REPORTS AND COMMENT

Northern Italian dam failure and mudflow, July 1985

David Alexander

Department of Geology and Geography University of Massachusetts Amherst MA 01003, U.S.A.

On 19th July 1985 Italy experienced its worst dam disaster since the Vajont reservoir overflow of 1963. The Val di Stava dam rupture and mudflow killed at least 199 people and caused damage valued at more than 8,500 million lire (U.S.S4.7 million). This report will describe the event and its setting, assess the relief effort and relate these respectively to their physical and social contexts.

THE SETTING

The Stava Valley contains a small northern affluent of the River Alvisio, which flows N W P E down the‘ Val di Fiemme, through the Prealpine Dolomites in the Province of Trento (Trentino-Alto Adige Region, northern Italy; Fig. 1). Towards the head of the valley, at 1,285 m above sea level, about 8 km from the town of Cavalese and 6 km from a smaller settlement called Tesero, a mining concern had set up a small works where calcareous strata of the Mesozoic Lower Trias outcrop above Permian deposits of the Paleozoic (age about 230 ma B.P.). The mine extracted fluorite (CaF?), from which flourine was obtained for use in optical, chemical and steel-making industries. About fifteen workers were employed in mining the deposit, and a similar number in separating the flourite from other minerals and depositing the residue in tip heaps. As flourite has a specific gravity of only 1.8 this was accomplished using settling tanks. About 60,000 kg of mined deposits were treated in this manner each day, such that more than 6,000 tonnes of waste had accumulated at the site.

The cleansing process took place in two trapezoidal basins, arranged in step form so that the upper one decanted into the lower. About 40,000 cubic metres of water were impounded behind the two steep-walled, terraced earth dams, which had at least partly been constructed from the detritus of the mining operation.

Friday, 19th July 1985, marked the start of the peak holiday season in the Dolomites, which was to continue for one month until after the traditional Italian summer recess offerragosto. The Stava Valley was also a tourist centre containing four hotels and a number of chalet residences. A full complement of tourists was present, including visitors from central Italy, Sardinia and Germany.

THE EVENT

The base of the upper dam was observed to be leaking during the week of 12-19th July 1985, yet no alarm was raised. At about 12.20 in the afternoon on 19th July the upper dam breached, creating a rift 120 by 80 m through which water and sediment poured into the lower basin, which also breached. About 40,OOO cubic metres of water were thus added to 150,OOO cubic metres of debris from the retaining walls of the dams, and this admixture picked up more than 100,OOO cubic metres of debris as it travelled down the Stava Valley. The material flowed about 5 km, spanning a width of 50-300 m and coming to rest as a body of sediment about 1,000 m long. Although not scientifically verified, eye-witness reports indicate that the speed of travel would place this into the “extremely rapid” (greater than 3 m/sec) category of Varnes’ (1978) mass movement classification. Eyewitnesses also demonstrated that the material had no bearing capacity (rescue workers sank into it) and it ponded water on its surface, indicating a high degree of saturation. However, several reports suggest that this was a very high density mudflow: the material (derived from crushing the mined deposits) was homo- geneous and rich in silt-size particles, after drainage it underwent some cementation, and during flow it proved capable of destroying reinforced concrete structures.

Alpine chalets, tents, houses and three of the hotels were carried away by the mudflow. The fourth hotel was seriously damaged. The destruction also encompassed fourteen homes, two businesses, several farm buildings, water, sewerage and electricity lines and 3 km of local road. Walls, trees and vehicles were carried away and, in all, 8.5 billion lire (U.S.W.7 million) of damage was caused.

At the conclusion of search-and-rescue, 199 bodies had been recovered and forty people were unaccounted for.

THE RELIEF EFFORT

About one hour after the event the Ministry for Civil Protection (in Rome) was informed and, after a brief conference, Sig. Giuseppe Zamberletti, the Minister, set out for the North. Under his auspices a relief operation was launched, reaching such proportions that the Turin newspaper La Stampa was moved to comment: “Partic- ipation as never before seen; co-ordinated and efficient.” The forces and equipment mobilized can be summarized as follows: Personnel: 5,442 men: 39.5% army

26.5% police 25.0% fire brigades 5.5% forestry corps 3.5% volunteers

72 floodlighting units 41 generator units 27 camp kitchens 26 ambulances

Equipment: 774 heavy plant 137 specialized plant

16 tall crane units 19 helicopters 144 radio transmitters

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REPORTS AND COMMENT 4

-- 100 km

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Fig. 1. Val di Stava location map.

