Morphological and climatic aspects of the initiation of the San Mango sul Calore debris avalanche in southern Italy

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1 Morphological and climatic aspects of the initiation of the San Mango sul Calore debris avalanche in southern Italy Guerriero Luigi 1, Revellino Paola 1, Diodato Nazzareno 1-2, Grelle Gerardo 1, De Vito Aldo 1, Guadagno Francesco Maria 1 1 University of Sannio, Department of Sciences and Technologies, 82100, Benevento, Italy 2 Met European Research Observatory, HyMex, GEWEX-Experiment (WCRP), 82100, Benevento, Italy Abstract: On the 10th November 2010 a high-velocity landslide occurred in the San Mango sul Calore municipality (Southern Italy). The event triggered from the North facing side of the Tuoro Mt. after a rainstorm, involved the pyroclastic and colluvial materials that covered part of the hill-slope. The debris avalanche destroyed an occupied house and damaged several service lines. Field surveys shown that it affected only the deforested part of the slope and its source area was located downslope a man-made cut. We analyzed rainfall data of the climatic station located about 1 km far from the debris avalanche at about 600 m above the sea level. The landslide occurred after about 63 hours of rainfall, after the rainstorm. The cumulative rain recorded during the three days storm was about 235 mm and the alert threshold of the rainstorm hazard index, has been exceeded. Keywords: debris avalanche, landslide initiation, rainstorm, pyroclastic deposit, Campania, Southern Italy 1. Introduction In Southern Italy, high-velocity landslides triggered by heavy rains are recurrent events in time. As an example, in May 1998 hundreds of landslides were triggered by a storm (Fiorillo et al. 2013) in the Sarno-Quindici area destroying houses and infrastructures and killing about 160 people (Del Prete et al. 1998); in December 1999 at Cervinara a debris avalanche following prolonged rainfall caused 6 deads and destroyed both houses and infrastructures (Fiorillo et al. 2001) in March 2005 a storm triggered a catastrophic landslide at Nocera Inferiore killing 3 people (Revellino et al. 2013).

2 2 On the 10th of November 2010 a landslide occurred at San Mango sul Calore (Campania Region, southern Italy, Fig.1). It partially destroyed an occupied house and damaged several service lines. It did not caused any casualties. The slope movement, classified as a debris avalanche (Hungr et al. 2001), was triggered by a rainstorm from the north-facing side of the Tuoro Mount and involved the pyroclastic air-fall material mantling part of the hill-slope. The source area was located just within a man-made cut as observed in many debris avalanches in Campania (Guadagno et al. 2005). The debris avalanche represent an example of rapid landslide which often occurred in Campania region involving the pyroclastic mantle after extreme rainfall. Most of these landslide events occur in pyroclastic deposits that cover the carbonate reliefs (Guadagno et al. 2011). Differently, this landslide event involved pyroclastic material that covered sandstone of the Castelvetere Formation. Our paper investigates morphological and climatic aspects of the debris avalanche triggering, and describe landslide features, geological setting and debrisavalanche effects. We based our description on field data acquired between December 2010 and January The meteorological event that triggered the debris avalanche was analyzed using rainfall data of the nearby meteorological station of San Mango sul Calore. 2. The Debris Avalanche The debris avalanche of the 10th November 2010 at San Mango sul Calore is located at about E and (UTM 33 N) along the northeastern side of the Tuoro Mountain at about 650 m above sea level (a.s.l). The mountainside is developed in SW-NE direction and is locally covered by weathered air-fall pyroclastic deposits emplaced during the so-called Eruption of Avellino (Rolandi et al. 1993). In the study area these deposits are composed mainly from ash and pumices. In the upper source area the thickness of the pyroclastic mantel reached almost 3 meters. We did not recognize a clear stratification of ash and pumices within the material exposed in the source area. The debris avalanche was triggered below a man-made track at 680 m a.s.l. (Fig.1). The event involved a calculated volume of 1500m 3 of weathered pyroclastic material, resulting as difference of DEMs. The DEM pre-event was obtained through a 5x5 m interpolation of a Triangular Irregular Network extracted from a 1:5000 numerical cartography. The DEM post-event has been obtained through a 3x3 m interpolation of a Triangular Irregular Network extracted from points collected during a Real Time Kinematic GPS survey carried out in December 2010 (Gili et al. 2000). We used the Kriging method (Oliver M.A. & Webster R. 2007) to execute the interpolation. The per-event DEM was resampled at 3x3 m in order to do the difference.

