that no large landslides would have occurred within times of technical interest (i.e., 100–150 years). Unloading the toe of the rock mass through landsliding and erosion along Val Pola seems to have been the key destabilizing element (cf. Hutchinson, 1987). It caused a sharp increase in the deformation rate of the mass, from 2–3 mm/yr (estimated average over 10 k.y.) to ~2 m/day (actual mean figure on July 26 and 27), with a rate increase of almost 3 × 105. As a consequence, the summit crack opened, thus revealing the progress of failure, and at that point only the Spur, though increasingly strained, was able to prevent total failure. When the Spur finally sheared, the failure took place catastrophically within 60 minutes.

Kinematics of the Rock Avalanche

The description presented in this section is partly based on eyewitness reports, and on air photographs taken ~6 and 28 h after the event.

At 6:20 a.m. on July 28 the Spur generated two rockfalls (described as “bursts” by the witnesses) larger than those of the previous days; a conspicuous cavity formed. About 1 h later, additional rockfalls enlarged the cavity upward and a number of blocks rolled down the slope. For the first time a high-tension pylon at 1180 m, i.e., more than 600 m below, was reached. A few seconds later, the main rock mass started moving northward: the first stage of motion was described as a sequence of brief pulses with progressively increasing speed. Upon impacting against the south side of the Sassavin-Motta ridge the mass fragmented, changed its direction from north to east, and plunged down onto the valley bottom, 650–900 m below (Fig. 11). Seven men working at the foot of the slope disappeared.

The debris mass crossed the valley bottom, and ran up the opposite slope preceded by an air blast. A dust cloud rose to 2000 m. The village of Morignone and the hamlets of Orcaccia, Plegne, Foliano, Castellaccio, Presure, and San Martino Serravalle, all previously evacuated, were overwhelmed.

At the rock prominence of Plaz the debris mass parted into two arms, while a small amount of material was piled up onto the Plaz prominence (Figs. 13, 14, and 15). The material in the southern arm crossed the slope south of Plaz, where part of it came to rest. Another part fell back, again crossed the valley, and ran up the source slope. Finally, a part of the mass was channeled southward along the valley bottom.

Figure 13.

Rock-avalanche (r.a.) morphology. Silhouette on the right shows subdivision by sectors.

Figure 13.

Rock-avalanche (r.a.) morphology. Silhouette on the right shows subdivision by sectors.

Figure 14.

Aerial view to southeast of debris accumulation (July 29, 1987). Plaz (Pl) and San Bartolomeo (SB) rock prominences are shown. Above them and right of Plaz, debris sprays can be seen along perimeter. Note denuded slopes at and between the two prominences, northern and southern reverse runup ridges (RN and RS, respectively). and new landslide lake (Ll). Photo courtesy of Azienda Energetica Municipale, Milan, Italy.

Figure 14.

Aerial view to southeast of debris accumulation (July 29, 1987). Plaz (Pl) and San Bartolomeo (SB) rock prominences are shown. Above them and right of Plaz, debris sprays can be seen along perimeter. Note denuded slopes at and between the two prominences, northern and southern reverse runup ridges (RN and RS, respectively). and new landslide lake (Ll). Photo courtesy of Azienda Energetica Municipale, Milan, Italy.

Figure 15.

Profiles of rock avalanche, with prelandslide and postlandslide topographic surfaces. Small circles mark postlandslide surface in positions where confusion might arise. Profile west-southwest–east-northeast goes from top of headscarp to point of maximum runup height uphill of Castellaccio. Rock prominence of Plaz is also shown. North-arm and south-arm profiles follow approximately buried Adda thalweg. In downstream part of south arm, note thin debris tongue projected toward Verzedo. Traces of profiles are in silhouette (A). Graph (B) shows runup datum at Val Pola plotted into runup diagram by Evans et al. (1994).

Figure 15.

Profiles of rock avalanche, with prelandslide and postlandslide topographic surfaces. Small circles mark postlandslide surface in positions where confusion might arise. Profile west-southwest–east-northeast goes from top of headscarp to point of maximum runup height uphill of Castellaccio. Rock prominence of Plaz is also shown. North-arm and south-arm profiles follow approximately buried Adda thalweg. In downstream part of south arm, note thin debris tongue projected toward Verzedo. Traces of profiles are in silhouette (A). Graph (B) shows runup datum at Val Pola plotted into runup diagram by Evans et al. (1994).

The debris in the northern arm ran across the reentrant between the Plaz and San Bartolomeo rock prominences. A large part of it stopped on this side of the valley, while another part ran back across the valley bottom and back up the source slope. The water in the preexisting lake was violently displaced: splashed water was hurled 140 m above the lake. A wave, initially 95 m high, swept north along the valley bottom for more than 2 km, inundating the villages of Sant'Antonio Morignone, Poz, Tirindrè, and Aquilone. The first three villages had been evacuated, but Aquilone had not, and 22 people were killed. Before coming to rest, water of the return wave flooded a considerable part of the north arm accumulation. After the end of the process, this arm presented a marshy area on its north side, which was followed by a new lake farther north (Figs. 13 and 14).

For details on the hydraulic aspects of the event, reference can be made to Govi (1988, 1989), Maione (1990), Costa (1991), Azzoni et al. (1992), Govi and Turitto (1992), Huber (1992), and Schuster (1994).

The landslide-generated seismic waves were recorded at several seismographic stations. The three stations closest to Val Pola are in Switzerland, at Davos, Val di Lei, and Ova Spin. Their seismograms (data courtesy of the Swiss Seismological Service) last between 71.5 and 117.5 seconds, and enable us to distinguish six phases in the development of the event (Table 5).

Table 5.