In addition, a group of sixty-two volunteers from the city of Bergamo brought thirty-eight vehicles, including ambu- lances and earth-moving equipment. The Italian Red Cross and Catholic relief agencies participated, and tire brigades were drafted in from other provinces and regions. Of these forces, 31% were at work within eight hours of the disaster, 86% within eleven hours and full capacity was reached before dawn on the morning after the impact.

Despite this bewildering array of resources, it was possible to extract only thirteen victims alive from the rubble and mud. Search-and-rescue lasted 55 hours, but was forced to concentrate mainly on locating bodies, using tracker dogs, probing rods and - in the case of water impoundments - skin divers. No live victims were retrieved more than 18 hours after the impact. Temporary morgues were set up in an elementary school and two churches, and in the refrigerated warehouse of a local fruit wholesaler. Helicopters were used to carry bodies to the morgues and equipment from the peripheral command centre to the operating locations at the scene of the mudflow. Fifty-two of the bodies were eventually given burial without being identified, such was the extent of disfigurement, and thirty-five local residents were interred simultaneously in a mass grave at the side of the parish church at Tesero, close beyond the foot of the mudflow. Mortuary operations were wound up on the third day after the disaster.

A relief appeal in favour of the survivors was set up and international donations were received. The semi- autonomous Trento provincial government voted 5 billion lire (U.S.S2.8 million) in relief funds, to be allotted as 60% in direct aid, 30% as subsidies to the survivors, and 10% for the repair of utilities and public works.

THE RESPONSIBILITY

Investigation of the disaster became the responsibility of the Public Prosecutor for the Province of Trento, in which Val di Stava is situated. This official, Dr. Francesco Simeoni, nominated several engineering geologists to a commission of enquiry, in order to investigate the cause of the dam failure. The mining company, Prealpi Mineraria, had recently been sold by the chemical consortium Montecatini-Edison to Samim, a subsidiary of the national petrochemical corporation ENI. In any event, the mining concession was run by two brothers, Giulio and Aldo Rota, who were indicted for multiple culpable homicide and for causing a disaster.

There are about 7,000 artificial lakes in Italy, but by no means all are subject to routine safety checks. The Val di Stava impoundments were not listed among the 530 dams regularly inspected by the Ministry of Public Works. or the 256 under the jurisdiction of ENEL, the national electrical power corporation. One possible reason for this is that the

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REPORTS AND COMMENT 5

Italian working definition of a dam implies a structure at least 10 m high that impounds more than 150,OOO cubic metres of water. The dams at Tesero were apparently too small to qualify for mandatory inspection (yet not too small to create a disaster). In any event, the catastrophe took place nine months after the most recent visit of a survey engineer to the site.

As it happened, some eight months before the disaster the local branch of Lista Verde, the Italian ecological party, had agitated for better regulation of the Prealpi Mineraria operations. The mine itself was almost completely worked out and flourite was being brought in for cleansink at Tesero from up to 150 km away. The stilling basins became overfull, and Prealpi Mineraria applied to the local authority to increase the size of its waste tip. Permission was refused, but granted instead by the other main local land- holder, the National Forestry Corps. Yet in November 1984 what concerned Lista Verde was more the dereliction of land than the risk of dam failure, especially as the waste dumps would not recolonize with native species. Thus the raising and reinforcing of the earth dams using material from the fluorite separation process seems to have gone unnoticed by the authorities until it was too late.

According to the public prosecutor, some links between the parent company and subcontractors for the mining enterprise were not properly documented in the company registration deeds, and in the document leasing the concession to the Rota brothers. Clearly there was negligence concerning the geotechnical and environmental impact of the mining operation in Val di Stava, and the press was quick to pick it up (La Stampu, Turin, 23rd July 1985; Panorama, Rome, 28th July 1985). Yet it is not a simple process to allocate the balance of responsibility among the various parties: the Municipality of Tesero, the Province of Trento (and their various officials responsible for industry, mining, and forests and watersheds), the Forestry Corps, the Mining Administration District, the Provincial Corps of Engineers, Prealpi Mineraria, Montecatini-Edison and Samin-ENI.

