Use of unmanned aerial vehicles in monitoring application and management of natural hazards

ABSTRACT The recent development of unmanned aerial vehicles (UAVs) has been increasing the number of technical solutions that can be used to monitor and map the effects of natural hazards. UAVs are generally cheaper and more versatile than traditional remote-sensing techniques, and they can be therefore considered as a good alternative for the acquisition of imagery and other physical parameters before, during and after a natural hazard event. This is an important added value especially for investigations over small areas (few km2). In the special issue ‘The use of Unmanned Aerial Vehicles in monitoring application and management of natural hazards’, we collected a number of case studies, aiming at providing a range of applications of monitoring and management of natural hazards assessed through the use of UAVs.


Introduction
In recent times, the use of spaceborne and airborne remote-sensing data has become a common practice to study natural hazards. This evolution has been mainly triggered by the increased availability of up-to-date imagery, as well as by the development in geospatial technologies (Joyce et al. 2009;Remondino 2011). These data have demonstrated to play an important role in the investigation of catastrophic natural events such as floods, earthquakes, landslides, subsidence, etc. The different acquisition modes and the capability to obtain high spatial and temporal resolutions allow deriving detailed information on landscape evolution over wide areas, and are an effective and complementary tool to field investigations.
In this context, unmanned aerial vehicles (UAVs) offer unprecedented spatial resolution and new mapping opportunities at local scales (Boccardo et al. 2015), where the survey covers few square kilometres and the use of aerial or satellite platforms could be considered too expensive. In addition, UAVs present various advantages, including: (1) the ability to fly at low altitudes (less than 150 m above ground level), (2) the ability to reach remote locations and capturing high resolution images, (3) the capability to host different sensors (cameras, laser scanners, navigation/inertial sensors, etc.), (4) the possibility to acquire imagery with different angles and (5) the flexibility of carrying out small, medium and large scale monitoring operations. Also for these reasons, UAV platforms are commonly used for assistance and management of emergencies. For example, UAVs can rapidly provide information on collapsed buildings after an earthquake, help to evaluate structural damages CONTACT D. Giordan daniele.giordan@irpi.cnr.it and to perform early impact assessments (Murphy et al. 2008;Pratt et al. 2009;Chou et al. 2010;Molina et al. 2012). This special issue is a collection of the contributions presented during a session hosted at the European Geoscience Union meeting in 2015. Several applications of UAVs in natural hazard contexts are presented, following three main phases of the hazard documentation and monitoring: preand post-event data acquisition, emergency support and monitoring. The articles focus on: (1) data acquisition of pre-and post-environmental/geomorphological events; (2) operational support during emergencies due to a catastrophic event and (3) monitoring of damages in critical infrastructures.

Acquisition of pre-and post-event data-set
As previously mentioned, one of the most useful characteristic of UAVs is the possibility to acquire on demand a data-set of a limited area (Koeva et al. 2016). This is particularly relevant when these systems are adopted to gain valuable information over a particular environment and/or geomorphological process. In geohazards, the effects of the evolution of a geological/geomorphological process can be often achieved by the comparison of pre-and post-event information in the study area. In the past, this approach was often supported by the use of terrestrial (e.g. Baldo et al. 2009 and references therein) or airborne LiDAR (e.g. Nissen et al. 2012 and references therein), but the introduction of UAV represents nowadays a valuable alternative for a multi-temporal acquisition of data-sets that can be used for the study of natural hazards. In this special issue, several applications in the field of geological mapping are presented and discussed focusing on the fractures' identification of marble quarries (Salvini et al. 2016), the geological mapping of mountain areas ) and the identification of interseismic shallow deformations (Deffontaines et al. 2016). Hydrogeological applications are instead related to the identification of subfluvial springs , measurements of open channel water surface velocities (Bolognesi et al. 2016) and measurement of flushed sediment in a reservoir (Pagliari et al. 2016).

Support during emergencies
The second typology of applications considered in this special issue is the use of UAV during emergencies. The use of these systems for supporting the management of emergencies can be critical particularly when the meteorological conditions are unfavourable. In this special issue, an application of UAVs for search and rescue operations for missing people in a natural environment (Jurecka & Niedzielski 2016) and a multipurpose UAV for mountain rescue operations (Silvagni et al. 2016) are presented. Jurecka and Niedzielski (2016) demonstrate that the use of UAVs for quick and reliable localization of lost persons in natural areas is a suitable approach. The application is based on the concept of a crow's flight distance travelled by a lost person and its probability distribution. Instead, Silvagni et al. (2016) present a multi-rotors flying platform and its embedded avionics designed to meet environmental requirements for mountainous terrain such as low temperatures, high altitude and strong winds. The system is able to host different payloads (separately or together) such as: (1) avalanche beacon (i.e. the ARTVA) with automatic signal recognition and path following algorithms for the rapid location of snow-covered body; (ii) cameras (visible and thermal) for search and rescue of missing persons on snow and in woods during day or night light conditions.

Monitoring damages of infrastructures
The last typology of applications considered is the use of UAV to monitor damage at infrastructures. The use of UAV for 3D reconstruction of anthropic structures, and in particular for monuments, represent the first applications developed and published during the last decade (Çabuk et al. 2007;Lambers et al. 2007;Sauerbier & Eisenbeiss 2010;Remondino et al. 2011). In this special issue, Dominici et al. (2016) presented their experience after L'Aquila earthquake (occurred in central Italy in 2009), by considering the identification and evaluation of damages of several historical settlements. Another case study is presented by Lazzari and Gioia (2017) and refers to the study of the effects of gravitational processes on the Uggiano castle, a highly degraded medieval archaeological site located in Basilicata (southern Italy).

Conclusion
The aim of this special issue is to present, through different case studies, different UAV-based methodologies in operative conditions for the assessment and monitoring of natural hazard scenarios and effects. Three main phases have been considered in the reported articles, where UAVs support the documentation and management of natural hazards: pre-and post-event data acquisition, emergency support and monitoring. Existing approaches are very promising and many researchers are continuously developing innovative solutions in this direction.