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

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.