Phases of the Event and Duration of Seismic Records

PhaseProcessDuration
1Failure and sliding northward9–15
2Rockfalls eastward7
3Impact against the Sassavin-Motta ridge4.5–10
4Avalanching8–12.5
5Runups, deflections, fallbacks, reverse runups, water wave generation19–31
6Water wave motion31–42
PhaseProcessDuration
1Failure and sliding northward9–15
2Rockfalls eastward7
3Impact against the Sassavin-Motta ridge4.5–10
4Avalanching8–12.5
5Runups, deflections, fallbacks, reverse runups, water wave generation19–31
6Water wave motion31–42

Morphology of the Rock Avalanche

Exposed debris consisted mainly of diorite fragments; gabbro and paragneiss were locally present (Chiesa and Azzoni, 1988). Although blocks to 10 m and more were fairly common (some of which consisted of a conglomerate of ice and rock), the grain size of the debris ranged mostly between 0.5 m and sand size. Finer debris was present locally. The blocks were frequently capped by finer material. At several places (Figs. 13 and 14), particularly between Plaz and San Bartolomeo, debris had been sprayed beyond the accumulation perimeter, similar to what has been found at other rock avalanche sites (e.g., Kojan and Hutchinson, 1978; Plafker and Ericksen, 1978; Eisbacher, 1979; Cruden and Hungr, 1986; Evans et al., 1994). Single blocks were hurled to 150 m beyond the accumulation limit (A. Frassoni, 1988, personal commun.).

Because ~3000 tall-trunk trees had been destroyed, abundant wood was scattered throughout; however, it was mostly in the form of splinters, while entire logs and sizable log segments were seen only locally. None of the 21 holes later drilled through the debris body encountered logs.

For convenience, the morphology of the rock avalanche will be described by sectors (Fig. 13). All sectors but A belong to the zone of accumulation, and sector boundaries are generally marked by some physiographic feature. Sectors B, C, and D belong to the south arm of the accumulation, and sectors E, F, and G belong to the north arm. Many of the landforms illustrated in the following no longer exist, or were deeply modified by natural or, particularly in the valley bottom, man-made changes.

Sector A

The upper part of sector A is the huge cavity left by the rock mass. Here can be seen the two planar surfaces delimiting the failed mass (Figs. 1 and 13). The apex of the headscarp is at ~2330 m. Downslope of the detachment zone is a denuded area where trees and soil were removed by the avalanche. However, a number of trees, toppled and completely debarked but still rooted, could be seen along both margins of the descent path at elevation 1150–1200 m. A large debris fan, the lobate upper margin of which was at 1300–1400 m, was deposited at the foot of the slope. In this fan, grain size decreased downslope and large blocks were perched at the top. Both sides of the descent path presented minor debris tongues; the tongue on the north side derived from material hurled beyond the crest of the Sassavin-Motta ridge following the impact of the main mass. In the same locality, the impact caused the ground to swell and rise by ~0.5 m over a length of ~100 m (A. Frassoni, 1988, personal commun.). An elongated portion of terrain remained substantially untouched close to the south margin.

Sector B

Apart from a bare scar, the debris accumulated in the north part of sector B reached a thickness of several tens of meters. Poorly developed fallback ridges formed southwest of the Plaz prominence. In the south part of the sector, which was originally steeper, only a comparatively thin layer of debris remained upon the slope, while a larger amount fell back. The southernmost part of the sector was left completely bare and presented a rim of fallen trees whose pattern can only be explained if they have been snapped off by airborne debris. In the middle of the sector, a mass of debris of some hundred thousand cubic meters was remobilized by water from a rivulet south of Plaz, and flowed downslope in the few hours immediately following the main event, creating a distinct landform. Grain size of surficial debris was generally smaller than in the adjacent sectors C and E, although coarse debris was noted locally. This probably reflected the sequence of fallback: coarser material at the surface of the runup wave fell back first and was then mostly buried by finer debris present in the basal layer of the wave. A similar process was described by Evans et al. (1994).

Sector C

This sector consisted of a reverse runup ridge. It was bounded on both sides by aligned depressions, which were linked by furrows showing frequent changes in their direction of drainage.

Evidently, part of the material that had run up the opposite slope and had fallen back still had sufficient energy to run back up the source slope to an elevation 110 m above the original valley bottom. At the southwest corner of the sector, a high-tension pylon located at 1080 m was torn out and, remaining at the surface, was raised to 1102 m. Surficial grain size was coarse (blocks of several meters in diameter were common), and some paragneiss debris was present in the north part (Chiesa and Azzoni, 1988). The maximum thickness of the accumulation, 90 m, was achieved in this sector and in sector E..

Sector D

The distal two-thirds of this tongue-like extension of the south portion presented flow features. These were developed as an intricate pattern of transverse and longitudinal ridges and furrows. Convexity of transverse features was downstream in the tip zone, upstream in the central section. The proximal one-third of the sector was characterized by an arcuate escarpment that interrupted the reverse runup ridge of sector C exactly where it attained its maximum height (Figs. 1 and 13). That ridge, consisting of loose debris, was probably affected by stability problems as it grew higher and higher, and was impacted on the eastern side by the debris still arriving from sector B. It then failed along its only free side, generating a flow of almost dry debris nearly 900 m long (Figs. 13 and 15); the village of Verzedo escaped destruction only by a narrow margin. The water-saturated alluvium in the Adda bed might have favored the flow process through a mechanism of undrained loading and undrained shearing (Sassa, 1988). Marginal furrows, constantly draining southward, rimmed both sides of this sector for more than half its length, and were locally accompanied by well-developed lateral levees. Surficial grain size was finer than in sector C, with a concentration of blocks at the distal tip. Logs, blocks, and water were sprayed ~150 m beyond the tip.

Sector E

In the eastern part of sector E, some debris was left upon the relatively flat area uphill of the Plaz–San Bartolomeo reentrant. In the ridge along the south side of the San Bartolomeo prominence, two different debris waves could be clearly distinguished. The first wave, generated by direct impact, came from the southwest, while the second, which had moved semicircularly across the Plaz–San Bartolomeo reentrant, arrived from the south and was apparently more mobile. The central part of this sector presents a series of slopes denuded by the avalanche. These were followed westward by a large accumulation of fallback debris. The southern part of this accumulation presented some curvilinear north-south alignments of logs and blocks. Some color banding, accompanied by a degree of granulometric sorting and by a flow-like pattern, could be seen in the northern part; the limits between adjacent bands were locally marked by furrows. Such features suggest differential motion between contiguous debris sheets (Kojan and Hutchinson, 1978). Debris in this sector was darker than elsewhere, perhaps because humusrich soil had been entrained between Plaz and San Bartolomeo. Coarse gabbro debris with abundant blocks was located in the west part of the sector (Chiesa and Azzoni, 1988). The maximum runup height (290 m) was in this sector.