GENERAL CONSIDERATIONS

The following section will compare the Val di Stava dam failure with similar events elsewhere, comment on the nature of debris flow processes in the valley and develop a theoretical context in which to place the relief effort.

The Val di Stava disaster was in terms of mortality the worst such event in Italy since the 1%3 Vajont catastrophe in the Piave Valley, 65 km to the east of Tesero (Quarantelli, 1979). However, although this event involved the avalanching of some saturated debris, as a result of a major landslide into the reservoir (Trollope, 19811, the concrete arch dam at Vajont did not rupture. The failure of the dam at Tesero could basically be attributed to poor construction, which appears to be a relatively uncommon cause: Gruner (1963) analyzed 308 dam failures occurring during the period 179G1944 and found that 40% were attributable to foundation failure, 23% to failure of the spillway and only 12% to poor construction.

Although perhaps not entirely without precedent in Italy, the Val di Stava disaster relates more closely to four similar events in the U.S.A. In 1938 at Fort Peck, Montana, a hydraulic fill dam collapsed. This involved the displacement of 7.6 million cubic metres of fill and foundation sands 425 m laterally in only three minutes (at an average of 8.5 kmlhr), and eighty lives were lost (Bishop, 1973). The Baldwin Hills reservoir, in Los Angeles County, California, breached on 14th December 1963 with the loss of five lives and incurring damages of $15 million (James, 1%8). This rolled earth impoundment bore some resemblance to the Val di Stava basins, and like them it failed by the catastro- phic acceleration of leakage, leading to inundation of a settled area with water and sediment. However, the Val di Stava dam rupture apparently did not involve foundational instability, whereas the foundations of the Fort Peck dam failed and those of the Baldwin Hills reservoir were affected by subsidence related to the fluid extraction.

The Teton dam, in Idaho, which collapsed in June 1976 upon filling of the reservoir behind it, had no foundational instability. It, too, was a multi-layered earth structure that suffered catastrophic breaching after excessive penetration by reservoir water (Boffey, 1977). However, one principal cause of this was the inadequacy of grouting in the fractured bedrock behind the dam (which should never been constructed at such a site). As with the Tesero dam, leakage and erosion of the compacted fill probably occurred for some time before eventual collapse, but were not noticed by engineers supervising the filling of the reservoir. Fourteen people died in the ensuing flood and damage was estimated at S400-1,OOO million.

Probably the closest analogy to the Val di Stava case was the failure in 1972 of the third in alignment of four colliery dams at Buffalo Creek, West Virginia, killing 118 people, leaving 4,000 homeless and causing $50 million of damage (Davies, 1973). About 0.5 million cubic metres of water and slurry were released, with 170,000 cubic metres of material from the colliery banks, and transported down-channel at a maximum of 6 m/sec (22km/hr). The foundation of the dam was not stripped and accelerated piping was the immediate cause of failure, with inadequate maintenance of the structure as the long-term cause.

Hence, in international circles there is a wealth of information from which to derive lessons concerning the construction, degradation and maintenance of earth dams (summarized in ASCE, 1472). The civil engineer Prof. Alan W. Bishop has argued that using mining debris as dam construction material is not in itself necessarily hazardous, providing that foundation strata are stable and not liable to piping erosion, and providing that consolidation and compaction of the material are carefully carried out. Foundation strata that are fissured should be very carefully grouted in order to stop water concentrating within the dam (Bishop, 1973).

Orombelli and Porter (1981) have shown that the risk of landslides in the Italian Alps is not to be underestimated, and Azzola and Tuia (1983) analyzed the flowslides of May 1983 in Valtellina, which killed eighteen people. However, little attention has been given to the risk of non-natural.

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mass movements such as the Val di Stava mudflow. Innes (1983) shows that this event would classify as a form of debris flow, and it would fall between his “catastrophic flow” and “valley-confined flow” categories, but would definitely occupy his “large scale” category (with greater than 100,000 cubic metres of material moved). Okuda et al. (1980) report that 3070 of fatalities during heavy rainstorms in Japan are caused by debris flows, and Bishop lists some notable flow-slides in mining spoil over the period 1938- 1972, including the collapse of 107,000 cubic metres of material at Aberfan, South Wales, in 1%6, which killed 144 people. Furthermore, Pomeroy (1980) has shown that in Appalachia mining dereliction, debris flows and urban development often seem to coincide.