3 3 As shown in the map in figure 1 the debris avalanche was about 185 meters long, covering an area of 5500 m 2. The total elevation difference from the upper source area to the lower end was about 50 m and the slope angle ranged from about 30 to 10. Before the event, along the source area the slope angle was about 15. The maximum width and maximum length of the source area was 25 m and 30 m respectively. Differently from similar debris avalanches (Revellino et al. 2004), the event did not produce erosion along the path. This was because downslope the source area, where the slope angle reached about 30, the thickness of the pyroclastic mantel that covered the sandstone of the Castelvetere Formation was only few centimeters. This geological condition inhibited the erosion and the consequent increasing of the volume typical of debris avalanches (Hungr et al. 2001). Fig. 1 Debris avalanche map. The elevation and the scale distance are in meters. The debris-avalanche deposit covered an area of 3000 m 2. The maximum thickness was about 3 m; it was reached near the northern side of the damaged house, within the ramp going down to the garage that was filled from debrisavalanche material. We measured the thickness of the debris-avalanche deposit in some points (Fig. 1). In most of these points the thickness ranged from 15 to 35 cm. The runup equation (Rickenmann 1999) allowed to estimates the velocity of the debris avalanche on the base of the height of the splash on the house of the debris-avalanche material (Fig.2). The resulting velocity estimated was about 7.7 m/s. The debris avalanche destroyed the first level of the house located about 100 m downslope the track-way that it started from. The second level was damaged on

4 4 the south facing side. The impact of the material on the house produced a splash on the wall about 3 meters high. It reached almost the roof of the house. After the event the gate was found about 70 m downslope its original position near a chestnut root. It was probably ripped from the root transported during the event. The Cesine local road was completely covered by the debris-avalanche material. Within the involved track-way several service line were damaged from the debris avalanche. 3. Debris avalanche initiation The debris avalanche was triggered in the early morning of the 10th of November 2010 after about 63 hours of rainfall. As in most of the cases of landslide event in Campania involving pyroclastic deposit (e.g. Revellino et al. 2004, 2008), the debris avalanche was triggered from a track-way. Only where the debris avalanche occurred, the track-way was cut in pyroclastic deposit. As widely discussed in literature, the presence of man-made cut influence the stability of slopes. In particular Guadagno et al (2003) demonstrated using numerical analysis, that track-way and geomorphological discontinuities had a negative impact on the stability of slopes. So, the track-ways were an important predisposing factor. During field survey, we observed that the source area was located in a morphological convergence area. An important amount of rain fell on the slope and on the track-way too, tended to move to the source area, infiltrating there. The local morphological condition of the track-way induced a forced infiltration immediately upslope the source area. Also, the sector of the slope affected from the debris avalanche has been deforested several years before the event. The debris avalanche involved only the deforested part of the slope. We analyzed rainfall data of the climatic station located about 1 km far from the debris avalanche at about 600 m a.s.l. The landslide moved after about 63 hours of rainfall at the end of the storm. The cumulative rain recorded during the storm was about 235 mm. In the three days of rain, the alert threshold of the rainstorm hazard index of Diodato et al. (2012), was exceeded. Taking into account the cumulated rainfall of the previous 30 days, we observed that only in the day of the debris avalanche occurrence both the rainstorm hazard index and the cumulated antecedent rainfall exceeded together the threshold values. This means that the possibility that an extreme rain event triggers a landslide depend from its intensity and also from its duration and from the total amount of rain fell in the previous 30 days. At this location the landslide was triggered as soon as the threshold of 400 mm of rain in 30 days was exceeded. The cumulative amount of rain fell during the rain event reached about 235 mm. From 1:00 AM to 6:00 AM on November 10, about 82 mm of rain fell. The day before the event, November 9, about 145 mm of rain fell. In the Avellino

5 5 province this climatic event caused many problems. A lot of families had to flee their flooded homes and many landslides occurred Rainstorm Hazard Index x 10 1 Alert Daily Precipitation (mm) 0 1-Oct 11-Oct 21-Oct 31-Oct 10-Nov 20-Nov 30-Nov Date (Year 2010) Fig. 2 Daily rainstorms depth (blue bars), simulated Rainstorm Hazard Index (orange bars) and 30-days cumulated rainfall (violet curve), during the 1 October 30 November Landslide activation (arrow) on the 10 November and thresholds of 1 (Alert in dashed line) are drawn too. 4. Conclusion The debris avalanche occurred at San Mango sul Calore, triggered in early morning of November 10, 2010, after an extreme meteorological event. Field survey showed that the geologic and morphologic conditions of the slope arrange in advance landslide occurrence. In particular, the presence of a localized thick-layer of pyroclastic material, the presence of a man-made cut and the deforestation operated before the event, concur to debris avalanche occurrence. Analysis of climatic data showed that the debris avalanche was triggered after 63 h of rainfall. During this meteorological event the alert threshold of the rainstorm hazard index was exceeded but the debris avalanche triggered only when the threshold of 400 mm of rain in 30 days was exceeded. This means that the possibility that an extreme rain event trigger a landslide depend from its intensity and also from its duration and from the total amount of rain fell in the previous 30 days.

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