Sector F

Sector F consisted basically of a reverse-runup ridge, created by material fallen back from the opposite slope. Elevation of the debris above the former valley bottom reached 110 m, as in sector C. The landforms in the northern side of the ridge suggest that this side, which was free, failed. The process was presumably similar to that described for sector D, but in this case the dynamic action of the return water wave could also have had a destabilizing role. Unfortunately the resulting landforms were largely concealed by flooding or submersion soon afterward. Remnants of a marginal furrow could be seen along the western limit. The south part of this sector, where relatively fine grained paragneiss fragments predominated, showed a thick sequence of tiny wrinkles, oriented north-south. This material was emplaced toward the end of the deposition process, and mantled the core of the ridge, where coarser fragments were present and locally exposed. The northern and central parts of the ridge were covered to a considerable extent by lobes of dark gray, silty-clayey materials of fluviolacustrine origin. Deposited by the return water wave, these sediments of the old valley bottom had probably been bulldozed northward by the rock-avalanche debris (Yarnold, 1993) and were then displaced back by the wave. The lobes were partly covered along the boundary with sector A by dry landslide debris of a later phase of deposition.

Sector G

Sector G of the debris accumulation was completely flooded by the return wave. The south part was mantled by a layer of mud locally intermixed with sediments similar to those mentioned in the preceding, and still preserved some relief, though much less pronounced than in the other sectors. At the eastern limit, below San Bartolomeo, two small fallback ridges with marginal furrow could be seen. The north portion consisted of a marshy flat from which a few island-like mounds protruded. These consisted of rock- avalanche debris and rubble from destroyed buildings. The new landslide lake extended farther north; only two islets appeared on the surface. In Figure 13, the north limit of the rock-avalanche accumulation has been placed, by inference, just north of the islet at the middle of the valley.

The rock avalanche dammed Valtellina from side to side, and extended considerably both upvalley and downvalley. Therefore, it is assigned to type III of the Costa and Schuster (1988) classification. It is interesting that no seepage was observed on the downstream side of the dam before the activation of the artificial spillway at the end of August.

Morphometry of the Rock Avalanche

Morphometrical data, with definitions and references, are listed in Table 6. In many cases, Abele's (1974) vast study of large alpine landslides, mostly rock avalanches, provides a useful comparison. However, three points need to be emphasized first.

1. The volume of the rock avalanches ranges, on Earth, between a few millions of cubic meters and 20 × 109 m3 (Saidmarreh, Iran; Watson and Wright, 1967). Therefore, by its volume (41 × 106 m3 of debris), the Val Pola rock avalanche is small compared to others in this category, and all the morphometrical considerations that follow have to be seen against this fundamental fact.

2. At Val Pola, the elevation difference was more than 1300 m, a high value in absolute terms for the Alps, in which maximum observed elevation differences were of ~2100 m, and decidedly a high value with respect to the volume (Abele, 1974). If one adds to this that the mass descended 650–900 m down an almost obstacle-free 32° slope, it must be concluded that the initial energetic input was great, as also noted by Antoine (1993). The high speed reached by the rock avalanche, which Costa (1991) calculated as between 76 and 108 m/s, fully supports this conclusion.

3. Both the mean thickness of the failed mass (64 m) and the relative hollow of the detachment cavity (0.328) were small; i.e., the mass was relatively tabular in shape.

Table 6.

Val Pola Rock Avalanche—Morphometrical Data

Planimetric surface of original rock mass (Sr) = 0.53 km
Volume of original rock mass (Vr) = 34 × 106 m3*
Maximum thickness of original rock mass = 150 m
Average thickness of original rock mass = 64 m
Planimetric surface of debris accumulation (Sd) = 2.05 km2
Volume of debris accumulation (Vd) = 41 × 106 m3*
Vd/Vr=1.2
Total planimetric surface (S) = 3.36 km2
Runout (L) = 3 900 m§
Length (D) = 2 790 m#
Elevation difference (H) = 1 342 m**
Excessive travel distance (Le = LH/tan 32°) = 1 752 m††
H/L = 0.344; Fahrböschung = arc tangent (H/L) = 19°§§
Le/L = 0.449§§
Length of accumulation (La) = 2 465 m##
Slope angle of accumulation = 10°***
Planimetric width at D/2 (Wm) = 1 055 m†††
Maximum planimetric width of accumulation (Wa) = 3 250 m†††
Wm/Wa = 0.325†††
Wm/L = 0.271†††
Wm/D = 0.378†††
Wa/L = 0.833†††
Wa/D = 1.165†††
Maximum thickness of accumulation = 90 m
Average thickness of accumulation (Vd/Sd) = 20 m
Relative thickness of accumulation [(Vd/Sd)/√Sd] = 0.014§§§
Spreading (Sd/Sr) = 3.9###
Relative hollow = 0.328****
Height of descent slope (h1)= 1 290 m
Height of runup (h2) = 290 m
Height of reverse runup (h3) = 110 m
h2/h1 = 0.225††††
h3/h2 = 0.379
Planimetric surface of original rock mass (Sr) = 0.53 km
Volume of original rock mass (Vr) = 34 × 106 m3*
Maximum thickness of original rock mass = 150 m
Average thickness of original rock mass = 64 m
Planimetric surface of debris accumulation (Sd) = 2.05 km2
Volume of debris accumulation (Vd) = 41 × 106 m3*
Vd/Vr=1.2
Total planimetric surface (S) = 3.36 km2
Runout (L) = 3 900 m§
Length (D) = 2 790 m#
Elevation difference (H) = 1 342 m**
Excessive travel distance (Le = LH/tan 32°) = 1 752 m††
H/L = 0.344; Fahrböschung = arc tangent (H/L) = 19°§§
Le/L = 0.449§§
Length of accumulation (La) = 2 465 m##
Slope angle of accumulation = 10°***
Planimetric width at D/2 (Wm) = 1 055 m†††
Maximum planimetric width of accumulation (Wa) = 3 250 m†††
Wm/Wa = 0.325†††
Wm/L = 0.271†††
Wm/D = 0.378†††
Wa/L = 0.833†††
Wa/D = 1.165†††
Maximum thickness of accumulation = 90 m
Average thickness of accumulation (Vd/Sd) = 20 m
Relative thickness of accumulation [(Vd/Sd)/√Sd] = 0.014§§§
Spreading (Sd/Sr) = 3.9###
Relative hollow = 0.328****
Height of descent slope (h1)= 1 290 m
Height of runup (h2) = 290 m
Height of reverse runup (h3) = 110 m
h2/h1 = 0.225††††
h3/h2 = 0.379