Several peculiarities of debris flow mechanisms are relevant to the Val di Stava case. Experiments by Okuda et al. (1980) registered debris flow impact pressures of at maximum 4 kN/cm2, which is more than two orders of magnitude greater than the force required to move a reinforced concrete structure. Such flows are also noted for their ability to transport buoyantly large-scale material (Fisher, 1971), including boulders of up to 3,000 tonnes, and to flow on very gentle slopes (Rodine and Johnson, 1976). However, much depends on the non-linear relationships among rheological properties, dispersive stresses, water content and shearing stresses. Dispersive stresses, shearing and bulk density increase with falling water content (Innes, 1983). For convenience, debris flow activity can be divided into a fluid, or continuous, phase and a dispersed phase. In the former, bulk density may be as low as 1.07-1.16, depending on the amount of clay and water present. In the latter, bulk density may increase to 2.Ck2.7, and the effective normal stress between particles will be low. Rheologically, the admixture may undergo quasi-plastic or modified Bingham plastic behaviour and will manifest some yield strength (Fisher, 1971). High bulk density, high clay content and poor sorting of particles all promote dispersive stresses, reduce the intergranular effective normal stresses, and thus allow flow to take place on low slopes. Hence, as the body of material drains, it tends to surge from the centre, with little lateral and basal movement: and the increased shearing promotes buoyancy among coarse particles, which is in turn reduced as the material thins by extension.

Although undrained loading is considered a fundamental mechanism of mudflows and debris flows (Hutchinson and Bhandari, 1971), the Val di Stava mass movement involved as much the pressure of hydraulic head in the two stilling basins, which may have increased the velocity of down- channel flow. During the fluid phase, bulk density was probably relatively low and flow mainly laminar, yet sufficient impact pressure still existed to cut easily through man-made structures, despite the low slope of the mudflow track (6-7’). Competence thus remained very high. In conclusion, much attention has been given to the identification, field mapping and process monitoring of natural debris flows (Hansen, 1984), and - understandably - rather less notice has been taken of those with artificial causes, or occurring in artificially reworked deposits. Yet

the risk to life and property that these pose is far from insignificant.

Finally, some evaluation of the relief effort is required. In terms of spatial organization, the theoretical zones postulated in the seminal study of disaster logistics by Wallace (1956) can be identified in practice. The impact area itself was localized to about 5 km of the Stava Valley and was obviously narrow and confined. The zone of marginal impact was even more restricted, principally to the lower fringes of the debris flow deposit. The zone of filtration, from which initial aid arrived, had a radius of less than 30 km and included Tesero, the nearest settlement, Predazzo (at 10 km), from which an alpine rescue team arrived, and Bolzano (distant 26 km directly or 46 km by main road), from which volunteer forces arrived. The zone of national aid involved northern Italian cities from Trento (52 km from the disaster area) to Turin (436 km), from which most of the aid arrived, and Rome (648 km), from which officials of the Ministry for Civil Protection and the Italian Red Cross arrived. The zone of international aid included Japan, whose construction industry made a donation to the relief appeal. There is this an approximately logarithmic progression in the radii of these zones.

A most startling feature of the Val di Stava disaster is the speed with which aid arrived and the scale on which the operation was mounted. Elsewhere (Alexander, in press), I have shown that the public and government in Italy remained highly sensitized to natural disaster after events such as the 1980 southern Apennine earthquake, the 1982 Ancona landslide and a swarm of thirty-six damaging earth tremors over 1982-1984, which culminated in two minor seismic disasters in central Italy. The legislative structure to deal with these catastrophes was acquired piecemeal, but the practical experience enabled full-scale deployment of resources - in the minimum feasible time - in July 1985. Although this represents “overkill,” public opinion would in any case not have been satisfied with anything less.

