Characteristics of May 5–6, 1998 volcaniclastic debris flows in the Sarno area (Campania, southern Italy): relationships to structural damage and hazard zonation

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Abstract

The destructive power of debris flows on alluvial fan surfaces was investigated by studying the event of May 5–6, 1998 in the Sarno area (Campania, southern Italy). We assessed some physical and dynamic properties of debris flows, such as volumes, peak discharge and mobility, in order to obtain velocity profiles along alluvial fan surfaces. Reconstructed peak velocities vary from 20 m s−1 to 14 m s−1, while the runouts from the alluvial fan apexes range between ∼900 m and ∼2000 m. Debris flow velocities exponentially decrease from the alluvial fan apex to the distal depositional zone but abruptly drop to zero after entering the densely inhabited areas. Based on velocity data and estimates of flow density, the values of impact pressure on rigid structures (i.e. buildings) were assessed through the calculation of hydrostatic pressure and dynamic overpressure. The data from the study area show that at flow velocities >4–5 m s−1 the dynamic overpressure accounts for most of the observed damage, whereas at lower velocities the contributions of the hydrostatic and dynamic pressures become similar. The comparisons between the calculated impact pressures and the damage on structures show that for loading >35 kPa (velocity >3 m s−1) most of buildings were destroyed or severely damaged. For values <35 kPa only minor damage occurred to the structures. Finally, an empirical law for the assessment of impact pressure on spreading areas was presented, useful for hazard zonation in similar geomorphologic settings.

Introduction

Steep hill-slopes are susceptible to several kinds of mass wasting. This can represent a serious hazard for the people living at the foot of such slopes. In these areas, the development of correct and appropriate hazard mitigation plans should be the main goal of the public administration and civil protection. Mountainous areas near explosive volcanoes are particularly prone to develop debris flows triggered by heavy and/or prolonged rainfall which causes the sudden remobilisation of the volcaniclastic cover. Although debris flows are particularly common during and following explosive eruptions, hazards associated with these phenomena are a permanent feature of circumvolcanic areas that are mantled by volcaniclastic deposits (e.g. Pareschi et al., 2000, Pareschi et al., 2002). The rugged and steep relief downwind of the Somma–Vesuvius and Phlegrean Fields volcanoes, in southern Campania (Italy) are areas where there is typically a formation of volcaniclastic debris flows. This was tragically shown on May 5–6, 1998 when the southern Campania region was struck by tens of debris flows initiated as soil slips triggered by prolonged rainfall (Braca and Onorati, 1998, Del Prete et al., 1998, Calcaterra et al., 1999, de Riso et al., 1999, Onorati et al., 1999, Brancaccio et al., 2000, Calcaterra et al., 2000, Cascini et al., 2000, Chirico et al., 2000, Pareschi et al., 2000, Pareschi et al., 2002). In the highland areas of Sarno (Fig. 1) some 34 small drainage basins generated a series of debris flows, which killed more than 150 people in downstream villages (Pareschi et al., 2000). This event was exceptional due to the number of failures and human deaths, but debris flows are not unusual in the area (Di Crescenzo and Santo, 1998, de Riso et al., 1999). For instance, historic data show how at least 40 debris flow episodes have occurred in the last four centuries in the Sarno–Episcopio area, mainly during spring or autumn (Fig. 2; Migale and Milone, 1998). Previous research has assessed the debris flow hazard potential in southern Campania on the basis of geomorphological data (e.g. Amanti et al., 1999, Pareschi et al., 2000, Pareschi et al., 2002). At the moment, no precise data are available for the assessment of the destructive power of these debris flows on alluvial fan surfaces. For this purpose determination of the physical behaviour of these natural phenomena is a main aim for tracing hazard zonation and promoting mitigation plans.

This paper illustrates and discusses the results obtained in assessing volumes, fluid and flow densities, maximum discharge, velocity and impact pressure for some debris flows which occurred on May 5–6, 1998 in the Sarno area (Fig. 1) and their relationship to structural damages. A digital elevation model (DEM; map scale 1:13 000, step 2 m) of the study area was produced in order to improve morphologic and morphometric analyses (Fig. 1). Finally, these data were used to infer an empirical law to evaluate velocities and related dynamic overpressures useful for hazard zonation in similar geomorphological settings.

Section snippets

Geological and geomorphological setting

The study area is located in the southeastern part of the Campanian Plain (Fig. 1) and is centred on the highlands of Sarno, a marginal portion of the Apennines belt east of the Somma–Vesuvius volcano. The relief is made up of Meso–Cenozoic carbonate successions (Pescatore and Ortolani, 1973) and is bordered by NW–SE and SW–NE trending normal faults that originated steep slopes (Brancaccio et al., 2000).

Second- and third-order deep gullies form the drainage network, but several hollows

The May 5–6, 1998 event

The debris flows of May 5–6, 1998 were generated by shallow soil slips mainly originating near the drainage basin apex boundary (∼800–950 m a.s.l.; Fig. 1, Fig. 3), generally in correspondence with steep slopes (>30–35°; de Riso et al., 1999, Pareschi et al., 2000, Cascini et al., 2000). The main triggering mechanism was related to the failure of the volcaniclastic cover, which in the disrupted areas ranges in thickness between 0.5 and 2 m (Pareschi et al., 2000, Calcaterra et al., 2000,

Characteristics of the source materials and deposits

The physical and mechanical properties of the source soils, such as grain-size distribution, porosity, Atterberg limits, internal friction angle and cohesion, strongly influence the origin and development of soil slips/debris flows (e.g. Ellen and Fleming, 1987, Iverson et al., 1997, Crosta, 1998). In order to investigate these parameters we collected several undisturbed samples using a cylindrical manual corer (diameter 8 cm, height 20 cm) from both slide scarps and from within the debris flow

Physical characteristics of the May 5–6, 1998 debris flows

Although some parameters are difficult to obtain from deposits in the absence of direct measurement, many physical parameters of debris flows can be estimated empirically. The parameters assessed in this way give only a rough approximation of the actual behaviour of debris flows, but they can be reasonably assumed for flow parameterisation (e.g. Pierson, 1985, Costa, 1997).

Impact pressure

The huge destructive power of debris flows is due to the action of three main forces: (1) hydrodynamic (i.e. the dynamic overpressure due to the frontal impact, the drag effect exerted by the flow running along the sides of a structure, etc.); (2) hydrostatic, which accounts for the weight of the flow; (3) collisional, due to individual objects carried by the flow. These forces mainly depend on peak discharge, velocity, volumes, sediment–water ratio and grain-size distribution of debris flows.

Discussion and conclusions

The values of Pt obtained from the Eq. 17 can be compared to the damages induced by the May 5–6, 1998 debris flows in the study area. Such damages range from the complete collapse of buildings to the blowing down of doors and windows. Obviously, the response of structures at a given location to loading from flows depends in detail on many variables. These include the shape of the structure, its orientation relative to the overpressure, the number of openings in the structure, and whether it is

Acknowledgements

This work was supported by GNV D.L. 29/12/1995 as part of the project ‘Emergenza Idrogeologica del 5 Maggio 1998 territorio Campano’, sponsored by the Department of Civil Protection and by the GNDCI–CNR project for the years 1998, 1999, and 2000. We are grateful to A. Armanini and P. Scotton for useful suggestions. We thank T.C. Pierson and an anonymous reviewer for suggestions and comments that improved the quality of the manuscript. S. Ranieri is greatly acknowledged for supplying some of the

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