Note: Data in this table update and complete those published in Nicoletti and Sorriso-Valvo (1991) under heading Monte Zandila.

*Godone(1988).

†Area of devastation was larger due to water wave, sprays, and air blast.

§L is the planimetric distance between apex of headscarp and farthest point of accumulation, measured following approximately the center-line of the debris stream (Heim, 1932; Abele, 1974). In this case maximum runout took place in the southern arm where, in addition, some debris and water were sprayed about 150 m further downstream. Runout of northern arm was 3550 m.

#D is the planimetric distance between apex of headscarp and topmost point of accumulation on the opposite slope (Nicoletti and Sorriso-Valvo, 1991).

**Measured between the two points that define L.

††Hsü, 1975.

§§Mobility indices.

##Planimetric distance between the topmost point of the accumulation and its farthest end, measured following approximately the center line of the debris stream.

***Measured between the two points defining La (Heim, 1932; Abele, 1974).

†††These figures define rock avalanche shape (Nicoletti and Sorriso-Valvo, 1991); Wa is measured between the northern and southern tips of the accumulation.

§§§This index allows to compare accumulations of different order of magnitude (Abele, 1974).

****Ratio between maximum hollow in the detachment zone and its width, measured at the same elevation (Abele, 1974). Reference elevation is 2100 m in this case.

††††Heim (1932); Evans (1989); Kaiser and Simmons (1990); Evans et al. (1994).

At the end of its descent path, the streaming debris crossed the narrow Adda Valley, impacted almost perpendicularly against the opposite slope, ran up, fell back, and finally ran back up the source slope. These complicated back and forth fluctuations, accompanied by a well-developed fragmentation of the material, took a large energy expenditure. Therefore, the Val Pola event is a typical example of a rock avalanche whose mobility, i.e., the capability of moving over a long distance, was hampered by the local morphology; i.e., it had high-energy-dissipative geomorphic control (Nicoletti and Sorriso-Valvo, 1991). As will be shown, the interaction between a strong initial energetic input and a high-energy-dissipative geomorphic control resulted in a rock avalanche, whose mobility was not spectacular, if measured through indicators such as runout, excessive travel distance, fahrböschung, and Le/L ratio, but whose complicated pattern of motion allowed the debris to spread to an unusual degree.

The runout of the rock avalanche (3900 m) was relatively moderate for its size, and fairly high if compared with cases of analogous geomorphic control (Abele, 1974; Nicoletti and Sorriso-Valvo, 1991). This depends obviously on the kinetic energy gained along the descent path. Less obviously, it also depends on the 900 m downstream offshoot in sector D which, although involving a mass of only ~1 × 106 m3, contributed significantly to the measured runout. Excessive travel distance (1752 m) and Le/L ratio (0.449) were moderate; fahrböschung (19°) was moderate with respect to volume, but low (i.e., indicating fair mobility) with respect to geomorphic control.

As concerns the areal parameters, the overall surface involved was 3.36 km2, excluding the zone devastated by the water wave, and the morphometrical indicators give the following results. The length of accumulation (2465 m) is conspicuous, all the more if the type of geomorphic control is taken into account. The slope angle of accumulation (10°) is smaller than those of comparable events. The parameter Wa (3250 m) has a considerable value; the Wm/Wa ratio (0.325) is low (i.e., the accumulation is much wider than the descent path), and the Wa/L (0.833) and Wa/D (1.165) ratios are high (i.e., the width of the accumulation is conspicuous with respect to both runout and length). The average thickness of the debris (20 m) is only ~30% of rock mass thickness, and ranks in the lowest of the three classes proposed by Abele (1974). The relative thickness (0.014) is small with respect to the volume (this ranks in the lowest of Abele's [1974] three classes), and is the smallest for rock avalanches with comparable geomorphic control in the Alps. Conversely, the spreading index (3.9) is high if considered as a function of volume, and decidedly high if considered in terms of geomorphic control. In conclusion, a great amount of energy was spent for spreading the debris rather than for achieving a spectacular runout.

The runup height (290 m) plots below the limit line h20.3 h1 that in the diagram by Evans et al. (1994) envelops nearly all known data (Fig. 15). We also looked for an analogous relationship for the reverse runup: h3/h2 (0.379) results in a much greater value than h2/h1 (0.225), which does not seem verisimilar. Perhaps the thickness of the debris deposited on the valley floor should be considered in such computations, e.g., if we assume a debris layer 50 m thick, then the height of secondary descent slope reduces to 240 m, the height of reverse runup reduces to 60 m, and h3/h2 reduces to 0.250, which looks somewhat more realistic. Unfortunately there are no comparative data from other rock avalanches.

