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8 maltatoday | SUNDAY • 1 MAY 2022 NEWS PHOTOGRAPHY DWEJRA.NET AN interdisciplinary group of researchers has completed a study of the underwater remains of the iconic Tieqa tad-Dwejra, or as it was commonly known, the Azure Window, which collapsed on 8 March, 2017, and has since become a popular diving site. The aim of the work, led by Prof. Jo- seph Caruana from the Department of Physics and Institute of Space Scienc- es and Astronomy of the University of Malta, was to gain a better understand- ing of the collapse event, and to record the present state of the site underwater, which is subject to continued change via erosion. Arches and stacks are very popular at- tractions around the world. However, the dynamics of stack formation and evolution remain poorly constrained. In the case of the Azure Window, the stack – consisting of the base that supported the pillar above sea level - is completely submerged, which presented an addi- tional challenge to its study. The peer-reviewed work, published in the academic journal Geomorphology, was carried out over nearly three years. "It is very unlikely that someone would witness an arch collapse at the mo- ment it happened, partly because such an event is bound to take place during rough weather, when fewer people are present on site. This means that a bet- ter understanding of such an event has to come from patient study of the sub- merged remains, mapping their loca- tion and measuring their sizes to enable a reconstruction of the collapse," Prof. Caruana said. This required multiple dives by a team of technical divers, led by Prof. Carua- na and John Wood. The team surveyed the site, taking thousands of high-res- olution photographs and carrying out measurements underwater. A detailed 3D-model of the seabed and the re- mains was then constructed via a tech- nique called photogrammetry. "The idea to carry out this study real- ly came up right as soon as the window collapsed. We knew the site very well – both above and underwater – and this event presented an opportunity to study the submerged remains, addressing in detail the question of how the col- lapse transpired, particularly through an analysis of the existing stack and the final resting place of several compo- nents," Prof. Caruana said. The work contributes insight into how such events occur, and helps assessing potential hazards, specifically in rela- tion to the prediction of coastal arch collapse. Using a purpose-built setup, with cameras and lights mounted on an un- derwater scooter, the researchers sur- veyed the entire site, which approxi- mately measures 8,000 square metres. They were able to confirm that erosion at the base of the pillar resulted in the formation of a notch on its north-facing side. "The base weakened to the extent that the pillar eventually gave way, falling to- wards the southwest and breaking into two main sections that separated along the lithological boundary – the parti- tion delineating two different types of rock. Large sections from atop the pillar broke along existing joints in the rock, remaining relatively intact. The bridge, which used to see many people crossing it, collapsed vertically downwards up- on losing the support of the pillar, and fragmented," Prof. Caruana said. The work required undertaking long dives using rebreathers, a type of div- ing apparatus that minimises both the volume of gases used and the amount of decompression that is required. The latter meant that the divers could de- vote more of their time underwater to carrying out the required work, while reducing the duration of the slow as- cent back to the surface. John Wood, who led the photogram- metry effort, described how the pro- ject entailed several unique challeng- es: "The area covered by the Azure Window's remains is many times the size of the large sites I had worked on previously, which include some of the largest shipwrecks in Maltese waters. This entailed multiple visits to the site, each visit focussing on data capture of a separate, relatively small area. This approach resulted in further complex- ities and very long processing times to build the 3D-model itself." An added challenge was the depth range to be covered. Spanning from just below the surface down to 60 me- tres, this made some specific dives technically quite challenging, Wood said. The study also documented chang- es at the site during the time that has elapsed since the collapse. Certain parts are still experiencing erosion, and particularly unstable sections have since collapsed as well. A 3D-model will now serve as a reference point: any future surveying can be compared against the present state of the site as recorded through this model, enabling the documentation of the site's trans- formation over time. Prof. Caruana said the 3D-model can also serve as a guide for the many di- vers that visit the site every year to ex- plore the Azure Window's remains. "It also allows the non-diving pub- lic to view the underwater site, which forms part of the islands' underwater Azure Window underwater study reconstructs collapse

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