Elsevier

Geomorphology

Volume 66, Issues 1–4, 1 March 2005, Pages 255-276
Geomorphology

Debris slides–rapid earth flows in the carbonate massifs of the Campania region (Southern Italy): morphological and morphometric data for evaluating triggering susceptibility

https://doi.org/10.1016/j.geomorph.2004.09.015Get rights and content

Abstract

This paper examines the morphological and morphometric properties of debris slides–rapid earth flows which have occurred in pyroclastic deposits of the Campania region over the last few centuries. The slopes bordering the main carbonate massif of western Campania are covered by several metres of pyroclastic deposits from the Somma-Vesuvio volcano that, due to the steepness of the slopes, are easily eroded by surface waters and periodic debris slides–rapid earth flows. These flows are sudden and unpredictable, and have caused both damage to the environment and loss of life over the last few decades.

After consulting scientific papers and historical documents, the areas most recently affected by debris slides–rapid earth flows were analysed through aerial photographs and a geological survey. An inventory of 172 landslides was produced, and landslides were mapped at a scale of 1:2000 for further geomorphological and morphometric analysis.

Morphological analysis was primarily concerned with predisposing factors; it confirmed that 86% of the slope failurecrowns developed in response to man-made cuts and tracks (44%), or due to the presence of cliffs (42%).

Two different kinds of slopes are affected by debris slides–rapid earth flows: slopes lacking drainage basins and slopes cut by one or several drainage basins. Typical triangular-shaped earth flows with a landslide mass located at the foot of the slope (unchannelled debris slides–rapid earth flows) were found along the former slopes. In contrast, channelled debris slides–rapid earth flows formed along the latter slopes. These originated in the drainage divide area, developed along the main water segment and invaded the alluvial plain far from the foot of the slope. An intermediate group of flows started as triangular shaped and then became channelled.

Morphometric analysis also investigated the parameters most commonly found in the literature (slope angle of the crown and sliding zone, crown altitude, areal extent of the landslide), together with other parameters (apical angle of the crown zone and landslide body, slope relief energy and potential zone of triggering).

This analysis identified critical values for the steepness of crown zones and for the steepness of the sliding zone. Critical values of the apical angle of the crown zone for mixed and unchannelled landslides were also established. Correlations among some of the investigated parameters were investigated; in particular, the relationships between slope relief energy and areal extent of landslides or the height of the landslide crown zone.

All the factors and parameters analysed in this paper can be used to identify areas which need to be stabilized or monitored, and could be important for planning defensive measures of debris slides–rapid earth flows.

Introduction

In the last few centuries, there have been many debris slides–rapid earth flows in the pyroclastic deposits that cover the steep slopes bordering the main carbonate massif of the western Campania region (Southern Italy).

These landslides were initially triggered by small falls or slides; they subsequently evolved through a slide and an amplification phase due to an “avalanche” effect, and lastly, a rapid flow and accumulation phase took place (Di Crescenzo and Santo, 1998, Di Crescenzo and Santo, 1999). Characterized by high velocity and fluidity, the flows determine highly hazardous situations. They have been widely studied from an applied geology, geomorphological, hydrogeological and geotechnical perspective.

In the literature, mass movements such those under investigation, involving a mixture of water and sediments, are defined as “mud flows”, “hyperconcentrated flows”, “granular flows”, and “debris flows” (Caine, 1980, Guida et al., 1986, Celico et al., 1986, Guadagno, 1991, Takahashi, 1991, WP/WLI, 1993, Pellegrino, 1994, Canuti and Esu, 1995, Cruden and Varnes, 1996, Dikau et al., 1996, Corominas and Moreno, 1998). This suggests that there is little general agreement on how to model and classify such processes. Some researchers (Pierson and Costa, 1987, Costa, 1988, Pierson, 1989, Coussot and Meunier, 1996) classify flows according to their rheological behaviour and therefore their “water/sediment” ratio, whereas others (Johnson, 1970, Johnson and Rodine, 1984, Fleming et al., 1989) consider “debris flow”a generic term for all the different types of flow regardless of the “water/sediment” ratio.

Di Crescenzo and Santo, 1998, Di Crescenzo and Santo, 1999 show that debris slides–rapid earth flows in Campania are always characterized by a limited area where detachment first occurs, a second area where translational sliding occurs and in which the landslide is greatly amplified, and finally, an area of channelization, flow and accumulation. The different phases occur in quick succession; because a rapid earth flow generally quickly follows the initial fall-slide phase, it would be more appropriate to speak of complex “debris slides–rapid earth flows”.

This paper assesses all published data on these debris slides–rapid earth flows and through new “allometric” studies (sensu Gould, 1966, Bull, 1975, Govi, 1977, Church and Mark, 1980, Pasuto et al., 1992), aims at identifying factors, especially morphometric indicators, which can be used to define potential zones of triggering. These parameters can be easily determined and can therefore greatly contribute to landslide hazard studies.

Section snippets

Methods

After consulting scientific papers and historical documents, the areas most recently affected by debris slides–rapid earth flows were analysed by means of aerial photo interpretation and a detailed geological survey. All landslides longer than 300 m which occurred in Campania in the last century (only a few date back to 1770 and 1800) were investigated. Among them, 172 landslides (Table 1) were selected for further geomorphological and morphometric analysis and were mapped at a 1:2000 scale.

General setting

The study area is located on the Tyrrhenian coast of Campania where, during Plio-Quaternary times, important regional faults linked to the extension of the Tyrrhenian area, defined a major tectonic depression named the “Campania graben”. The structural heights delimiting this graben consist of the carbonate peaks of the Sorrento Peninsula–Lattari Hills ridge, the Partenio group Hills, the Caserta Hills, of Pizzo D'Alvano and Mt. Maggiore (Fig. 5). These peaks consist of more than 1500-m-thick

Predisposing factors

The geological and geomorphological survey evidenced a repetitiveness of events along road cuts and rocky cliffs, suggesting the need for a detailed inventory of their number and location (Guadagno and Perriello Zampelli, 2000). As seen in Fig. 7a, about 86% of the 172 investigated landslides developed above or below roads/tracks and rocky cliffs. In particular, the predisposing factor for most channelled landslides was the presence of rocky cliffs, whereas there is a substantial balance in the

Conclusions and perspectives

The analysed landslides are always characterized by a limited area where detachment first occurs, a second area where translational sliding occurs and in which the landslide is greatly amplified, and finally, an area of channelization, flow and accumulation. The different phases occur in quick succession. Because a rapid earth flow generally quickly follows the initial fall or slide phase, it would be more appropriate to speak of complex “debris slides–rapid earth flows”. The study of 172

Acknowledgements

Authors would like to thank the anonymous reviewers, the Guest Editors, Prof. R. de Riso and Prof. N. Santangelo for their stimulating and useful advices.

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