Discussion and Conclusions

The Val Pola rock avalanche was the final stage of a process begun, with the sagging of the slope, presumably at or shortly after the end of the Würm deglaciation. The sagging continued for millennia, with a mean deformation rate of 2–3 mm/year. For a long time, the only visible aspects of the process were the trench between the two alignment of pinnacles, and the rockfall activity from both the 2000–2100 m escarpment and the Spur. However, bulging along the right side of Val Pola also may have occurred, but because it was very slow, and was concealed by both forest cover and other geomorphic processes, it went unnoticed. In this way, the factor of safety of the sagging mass may have reached close to 1.0. In mid-July 1987, heavy rainfalls took place. Their local effect resulted in shallow landsliding in the debris cover and in debris flows, which widened and deepened the Val Pola canyon and, consequently, unloaded the toe of the sagging mass. The failure process accelerated sharply and, after the resistance by the Spur was overcome, progressed catastrophically.

From a geomorphological standpoint, this rock avalanche shows that even when local morphology does not favor the mobility of the debris mass, i.e., its capability of moving over a long distance, the overall effects may be really conspicuous in terms of areal devastation. Because the research effort has been mostly devoted to “linear” mobility, this is a particularly critical point. We note that minor offshoots, like that in sector D, may become very dangerous as a consequence of local topomorphic and dynamic conditions, despite their small mass.

From a risk prevention point of view, the hazard represented by water bodies lying within the range of the moving debris mass has to be stressed once more. In this case there was a small lake, only 5 m in average depth, and evacuation measures had been taken upstream. Unfortunately, the landslide-generated water wave proved more powerful than the pessimism of authorities and 22 people were killed at Aquilone. Unpleasantly enough, the lesson seems to be: Be more pessimistic.

The lesson to be drawn from this experience is that sagging slopes, although often showing very few or no symptoms of activity, may respond very dangerously to triggers, as a result of the crisis of an internal equilibrium very difficult to envisage, as well as expensive to investigate in the usual geotechnical terms. It therefore seems strongly advisable (1) to systematically identify such slopes, at least in densely populated regions, and (2) to define and validate reference geotechnical models that enable decision makers to plan prevention measures and/or to rationally manage the state of emergency created.

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Acknowledgments

We thank Licinio Ferretti, Head of Compagnia Generale Ripreseaeree (Parma, Italy), for kindly providing copies of unpublished postlandslide topographic maps, Maurizio Ponte for general support and data processing, and Pier Giuseppe Trebò (Consiglio Nazionale delle Ricerche-Istituto di Ricerca per la Protezione Idrogeologica, Turin, Italy) for his skillful work of photo reproduction. We also thank Jerome V. De Graff, Stephen J. Evans, and Douglas A. Sprinkel for thoughtful reviews and fruitful suggestions.

Figures & Tables

Figure 1.

Aerial view to west of Val Pola rock avalanche, showing source, descent path, and most of accumulation (August 1, 1987). Several elements mentioned in text can be seen: two surfaces delimiting tailed mass (dip direction/dip, respectively, 070°/45° and 350°/35°); gneiss (G) of Cristallino del Tonale; rock pinnacles of southern alignment (PS); northern and southern reverse runup ridges (RN and RS, respectively); escarpment caused by failure of downstream side of southern reverse runup ridge (left of RS); lacustrine sediments abandoned by return water wave, and deposited as lobes (L) or as mud layer (M); and rock prominence of Plaz (Pl). Photo courtesy of Foto Benini. Sondrio, Italy.

Figure 1.

Aerial view to west of Val Pola rock avalanche, showing source, descent path, and most of accumulation (August 1, 1987). Several elements mentioned in text can be seen: two surfaces delimiting tailed mass (dip direction/dip, respectively, 070°/45° and 350°/35°); gneiss (G) of Cristallino del Tonale; rock pinnacles of southern alignment (PS); northern and southern reverse runup ridges (RN and RS, respectively); escarpment caused by failure of downstream side of southern reverse runup ridge (left of RS); lacustrine sediments abandoned by return water wave, and deposited as lobes (L) or as mud layer (M); and rock prominence of Plaz (Pl). Photo courtesy of Foto Benini. Sondrio, Italy.

Figure 2.

Map of Val Pola area in Valtellina (valley of Adda River, in northern Italy), showing villages and physiographic features mentioned in text. Zone devastated by landslide and landslide-generated water wave is outlined.

Figure 2.

Map of Val Pola area in Valtellina (valley of Adda River, in northern Italy), showing villages and physiographic features mentioned in text. Zone devastated by landslide and landslide-generated water wave is outlined.

Figure 3.

Stereopair of Valtellina-Val Pola area (air photographs 1981). Note northern and southern pinnacle alignments (PN and PS, respectively), Spur (S). and Val Pola canyon (VP). Valley-bottom villages and hamlets are shown: Verzedo (Ve), San Martino Serravallc (SM), Orcaccia (Or), Plaz (Pl), Presure (Pr), Plegne (Pe), Castellaccio (Ca), Morignone (Mo), Foliano (Fo), San Bartolomeo (SB), Sant'Antonio Morignone (SA), Poz (Po), Tirindré (Ti), and Aquilone (Aq). Scale bar is approximate. Photo courtesy of Regione Lombardia (Milan, Italy), published by permission of the Italian Air Force (Concessione Stato Maggiore Aeronautica n. 319 del 9 luglio 1996).

Figure 3.

Stereopair of Valtellina-Val Pola area (air photographs 1981). Note northern and southern pinnacle alignments (PN and PS, respectively), Spur (S). and Val Pola canyon (VP). Valley-bottom villages and hamlets are shown: Verzedo (Ve), San Martino Serravallc (SM), Orcaccia (Or), Plaz (Pl), Presure (Pr), Plegne (Pe), Castellaccio (Ca), Morignone (Mo), Foliano (Fo), San Bartolomeo (SB), Sant'Antonio Morignone (SA), Poz (Po), Tirindré (Ti), and Aquilone (Aq). Scale bar is approximate. Photo courtesy of Regione Lombardia (Milan, Italy), published by permission of the Italian Air Force (Concessione Stato Maggiore Aeronautica n. 319 del 9 luglio 1996).

