Stability Assessment and Geomorphological Evolution of Sea Natural Arches by Geophysical Measurement: The Case Study of Wied Il-Mielah Window (Gozo, Malta)

The Wied il-Miela? Window (Gozo-Malta) is a limestone natural arch on thenorth-western coast of the island of Gozo in Malta. It is located at the end of the Wied il-Miela?valley north of the village of G?arb. This natural arch is less well known than the Azure Window,which collapsed in March 2017 following a heavy storm, but notwithstanding, it is an imposingand important natural monument too. In the past, the Wied il-Mielah valley was responsible fordischarging wastewater from the surrounding localities to the Mediterranean directly at the Wiedil-Mielah Window. The sewage flag was often clearly visible underneath the archway into the opensea. The natural features of the arch provide an outstanding touristic attraction. To avoid whathappened to the Azure Window, a methodology for the evaluation of the collapse hazard, combiningpassive seismic, ground penetrating radar (GPR), geological/geomorphological surveys andmine engineering methods, is here proposed. In this study, a methodological approach was applied,based on the following: (i) passive seismic method to study the physical-mechanical characteristicsof the rock mass that constitutes the window; (ii) GPR method in order to demonstratethe conservation state (i.e., the intensity of fracturing); (iii) geological/geomorphological surveys inorder to obtain a crack pattern; and (iv) scaled span empirical analysis in order to evaluate the stabilityof the arch. The calculation of the safety factor, with a static method, gave a value equal to3.75 with a probability of collapse of the marine arch within 50 and 100 years.