Subaerial and subaqueous dynamics of coastal rockfalls Fabio Vittorio De Blasio a, ⁎, Paolo Mazzanti b a Department of Geosciences, University of Oslo, P.O. Box 1047 Blindern, N-0316 Oslo, Norway b Department of Earth Sciences, University of Rome “Sapienza”, P.le Aldo Moro 5, 00185, Roma, Italy abstract article info Article history: Received 30 April 2009 Received in revised form 1 September 2009 Accepted 3 September 2009 Available online 12 September 2009 Keywords: Rockfalls Coastal rockfalls Mixed Rockfall (MRF) Scilla-Palmi coast Lake Albano In spite of the hazard represented by rockfalls in coastal areas, few investigators have studied the movement of rocks falling in water. This work describes a model for the propagations of coastal rockfalls, i.e., blocks detaching from a subaerial cliff, propagating initially in air, impacting against the water surface, and finally coming to rest in the water basin. Application of the model to two real cases in Italy shows a satisfactory agreement between the predicted rockfall run-out and the field data. Some scattering observed in the data is explained as the consequence of different heights of the source area. It is shown that largest boulders usually reach a longer distance. However, the schematic shape for blocks adopted in the model likely results in a much more regular behaviour compared to reality. © 2009 Elsevier B.V. All rights reserved. 1. Introduction Landslides starting subaerially and ending in a water basin represent a peculiar class of mass wasting where the characters of subaerial and subaqueous movements are both important. Described cases are limited but relatively well studied. They include subaerial slides plunging into fjords (Longva et al., 2003), lakes (Mazzanti et al., 2007; Bozzano et al., 2009) or the sea (Chiocci et al., 2008; Bozzano et al., 2008; Mazzanti, 2008). The mechanical problems posed by this kind of events are fairly novel and have relevant applications. The impact with water is capable of generating particularly high tsunamis compared to entirely subaqueous slides (Miller, 1960; Harbitz et al., 1993; Mazzanti, 2008). Along this research line, the present paper addresses a smaller-scale problem, i.e., the dynamics of isolated blocks detaching from a subaerial cliff, impacting against a water surface, and finally coming to rest in the water basin. Due to the relevant threat represented by rockfalls in mountain areas, many models have been developed that simulate rocks falling subaerially (Bozzolo and Pamini, 1986; Pfeiffer and Bowen, 1989; Giani, 1992; Evans and Hungr, 1993; Azzoni and De Freitas, 1995; Jones et al., 2000; Guzzetti et al., 2002; Dorren, 2003). Studies addressing the rockfall hazard in open sea structures or ships have been restricted to the subaerial trajectory only (Crosta et al., 2007). Surprisingly, few studies have considered the rockfall problem when the end point of blocks occurs in water, accounting also for the submerged trajectory of the block. A better understanding of the subaqueous dynamics is highly desirable at least from three view- points. Firstly, the subaqueous deposits from rockfalls may reach in some cases relevant volumes; this makes the problem of the subaqueous blocks distribution of interest for sedimentology and geomorphology. Secondly, the documented cases of rockfalls in the sea, fjords or lakes may put subaqueous structures at risk (Beranger et al., 1998). Finally, the dynamics of a block moving in water and the impact against the water surface are motivating problems in themselves, relevant for geophysics and fluid mechanics. Turmel and Locat (2007) have preliminarily estimated the forces acting on a block moving in water without calculating the block trajectories. These authors focused only on the subaqueous part, thus ignoring the impact with water. In this paper we present a model for rockfalls starting subaerially, plunging into a water basin, and then moving in water. For brevity we will call Mixed Rockfall (MRF) the event comprised between the detachment of the block on land to the final stop in the water basin. There are many physical issues involved in the study of MRFs. In addition to the propagation of the rock in air, addressed in the mentioned investigations, in the watery phase both lift and drag forces become important. The impact with the bottom and the rolling friction along the subaqueous floor may also disclose a different character from the subaerial case due to the different types of sediment and the saturation conditions of the soil. Moreover, the impact with the water surface will deprive a block of part of its kinetic energy. Specifically, we calculate the forces acting on blocks of various sizes and integrate the equation of motion to calculate their velocity and trajectory. We predict trajectories and run-outs for two actual cases: the Lake Albano in central Italy, and the sea offshore Scilla in southern Italy. Geomorphology 115 (2010) 188–193 ⁎ Corresponding author. E-mail address: fvblasio@geologi.uio.no (F.V. De Blasio). 0169-555X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.geomorph.2009.09.024 Contents lists available at ScienceDirect Geomorphology journal homepage: www.elsevier.com/locate/geomorph