Figure 4.

Geological map of Val Pola area (after Chiesa and Azzoni, 1988; Azzoni et al. 1992 [modified]). Prelandslide configuration is shown (except for boreholes, which were drilled between October and November 1987).

Figure 4.

Geological map of Val Pola area (after Chiesa and Azzoni, 1988; Azzoni et al. 1992 [modified]). Prelandslide configuration is shown (except for boreholes, which were drilled between October and November 1987).

Figure 5.

Close-up view of Spur (S) (July 27, 1987; view to northwest). Traces of recent rockfall activity from and around it are evident.

Figure 5.

Close-up view of Spur (S) (July 27, 1987; view to northwest). Traces of recent rockfall activity from and around it are evident.

Figure 6.

Cumulative rainfall recorded at Arnoga rain gauge from January to July in period 1962–1987.

Figure 6.

Cumulative rainfall recorded at Arnoga rain gauge from January to July in period 1962–1987.

Figure 7.

Daily and cumulative rainfall patterns at Arnoga station from January to July, 1987.

Figure 7.

Daily and cumulative rainfall patterns at Arnoga station from January to July, 1987.

Figure 8.

Morphology and mass movement before July 1987.

Figure 8.

Morphology and mass movement before July 1987.

Figure 9.

Area on east flank of Mount Zandila involved in 28 July rock avalanche, as it appeared 24 h earlier (view to northwest). Val Pola (VP) canyon is in middle of image, and traces of landslide and erosion processes of previous days are clearly seen (cf. Fig. 3). Such processes resulted in emplacement of debris fan (DF) across Adda; as consequence, small lake is being impounded (I) and Morignone (Mo) is partly flooded. Large Y-shaped scree that originated from rockfall accumulation is visible left of Val Pola. Stem of Y is particularly pronounced, as consequence of large number of rockfalls generated in those days by Spur (S) (pegmatite body, lozenge shaped in exposure. crops out above stem top). Approximate outline of rock mass that failed on July 28 is shown. In foreground are rock prominence of Plaz (Pl) and houses of San Martino Serravalle. Photo courtesy of Azienda Energetica Municipale, Milan. Italy.

Figure 9.

Area on east flank of Mount Zandila involved in 28 July rock avalanche, as it appeared 24 h earlier (view to northwest). Val Pola (VP) canyon is in middle of image, and traces of landslide and erosion processes of previous days are clearly seen (cf. Fig. 3). Such processes resulted in emplacement of debris fan (DF) across Adda; as consequence, small lake is being impounded (I) and Morignone (Mo) is partly flooded. Large Y-shaped scree that originated from rockfall accumulation is visible left of Val Pola. Stem of Y is particularly pronounced, as consequence of large number of rockfalls generated in those days by Spur (S) (pegmatite body, lozenge shaped in exposure. crops out above stem top). Approximate outline of rock mass that failed on July 28 is shown. In foreground are rock prominence of Plaz (Pl) and houses of San Martino Serravalle. Photo courtesy of Azienda Energetica Municipale, Milan. Italy.

Figure 10.

Longitudinal section of Val Pola sagging. Drawing shows northern and southern alignments of pinnacles (PN and PS, respectively). original topographic surface (T), postslide topographic surface. which largely coincides with slip surface (Ss; dashed when inferred because buried by debris), material removed from Val Pola prior to July 28 failure (gray tone), and relevant morphometrical parameters defined in Table 1 (Hutchinson, 1995). See Figure 8 for location.

Figure 10.

Longitudinal section of Val Pola sagging. Drawing shows northern and southern alignments of pinnacles (PN and PS, respectively). original topographic surface (T), postslide topographic surface. which largely coincides with slip surface (Ss; dashed when inferred because buried by debris), material removed from Val Pola prior to July 28 failure (gray tone), and relevant morphometrical parameters defined in Table 1 (Hutchinson, 1995). See Figure 8 for location.

Figure 11.

State of sites in early morning of July 28, before rock avalanche, and rock-avalanche kinematics.

Figure 11.

State of sites in early morning of July 28, before rock avalanche, and rock-avalanche kinematics.

Figure 12.

Uppermost part of rock mass (July 25, 1987; view north). Pale line zigzagging along escarpment foot (arrowed) is perimetrical crack observed that day for first time, when it was 600 m long and 1 m (maximum) wide; it represented clear symptom of destabilization.

Figure 12.

Uppermost part of rock mass (July 25, 1987; view north). Pale line zigzagging along escarpment foot (arrowed) is perimetrical crack observed that day for first time, when it was 600 m long and 1 m (maximum) wide; it represented clear symptom of destabilization.

Figure 13.

Rock-avalanche (r.a.) morphology. Silhouette on the right shows subdivision by sectors.

Figure 13.

Rock-avalanche (r.a.) morphology. Silhouette on the right shows subdivision by sectors.

Figure 14.

Aerial view to southeast of debris accumulation (July 29, 1987). Plaz (Pl) and San Bartolomeo (SB) rock prominences are shown. Above them and right of Plaz, debris sprays can be seen along perimeter. Note denuded slopes at and between the two prominences, northern and southern reverse runup ridges (RN and RS, respectively). and new landslide lake (Ll). Photo courtesy of Azienda Energetica Municipale, Milan, Italy.

Figure 14.

Aerial view to southeast of debris accumulation (July 29, 1987). Plaz (Pl) and San Bartolomeo (SB) rock prominences are shown. Above them and right of Plaz, debris sprays can be seen along perimeter. Note denuded slopes at and between the two prominences, northern and southern reverse runup ridges (RN and RS, respectively). and new landslide lake (Ll). Photo courtesy of Azienda Energetica Municipale, Milan, Italy.

Figure 15.

Profiles of rock avalanche, with prelandslide and postlandslide topographic surfaces. Small circles mark postlandslide surface in positions where confusion might arise. Profile west-southwest–east-northeast goes from top of headscarp to point of maximum runup height uphill of Castellaccio. Rock prominence of Plaz is also shown. North-arm and south-arm profiles follow approximately buried Adda thalweg. In downstream part of south arm, note thin debris tongue projected toward Verzedo. Traces of profiles are in silhouette (A). Graph (B) shows runup datum at Val Pola plotted into runup diagram by Evans et al. (1994).

