Casagli, Nicola

Falorni, Giacomo

Farina, Paolo

Lombardi, Luca

Nocentini, Massimiliano

Righini, Gaia

Tofani, Veronica

Vannocci, Pietro

English

Florence Research

Geophysical Research Abstracts, Vol. 9, 03286, 2007SRef-ID: 1607-7962/gra/EGU2007-A-03286© European Geosciences Union 2007Geotechnical investigation and dynamic modelling ofthe 30 April 2006 debris flows on Ischia IslandN. Casagli (1), M. Nocentini (1), G. Falorni (1), P. Farina (1), L. Lombardi (1), G.Righini (1), V. Tofani (1), P. Vannocci (1)(1) Department of Earth Sciences, University of Florence (nicola.casagli@unifi.it)Ischia is an active volcanic island located in the Tyrrhenian Sea, approximately 30 kmWSW from the city of Naples in Southern Italy. On 30 April 2006, following severalhours of rainfall, four small soil slips-debris flows were triggered on the slopes of Mt.Vezzi (ca. 400 m a.s.l.), in the SE portion of the island. Notwithstanding their modestsize, the flows caused the deaths of 4 people, forced the evacuation of another 250 in-habitants and destroyed several buildings. These types of events are aggravated by thelargely uncontrolled urban expansion on the island, with buildings constructed at thebase of steep slopes and the transformation of many stream channels into access roadsleading to the buildings. Coupled with the periodical recurrence of soil slip - debrisflow type events, the entire island is exposed to high risk conditions. The steep slopesof Mt. Vezzi are conditioned by the volcanic activity of the last 10.000 years: a 1 - 2m thick soil derived from the weathering of pyroclastic material overlies thick falloutdeposits produced by the last eruptions. The debris flows initiated as soil slips at thesoil - fallout interface, in hollows with slope gradients ranging between 35◦ - 40◦,and quickly transformed into flows that reached the floodplain at the base of the hill,finally coming to a stop in a low gradient road. The flows started at elevations rangingbetween 310 and 350 m and stopped at approximately 80 m, corresponding to travelangles of 20-24◦. The volumes of the single events ranged from 2000 - 5000 m3. Twofield campaigns carried out immediately following the events were focused on collect-ing geotechnical data in the source area (angle of internal friction, unit weight, watercontent, grain size, etc.). Material shear strength was determined both in the labora-tory on reconstituted samples and in-situ by means of Borehole Shear Tests (BST)coupled with a tensiometer for the measurement of matric suction. Soil strength wasassessed both in partially and totally saturated conditions. Soil hydraulic conductiv-ity was quantified by means of in-situ constant head permeameter tests. These high-lighted a marked contrast in permeability between the soil and the underlying volcanicash layer. Saturated hydraulic conductivity values ranged from 1.2 x 10-5 m/s in theformer to 6.2 x 10-8 m/s in the latter. Soil slip triggering conditions were analyzedon the basis of precipitation data related to the 24 hour period prior to the event us-ing a coupled seepage-stability analysis. Back-analyses of the debris flows were usedto perform dynamic modelling of the event. DAN-W and FLO-2D were the modelsselected for this purpose. Calibration was based on deposit thickness and areal exten-sion, and on flow runout and velocity. This data was used to model potential events thesource areas of which were identified from fieldwork in order to delineate inundationareas, velocities and deposit thicknesses. The rheological parameters derived from themodelling were comparable to those obtained by Revellino et al (2004) for the Sarnodebris flows of May 1998, events which are quite similar to those observed at Ischia.

Geotechnical investigation and dynamic modelling of the 30 April 2006 debris flows on Ischia Island

https://flore.unifi.it/bitstream/2158/385150/1/Casagli%20et%20al%20GRA%20vol%209%20EGU%202007.pdf

Geotechnical investigation and dynamic modelling of the 30 April 2006 debris flows on Ischia Island

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