Unmanned Aerial Vehicle (UAV) time-lapse monitoring of the instability processes affecting Varano Hill: A case study of the ancient Roman site of Villa Arianna

The archaeological site of Villa Arianna - located on Varano Hill, south of Vesuvius - offer tantalizing information regarding first-century AD resilience to hydrogeological risk. Additionally, the site provides an important test case for mitigation efforts of current and future geo-hazard. Villa Arianna, notable in particular for its wall frescoes, is part of a complex of Roman villas built between 89 BC and AD 79 in the ancient coastal resort area of Stabiae. This villa complex is located on a morphological terrace that separates the ruins from the present-day urban center of Castellammare di Stabia. The Varano hill is formed of alternating pyroclastic deposits, from the Vesuvius Complex, and alluvial sediments, from the Sarno River. The area, in AD 79, was completely covered by PDCs from the Plinian eruption of Vesuvius. Due to the geomorphological structure the slope is prone to slope instability phenomena that are mainly represented by earth and debris flows, usually triggered by heavy rainfall. The susceptibility is worsened by changes in hydraulic and land-use conditions mainly caused by lack of maintenance of mitigation works. Villa Arianna is the subject of a joint pilot project of the INGV-ENEA-ISPRA that includes non-invasive monitoring techniques such as the use of UAVs to study the areas of the slope at higher risk of instability. The project, in particular, seeks to implement innovative mitigation solutions that are non-destructive to the cultural heritage. UAVs represent the fastest way to produce high-resolution 3D models of large sites and allow archaeologists to collect accurate spatial data that can be used for 3D GIS analyses. Through this pilot project, we have used detailed 3D models and high-resolution ortho-images for new analyses and documentation of the site and to map the slope instabilities that threatens the Villa Arianna site. Through multi-temporal analyses of different data acquisitions, we intend to define the detailed morphological evolution of the entire Varano slope. These analyses will allow us to highlight priority areas for future low-impact mitigation interventions.