Regarding the prospects for reconstruction, one must reserve judgement. With respect to another northern Italian disaster, the 1976 Friuli earthquake, Hogg (1980, p. 184) argued that “the resources available for reconstruction vary in proportion to the size of settlement;” and Alexander (1983) found in southern Italy that smaller settlements affected by landslide disaster tend to have much less political power than larger ones, and thus to obtain a much smaller proportion of the funds for reconstruction that they request from central government. These observations call into question the applicability in this case of general reconstruction models such as those propounded by Kates and Pijawka (1977).

REFERENCES

Alexander D.E., The landslide of 13th December 1982 at Ancona, central Italy, Report to the International Disaster Institute, London (1983).

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Alexander D.E., The Italian experience with earthquake disaster relief, in: Housing and Urban Development After Natural Disasters; Mitigating Future Losses, Proceedings of the Miami Symposium, October 23-26 1985, American Bar Association, Washington D.C. (in press).

ASCE, Proceedings of the Specialty Conference on the Performance of Earth and Earth-supported Structures, Purdue University, Lafayette, Indiana, American Society of Civil Engineers, New York, 2 Volumes (1972). Azzola M. and Tuia T., Osservazioni sui movimenti franosi che hanno interessato i vigneti terrazzati a monte Tresenda nel maggio 1983, Geologia Tecnica 30(4), 23-36 (1983). Bishop A.W., The stability of tips and spoil heaps, Q. J. Engng Geology 6(3-4), 3 3 S 3 7 7 (1973). Boffey P.M., Teton dam verdict: A foul-up by the engineers, Science 195, 270-272 (1977). Davies W.E., Buffalo Creek dam disaster: why it happened, Civil Engng 43(7), 69-72 (1973). Fisher R.V., Features of coarse-grained, high-concentration fluids and their deposits, J. Sedimentary Petrology 41(4),

Gruner E., Dam disasters, Proc. Inst. Civil Engrs 24, 4 7 - 6 0 (1963). Hansen A., Landslide hazard analysis, in: Slope Instability (Edited by D. Brunsden and D.B. Prior), pp. 523-602. Wiley-Interscience, Chichester, U.K. (1984). Hogg S.J., Reconstruction following seismic disaster in Venzone, Friuli, Disasters 4(2), 173-185 (1980).

916-927 (1971).

Hutchinson J.N. and Bhandari R.K., Undrained loading, a fundamental mechanism of mudflows and other mass movements, Geotechnique 21, 353-358 (1971).

Innes J.L., Debris flows, Progress in Physical Geography 7(4), 469-501 (1 983).

James L.B., Failure of Baldwin Hills reservoir, Lm Angeles, California, Engineering Geology Case Histories, N 0. 5. pp. 1-1 1. Geological Society of America, Boulder, Colorado (1 968).

Kates R.W. and Pijawka D., From rubble to monument: The pace of reconstruction, in: Disaster and Reconstruction (Edited by J.E. Haas, R.W. Kates and M.J. Bowden), pp. 1-23. M.I.T. Press, Cambridge MA (1977).

Okuda S., Suwa H., Okunishi K., Yokoyama K. and Nakano M., Observations on the motion of a debris flow and its geomorphological effects, Zeitschrift fur Geomor- phologie Supplementband 35, 142-163 (1980). Orombelli G. and Porter S.C., I1 rischio di frane nelle Alpi, Le Scienze 156, 68-79 (1981).

Pomeroy J.S., Storm-induced debris avalanching and related phenomena in the Johnstown area Pennsylvania, with reference to other studies in the Appalachians, U.S. Geological Survey Professional Paper N 0. 1191, 1-24 (1980).

Quarantelli E.L., The Vajont Dam overflow: A case study of extra-community responses in massive disasters, Disasters 3(2), 199-212 (1979). Rodine J.D. and Johnson A.M., The ability of debris, heavily freighted with coarse clastic materials, to flow on gentle slopes, Sedimentology 23, 213-234 (1976). Trollope D.H., The Vajont slide failure, Rock Mechanics 13(2), 7 1 - 8 8 (1981). Varnes D.J., Slope movement types and processes, in: Landslides; Analysis and Control (Edited by R.L. Schuster and R.J. Krizek), pp. 11-33. Transporation Research Board, Washington, D.C. (1978). Wallace A.F.C., Human Behaviour in Extreme Situations, Disaster Study No. 1, pp, 1-35. National Academy of Sciences Disaster Research Group, Washington, D.C. (1 956).

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