Figure 15.

Profiles of rock avalanche, with prelandslide and postlandslide topographic surfaces. Small circles mark postlandslide surface in positions where confusion might arise. Profile west-southwest–east-northeast goes from top of headscarp to point of maximum runup height uphill of Castellaccio. Rock prominence of Plaz is also shown. North-arm and south-arm profiles follow approximately buried Adda thalweg. In downstream part of south arm, note thin debris tongue projected toward Verzedo. Traces of profiles are in silhouette (A). Graph (B) shows runup datum at Val Pola plotted into runup diagram by Evans et al. (1994).

Table 1.

Val Pola Sagging—Morphometrical Data

Vmax (m)*α (°)δ (m)§L (m)#δ/L**
16528858850.096
Vmax (m)*α (°)δ (m)§L (m)#δ/L**
16528858850.096

Note: See Figure 10. All definitions below are from Hutchinson (1995).

*Deep-seatedness measure: it is the maximum vertical depth from ground surface to basal slip surface. 165 m is the maximum depth in the section considered, which is that with the highest δ/L ratio, but absolute maximum vertical depth (190 m) is in another section,

§Maximum depth.

Angle to the horizontal of the line connecting crown and toe of the sagging mass. Maximum depth of slip surface below the line defined above.

#Length of the line defined above.

**Form index of deep slides: the higher the index, the deeper the form.

Table 2.

Main Joint Sets

Joint setDip directionDipJoint Roughness Coefficient
S1050°–070°45°7
S2350°35°–50°7.5
Joint setDip directionDipJoint Roughness Coefficient
S1050°–070°45°7
S2350°35°–50°7.5
Table 3.

Rock Quality Designation Data From Boreholes

BoreholeClass 0–25Class 26–50Class 51–75Class 76–90Class 91–100TotalMinimumMaximumMeanStandard deviationWeight average
(n)(%)(n)(%)(n)(%)(n)(%)(n)(%)(n)(%)(%)(%)(%)(%)
110141217182515211623710100633267
213151011202225282124890100663189
1 + 2231522143823402537231600100653178
BoreholeClass 0–25Class 26–50Class 51–75Class 76–90Class 91–100TotalMinimumMaximumMeanStandard deviationWeight average
(n)(%)(n)(%)(n)(%)(n)(%)(n)(%)(n)(%)(%)(%)(%)(%)
110141217182515211623710100633267
213151011202225282124890100663189
1 + 2231522143823402537231600100653178
Table 4.

Geotechnical Parameters of Reference Rock Types

ParameterValueGabbro*Diorite*Quartzite*Granite*Plutonic r.§
γ(kN/m3)Min.26.519.925.123.525.6
Max.31.329.739.927.031.0
Mean29.327.125.726.027.1
E Lab. (GPa)Min.32.521.410.74.67.8
Max.125.0106.7119.078.399.4
Mean81.365.759.644.956.6
E Site (GPa)Min.N.D.2.61.90.3N.D.
Max.N.D.17.147.968.9N.D.
MeanN.D.8.920.714.2N.D.
G (Gpa)Min.14.912.80.27.1N.D.
Max.48.044.340.655.0N.D.
Mean34.631.627.619.4N.D.
vMin.0.110.010.080.020.05
Max.0.380.320.330.390.39
Mean0.220.220.200.190.20
sc(Mpa)Min.126.264.177.212.848.8
Max.317.0333.0357.0324.0324.0
Mean216.1192.5216.2157.8146.4
st (MPa)Min.13.85.23.52.62.6
Max.16.450.335.338.612.2
Mean15.131.423.412.49.1
c Lab. (MPa)Min.N.D.56.220.00.116.5
Max.N.D.176.070.656.0176.0
MeanN.D.N.D.N.D.N.D.56.1
c Site (MPa)Min.N.D.N.D.0.40.1N.D.
Max.N.D.N.D.2.02.5N.D.
MeanN.D.N.D.N.D.N.D.N.D.
φ Lab. (°)Min.N.D.24262923.8
Max.N.D.55646456.0
MeanN.D.N.D.N.D.N.D.45.6
φ Site (°)Min.N.D.N.D.4341N.D.
Max.N.D.N.D.5662N.D.
MeanN.D.N.D.N.D.N.D.N.D.
ParameterValueGabbro*Diorite*Quartzite*Granite*Plutonic r.§
γ(kN/m3)Min.26.519.925.123.525.6
Max.31.329.739.927.031.0
Mean29.327.125.726.027.1
E Lab. (GPa)Min.32.521.410.74.67.8
Max.125.0106.7119.078.399.4
Mean81.365.759.644.956.6
E Site (GPa)Min.N.D.2.61.90.3N.D.
Max.N.D.17.147.968.9N.D.
MeanN.D.8.920.714.2N.D.
G (Gpa)Min.14.912.80.27.1N.D.
Max.48.044.340.655.0N.D.
Mean34.631.627.619.4N.D.
vMin.0.110.010.080.020.05
Max.0.380.320.330.390.39
Mean0.220.220.200.190.20
sc(Mpa)Min.126.264.177.212.848.8
Max.317.0333.0357.0324.0324.0
Mean216.1192.5216.2157.8146.4
st (MPa)Min.13.85.23.52.62.6
Max.16.450.335.338.612.2
Mean15.131.423.412.49.1
c Lab. (MPa)Min.N.D.56.220.00.116.5
Max.N.D.176.070.656.0176.0
MeanN.D.N.D.N.D.N.D.56.1
c Site (MPa)Min.N.D.N.D.0.40.1N.D.
Max.N.D.N.D.2.02.5N.D.
MeanN.D.N.D.N.D.N.D.N.D.
φ Lab. (°)Min.N.D.24262923.8
Max.N.D.55646456.0
MeanN.D.N.D.N.D.N.D.45.6
φ Site (°)Min.N.D.N.D.4341N.D.
Max.N.D.N.D.5662N.D.
MeanN.D.N.D.N.D.N.D.N.D.

Note: Lab = Laboratory; Min. = minimum; Max. = maximum; N.D. = no data.

*Basic data after Lama and Vutukuri, 1978.

†The quartz-rich rock body forming the Spur is no longer exposed. It has been assumed that quartzite is sufficiently similar and that its mechanical properties may be representative of those of the Spur.

§Basic data after Kulhawy, 1975, in Lama and Vutukuri, 1978.

Table 5.

Phases of the Event and Duration of Seismic Records

PhaseProcessDuration
1Failure and sliding northward9–15
2Rockfalls eastward7
3Impact against the Sassavin-Motta ridge4.5–10
4Avalanching8–12.5
5Runups, deflections, fallbacks, reverse runups, water wave generation19–31
6Water wave motion31–42
PhaseProcessDuration
1Failure and sliding northward9–15
2Rockfalls eastward7
3Impact against the Sassavin-Motta ridge4.5–10
4Avalanching8–12.5
5Runups, deflections, fallbacks, reverse runups, water wave generation19–31
6Water wave motion31–42
Table 6.

Val Pola Rock Avalanche—Morphometrical Data

Planimetric surface of original rock mass (Sr) = 0.53 km
Volume of original rock mass (Vr) = 34 × 106 m3*
Maximum thickness of original rock mass = 150 m
Average thickness of original rock mass = 64 m
Planimetric surface of debris accumulation (Sd) = 2.05 km2
Volume of debris accumulation (Vd) = 41 × 106 m3*
Vd/Vr=1.2
Total planimetric surface (S) = 3.36 km2
Runout (L) = 3 900 m§
Length (D) = 2 790 m#
Elevation difference (H) = 1 342 m**
Excessive travel distance (Le = LH/tan 32°) = 1 752 m††
H/L = 0.344; Fahrböschung = arc tangent (H/L) = 19°§§
Le/L = 0.449§§
Length of accumulation (La) = 2 465 m##
Slope angle of accumulation = 10°***
Planimetric width at D/2 (Wm) = 1 055 m†††
Maximum planimetric width of accumulation (Wa) = 3 250 m†††
Wm/Wa = 0.325†††
Wm/L = 0.271†††
Wm/D = 0.378†††
Wa/L = 0.833†††
Wa/D = 1.165†††
Maximum thickness of accumulation = 90 m
Average thickness of accumulation (Vd/Sd) = 20 m
Relative thickness of accumulation [(Vd/Sd)/√Sd] = 0.014§§§
Spreading (Sd/Sr) = 3.9###
Relative hollow = 0.328****
Height of descent slope (h1)= 1 290 m
Height of runup (h2) = 290 m
Height of reverse runup (h3) = 110 m
h2/h1 = 0.225††††
h3/h2 = 0.379
Planimetric surface of original rock mass (Sr) = 0.53 km
Volume of original rock mass (Vr) = 34 × 106 m3*
Maximum thickness of original rock mass = 150 m
Average thickness of original rock mass = 64 m
Planimetric surface of debris accumulation (Sd) = 2.05 km2
Volume of debris accumulation (Vd) = 41 × 106 m3*
Vd/Vr=1.2
Total planimetric surface (S) = 3.36 km2
Runout (L) = 3 900 m§
Length (D) = 2 790 m#
Elevation difference (H) = 1 342 m**
Excessive travel distance (Le = LH/tan 32°) = 1 752 m††
H/L = 0.344; Fahrböschung = arc tangent (H/L) = 19°§§
Le/L = 0.449§§
Length of accumulation (La) = 2 465 m##
Slope angle of accumulation = 10°***
Planimetric width at D/2 (Wm) = 1 055 m†††
Maximum planimetric width of accumulation (Wa) = 3 250 m†††
Wm/Wa = 0.325†††
Wm/L = 0.271†††
Wm/D = 0.378†††
Wa/L = 0.833†††
Wa/D = 1.165†††
Maximum thickness of accumulation = 90 m
Average thickness of accumulation (Vd/Sd) = 20 m
Relative thickness of accumulation [(Vd/Sd)/√Sd] = 0.014§§§
Spreading (Sd/Sr) = 3.9###
Relative hollow = 0.328****
Height of descent slope (h1)= 1 290 m
Height of runup (h2) = 290 m
Height of reverse runup (h3) = 110 m
h2/h1 = 0.225††††
h3/h2 = 0.379

Note: Data in this table update and complete those published in Nicoletti and Sorriso-Valvo (1991) under heading Monte Zandila.

*Godone(1988).

†Area of devastation was larger due to water wave, sprays, and air blast.

§L is the planimetric distance between apex of headscarp and farthest point of accumulation, measured following approximately the center-line of the debris stream (Heim, 1932; Abele, 1974). In this case maximum runout took place in the southern arm where, in addition, some debris and water were sprayed about 150 m further downstream. Runout of northern arm was 3550 m.

#D is the planimetric distance between apex of headscarp and topmost point of accumulation on the opposite slope (Nicoletti and Sorriso-Valvo, 1991).

**Measured between the two points that define L.

††Hsü, 1975.

§§Mobility indices.

##Planimetric distance between the topmost point of the accumulation and its farthest end, measured following approximately the center line of the debris stream.

***Measured between the two points defining La (Heim, 1932; Abele, 1974).

†††These figures define rock avalanche shape (Nicoletti and Sorriso-Valvo, 1991); Wa is measured between the northern and southern tips of the accumulation.

§§§This index allows to compare accumulations of different order of magnitude (Abele, 1974).

****Ratio between maximum hollow in the detachment zone and its width, measured at the same elevation (Abele, 1974). Reference elevation is 2100 m in this case.

††††Heim (1932); Evans (1989); Kaiser and Simmons (1990); Evans et al. (1994).

Contents

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